/*
 * Recursive Merge algorithm stolen from git-merge-recursive.py by
 * Fredrik Kuivinen.
 * The thieves were Alex Riesen and Johannes Schindelin, in June/July 2006
 */
#include "cache.h"
#include "config.h"
#include "advice.h"
#include "lockfile.h"
#include "cache-tree.h"
#include "commit.h"
#include "blob.h"
#include "builtin.h"
#include "tree-walk.h"
#include "diff.h"
#include "diffcore.h"
#include "tag.h"
#include "unpack-trees.h"
#include "string-list.h"
#include "xdiff-interface.h"
#include "ll-merge.h"
#include "attr.h"
#include "merge-recursive.h"
#include "dir.h"
#include "submodule.h"

struct path_hashmap_entry {
        struct hashmap_entry e;
        char path[FLEX_ARRAY];
};

static int path_hashmap_cmp(const void *cmp_data,
                            const void *entry,
                            const void *entry_or_key,
                            const void *keydata)
{
        const struct path_hashmap_entry *a = entry;
        const struct path_hashmap_entry *b = entry_or_key;
        const char *key = keydata;

        if (ignore_case)
                return strcasecmp(a->path, key ? key : b->path);
        else
                return strcmp(a->path, key ? key : b->path);
}

static unsigned int path_hash(const char *path)
{
        return ignore_case ? strihash(path) : strhash(path);
}

static struct dir_rename_entry *dir_rename_find_entry(struct hashmap *hashmap,
                                                      char *dir)
{
        struct dir_rename_entry key;

        if (dir == NULL)
                return NULL;
        hashmap_entry_init(&key, strhash(dir));
        key.dir = dir;
        return hashmap_get(hashmap, &key, NULL);
}

static int dir_rename_cmp(const void *unused_cmp_data,
                          const void *entry,
                          const void *entry_or_key,
                          const void *unused_keydata)
{
        const struct dir_rename_entry *e1 = entry;
        const struct dir_rename_entry *e2 = entry_or_key;

        return strcmp(e1->dir, e2->dir);
}

static void dir_rename_init(struct hashmap *map)
{
        hashmap_init(map, dir_rename_cmp, NULL, 0);
}

static void dir_rename_entry_init(struct dir_rename_entry *entry,
                                  char *directory)
{
        hashmap_entry_init(entry, strhash(directory));
        entry->dir = directory;
        entry->non_unique_new_dir = 0;
        strbuf_init(&entry->new_dir, 0);
        string_list_init(&entry->possible_new_dirs, 0);
}

static struct collision_entry *collision_find_entry(struct hashmap *hashmap,
                                                    char *target_file)
{
        struct collision_entry key;

        hashmap_entry_init(&key, strhash(target_file));
        key.target_file = target_file;
        return hashmap_get(hashmap, &key, NULL);
}

static int collision_cmp(void *unused_cmp_data,
                         const struct collision_entry *e1,
                         const struct collision_entry *e2,
                         const void *unused_keydata)
{
        return strcmp(e1->target_file, e2->target_file);
}

static void collision_init(struct hashmap *map)
{
        hashmap_init(map, (hashmap_cmp_fn) collision_cmp, NULL, 0);
}

static void flush_output(struct merge_options *o)
{
        if (o->buffer_output < 2 && o->obuf.len) {
                fputs(o->obuf.buf, stdout);
                strbuf_reset(&o->obuf);
        }
}

static int err(struct merge_options *o, const char *err, ...)
{
        va_list params;

        if (o->buffer_output < 2)
                flush_output(o);
        else {
                strbuf_complete(&o->obuf, '\n');
                strbuf_addstr(&o->obuf, "error: ");
        }
        va_start(params, err);
        strbuf_vaddf(&o->obuf, err, params);
        va_end(params);
        if (o->buffer_output > 1)
                strbuf_addch(&o->obuf, '\n');
        else {
                error("%s", o->obuf.buf);
                strbuf_reset(&o->obuf);
        }

        return -1;
}

static struct tree *shift_tree_object(struct tree *one, struct tree *two,
                                      const char *subtree_shift)
{
        struct object_id shifted;

        if (!*subtree_shift) {
                shift_tree(&one->object.oid, &two->object.oid, &shifted, 0);
        } else {
                shift_tree_by(&one->object.oid, &two->object.oid, &shifted,
                              subtree_shift);
        }
        if (!oidcmp(&two->object.oid, &shifted))
                return two;
        return lookup_tree(&shifted);
}

static struct commit *make_virtual_commit(struct tree *tree, const char *comment)
{
        struct commit *commit = alloc_commit_node();

        set_merge_remote_desc(commit, comment, (struct object *)commit);
        commit->maybe_tree = tree;
        commit->object.parsed = 1;
        return commit;
}

/*
 * Since we use get_tree_entry(), which does not put the read object into
 * the object pool, we cannot rely on a == b.
 */
static int oid_eq(const struct object_id *a, const struct object_id *b)
{
        if (!a && !b)
                return 2;
        return a && b && oidcmp(a, b) == 0;
}

enum rename_type {
        RENAME_NORMAL = 0,
        RENAME_DIR,
        RENAME_DELETE,
        RENAME_ONE_FILE_TO_ONE,
        RENAME_ONE_FILE_TO_TWO,
        RENAME_TWO_FILES_TO_ONE
};

struct rename_conflict_info {
        enum rename_type rename_type;
        struct diff_filepair *pair1;
        struct diff_filepair *pair2;
        const char *branch1;
        const char *branch2;
        struct stage_data *dst_entry1;
        struct stage_data *dst_entry2;
        struct diff_filespec ren1_other;
        struct diff_filespec ren2_other;
};

/*
 * Since we want to write the index eventually, we cannot reuse the index
 * for these (temporary) data.
 */
struct stage_data {
        struct {
                unsigned mode;
                struct object_id oid;
        } stages[4];
        struct rename_conflict_info *rename_conflict_info;
        unsigned processed:1;
};

static inline void setup_rename_conflict_info(enum rename_type rename_type,
                                              struct diff_filepair *pair1,
                                              struct diff_filepair *pair2,
                                              const char *branch1,
                                              const char *branch2,
                                              struct stage_data *dst_entry1,
                                              struct stage_data *dst_entry2,
                                              struct merge_options *o,
                                              struct stage_data *src_entry1,
                                              struct stage_data *src_entry2)
{
        struct rename_conflict_info *ci = xcalloc(1, sizeof(struct rename_conflict_info));
        ci->rename_type = rename_type;
        ci->pair1 = pair1;
        ci->branch1 = branch1;
        ci->branch2 = branch2;

        ci->dst_entry1 = dst_entry1;
        dst_entry1->rename_conflict_info = ci;
        dst_entry1->processed = 0;

        assert(!pair2 == !dst_entry2);
        if (dst_entry2) {
                ci->dst_entry2 = dst_entry2;
                ci->pair2 = pair2;
                dst_entry2->rename_conflict_info = ci;
        }

        if (rename_type == RENAME_TWO_FILES_TO_ONE) {
                /*
                 * For each rename, there could have been
                 * modifications on the side of history where that
                 * file was not renamed.
                 */
                int ostage1 = o->branch1 == branch1 ? 3 : 2;
                int ostage2 = ostage1 ^ 1;

                ci->ren1_other.path = pair1->one->path;
                oidcpy(&ci->ren1_other.oid, &src_entry1->stages[ostage1].oid);
                ci->ren1_other.mode = src_entry1->stages[ostage1].mode;

                ci->ren2_other.path = pair2->one->path;
                oidcpy(&ci->ren2_other.oid, &src_entry2->stages[ostage2].oid);
                ci->ren2_other.mode = src_entry2->stages[ostage2].mode;
        }
}

static int show(struct merge_options *o, int v)
{
        return (!o->call_depth && o->verbosity >= v) || o->verbosity >= 5;
}

__attribute__((format (printf, 3, 4)))
static void output(struct merge_options *o, int v, const char *fmt, ...)
{
        va_list ap;

        if (!show(o, v))
                return;

        strbuf_addchars(&o->obuf, ' ', o->call_depth * 2);

        va_start(ap, fmt);
        strbuf_vaddf(&o->obuf, fmt, ap);
        va_end(ap);

        strbuf_addch(&o->obuf, '\n');
        if (!o->buffer_output)
                flush_output(o);
}

static void output_commit_title(struct merge_options *o, struct commit *commit)
{
        strbuf_addchars(&o->obuf, ' ', o->call_depth * 2);
        if (commit->util)
                strbuf_addf(&o->obuf, "virtual %s\n",
                        merge_remote_util(commit)->name);
        else {
                strbuf_add_unique_abbrev(&o->obuf, &commit->object.oid,
                                         DEFAULT_ABBREV);
                strbuf_addch(&o->obuf, ' ');
                if (parse_commit(commit) != 0)
                        strbuf_addstr(&o->obuf, _("(bad commit)\n"));
                else {
                        const char *title;
                        const char *msg = get_commit_buffer(commit, NULL);
                        int len = find_commit_subject(msg, &title);
                        if (len)
                                strbuf_addf(&o->obuf, "%.*s\n", len, title);
                        unuse_commit_buffer(commit, msg);
                }
        }
        flush_output(o);
}

static int add_cacheinfo(struct merge_options *o,
                unsigned int mode, const struct object_id *oid,
                const char *path, int stage, int refresh, int options)
{
        struct cache_entry *ce;
        int ret;

        ce = make_cache_entry(mode, oid ? oid->hash : null_sha1, path, stage, 0);
        if (!ce)
                return err(o, _("add_cacheinfo failed for path '%s'; merge aborting."), path);

        ret = add_cache_entry(ce, options);
        if (refresh) {
                struct cache_entry *nce;

                nce = refresh_cache_entry(ce, CE_MATCH_REFRESH | CE_MATCH_IGNORE_MISSING);
                if (!nce)
                        return err(o, _("add_cacheinfo failed to refresh for path '%s'; merge aborting."), path);
                if (nce != ce)
                        ret = add_cache_entry(nce, options);
        }
        return ret;
}

static void init_tree_desc_from_tree(struct tree_desc *desc, struct tree *tree)
{
        parse_tree(tree);
        init_tree_desc(desc, tree->buffer, tree->size);
}

static int git_merge_trees(struct merge_options *o,
                           struct tree *common,
                           struct tree *head,
                           struct tree *merge)
{
        int rc;
        struct tree_desc t[3];
        struct index_state tmp_index = { NULL };

        memset(&o->unpack_opts, 0, sizeof(o->unpack_opts));
        if (o->call_depth)
                o->unpack_opts.index_only = 1;
        else
                o->unpack_opts.update = 1;
        o->unpack_opts.merge = 1;
        o->unpack_opts.head_idx = 2;
        o->unpack_opts.fn = threeway_merge;
        o->unpack_opts.src_index = &the_index;
        o->unpack_opts.dst_index = &tmp_index;
        o->unpack_opts.aggressive = !merge_detect_rename(o);
        setup_unpack_trees_porcelain(&o->unpack_opts, "merge");

        init_tree_desc_from_tree(t+0, common);
        init_tree_desc_from_tree(t+1, head);
        init_tree_desc_from_tree(t+2, merge);

        rc = unpack_trees(3, t, &o->unpack_opts);
        cache_tree_free(&active_cache_tree);

        /*
         * Update the_index to match the new results, AFTER saving a copy
         * in o->orig_index.  Update src_index to point to the saved copy.
         * (verify_uptodate() checks src_index, and the original index is
         * the one that had the necessary modification timestamps.)
         */
        o->orig_index = the_index;
        the_index = tmp_index;
        o->unpack_opts.src_index = &o->orig_index;

        return rc;
}

struct tree *write_tree_from_memory(struct merge_options *o)
{
        struct tree *result = NULL;

        if (unmerged_cache()) {
                int i;
                fprintf(stderr, "BUG: There are unmerged index entries:\n");
                for (i = 0; i < active_nr; i++) {
                        const struct cache_entry *ce = active_cache[i];
                        if (ce_stage(ce))
                                fprintf(stderr, "BUG: %d %.*s\n", ce_stage(ce),
                                        (int)ce_namelen(ce), ce->name);
                }
                die("BUG: unmerged index entries in merge-recursive.c");
        }

        if (!active_cache_tree)
                active_cache_tree = cache_tree();

        if (!cache_tree_fully_valid(active_cache_tree) &&
            cache_tree_update(&the_index, 0) < 0) {
                err(o, _("error building trees"));
                return NULL;
        }

        result = lookup_tree(&active_cache_tree->oid);

        return result;
}

static int save_files_dirs(const struct object_id *oid,
                struct strbuf *base, const char *path,
                unsigned int mode, int stage, void *context)
{
        struct path_hashmap_entry *entry;
        int baselen = base->len;
        struct merge_options *o = context;

        strbuf_addstr(base, path);

        FLEX_ALLOC_MEM(entry, path, base->buf, base->len);
        hashmap_entry_init(entry, path_hash(entry->path));
        hashmap_add(&o->current_file_dir_set, entry);

        strbuf_setlen(base, baselen);
        return (S_ISDIR(mode) ? READ_TREE_RECURSIVE : 0);
}

static void get_files_dirs(struct merge_options *o, struct tree *tree)
{
        struct pathspec match_all;
        memset(&match_all, 0, sizeof(match_all));
        read_tree_recursive(tree, "", 0, 0, &match_all, save_files_dirs, o);
}

static int get_tree_entry_if_blob(const struct object_id *tree,
                                  const char *path,
                                  struct object_id *hashy,
                                  unsigned int *mode_o)
{
        int ret;

        ret = get_tree_entry(tree, path, hashy, mode_o);
        if (S_ISDIR(*mode_o)) {
                oidcpy(hashy, &null_oid);
                *mode_o = 0;
        }
        return ret;
}

/*
 * Returns an index_entry instance which doesn't have to correspond to
 * a real cache entry in Git's index.
 */
static struct stage_data *insert_stage_data(const char *path,
                struct tree *o, struct tree *a, struct tree *b,
                struct string_list *entries)
{
        struct string_list_item *item;
        struct stage_data *e = xcalloc(1, sizeof(struct stage_data));
        get_tree_entry_if_blob(&o->object.oid, path,
                               &e->stages[1].oid, &e->stages[1].mode);
        get_tree_entry_if_blob(&a->object.oid, path,
                               &e->stages[2].oid, &e->stages[2].mode);
        get_tree_entry_if_blob(&b->object.oid, path,
                               &e->stages[3].oid, &e->stages[3].mode);
        item = string_list_insert(entries, path);
        item->util = e;
        return e;
}

/*
 * Create a dictionary mapping file names to stage_data objects. The
 * dictionary contains one entry for every path with a non-zero stage entry.
 */
static struct string_list *get_unmerged(void)
{
        struct string_list *unmerged = xcalloc(1, sizeof(struct string_list));
        int i;

        unmerged->strdup_strings = 1;

        for (i = 0; i < active_nr; i++) {
                struct string_list_item *item;
                struct stage_data *e;
                const struct cache_entry *ce = active_cache[i];
                if (!ce_stage(ce))
                        continue;

                item = string_list_lookup(unmerged, ce->name);
                if (!item) {
                        item = string_list_insert(unmerged, ce->name);
                        item->util = xcalloc(1, sizeof(struct stage_data));
                }
                e = item->util;
                e->stages[ce_stage(ce)].mode = ce->ce_mode;
                oidcpy(&e->stages[ce_stage(ce)].oid, &ce->oid);
        }

        return unmerged;
}

static int string_list_df_name_compare(const char *one, const char *two)
{
        int onelen = strlen(one);
        int twolen = strlen(two);
        /*
         * Here we only care that entries for D/F conflicts are
         * adjacent, in particular with the file of the D/F conflict
         * appearing before files below the corresponding directory.
         * The order of the rest of the list is irrelevant for us.
         *
         * To achieve this, we sort with df_name_compare and provide
         * the mode S_IFDIR so that D/F conflicts will sort correctly.
         * We use the mode S_IFDIR for everything else for simplicity,
         * since in other cases any changes in their order due to
         * sorting cause no problems for us.
         */
        int cmp = df_name_compare(one, onelen, S_IFDIR,
                                  two, twolen, S_IFDIR);
        /*
         * Now that 'foo' and 'foo/bar' compare equal, we have to make sure
         * that 'foo' comes before 'foo/bar'.
         */
        if (cmp)
                return cmp;
        return onelen - twolen;
}

static void record_df_conflict_files(struct merge_options *o,
                                     struct string_list *entries)
{
        /* If there is a D/F conflict and the file for such a conflict
         * currently exist in the working tree, we want to allow it to be
         * removed to make room for the corresponding directory if needed.
         * The files underneath the directories of such D/F conflicts will
         * be processed before the corresponding file involved in the D/F
         * conflict.  If the D/F directory ends up being removed by the
         * merge, then we won't have to touch the D/F file.  If the D/F
         * directory needs to be written to the working copy, then the D/F
         * file will simply be removed (in make_room_for_path()) to make
         * room for the necessary paths.  Note that if both the directory
         * and the file need to be present, then the D/F file will be
         * reinstated with a new unique name at the time it is processed.
         */
        struct string_list df_sorted_entries = STRING_LIST_INIT_NODUP;
        const char *last_file = NULL;
        int last_len = 0;
        int i;

        /*
         * If we're merging merge-bases, we don't want to bother with
         * any working directory changes.
         */
        if (o->call_depth)
                return;

        /* Ensure D/F conflicts are adjacent in the entries list. */
        for (i = 0; i < entries->nr; i++) {
                struct string_list_item *next = &entries->items[i];
                string_list_append(&df_sorted_entries, next->string)->util =
                                   next->util;
        }
        df_sorted_entries.cmp = string_list_df_name_compare;
        string_list_sort(&df_sorted_entries);

        string_list_clear(&o->df_conflict_file_set, 1);
        for (i = 0; i < df_sorted_entries.nr; i++) {
                const char *path = df_sorted_entries.items[i].string;
                int len = strlen(path);
                struct stage_data *e = df_sorted_entries.items[i].util;

                /*
                 * Check if last_file & path correspond to a D/F conflict;
                 * i.e. whether path is last_file+'/'+<something>.
                 * If so, record that it's okay to remove last_file to make
                 * room for path and friends if needed.
                 */
                if (last_file &&
                    len > last_len &&
                    memcmp(path, last_file, last_len) == 0 &&
                    path[last_len] == '/') {
                        string_list_insert(&o->df_conflict_file_set, last_file);
                }

                /*
                 * Determine whether path could exist as a file in the
                 * working directory as a possible D/F conflict.  This
                 * will only occur when it exists in stage 2 as a
                 * file.
                 */
                if (S_ISREG(e->stages[2].mode) || S_ISLNK(e->stages[2].mode)) {
                        last_file = path;
                        last_len = len;
                } else {
                        last_file = NULL;
                }
        }
        string_list_clear(&df_sorted_entries, 0);
}

struct rename {
        struct diff_filepair *pair;
        /*
         * Purpose of src_entry and dst_entry:
         *
         * If 'before' is renamed to 'after' then src_entry will contain
         * the versions of 'before' from the merge_base, HEAD, and MERGE in
         * stages 1, 2, and 3; dst_entry will contain the respective
         * versions of 'after' in corresponding locations.  Thus, we have a
         * total of six modes and oids, though some will be null.  (Stage 0
         * is ignored; we're interested in handling conflicts.)
         *
         * Since we don't turn on break-rewrites by default, neither
         * src_entry nor dst_entry can have all three of their stages have
         * non-null oids, meaning at most four of the six will be non-null.
         * Also, since this is a rename, both src_entry and dst_entry will
         * have at least one non-null oid, meaning at least two will be
         * non-null.  Of the six oids, a typical rename will have three be
         * non-null.  Only two implies a rename/delete, and four implies a
         * rename/add.
         */
        struct stage_data *src_entry;
        struct stage_data *dst_entry;
        unsigned add_turned_into_rename:1;
        unsigned processed:1;
};

static int update_stages(struct merge_options *opt, const char *path,
                         const struct diff_filespec *o,
                         const struct diff_filespec *a,
                         const struct diff_filespec *b)
{

        /*
         * NOTE: It is usually a bad idea to call update_stages on a path
         * before calling update_file on that same path, since it can
         * sometimes lead to spurious "refusing to lose untracked file..."
         * messages from update_file (via make_room_for path via
         * would_lose_untracked).  Instead, reverse the order of the calls
         * (executing update_file first and then update_stages).
         */
        int clear = 1;
        int options = ADD_CACHE_OK_TO_ADD | ADD_CACHE_SKIP_DFCHECK;
        if (clear)
                if (remove_file_from_cache(path))
                        return -1;
        if (o)
                if (add_cacheinfo(opt, o->mode, &o->oid, path, 1, 0, options))
                        return -1;
        if (a)
                if (add_cacheinfo(opt, a->mode, &a->oid, path, 2, 0, options))
                        return -1;
        if (b)
                if (add_cacheinfo(opt, b->mode, &b->oid, path, 3, 0, options))
                        return -1;
        return 0;
}

static int update_stages_for_stage_data(struct merge_options *opt,
                                        const char *path,
                                        const struct stage_data *stage_data)
{
        struct diff_filespec o, a, b;

        o.mode = stage_data->stages[1].mode;
        oidcpy(&o.oid, &stage_data->stages[1].oid);

        a.mode = stage_data->stages[2].mode;
        oidcpy(&a.oid, &stage_data->stages[2].oid);

        b.mode = stage_data->stages[3].mode;
        oidcpy(&b.oid, &stage_data->stages[3].oid);

        return update_stages(opt, path,
                             is_null_oid(&o.oid) ? NULL : &o,
                             is_null_oid(&a.oid) ? NULL : &a,
                             is_null_oid(&b.oid) ? NULL : &b);
}

static void update_entry(struct stage_data *entry,
                         struct diff_filespec *o,
                         struct diff_filespec *a,
                         struct diff_filespec *b)
{
        entry->processed = 0;
        entry->stages[1].mode = o->mode;
        entry->stages[2].mode = a->mode;
        entry->stages[3].mode = b->mode;
        oidcpy(&entry->stages[1].oid, &o->oid);
        oidcpy(&entry->stages[2].oid, &a->oid);
        oidcpy(&entry->stages[3].oid, &b->oid);
}

static int remove_file(struct merge_options *o, int clean,
                       const char *path, int no_wd)
{
        int update_cache = o->call_depth || clean;
        int update_working_directory = !o->call_depth && !no_wd;

        if (update_cache) {
                if (remove_file_from_cache(path))
                        return -1;
        }
        if (update_working_directory) {
                if (ignore_case) {
                        struct cache_entry *ce;
                        ce = cache_file_exists(path, strlen(path), ignore_case);
                        if (ce && ce_stage(ce) == 0 && strcmp(path, ce->name))
                                return 0;
                }
                if (remove_path(path))
                        return -1;
        }
        return 0;
}

/* add a string to a strbuf, but converting "/" to "_" */
static void add_flattened_path(struct strbuf *out, const char *s)
{
        size_t i = out->len;
        strbuf_addstr(out, s);
        for (; i < out->len; i++)
                if (out->buf[i] == '/')
                        out->buf[i] = '_';
}

static char *unique_path(struct merge_options *o, const char *path, const char *branch)
{
        struct path_hashmap_entry *entry;
        struct strbuf newpath = STRBUF_INIT;
        int suffix = 0;
        size_t base_len;

        strbuf_addf(&newpath, "%s~", path);
        add_flattened_path(&newpath, branch);

        base_len = newpath.len;
        while (hashmap_get_from_hash(&o->current_file_dir_set,
                                     path_hash(newpath.buf), newpath.buf) ||
               (!o->call_depth && file_exists(newpath.buf))) {
                strbuf_setlen(&newpath, base_len);
                strbuf_addf(&newpath, "_%d", suffix++);
        }

        FLEX_ALLOC_MEM(entry, path, newpath.buf, newpath.len);
        hashmap_entry_init(entry, path_hash(entry->path));
        hashmap_add(&o->current_file_dir_set, entry);
        return strbuf_detach(&newpath, NULL);
}

/**
 * Check whether a directory in the index is in the way of an incoming
 * file.  Return 1 if so.  If check_working_copy is non-zero, also
 * check the working directory.  If empty_ok is non-zero, also return
 * 0 in the case where the working-tree dir exists but is empty.
 */
static int dir_in_way(const char *path, int check_working_copy, int empty_ok)
{
        int pos;
        struct strbuf dirpath = STRBUF_INIT;
        struct stat st;

        strbuf_addstr(&dirpath, path);
        strbuf_addch(&dirpath, '/');

        pos = cache_name_pos(dirpath.buf, dirpath.len);

        if (pos < 0)
                pos = -1 - pos;
        if (pos < active_nr &&
            !strncmp(dirpath.buf, active_cache[pos]->name, dirpath.len)) {
                strbuf_release(&dirpath);
                return 1;
        }

        strbuf_release(&dirpath);
        return check_working_copy && !lstat(path, &st) && S_ISDIR(st.st_mode) &&
                !(empty_ok && is_empty_dir(path));
}

/*
 * Returns whether path was tracked in the index before the merge started,
 * and its oid and mode match the specified values
 */
static int was_tracked_and_matches(struct merge_options *o, const char *path,
                                   const struct object_id *oid, unsigned mode)
{
        int pos = index_name_pos(&o->orig_index, path, strlen(path));
        struct cache_entry *ce;

        if (0 > pos)
                /* we were not tracking this path before the merge */
                return 0;

        /* See if the file we were tracking before matches */
        ce = o->orig_index.cache[pos];
        return (oid_eq(&ce->oid, oid) && ce->ce_mode == mode);
}

/*
 * Returns whether path was tracked in the index before the merge started
 */
static int was_tracked(struct merge_options *o, const char *path)
{
        int pos = index_name_pos(&o->orig_index, path, strlen(path));

        if (0 <= pos)
                /* we were tracking this path before the merge */
                return 1;

        return 0;
}

static int would_lose_untracked(const char *path)
{
        /*
         * This may look like it can be simplified to:
         *   return !was_tracked(o, path) && file_exists(path)
         * but it can't.  This function needs to know whether path was in
         * the working tree due to EITHER having been tracked in the index
         * before the merge OR having been put into the working copy and
         * index by unpack_trees().  Due to that either-or requirement, we
         * check the current index instead of the original one.
         *
         * Note that we do not need to worry about merge-recursive itself
         * updating the index after unpack_trees() and before calling this
         * function, because we strictly require all code paths in
         * merge-recursive to update the working tree first and the index
         * second.  Doing otherwise would break
         * update_file()/would_lose_untracked(); see every comment in this
         * file which mentions "update_stages".
         */
        int pos = cache_name_pos(path, strlen(path));

        if (pos < 0)
                pos = -1 - pos;
        while (pos < active_nr &&
               !strcmp(path, active_cache[pos]->name)) {
                /*
                 * If stage #0, it is definitely tracked.
                 * If it has stage #2 then it was tracked
                 * before this merge started.  All other
                 * cases the path was not tracked.
                 */
                switch (ce_stage(active_cache[pos])) {
                case 0:
                case 2:
                        return 0;
                }
                pos++;
        }
        return file_exists(path);
}

static int was_dirty(struct merge_options *o, const char *path)
{
        struct cache_entry *ce;
        int dirty = 1;

        if (o->call_depth || !was_tracked(o, path))
                return !dirty;

        ce = index_file_exists(o->unpack_opts.src_index,
                               path, strlen(path), ignore_case);
        dirty = verify_uptodate(ce, &o->unpack_opts) != 0;
        return dirty;
}

static int make_room_for_path(struct merge_options *o, const char *path)
{
        int status, i;
        const char *msg = _("failed to create path '%s'%s");

        /* Unlink any D/F conflict files that are in the way */
        for (i = 0; i < o->df_conflict_file_set.nr; i++) {
                const char *df_path = o->df_conflict_file_set.items[i].string;
                size_t pathlen = strlen(path);
                size_t df_pathlen = strlen(df_path);
                if (df_pathlen < pathlen &&
                    path[df_pathlen] == '/' &&
                    strncmp(path, df_path, df_pathlen) == 0) {
                        output(o, 3,
                               _("Removing %s to make room for subdirectory\n"),
                               df_path);
                        unlink(df_path);
                        unsorted_string_list_delete_item(&o->df_conflict_file_set,
                                                         i, 0);
                        break;
                }
        }

        /* Make sure leading directories are created */
        status = safe_create_leading_directories_const(path);
        if (status) {
                if (status == SCLD_EXISTS)
                        /* something else exists */
                        return err(o, msg, path, _(": perhaps a D/F conflict?"));
                return err(o, msg, path, "");
        }

        /*
         * Do not unlink a file in the work tree if we are not
         * tracking it.
         */
        if (would_lose_untracked(path))
                return err(o, _("refusing to lose untracked file at '%s'"),
                             path);

        /* Successful unlink is good.. */
        if (!unlink(path))
                return 0;
        /* .. and so is no existing file */
        if (errno == ENOENT)
                return 0;
        /* .. but not some other error (who really cares what?) */
        return err(o, msg, path, _(": perhaps a D/F conflict?"));
}

static int update_file_flags(struct merge_options *o,
                             const struct object_id *oid,
                             unsigned mode,
                             const char *path,
                             int update_cache,
                             int update_wd)
{
        int ret = 0;

        if (o->call_depth)
                update_wd = 0;

        if (update_wd) {
                enum object_type type;
                void *buf;
                unsigned long size;

                if (S_ISGITLINK(mode)) {
                        /*
                         * We may later decide to recursively descend into
                         * the submodule directory and update its index
                         * and/or work tree, but we do not do that now.
                         */
                        update_wd = 0;
                        goto update_index;
                }

                buf = read_object_file(oid, &type, &size);
                if (!buf)
                        return err(o, _("cannot read object %s '%s'"), oid_to_hex(oid), path);
                if (type != OBJ_BLOB) {
                        ret = err(o, _("blob expected for %s '%s'"), oid_to_hex(oid), path);
                        goto free_buf;
                }
                if (S_ISREG(mode)) {
                        struct strbuf strbuf = STRBUF_INIT;
                        if (convert_to_working_tree(path, buf, size, &strbuf)) {
                                free(buf);
                                size = strbuf.len;
                                buf = strbuf_detach(&strbuf, NULL);
                        }
                }

                if (make_room_for_path(o, path) < 0) {
                        update_wd = 0;
                        goto free_buf;
                }
                if (S_ISREG(mode) || (!has_symlinks && S_ISLNK(mode))) {
                        int fd;
                        if (mode & 0100)
                                mode = 0777;
                        else
                                mode = 0666;
                        fd = open(path, O_WRONLY | O_TRUNC | O_CREAT, mode);
                        if (fd < 0) {
                                ret = err(o, _("failed to open '%s': %s"),
                                          path, strerror(errno));
                                goto free_buf;
                        }
                        write_in_full(fd, buf, size);
                        close(fd);
                } else if (S_ISLNK(mode)) {
                        char *lnk = xmemdupz(buf, size);
                        safe_create_leading_directories_const(path);
                        unlink(path);
                        if (symlink(lnk, path))
                                ret = err(o, _("failed to symlink '%s': %s"),
                                        path, strerror(errno));
                        free(lnk);
                } else
                        ret = err(o,
                                  _("do not know what to do with %06o %s '%s'"),
                                  mode, oid_to_hex(oid), path);
 free_buf:
                free(buf);
        }
 update_index:
        if (!ret && update_cache)
                if (add_cacheinfo(o, mode, oid, path, 0, update_wd,
                                  ADD_CACHE_OK_TO_ADD))
                        return -1;
        return ret;
}

static int update_file(struct merge_options *o,
                       int clean,
                       const struct object_id *oid,
                       unsigned mode,
                       const char *path)
{
        return update_file_flags(o, oid, mode, path, o->call_depth || clean, !o->call_depth);
}

/* Low level file merging, update and removal */

struct merge_file_info {
        struct object_id oid;
        unsigned mode;
        unsigned clean:1,
                 merge:1;
};

static int merge_3way(struct merge_options *o,
                      mmbuffer_t *result_buf,
                      const struct diff_filespec *one,
                      const struct diff_filespec *a,
                      const struct diff_filespec *b,
                      const char *branch1,
                      const char *branch2)
{
        mmfile_t orig, src1, src2;
        struct ll_merge_options ll_opts = {0};
        char *base_name, *name1, *name2;
        int merge_status;

        ll_opts.renormalize = o->renormalize;
        ll_opts.xdl_opts = o->xdl_opts;

        if (o->call_depth) {
                ll_opts.virtual_ancestor = 1;
                ll_opts.variant = 0;
        } else {
                switch (o->recursive_variant) {
                case MERGE_RECURSIVE_OURS:
                        ll_opts.variant = XDL_MERGE_FAVOR_OURS;
                        break;
                case MERGE_RECURSIVE_THEIRS:
                        ll_opts.variant = XDL_MERGE_FAVOR_THEIRS;
                        break;
                default:
                        ll_opts.variant = 0;
                        break;
                }
        }

        if (strcmp(a->path, b->path) ||
            (o->ancestor != NULL && strcmp(a->path, one->path) != 0)) {
                base_name = o->ancestor == NULL ? NULL :
                        mkpathdup("%s:%s", o->ancestor, one->path);
                name1 = mkpathdup("%s:%s", branch1, a->path);
                name2 = mkpathdup("%s:%s", branch2, b->path);
        } else {
                base_name = o->ancestor == NULL ? NULL :
                        mkpathdup("%s", o->ancestor);
                name1 = mkpathdup("%s", branch1);
                name2 = mkpathdup("%s", branch2);
        }

        read_mmblob(&orig, &one->oid);
        read_mmblob(&src1, &a->oid);
        read_mmblob(&src2, &b->oid);

        merge_status = ll_merge(result_buf, a->path, &orig, base_name,
                                &src1, name1, &src2, name2, &ll_opts);

        free(base_name);
        free(name1);
        free(name2);
        free(orig.ptr);
        free(src1.ptr);
        free(src2.ptr);
        return merge_status;
}

static int merge_file_1(struct merge_options *o,
                        const struct diff_filespec *one,
                        const struct diff_filespec *a,
                        const struct diff_filespec *b,
                        const char *filename,
                        const char *branch1,
                        const char *branch2,
                        struct merge_file_info *result)
{
        result->merge = 0;
        result->clean = 1;

        if ((S_IFMT & a->mode) != (S_IFMT & b->mode)) {
                result->clean = 0;
                if (S_ISREG(a->mode)) {
                        result->mode = a->mode;
                        oidcpy(&result->oid, &a->oid);
                } else {
                        result->mode = b->mode;
                        oidcpy(&result->oid, &b->oid);
                }
        } else {
                if (!oid_eq(&a->oid, &one->oid) && !oid_eq(&b->oid, &one->oid))
                        result->merge = 1;

                /*
                 * Merge modes
                 */
                if (a->mode == b->mode || a->mode == one->mode)
                        result->mode = b->mode;
                else {
                        result->mode = a->mode;
                        if (b->mode != one->mode) {
                                result->clean = 0;
                                result->merge = 1;
                        }
                }

                if (oid_eq(&a->oid, &b->oid) || oid_eq(&a->oid, &one->oid))
                        oidcpy(&result->oid, &b->oid);
                else if (oid_eq(&b->oid, &one->oid))
                        oidcpy(&result->oid, &a->oid);
                else if (S_ISREG(a->mode)) {
                        mmbuffer_t result_buf;
                        int ret = 0, merge_status;

                        merge_status = merge_3way(o, &result_buf, one, a, b,
                                                  branch1, branch2);

                        if ((merge_status < 0) || !result_buf.ptr)
                                ret = err(o, _("Failed to execute internal merge"));

                        if (!ret &&
                            write_object_file(result_buf.ptr, result_buf.size,
                                              blob_type, &result->oid))
                                ret = err(o, _("Unable to add %s to database"),
                                          a->path);

                        free(result_buf.ptr);
                        if (ret)
                                return ret;
                        result->clean = (merge_status == 0);
                } else if (S_ISGITLINK(a->mode)) {
                        result->clean = merge_submodule(&result->oid,
                                                       one->path,
                                                       &one->oid,
                                                       &a->oid,
                                                       &b->oid,
                                                       !o->call_depth);
                } else if (S_ISLNK(a->mode)) {
                        switch (o->recursive_variant) {
                        case MERGE_RECURSIVE_NORMAL:
                                oidcpy(&result->oid, &a->oid);
                                if (!oid_eq(&a->oid, &b->oid))
                                        result->clean = 0;
                                break;
                        case MERGE_RECURSIVE_OURS:
                                oidcpy(&result->oid, &a->oid);
                                break;
                        case MERGE_RECURSIVE_THEIRS:
                                oidcpy(&result->oid, &b->oid);
                                break;
                        }
                } else
                        die("BUG: unsupported object type in the tree");
        }

        if (result->merge)
                output(o, 2, _("Auto-merging %s"), filename);

        return 0;
}

static int merge_file_special_markers(struct merge_options *o,
                                      const struct diff_filespec *one,
                                      const struct diff_filespec *a,
                                      const struct diff_filespec *b,
                                      const char *target_filename,
                                      const char *branch1,
                                      const char *filename1,
                                      const char *branch2,
                                      const char *filename2,
                                      struct merge_file_info *mfi)
{
        char *side1 = NULL;
        char *side2 = NULL;
        int ret;

        if (filename1)
                side1 = xstrfmt("%s:%s", branch1, filename1);
        if (filename2)
                side2 = xstrfmt("%s:%s", branch2, filename2);

        ret = merge_file_1(o, one, a, b, target_filename,
                           side1 ? side1 : branch1,
                           side2 ? side2 : branch2, mfi);

        free(side1);
        free(side2);
        return ret;
}

static int merge_file_one(struct merge_options *o,
                          const char *path,
                          const struct object_id *o_oid, int o_mode,
                          const struct object_id *a_oid, int a_mode,
                          const struct object_id *b_oid, int b_mode,
                          const char *branch1,
                          const char *branch2,
                          struct merge_file_info *mfi)
{
        struct diff_filespec one, a, b;

        one.path = a.path = b.path = (char *)path;
        oidcpy(&one.oid, o_oid);
        one.mode = o_mode;
        oidcpy(&a.oid, a_oid);
        a.mode = a_mode;
        oidcpy(&b.oid, b_oid);
        b.mode = b_mode;
        return merge_file_1(o, &one, &a, &b, path, branch1, branch2, mfi);
}

static int conflict_rename_dir(struct merge_options *o,
                               struct diff_filepair *pair,
                               const char *rename_branch,
                               const char *other_branch)
{
        const struct diff_filespec *dest = pair->two;

        if (!o->call_depth && would_lose_untracked(dest->path)) {
                char *alt_path = unique_path(o, dest->path, rename_branch);

                output(o, 1, _("Error: Refusing to lose untracked file at %s; "
                               "writing to %s instead."),
                       dest->path, alt_path);
                /*
                 * Write the file in worktree at alt_path, but not in the
                 * index.  Instead, write to dest->path for the index but
                 * only at the higher appropriate stage.
                 */
                if (update_file(o, 0, &dest->oid, dest->mode, alt_path))
                        return -1;
                free(alt_path);
                return update_stages(o, dest->path, NULL,
                                     rename_branch == o->branch1 ? dest : NULL,
                                     rename_branch == o->branch1 ? NULL : dest);
        }

        /* Update dest->path both in index and in worktree */
        if (update_file(o, 1, &dest->oid, dest->mode, dest->path))
                return -1;
        return 0;
}

static int handle_change_delete(struct merge_options *o,
                                 const char *path, const char *old_path,
                                 const struct object_id *o_oid, int o_mode,
                                 const struct object_id *changed_oid,
                                 int changed_mode,
                                 const char *change_branch,
                                 const char *delete_branch,
                                 const char *change, const char *change_past)
{
        char *alt_path = NULL;
        const char *update_path = path;
        int ret = 0;

        if (dir_in_way(path, !o->call_depth, 0) ||
            (!o->call_depth && would_lose_untracked(path))) {
                update_path = alt_path = unique_path(o, path, change_branch);
        }

        if (o->call_depth) {
                /*
                 * We cannot arbitrarily accept either a_sha or b_sha as
                 * correct; since there is no true "middle point" between
                 * them, simply reuse the base version for virtual merge base.
                 */
                ret = remove_file_from_cache(path);
                if (!ret)
                        ret = update_file(o, 0, o_oid, o_mode, update_path);
        } else {
                if (!alt_path) {
                        if (!old_path) {
                                output(o, 1, _("CONFLICT (%s/delete): %s deleted in %s "
                                       "and %s in %s. Version %s of %s left in tree."),
                                       change, path, delete_branch, change_past,
                                       change_branch, change_branch, path);
                        } else {
                                output(o, 1, _("CONFLICT (%s/delete): %s deleted in %s "
                                       "and %s to %s in %s. Version %s of %s left in tree."),
                                       change, old_path, delete_branch, change_past, path,
                                       change_branch, change_branch, path);
                        }
                } else {
                        if (!old_path) {
                                output(o, 1, _("CONFLICT (%s/delete): %s deleted in %s "
                                       "and %s in %s. Version %s of %s left in tree at %s."),
                                       change, path, delete_branch, change_past,
                                       change_branch, change_branch, path, alt_path);
                        } else {
                                output(o, 1, _("CONFLICT (%s/delete): %s deleted in %s "
                                       "and %s to %s in %s. Version %s of %s left in tree at %s."),
                                       change, old_path, delete_branch, change_past, path,
                                       change_branch, change_branch, path, alt_path);
                        }
                }
                /*
                 * No need to call update_file() on path when change_branch ==
                 * o->branch1 && !alt_path, since that would needlessly touch
                 * path.  We could call update_file_flags() with update_cache=0
                 * and update_wd=0, but that's a no-op.
                 */
                if (change_branch != o->branch1 || alt_path)
                        ret = update_file(o, 0, changed_oid, changed_mode, update_path);
        }
        free(alt_path);

        return ret;
}

static int conflict_rename_delete(struct merge_options *o,
                                   struct diff_filepair *pair,
                                   const char *rename_branch,
                                   const char *delete_branch)
{
        const struct diff_filespec *orig = pair->one;
        const struct diff_filespec *dest = pair->two;

        if (handle_change_delete(o,
                                 o->call_depth ? orig->path : dest->path,
                                 o->call_depth ? NULL : orig->path,
                                 &orig->oid, orig->mode,
                                 &dest->oid, dest->mode,
                                 rename_branch, delete_branch,
                                 _("rename"), _("renamed")))
                return -1;

        if (o->call_depth)
                return remove_file_from_cache(dest->path);
        else
                return update_stages(o, dest->path, NULL,
                                     rename_branch == o->branch1 ? dest : NULL,
                                     rename_branch == o->branch1 ? NULL : dest);
}

static struct diff_filespec *filespec_from_entry(struct diff_filespec *target,
                                                 struct stage_data *entry,
                                                 int stage)
{
        struct object_id *oid = &entry->stages[stage].oid;
        unsigned mode = entry->stages[stage].mode;
        if (mode == 0 || is_null_oid(oid))
                return NULL;
        oidcpy(&target->oid, oid);
        target->mode = mode;
        return target;
}

static int handle_file(struct merge_options *o,
                        struct diff_filespec *rename,
                        int stage,
                        struct rename_conflict_info *ci)
{
        char *dst_name = rename->path;
        struct stage_data *dst_entry;
        const char *cur_branch, *other_branch;
        struct diff_filespec other;
        struct diff_filespec *add;
        int ret;

        if (stage == 2) {
                dst_entry = ci->dst_entry1;
                cur_branch = ci->branch1;
                other_branch = ci->branch2;
        } else {
                dst_entry = ci->dst_entry2;
                cur_branch = ci->branch2;
                other_branch = ci->branch1;
        }

        add = filespec_from_entry(&other, dst_entry, stage ^ 1);
        if (add) {
                int ren_src_was_dirty = was_dirty(o, rename->path);
                char *add_name = unique_path(o, rename->path, other_branch);
                if (update_file(o, 0, &add->oid, add->mode, add_name))
                        return -1;

                if (ren_src_was_dirty) {
                        output(o, 1, _("Refusing to lose dirty file at %s"),
                               rename->path);
                }
                /*
                 * Because the double negatives somehow keep confusing me...
                 *    1) update_wd iff !ren_src_was_dirty.
                 *    2) no_wd iff !update_wd
                 *    3) so, no_wd == !!ren_src_was_dirty == ren_src_was_dirty
                 */
                remove_file(o, 0, rename->path, ren_src_was_dirty);
                dst_name = unique_path(o, rename->path, cur_branch);
        } else {
                if (dir_in_way(rename->path, !o->call_depth, 0)) {
                        dst_name = unique_path(o, rename->path, cur_branch);
                        output(o, 1, _("%s is a directory in %s adding as %s instead"),
                               rename->path, other_branch, dst_name);
                } else if (!o->call_depth &&
                           would_lose_untracked(rename->path)) {
                        dst_name = unique_path(o, rename->path, cur_branch);
                        output(o, 1, _("Refusing to lose untracked file at %s; "
                                       "adding as %s instead"),
                               rename->path, dst_name);
                }
        }
        if ((ret = update_file(o, 0, &rename->oid, rename->mode, dst_name)))
                ; /* fall through, do allow dst_name to be released */
        else if (stage == 2)
                ret = update_stages(o, rename->path, NULL, rename, add);
        else
                ret = update_stages(o, rename->path, NULL, add, rename);

        if (dst_name != rename->path)
                free(dst_name);

        return ret;
}

static int conflict_rename_rename_1to2(struct merge_options *o,
                                        struct rename_conflict_info *ci)
{
        /* One file was renamed in both branches, but to different names. */
        struct diff_filespec *one = ci->pair1->one;
        struct diff_filespec *a = ci->pair1->two;
        struct diff_filespec *b = ci->pair2->two;

        output(o, 1, _("CONFLICT (rename/rename): "
               "Rename \"%s\"->\"%s\" in branch \"%s\" "
               "rename \"%s\"->\"%s\" in \"%s\"%s"),
               one->path, a->path, ci->branch1,
               one->path, b->path, ci->branch2,
               o->call_depth ? _(" (left unresolved)") : "");
        if (o->call_depth) {
                struct merge_file_info mfi;
                struct diff_filespec other;
                struct diff_filespec *add;
                if (merge_file_one(o, one->path,
                                 &one->oid, one->mode,
                                 &a->oid, a->mode,
                                 &b->oid, b->mode,
                                 ci->branch1, ci->branch2, &mfi))
                        return -1;

                /*
                 * FIXME: For rename/add-source conflicts (if we could detect
                 * such), this is wrong.  We should instead find a unique
                 * pathname and then either rename the add-source file to that
                 * unique path, or use that unique path instead of src here.
                 */
                if (update_file(o, 0, &mfi.oid, mfi.mode, one->path))
                        return -1;

                /*
                 * Above, we put the merged content at the merge-base's
                 * path.  Now we usually need to delete both a->path and
                 * b->path.  However, the rename on each side of the merge
                 * could also be involved in a rename/add conflict.  In
                 * such cases, we should keep the added file around,
                 * resolving the conflict at that path in its favor.
                 */
                add = filespec_from_entry(&other, ci->dst_entry1, 2 ^ 1);
                if (add) {
                        if (update_file(o, 0, &add->oid, add->mode, a->path))
                                return -1;
                }
                else
                        remove_file_from_cache(a->path);
                add = filespec_from_entry(&other, ci->dst_entry2, 3 ^ 1);
                if (add) {
                        if (update_file(o, 0, &add->oid, add->mode, b->path))
                                return -1;
                }
                else
                        remove_file_from_cache(b->path);
        } else if (handle_file(o, a, 2, ci) || handle_file(o, b, 3, ci))
                return -1;

        return 0;
}

static int conflict_rename_rename_2to1(struct merge_options *o,
                                        struct rename_conflict_info *ci)
{
        /* Two files, a & b, were renamed to the same thing, c. */
        struct diff_filespec *a = ci->pair1->one;
        struct diff_filespec *b = ci->pair2->one;
        struct diff_filespec *c1 = ci->pair1->two;
        struct diff_filespec *c2 = ci->pair2->two;
        char *path = c1->path; /* == c2->path */
        char *path_side_1_desc;
        char *path_side_2_desc;
        struct merge_file_info mfi_c1;
        struct merge_file_info mfi_c2;
        int ret;

        output(o, 1, _("CONFLICT (rename/rename): "
               "Rename %s->%s in %s. "
               "Rename %s->%s in %s"),
               a->path, c1->path, ci->branch1,
               b->path, c2->path, ci->branch2);

        remove_file(o, 1, a->path, o->call_depth || would_lose_untracked(a->path));
        remove_file(o, 1, b->path, o->call_depth || would_lose_untracked(b->path));

        path_side_1_desc = xstrfmt("%s (was %s)", path, a->path);
        path_side_2_desc = xstrfmt("%s (was %s)", path, b->path);
        if (merge_file_special_markers(o, a, c1, &ci->ren1_other,
                                       path_side_1_desc,
                                       o->branch1, c1->path,
                                       o->branch2, ci->ren1_other.path, &mfi_c1) ||
            merge_file_special_markers(o, b, &ci->ren2_other, c2,
                                       path_side_2_desc,
                                       o->branch1, ci->ren2_other.path,
                                       o->branch2, c2->path, &mfi_c2))
                return -1;
        free(path_side_1_desc);
        free(path_side_2_desc);

        if (o->call_depth) {
                /*
                 * If mfi_c1.clean && mfi_c2.clean, then it might make
                 * sense to do a two-way merge of those results.  But, I
                 * think in all cases, it makes sense to have the virtual
                 * merge base just undo the renames; they can be detected
                 * again later for the non-recursive merge.
                 */
                remove_file(o, 0, path, 0);
                ret = update_file(o, 0, &mfi_c1.oid, mfi_c1.mode, a->path);
                if (!ret)
                        ret = update_file(o, 0, &mfi_c2.oid, mfi_c2.mode,
                                          b->path);
        } else {
                char *new_path1 = unique_path(o, path, ci->branch1);
                char *new_path2 = unique_path(o, path, ci->branch2);
                output(o, 1, _("Renaming %s to %s and %s to %s instead"),
                       a->path, new_path1, b->path, new_path2);
                if (was_dirty(o, path))
                        output(o, 1, _("Refusing to lose dirty file at %s"),
                               path);
                else if (would_lose_untracked(path))
                        /*
                         * Only way we get here is if both renames were from
                         * a directory rename AND user had an untracked file
                         * at the location where both files end up after the
                         * two directory renames.  See testcase 10d of t6043.
                         */
                        output(o, 1, _("Refusing to lose untracked file at "
                                       "%s, even though it's in the way."),
                               path);
                else
                        remove_file(o, 0, path, 0);
                ret = update_file(o, 0, &mfi_c1.oid, mfi_c1.mode, new_path1);
                if (!ret)
                        ret = update_file(o, 0, &mfi_c2.oid, mfi_c2.mode,
                                          new_path2);
                /*
                 * unpack_trees() actually populates the index for us for
                 * "normal" rename/rename(2to1) situtations so that the
                 * correct entries are at the higher stages, which would
                 * make the call below to update_stages_for_stage_data
                 * unnecessary.  However, if either of the renames came
                 * from a directory rename, then unpack_trees() will not
                 * have gotten the right data loaded into the index, so we
                 * need to do so now.  (While it'd be tempting to move this
                 * call to update_stages_for_stage_data() to
                 * apply_directory_rename_modifications(), that would break
                 * our intermediate calls to would_lose_untracked() since
                 * those rely on the current in-memory index.  See also the
                 * big "NOTE" in update_stages()).
                 */
                if (update_stages_for_stage_data(o, path, ci->dst_entry1))
                        ret = -1;

                free(new_path2);
                free(new_path1);
        }

        return ret;
}

/*
 * Get the diff_filepairs changed between o_tree and tree.
 */
static struct diff_queue_struct *get_diffpairs(struct merge_options *o,
                                               struct tree *o_tree,
                                               struct tree *tree)
{
        struct diff_queue_struct *ret;
        struct diff_options opts;

        diff_setup(&opts);
        opts.flags.recursive = 1;
        opts.flags.rename_empty = 0;
        opts.detect_rename = merge_detect_rename(o);
        /*
         * We do not have logic to handle the detection of copies.  In
         * fact, it may not even make sense to add such logic: would we
         * really want a change to a base file to be propagated through
         * multiple other files by a merge?
         */
        if (opts.detect_rename > DIFF_DETECT_RENAME)
                opts.detect_rename = DIFF_DETECT_RENAME;
        opts.rename_limit = o->merge_rename_limit >= 0 ? o->merge_rename_limit :
                            o->diff_rename_limit >= 0 ? o->diff_rename_limit :
                            1000;
        opts.rename_score = o->rename_score;
        opts.show_rename_progress = o->show_rename_progress;
        opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        diff_setup_done(&opts);
        diff_tree_oid(&o_tree->object.oid, &tree->object.oid, "", &opts);
        diffcore_std(&opts);
        if (opts.needed_rename_limit > o->needed_rename_limit)
                o->needed_rename_limit = opts.needed_rename_limit;

        ret = xmalloc(sizeof(*ret));
        *ret = diff_queued_diff;

        opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        diff_queued_diff.nr = 0;
        diff_queued_diff.queue = NULL;
        diff_flush(&opts);
        return ret;
}

static int tree_has_path(struct tree *tree, const char *path)
{
        struct object_id hashy;
        unsigned int mode_o;

        return !get_tree_entry(&tree->object.oid, path,
                               &hashy, &mode_o);
}

/*
 * Return a new string that replaces the beginning portion (which matches
 * entry->dir), with entry->new_dir.  In perl-speak:
 *   new_path_name = (old_path =~ s/entry->dir/entry->new_dir/);
 * NOTE:
 *   Caller must ensure that old_path starts with entry->dir + '/'.
 */
static char *apply_dir_rename(struct dir_rename_entry *entry,
                              const char *old_path)
{
        struct strbuf new_path = STRBUF_INIT;
        int oldlen, newlen;

        if (entry->non_unique_new_dir)
                return NULL;

        oldlen = strlen(entry->dir);
        newlen = entry->new_dir.len + (strlen(old_path) - oldlen) + 1;
        strbuf_grow(&new_path, newlen);
        strbuf_addbuf(&new_path, &entry->new_dir);
        strbuf_addstr(&new_path, &old_path[oldlen]);

        return strbuf_detach(&new_path, NULL);
}

static void get_renamed_dir_portion(const char *old_path, const char *new_path,
                                    char **old_dir, char **new_dir)
{
        char *end_of_old, *end_of_new;
        int old_len, new_len;

        *old_dir = NULL;
        *new_dir = NULL;

        /*
         * For
         *    "a/b/c/d/e/foo.c" -> "a/b/some/thing/else/e/foo.c"
         * the "e/foo.c" part is the same, we just want to know that
         *    "a/b/c/d" was renamed to "a/b/some/thing/else"
         * so, for this example, this function returns "a/b/c/d" in
         * *old_dir and "a/b/some/thing/else" in *new_dir.
         *
         * Also, if the basename of the file changed, we don't care.  We
         * want to know which portion of the directory, if any, changed.
         */
        end_of_old = strrchr(old_path, '/');
        end_of_new = strrchr(new_path, '/');

        if (end_of_old == NULL || end_of_new == NULL)
                return;
        while (*--end_of_new == *--end_of_old &&
               end_of_old != old_path &&
               end_of_new != new_path)
                ; /* Do nothing; all in the while loop */
        /*
         * We've found the first non-matching character in the directory
         * paths.  That means the current directory we were comparing
         * represents the rename.  Move end_of_old and end_of_new back
         * to the full directory name.
         */
        if (*end_of_old == '/')
                end_of_old++;
        if (*end_of_old != '/')
                end_of_new++;
        end_of_old = strchr(end_of_old, '/');
        end_of_new = strchr(end_of_new, '/');

        /*
         * It may have been the case that old_path and new_path were the same
         * directory all along.  Don't claim a rename if they're the same.
         */
        old_len = end_of_old - old_path;
        new_len = end_of_new - new_path;

        if (old_len != new_len || strncmp(old_path, new_path, old_len)) {
                *old_dir = xstrndup(old_path, old_len);
                *new_dir = xstrndup(new_path, new_len);
        }
}

static void remove_hashmap_entries(struct hashmap *dir_renames,
                                   struct string_list *items_to_remove)
{
        int i;
        struct dir_rename_entry *entry;

        for (i = 0; i < items_to_remove->nr; i++) {
                entry = items_to_remove->items[i].util;
                hashmap_remove(dir_renames, entry, NULL);
        }
        string_list_clear(items_to_remove, 0);
}

/*
 * See if there is a directory rename for path, and if there are any file
 * level conflicts for the renamed location.  If there is a rename and
 * there are no conflicts, return the new name.  Otherwise, return NULL.
 */
static char *handle_path_level_conflicts(struct merge_options *o,
                                         const char *path,
                                         struct dir_rename_entry *entry,
                                         struct hashmap *collisions,
                                         struct tree *tree)
{
        char *new_path = NULL;
        struct collision_entry *collision_ent;
        int clean = 1;
        struct strbuf collision_paths = STRBUF_INIT;

        /*
         * entry has the mapping of old directory name to new directory name
         * that we want to apply to path.
         */
        new_path = apply_dir_rename(entry, path);

        if (!new_path) {
                /* This should only happen when entry->non_unique_new_dir set */
                if (!entry->non_unique_new_dir)
                        BUG("entry->non_unqiue_dir not set and !new_path");
                output(o, 1, _("CONFLICT (directory rename split): "
                               "Unclear where to place %s because directory "
                               "%s was renamed to multiple other directories, "
                               "with no destination getting a majority of the "
                               "files."),
                       path, entry->dir);
                clean = 0;
                return NULL;
        }

        /*
         * The caller needs to have ensured that it has pre-populated
         * collisions with all paths that map to new_path.  Do a quick check
         * to ensure that's the case.
         */
        collision_ent = collision_find_entry(collisions, new_path);
        if (collision_ent == NULL)
                BUG("collision_ent is NULL");

        /*
         * Check for one-sided add/add/.../add conflicts, i.e.
         * where implicit renames from the other side doing
         * directory rename(s) can affect this side of history
         * to put multiple paths into the same location.  Warn
         * and bail on directory renames for such paths.
         */
        if (collision_ent->reported_already) {
                clean = 0;
        } else if (tree_has_path(tree, new_path)) {
                collision_ent->reported_already = 1;
                strbuf_add_separated_string_list(&collision_paths, ", ",
                                                 &collision_ent->source_files);
                output(o, 1, _("CONFLICT (implicit dir rename): Existing "
                               "file/dir at %s in the way of implicit "
                               "directory rename(s) putting the following "
                               "path(s) there: %s."),
                       new_path, collision_paths.buf);
                clean = 0;
        } else if (collision_ent->source_files.nr > 1) {
                collision_ent->reported_already = 1;
                strbuf_add_separated_string_list(&collision_paths, ", ",
                                                 &collision_ent->source_files);
                output(o, 1, _("CONFLICT (implicit dir rename): Cannot map "
                               "more than one path to %s; implicit directory "
                               "renames tried to put these paths there: %s"),
                       new_path, collision_paths.buf);
                clean = 0;
        }

        /* Free memory we no longer need */
        strbuf_release(&collision_paths);
        if (!clean && new_path) {
                free(new_path);
                return NULL;
        }

        return new_path;
}

/*
 * There are a couple things we want to do at the directory level:
 *   1. Check for both sides renaming to the same thing, in order to avoid
 *      implicit renaming of files that should be left in place.  (See
 *      testcase 6b in t6043 for details.)
 *   2. Prune directory renames if there are still files left in the
 *      the original directory.  These represent a partial directory rename,
 *      i.e. a rename where only some of the files within the directory
 *      were renamed elsewhere.  (Technically, this could be done earlier
 *      in get_directory_renames(), except that would prevent us from
 *      doing the previous check and thus failing testcase 6b.)
 *   3. Check for rename/rename(1to2) conflicts (at the directory level).
 *      In the future, we could potentially record this info as well and
 *      omit reporting rename/rename(1to2) conflicts for each path within
 *      the affected directories, thus cleaning up the merge output.
 *   NOTE: We do NOT check for rename/rename(2to1) conflicts at the
 *         directory level, because merging directories is fine.  If it
 *         causes conflicts for files within those merged directories, then
 *         that should be detected at the individual path level.
 */
static void handle_directory_level_conflicts(struct merge_options *o,
                                             struct hashmap *dir_re_head,
                                             struct tree *head,
                                             struct hashmap *dir_re_merge,
                                             struct tree *merge)
{
        struct hashmap_iter iter;
        struct dir_rename_entry *head_ent;
        struct dir_rename_entry *merge_ent;

        struct string_list remove_from_head = STRING_LIST_INIT_NODUP;
        struct string_list remove_from_merge = STRING_LIST_INIT_NODUP;

        hashmap_iter_init(dir_re_head, &iter);
        while ((head_ent = hashmap_iter_next(&iter))) {
                merge_ent = dir_rename_find_entry(dir_re_merge, head_ent->dir);
                if (merge_ent &&
                    !head_ent->non_unique_new_dir &&
                    !merge_ent->non_unique_new_dir &&
                    !strbuf_cmp(&head_ent->new_dir, &merge_ent->new_dir)) {
                        /* 1. Renamed identically; remove it from both sides */
                        string_list_append(&remove_from_head,
                                           head_ent->dir)->util = head_ent;
                        strbuf_release(&head_ent->new_dir);
                        string_list_append(&remove_from_merge,
                                           merge_ent->dir)->util = merge_ent;
                        strbuf_release(&merge_ent->new_dir);
                } else if (tree_has_path(head, head_ent->dir)) {
                        /* 2. This wasn't a directory rename after all */
                        string_list_append(&remove_from_head,
                                           head_ent->dir)->util = head_ent;
                        strbuf_release(&head_ent->new_dir);
                }
        }

        remove_hashmap_entries(dir_re_head, &remove_from_head);
        remove_hashmap_entries(dir_re_merge, &remove_from_merge);

        hashmap_iter_init(dir_re_merge, &iter);
        while ((merge_ent = hashmap_iter_next(&iter))) {
                head_ent = dir_rename_find_entry(dir_re_head, merge_ent->dir);
                if (tree_has_path(merge, merge_ent->dir)) {
                        /* 2. This wasn't a directory rename after all */
                        string_list_append(&remove_from_merge,
                                           merge_ent->dir)->util = merge_ent;
                } else if (head_ent &&
                           !head_ent->non_unique_new_dir &&
                           !merge_ent->non_unique_new_dir) {
                        /* 3. rename/rename(1to2) */
                        /*
                         * We can assume it's not rename/rename(1to1) because
                         * that was case (1), already checked above.  So we
                         * know that head_ent->new_dir and merge_ent->new_dir
                         * are different strings.
                         */
                        output(o, 1, _("CONFLICT (rename/rename): "
                                       "Rename directory %s->%s in %s. "
                                       "Rename directory %s->%s in %s"),
                               head_ent->dir, head_ent->new_dir.buf, o->branch1,
                               head_ent->dir, merge_ent->new_dir.buf, o->branch2);
                        string_list_append(&remove_from_head,
                                           head_ent->dir)->util = head_ent;
                        strbuf_release(&head_ent->new_dir);
                        string_list_append(&remove_from_merge,
                                           merge_ent->dir)->util = merge_ent;
                        strbuf_release(&merge_ent->new_dir);
                }
        }

        remove_hashmap_entries(dir_re_head, &remove_from_head);
        remove_hashmap_entries(dir_re_merge, &remove_from_merge);
}

static struct hashmap *get_directory_renames(struct diff_queue_struct *pairs,
                                             struct tree *tree)
{
        struct hashmap *dir_renames;
        struct hashmap_iter iter;
        struct dir_rename_entry *entry;
        int i;

        /*
         * Typically, we think of a directory rename as all files from a
         * certain directory being moved to a target directory.  However,
         * what if someone first moved two files from the original
         * directory in one commit, and then renamed the directory
         * somewhere else in a later commit?  At merge time, we just know
         * that files from the original directory went to two different
         * places, and that the bulk of them ended up in the same place.
         * We want each directory rename to represent where the bulk of the
         * files from that directory end up; this function exists to find
         * where the bulk of the files went.
         *
         * The first loop below simply iterates through the list of file
         * renames, finding out how often each directory rename pair
         * possibility occurs.
         */
        dir_renames = xmalloc(sizeof(*dir_renames));
        dir_rename_init(dir_renames);
        for (i = 0; i < pairs->nr; ++i) {
                struct string_list_item *item;
                int *count;
                struct diff_filepair *pair = pairs->queue[i];
                char *old_dir, *new_dir;

                /* File not part of directory rename if it wasn't renamed */
                if (pair->status != 'R')
                        continue;

                get_renamed_dir_portion(pair->one->path, pair->two->path,
                                        &old_dir,        &new_dir);
                if (!old_dir)
                        /* Directory didn't change at all; ignore this one. */
                        continue;

                entry = dir_rename_find_entry(dir_renames, old_dir);
                if (!entry) {
                        entry = xmalloc(sizeof(*entry));
                        dir_rename_entry_init(entry, old_dir);
                        hashmap_put(dir_renames, entry);
                } else {
                        free(old_dir);
                }
                item = string_list_lookup(&entry->possible_new_dirs, new_dir);
                if (!item) {
                        item = string_list_insert(&entry->possible_new_dirs,
                                                  new_dir);
                        item->util = xcalloc(1, sizeof(int));
                } else {
                        free(new_dir);
                }
                count = item->util;
                *count += 1;
        }

        /*
         * For each directory with files moved out of it, we find out which
         * target directory received the most files so we can declare it to
         * be the "winning" target location for the directory rename.  This
         * winner gets recorded in new_dir.  If there is no winner
         * (multiple target directories received the same number of files),
         * we set non_unique_new_dir.  Once we've determined the winner (or
         * that there is no winner), we no longer need possible_new_dirs.
         */
        hashmap_iter_init(dir_renames, &iter);
        while ((entry = hashmap_iter_next(&iter))) {
                int max = 0;
                int bad_max = 0;
                char *best = NULL;

                for (i = 0; i < entry->possible_new_dirs.nr; i++) {
                        int *count = entry->possible_new_dirs.items[i].util;

                        if (*count == max)
                                bad_max = max;
                        else if (*count > max) {
                                max = *count;
                                best = entry->possible_new_dirs.items[i].string;
                        }
                }
                if (bad_max == max)
                        entry->non_unique_new_dir = 1;
                else {
                        assert(entry->new_dir.len == 0);
                        strbuf_addstr(&entry->new_dir, best);
                }
                /*
                 * The relevant directory sub-portion of the original full
                 * filepaths were xstrndup'ed before inserting into
                 * possible_new_dirs, and instead of manually iterating the
                 * list and free'ing each, just lie and tell
                 * possible_new_dirs that it did the strdup'ing so that it
                 * will free them for us.
                 */
                entry->possible_new_dirs.strdup_strings = 1;
                string_list_clear(&entry->possible_new_dirs, 1);
        }

        return dir_renames;
}

static struct dir_rename_entry *check_dir_renamed(const char *path,
                                                  struct hashmap *dir_renames)
{
        char temp[PATH_MAX];
        char *end;
        struct dir_rename_entry *entry;

        strcpy(temp, path);
        while ((end = strrchr(temp, '/'))) {
                *end = '\0';
                entry = dir_rename_find_entry(dir_renames, temp);
                if (entry)
                        return entry;
        }
        return NULL;
}

static void compute_collisions(struct hashmap *collisions,
                               struct hashmap *dir_renames,
                               struct diff_queue_struct *pairs)
{
        int i;

        /*
         * Multiple files can be mapped to the same path due to directory
         * renames done by the other side of history.  Since that other
         * side of history could have merged multiple directories into one,
         * if our side of history added the same file basename to each of
         * those directories, then all N of them would get implicitly
         * renamed by the directory rename detection into the same path,
         * and we'd get an add/add/.../add conflict, and all those adds
         * from *this* side of history.  This is not representable in the
         * index, and users aren't going to easily be able to make sense of
         * it.  So we need to provide a good warning about what's
         * happening, and fall back to no-directory-rename detection
         * behavior for those paths.
         *
         * See testcases 9e and all of section 5 from t6043 for examples.
         */
        collision_init(collisions);

        for (i = 0; i < pairs->nr; ++i) {
                struct dir_rename_entry *dir_rename_ent;
                struct collision_entry *collision_ent;
                char *new_path;
                struct diff_filepair *pair = pairs->queue[i];

                if (pair->status != 'A' && pair->status != 'R')
                        continue;
                dir_rename_ent = check_dir_renamed(pair->two->path,
                                                   dir_renames);
                if (!dir_rename_ent)
                        continue;

                new_path = apply_dir_rename(dir_rename_ent, pair->two->path);
                if (!new_path)
                        /*
                         * dir_rename_ent->non_unique_new_path is true, which
                         * means there is no directory rename for us to use,
                         * which means it won't cause us any additional
                         * collisions.
                         */
                        continue;
                collision_ent = collision_find_entry(collisions, new_path);
                if (!collision_ent) {
                        collision_ent = xcalloc(1,
                                                sizeof(struct collision_entry));
                        hashmap_entry_init(collision_ent, strhash(new_path));
                        hashmap_put(collisions, collision_ent);
                        collision_ent->target_file = new_path;
                } else {
                        free(new_path);
                }
                string_list_insert(&collision_ent->source_files,
                                   pair->two->path);
        }
}

static char *check_for_directory_rename(struct merge_options *o,
                                        const char *path,
                                        struct tree *tree,
                                        struct hashmap *dir_renames,
                                        struct hashmap *dir_rename_exclusions,
                                        struct hashmap *collisions,
                                        int *clean_merge)
{
        char *new_path = NULL;
        struct dir_rename_entry *entry = check_dir_renamed(path, dir_renames);
        struct dir_rename_entry *oentry = NULL;

        if (!entry)
                return new_path;

        /*
         * This next part is a little weird.  We do not want to do an
         * implicit rename into a directory we renamed on our side, because
         * that will result in a spurious rename/rename(1to2) conflict.  An
         * example:
         *   Base commit: dumbdir/afile, otherdir/bfile
         *   Side 1:      smrtdir/afile, otherdir/bfile
         *   Side 2:      dumbdir/afile, dumbdir/bfile
         * Here, while working on Side 1, we could notice that otherdir was
         * renamed/merged to dumbdir, and change the diff_filepair for
         * otherdir/bfile into a rename into dumbdir/bfile.  However, Side
         * 2 will notice the rename from dumbdir to smrtdir, and do the
         * transitive rename to move it from dumbdir/bfile to
         * smrtdir/bfile.  That gives us bfile in dumbdir vs being in
         * smrtdir, a rename/rename(1to2) conflict.  We really just want
         * the file to end up in smrtdir.  And the way to achieve that is
         * to not let Side1 do the rename to dumbdir, since we know that is
         * the source of one of our directory renames.
         *
         * That's why oentry and dir_rename_exclusions is here.
         *
         * As it turns out, this also prevents N-way transient rename
         * confusion; See testcases 9c and 9d of t6043.
         */
        oentry = dir_rename_find_entry(dir_rename_exclusions, entry->new_dir.buf);
        if (oentry) {
                output(o, 1, _("WARNING: Avoiding applying %s -> %s rename "
                               "to %s, because %s itself was renamed."),
                       entry->dir, entry->new_dir.buf, path, entry->new_dir.buf);
        } else {
                new_path = handle_path_level_conflicts(o, path, entry,
                                                       collisions, tree);
                *clean_merge &= (new_path != NULL);
        }

        return new_path;
}

static void apply_directory_rename_modifications(struct merge_options *o,
                                                 struct diff_filepair *pair,
                                                 char *new_path,
                                                 struct rename *re,
                                                 struct tree *tree,
                                                 struct tree *o_tree,
                                                 struct tree *a_tree,
                                                 struct tree *b_tree,
                                                 struct string_list *entries,
                                                 int *clean)
{
        struct string_list_item *item;
        int stage = (tree == a_tree ? 2 : 3);
        int update_wd;

        /*
         * In all cases where we can do directory rename detection,
         * unpack_trees() will have read pair->two->path into the
         * index and the working copy.  We need to remove it so that
         * we can instead place it at new_path.  It is guaranteed to
         * not be untracked (unpack_trees() would have errored out
         * saying the file would have been overwritten), but it might
         * be dirty, though.
         */
        update_wd = !was_dirty(o, pair->two->path);
        if (!update_wd)
                output(o, 1, _("Refusing to lose dirty file at %s"),
                       pair->two->path);
        remove_file(o, 1, pair->two->path, !update_wd);

        /* Find or create a new re->dst_entry */
        item = string_list_lookup(entries, new_path);
        if (item) {
                /*
                 * Since we're renaming on this side of history, and it's
                 * due to a directory rename on the other side of history
                 * (which we only allow when the directory in question no
                 * longer exists on the other side of history), the
                 * original entry for re->dst_entry is no longer
                 * necessary...
                 */
                re->dst_entry->processed = 1;

                /*
                 * ...because we'll be using this new one.
                 */
                re->dst_entry = item->util;
        } else {
                /*
                 * re->dst_entry is for the before-dir-rename path, and we
                 * need it to hold information for the after-dir-rename
                 * path.  Before creating a new entry, we need to mark the
                 * old one as unnecessary (...unless it is shared by
                 * src_entry, i.e. this didn't use to be a rename, in which
                 * case we can just allow the normal processing to happen
                 * for it).
                 */
                if (pair->status == 'R')
                        re->dst_entry->processed = 1;

                re->dst_entry = insert_stage_data(new_path,
                                                  o_tree, a_tree, b_tree,
                                                  entries);
                item = string_list_insert(entries, new_path);
                item->util = re->dst_entry;
        }

        /*
         * Update the stage_data with the information about the path we are
         * moving into place.  That slot will be empty and available for us
         * to write to because of the collision checks in
         * handle_path_level_conflicts().  In other words,
         * re->dst_entry->stages[stage].oid will be the null_oid, so it's
         * open for us to write to.
         *
         * It may be tempting to actually update the index at this point as
         * well, using update_stages_for_stage_data(), but as per the big
         * "NOTE" in update_stages(), doing so will modify the current
         * in-memory index which will break calls to would_lose_untracked()
         * that we need to make.  Instead, we need to just make sure that
         * the various conflict_rename_*() functions update the index
         * explicitly rather than relying on unpack_trees() to have done it.
         */
        get_tree_entry(&tree->object.oid,
                       pair->two->path,
                       &re->dst_entry->stages[stage].oid,
                       &re->dst_entry->stages[stage].mode);

        /* Update pair status */
        if (pair->status == 'A') {
                /*
                 * Recording rename information for this add makes it look
                 * like a rename/delete conflict.  Make sure we can
                 * correctly handle this as an add that was moved to a new
                 * directory instead of reporting a rename/delete conflict.
                 */
                re->add_turned_into_rename = 1;
        }
        /*
         * We don't actually look at pair->status again, but it seems
         * pedagogically correct to adjust it.
         */
        pair->status = 'R';

        /*
         * Finally, record the new location.
         */
        pair->two->path = new_path;
}

/*
 * Get information of all renames which occurred in 'pairs', making use of
 * any implicit directory renames inferred from the other side of history.
 * We need the three trees in the merge ('o_tree', 'a_tree' and 'b_tree')
 * to be able to associate the correct cache entries with the rename
 * information; tree is always equal to either a_tree or b_tree.
 */
static struct string_list *get_renames(struct merge_options *o,
                                       struct diff_queue_struct *pairs,
                                       struct hashmap *dir_renames,
                                       struct hashmap *dir_rename_exclusions,
                                       struct tree *tree,
                                       struct tree *o_tree,
                                       struct tree *a_tree,
                                       struct tree *b_tree,
                                       struct string_list *entries,
                                       int *clean_merge)
{
        int i;
        struct hashmap collisions;
        struct hashmap_iter iter;
        struct collision_entry *e;
        struct string_list *renames;

        compute_collisions(&collisions, dir_renames, pairs);
        renames = xcalloc(1, sizeof(struct string_list));

        for (i = 0; i < pairs->nr; ++i) {
                struct string_list_item *item;
                struct rename *re;
                struct diff_filepair *pair = pairs->queue[i];
                char *new_path; /* non-NULL only with directory renames */

                if (pair->status != 'A' && pair->status != 'R') {
                        diff_free_filepair(pair);
                        continue;
                }
                new_path = check_for_directory_rename(o, pair->two->path, tree,
                                                      dir_renames,
                                                      dir_rename_exclusions,
                                                      &collisions,
                                                      clean_merge);
                if (pair->status != 'R' && !new_path) {
                        diff_free_filepair(pair);
                        continue;
                }

                re = xmalloc(sizeof(*re));
                re->processed = 0;
                re->add_turned_into_rename = 0;
                re->pair = pair;
                item = string_list_lookup(entries, re->pair->one->path);
                if (!item)
                        re->src_entry = insert_stage_data(re->pair->one->path,
                                        o_tree, a_tree, b_tree, entries);
                else
                        re->src_entry = item->util;

                item = string_list_lookup(entries, re->pair->two->path);
                if (!item)
                        re->dst_entry = insert_stage_data(re->pair->two->path,
                                        o_tree, a_tree, b_tree, entries);
                else
                        re->dst_entry = item->util;
                item = string_list_insert(renames, pair->one->path);
                item->util = re;
                if (new_path)
                        apply_directory_rename_modifications(o, pair, new_path,
                                                             re, tree, o_tree,
                                                             a_tree, b_tree,
                                                             entries,
                                                             clean_merge);
        }

        hashmap_iter_init(&collisions, &iter);
        while ((e = hashmap_iter_next(&iter))) {
                free(e->target_file);
                string_list_clear(&e->source_files, 0);
        }
        hashmap_free(&collisions, 1);
        return renames;
}

static int process_renames(struct merge_options *o,
                           struct string_list *a_renames,
                           struct string_list *b_renames)
{
        int clean_merge = 1, i, j;
        struct string_list a_by_dst = STRING_LIST_INIT_NODUP;
        struct string_list b_by_dst = STRING_LIST_INIT_NODUP;
        const struct rename *sre;

        for (i = 0; i < a_renames->nr; i++) {
                sre = a_renames->items[i].util;
                string_list_insert(&a_by_dst, sre->pair->two->path)->util
                        = (void *)sre;
        }
        for (i = 0; i < b_renames->nr; i++) {
                sre = b_renames->items[i].util;
                string_list_insert(&b_by_dst, sre->pair->two->path)->util
                        = (void *)sre;
        }

        for (i = 0, j = 0; i < a_renames->nr || j < b_renames->nr;) {
                struct string_list *renames1, *renames2Dst;
                struct rename *ren1 = NULL, *ren2 = NULL;
                const char *branch1, *branch2;
                const char *ren1_src, *ren1_dst;
                struct string_list_item *lookup;

                if (i >= a_renames->nr) {
                        ren2 = b_renames->items[j++].util;
                } else if (j >= b_renames->nr) {
                        ren1 = a_renames->items[i++].util;
                } else {
                        int compare = strcmp(a_renames->items[i].string,
                                             b_renames->items[j].string);
                        if (compare <= 0)
                                ren1 = a_renames->items[i++].util;
                        if (compare >= 0)
                                ren2 = b_renames->items[j++].util;
                }

                /* TODO: refactor, so that 1/2 are not needed */
                if (ren1) {
                        renames1 = a_renames;
                        renames2Dst = &b_by_dst;
                        branch1 = o->branch1;
                        branch2 = o->branch2;
                } else {
                        renames1 = b_renames;
                        renames2Dst = &a_by_dst;
                        branch1 = o->branch2;
                        branch2 = o->branch1;
                        SWAP(ren2, ren1);
                }

                if (ren1->processed)
                        continue;
                ren1->processed = 1;
                ren1->dst_entry->processed = 1;
                /* BUG: We should only mark src_entry as processed if we
                 * are not dealing with a rename + add-source case.
                 */
                ren1->src_entry->processed = 1;

                ren1_src = ren1->pair->one->path;
                ren1_dst = ren1->pair->two->path;

                if (ren2) {
                        /* One file renamed on both sides */
                        const char *ren2_src = ren2->pair->one->path;
                        const char *ren2_dst = ren2->pair->two->path;
                        enum rename_type rename_type;
                        if (strcmp(ren1_src, ren2_src) != 0)
                                die("BUG: ren1_src != ren2_src");
                        ren2->dst_entry->processed = 1;
                        ren2->processed = 1;
                        if (strcmp(ren1_dst, ren2_dst) != 0) {
                                rename_type = RENAME_ONE_FILE_TO_TWO;
                                clean_merge = 0;
                        } else {
                                rename_type = RENAME_ONE_FILE_TO_ONE;
                                /* BUG: We should only remove ren1_src in
                                 * the base stage (think of rename +
                                 * add-source cases).
                                 */
                                remove_file(o, 1, ren1_src, 1);
                                update_entry(ren1->dst_entry,
                                             ren1->pair->one,
                                             ren1->pair->two,
                                             ren2->pair->two);
                        }
                        setup_rename_conflict_info(rename_type,
                                                   ren1->pair,
                                                   ren2->pair,
                                                   branch1,
                                                   branch2,
                                                   ren1->dst_entry,
                                                   ren2->dst_entry,
                                                   o,
                                                   NULL,
                                                   NULL);
                } else if ((lookup = string_list_lookup(renames2Dst, ren1_dst))) {
                        /* Two different files renamed to the same thing */
                        char *ren2_dst;
                        ren2 = lookup->util;
                        ren2_dst = ren2->pair->two->path;
                        if (strcmp(ren1_dst, ren2_dst) != 0)
                                die("BUG: ren1_dst != ren2_dst");

                        clean_merge = 0;
                        ren2->processed = 1;
                        /*
                         * BUG: We should only mark src_entry as processed
                         * if we are not dealing with a rename + add-source
                         * case.
                         */
                        ren2->src_entry->processed = 1;

                        setup_rename_conflict_info(RENAME_TWO_FILES_TO_ONE,
                                                   ren1->pair,
                                                   ren2->pair,
                                                   branch1,
                                                   branch2,
                                                   ren1->dst_entry,
                                                   ren2->dst_entry,
                                                   o,
                                                   ren1->src_entry,
                                                   ren2->src_entry);

                } else {
                        /* Renamed in 1, maybe changed in 2 */
                        /* we only use sha1 and mode of these */
                        struct diff_filespec src_other, dst_other;
                        int try_merge;

                        /*
                         * unpack_trees loads entries from common-commit
                         * into stage 1, from head-commit into stage 2, and
                         * from merge-commit into stage 3.  We keep track
                         * of which side corresponds to the rename.
                         */
                        int renamed_stage = a_renames == renames1 ? 2 : 3;
                        int other_stage =   a_renames == renames1 ? 3 : 2;

                        /* BUG: We should only remove ren1_src in the base
                         * stage and in other_stage (think of rename +
                         * add-source case).
                         */
                        remove_file(o, 1, ren1_src,
                                    renamed_stage == 2 || !was_tracked(o, ren1_src));

                        oidcpy(&src_other.oid,
                               &ren1->src_entry->stages[other_stage].oid);
                        src_other.mode = ren1->src_entry->stages[other_stage].mode;
                        oidcpy(&dst_other.oid,
                               &ren1->dst_entry->stages[other_stage].oid);
                        dst_other.mode = ren1->dst_entry->stages[other_stage].mode;
                        try_merge = 0;

                        if (oid_eq(&src_other.oid, &null_oid) &&
                            ren1->add_turned_into_rename) {
                                setup_rename_conflict_info(RENAME_DIR,
                                                           ren1->pair,
                                                           NULL,
                                                           branch1,
                                                           branch2,
                                                           ren1->dst_entry,
                                                           NULL,
                                                           o,
                                                           NULL,
                                                           NULL);
                        } else if (oid_eq(&src_other.oid, &null_oid)) {
                                setup_rename_conflict_info(RENAME_DELETE,
                                                           ren1->pair,
                                                           NULL,
                                                           branch1,
                                                           branch2,
                                                           ren1->dst_entry,
                                                           NULL,
                                                           o,
                                                           NULL,
                                                           NULL);
                        } else if ((dst_other.mode == ren1->pair->two->mode) &&
                                   oid_eq(&dst_other.oid, &ren1->pair->two->oid)) {
                                /*
                                 * Added file on the other side identical to
                                 * the file being renamed: clean merge.
                                 * Also, there is no need to overwrite the
                                 * file already in the working copy, so call
                                 * update_file_flags() instead of
                                 * update_file().
                                 */
                                if (update_file_flags(o,
                                                      &ren1->pair->two->oid,
                                                      ren1->pair->two->mode,
                                                      ren1_dst,
                                                      1, /* update_cache */
                                                      0  /* update_wd    */))
                                        clean_merge = -1;
                        } else if (!oid_eq(&dst_other.oid, &null_oid)) {
                                clean_merge = 0;
                                try_merge = 1;
                                output(o, 1, _("CONFLICT (rename/add): Rename %s->%s in %s. "
                                       "%s added in %s"),
                                       ren1_src, ren1_dst, branch1,
                                       ren1_dst, branch2);
                                if (o->call_depth) {
                                        struct merge_file_info mfi;
                                        if (merge_file_one(o, ren1_dst, &null_oid, 0,
                                                           &ren1->pair->two->oid,
                                                           ren1->pair->two->mode,
                                                           &dst_other.oid,
                                                           dst_other.mode,
                                                           branch1, branch2, &mfi)) {
                                                clean_merge = -1;
                                                goto cleanup_and_return;
                                        }
                                        output(o, 1, _("Adding merged %s"), ren1_dst);
                                        if (update_file(o, 0, &mfi.oid,
                                                        mfi.mode, ren1_dst))
                                                clean_merge = -1;
                                        try_merge = 0;
                                } else {
                                        char *new_path = unique_path(o, ren1_dst, branch2);
                                        output(o, 1, _("Adding as %s instead"), new_path);
                                        if (update_file(o, 0, &dst_other.oid,
                                                        dst_other.mode, new_path))
                                                clean_merge = -1;
                                        free(new_path);
                                }
                        } else
                                try_merge = 1;

                        if (clean_merge < 0)
                                goto cleanup_and_return;
                        if (try_merge) {
                                struct diff_filespec *one, *a, *b;
                                src_other.path = (char *)ren1_src;

                                one = ren1->pair->one;
                                if (a_renames == renames1) {
                                        a = ren1->pair->two;
                                        b = &src_other;
                                } else {
                                        b = ren1->pair->two;
                                        a = &src_other;
                                }
                                update_entry(ren1->dst_entry, one, a, b);
                                setup_rename_conflict_info(RENAME_NORMAL,
                                                           ren1->pair,
                                                           NULL,
                                                           branch1,
                                                           NULL,
                                                           ren1->dst_entry,
                                                           NULL,
                                                           o,
                                                           NULL,
                                                           NULL);
                        }
                }
        }
cleanup_and_return:
        string_list_clear(&a_by_dst, 0);
        string_list_clear(&b_by_dst, 0);

        return clean_merge;
}

struct rename_info {
        struct string_list *head_renames;
        struct string_list *merge_renames;
};

static void initial_cleanup_rename(struct diff_queue_struct *pairs,
                                   struct hashmap *dir_renames)
{
        struct hashmap_iter iter;
        struct dir_rename_entry *e;

        hashmap_iter_init(dir_renames, &iter);
        while ((e = hashmap_iter_next(&iter))) {
                free(e->dir);
                strbuf_release(&e->new_dir);
                /* possible_new_dirs already cleared in get_directory_renames */
        }
        hashmap_free(dir_renames, 1);
        free(dir_renames);

        free(pairs->queue);
        free(pairs);
}

static int handle_renames(struct merge_options *o,
                          struct tree *common,
                          struct tree *head,
                          struct tree *merge,
                          struct string_list *entries,
                          struct rename_info *ri)
{
        struct diff_queue_struct *head_pairs, *merge_pairs;
        struct hashmap *dir_re_head, *dir_re_merge;
        int clean = 1;

        ri->head_renames = NULL;
        ri->merge_renames = NULL;

        if (!merge_detect_rename(o))
                return 1;

        head_pairs = get_diffpairs(o, common, head);
        merge_pairs = get_diffpairs(o, common, merge);

        dir_re_head = get_directory_renames(head_pairs, head);
        dir_re_merge = get_directory_renames(merge_pairs, merge);

        handle_directory_level_conflicts(o,
                                         dir_re_head, head,
                                         dir_re_merge, merge);

        ri->head_renames  = get_renames(o, head_pairs,
                                        dir_re_merge, dir_re_head, head,
                                        common, head, merge, entries,
                                        &clean);
        if (clean < 0)
                goto cleanup;
        ri->merge_renames = get_renames(o, merge_pairs,
                                        dir_re_head, dir_re_merge, merge,
                                        common, head, merge, entries,
                                        &clean);
        if (clean < 0)
                goto cleanup;
        clean &= process_renames(o, ri->head_renames, ri->merge_renames);

cleanup:
        /*
         * Some cleanup is deferred until cleanup_renames() because the
         * data structures are still needed and referenced in
         * process_entry().  But there are a few things we can free now.
         */
        initial_cleanup_rename(head_pairs, dir_re_head);
        initial_cleanup_rename(merge_pairs, dir_re_merge);

        return clean;
}

static void final_cleanup_rename(struct string_list *rename)
{
        const struct rename *re;
        int i;

        if (rename == NULL)
                return;

        for (i = 0; i < rename->nr; i++) {
                re = rename->items[i].util;
                diff_free_filepair(re->pair);
        }
        string_list_clear(rename, 1);
        free(rename);
}

static void final_cleanup_renames(struct rename_info *re_info)
{
        final_cleanup_rename(re_info->head_renames);
        final_cleanup_rename(re_info->merge_renames);
}

static struct object_id *stage_oid(const struct object_id *oid, unsigned mode)
{
        return (is_null_oid(oid) || mode == 0) ? NULL: (struct object_id *)oid;
}

static int read_oid_strbuf(struct merge_options *o,
        const struct object_id *oid, struct strbuf *dst)
{
        void *buf;
        enum object_type type;
        unsigned long size;
        buf = read_object_file(oid, &type, &size);
        if (!buf)
                return err(o, _("cannot read object %s"), oid_to_hex(oid));
        if (type != OBJ_BLOB) {
                free(buf);
                return err(o, _("object %s is not a blob"), oid_to_hex(oid));
        }
        strbuf_attach(dst, buf, size, size + 1);
        return 0;
}

static int blob_unchanged(struct merge_options *opt,
                          const struct object_id *o_oid,
                          unsigned o_mode,
                          const struct object_id *a_oid,
                          unsigned a_mode,
                          int renormalize, const char *path)
{
        struct strbuf o = STRBUF_INIT;
        struct strbuf a = STRBUF_INIT;
        int ret = 0; /* assume changed for safety */

        if (a_mode != o_mode)
                return 0;
        if (oid_eq(o_oid, a_oid))
                return 1;
        if (!renormalize)
                return 0;

        assert(o_oid && a_oid);
        if (read_oid_strbuf(opt, o_oid, &o) || read_oid_strbuf(opt, a_oid, &a))
                goto error_return;
        /*
         * Note: binary | is used so that both renormalizations are
         * performed.  Comparison can be skipped if both files are
         * unchanged since their sha1s have already been compared.
         */
        if (renormalize_buffer(&the_index, path, o.buf, o.len, &o) |
            renormalize_buffer(&the_index, path, a.buf, a.len, &a))
                ret = (o.len == a.len && !memcmp(o.buf, a.buf, o.len));

error_return:
        strbuf_release(&o);
        strbuf_release(&a);
        return ret;
}

static int handle_modify_delete(struct merge_options *o,
                                 const char *path,
                                 struct object_id *o_oid, int o_mode,
                                 struct object_id *a_oid, int a_mode,
                                 struct object_id *b_oid, int b_mode)
{
        const char *modify_branch, *delete_branch;
        struct object_id *changed_oid;
        int changed_mode;

        if (a_oid) {
                modify_branch = o->branch1;
                delete_branch = o->branch2;
                changed_oid = a_oid;
                changed_mode = a_mode;
        } else {
                modify_branch = o->branch2;
                delete_branch = o->branch1;
                changed_oid = b_oid;
                changed_mode = b_mode;
        }

        return handle_change_delete(o,
                                    path, NULL,
                                    o_oid, o_mode,
                                    changed_oid, changed_mode,
                                    modify_branch, delete_branch,
                                    _("modify"), _("modified"));
}

static int merge_content(struct merge_options *o,
                         const char *path,
                         int is_dirty,
                         struct object_id *o_oid, int o_mode,
                         struct object_id *a_oid, int a_mode,
                         struct object_id *b_oid, int b_mode,
                         struct rename_conflict_info *rename_conflict_info)
{
        const char *reason = _("content");
        const char *path1 = NULL, *path2 = NULL;
        struct merge_file_info mfi;
        struct diff_filespec one, a, b;
        unsigned df_conflict_remains = 0;

        if (!o_oid) {
                reason = _("add/add");
                o_oid = (struct object_id *)&null_oid;
        }
        one.path = a.path = b.path = (char *)path;
        oidcpy(&one.oid, o_oid);
        one.mode = o_mode;
        oidcpy(&a.oid, a_oid);
        a.mode = a_mode;
        oidcpy(&b.oid, b_oid);
        b.mode = b_mode;

        if (rename_conflict_info) {
                struct diff_filepair *pair1 = rename_conflict_info->pair1;

                path1 = (o->branch1 == rename_conflict_info->branch1) ?
                        pair1->two->path : pair1->one->path;
                /* If rename_conflict_info->pair2 != NULL, we are in
                 * RENAME_ONE_FILE_TO_ONE case.  Otherwise, we have a
                 * normal rename.
                 */
                path2 = (rename_conflict_info->pair2 ||
                         o->branch2 == rename_conflict_info->branch1) ?
                        pair1->two->path : pair1->one->path;

                if (dir_in_way(path, !o->call_depth,
                               S_ISGITLINK(pair1->two->mode)))
                        df_conflict_remains = 1;
        }
        if (merge_file_special_markers(o, &one, &a, &b, path,
                                       o->branch1, path1,
                                       o->branch2, path2, &mfi))
                return -1;

        /*
         * We can skip updating the working tree file iff:
         *   a) The merge is clean
         *   b) The merge matches what was in HEAD (content, mode, pathname)
         *   c) The target path is usable (i.e. not involved in D/F conflict)
         */
        if (mfi.clean &&
            was_tracked_and_matches(o, path, &mfi.oid, mfi.mode) &&
            !df_conflict_remains) {
                output(o, 3, _("Skipped %s (merged same as existing)"), path);
                if (add_cacheinfo(o, mfi.mode, &mfi.oid, path,
                                  0, (!o->call_depth && !is_dirty), 0))
                        return -1;
                return mfi.clean;
        }

        if (!mfi.clean) {
                if (S_ISGITLINK(mfi.mode))
                        reason = _("submodule");
                output(o, 1, _("CONFLICT (%s): Merge conflict in %s"),
                                reason, path);
                if (rename_conflict_info && !df_conflict_remains)
                        if (update_stages(o, path, &one, &a, &b))
                                return -1;
        }

        if (df_conflict_remains || is_dirty) {
                char *new_path;
                if (o->call_depth) {
                        remove_file_from_cache(path);
                } else {
                        if (!mfi.clean) {
                                if (update_stages(o, path, &one, &a, &b))
                                        return -1;
                        } else {
                                int file_from_stage2 = was_tracked(o, path);
                                struct diff_filespec merged;
                                oidcpy(&merged.oid, &mfi.oid);
                                merged.mode = mfi.mode;

                                if (update_stages(o, path, NULL,
                                                  file_from_stage2 ? &merged : NULL,
                                                  file_from_stage2 ? NULL : &merged))
                                        return -1;
                        }

                }
                new_path = unique_path(o, path, rename_conflict_info->branch1);
                if (is_dirty) {
                        output(o, 1, _("Refusing to lose dirty file at %s"),
                               path);
                }
                output(o, 1, _("Adding as %s instead"), new_path);
                if (update_file(o, 0, &mfi.oid, mfi.mode, new_path)) {
                        free(new_path);
                        return -1;
                }
                free(new_path);
                mfi.clean = 0;
        } else if (update_file(o, mfi.clean, &mfi.oid, mfi.mode, path))
                return -1;
        return !is_dirty && mfi.clean;
}

static int conflict_rename_normal(struct merge_options *o,
                                  const char *path,
                                  struct object_id *o_oid, unsigned int o_mode,
                                  struct object_id *a_oid, unsigned int a_mode,
                                  struct object_id *b_oid, unsigned int b_mode,
                                  struct rename_conflict_info *ci)
{
        /* Merge the content and write it out */
        return merge_content(o, path, was_dirty(o, path),
                             o_oid, o_mode, a_oid, a_mode, b_oid, b_mode,
                             ci);
}

/* Per entry merge function */
static int process_entry(struct merge_options *o,
                         const char *path, struct stage_data *entry)
{
        int clean_merge = 1;
        int normalize = o->renormalize;
        unsigned o_mode = entry->stages[1].mode;
        unsigned a_mode = entry->stages[2].mode;
        unsigned b_mode = entry->stages[3].mode;
        struct object_id *o_oid = stage_oid(&entry->stages[1].oid, o_mode);
        struct object_id *a_oid = stage_oid(&entry->stages[2].oid, a_mode);
        struct object_id *b_oid = stage_oid(&entry->stages[3].oid, b_mode);

        entry->processed = 1;
        if (entry->rename_conflict_info) {
                struct rename_conflict_info *conflict_info = entry->rename_conflict_info;
                switch (conflict_info->rename_type) {
                case RENAME_NORMAL:
                case RENAME_ONE_FILE_TO_ONE:
                        clean_merge = conflict_rename_normal(o,
                                                             path,
                                                             o_oid, o_mode,
                                                             a_oid, a_mode,
                                                             b_oid, b_mode,
                                                             conflict_info);
                        break;
                case RENAME_DIR:
                        clean_merge = 1;
                        if (conflict_rename_dir(o,
                                                conflict_info->pair1,
                                                conflict_info->branch1,
                                                conflict_info->branch2))
                                clean_merge = -1;
                        break;
                case RENAME_DELETE:
                        clean_merge = 0;
                        if (conflict_rename_delete(o,
                                                   conflict_info->pair1,
                                                   conflict_info->branch1,
                                                   conflict_info->branch2))
                                clean_merge = -1;
                        break;
                case RENAME_ONE_FILE_TO_TWO:
                        clean_merge = 0;
                        if (conflict_rename_rename_1to2(o, conflict_info))
                                clean_merge = -1;
                        break;
                case RENAME_TWO_FILES_TO_ONE:
                        clean_merge = 0;
                        if (conflict_rename_rename_2to1(o, conflict_info))
                                clean_merge = -1;
                        break;
                default:
                        entry->processed = 0;
                        break;
                }
        } else if (o_oid && (!a_oid || !b_oid)) {
                /* Case A: Deleted in one */
                if ((!a_oid && !b_oid) ||
                    (!b_oid && blob_unchanged(o, o_oid, o_mode, a_oid, a_mode, normalize, path)) ||
                    (!a_oid && blob_unchanged(o, o_oid, o_mode, b_oid, b_mode, normalize, path))) {
                        /* Deleted in both or deleted in one and
                         * unchanged in the other */
                        if (a_oid)
                                output(o, 2, _("Removing %s"), path);
                        /* do not touch working file if it did not exist */
                        remove_file(o, 1, path, !a_oid);
                } else {
                        /* Modify/delete; deleted side may have put a directory in the way */
                        clean_merge = 0;
                        if (handle_modify_delete(o, path, o_oid, o_mode,
                                                 a_oid, a_mode, b_oid, b_mode))
                                clean_merge = -1;
                }
        } else if ((!o_oid && a_oid && !b_oid) ||
                   (!o_oid && !a_oid && b_oid)) {
                /* Case B: Added in one. */
                /* [nothing|directory] -> ([nothing|directory], file) */

                const char *add_branch;
                const char *other_branch;
                unsigned mode;
                const struct object_id *oid;
                const char *conf;

                if (a_oid) {
                        add_branch = o->branch1;
                        other_branch = o->branch2;
                        mode = a_mode;
                        oid = a_oid;
                        conf = _("file/directory");
                } else {
                        add_branch = o->branch2;
                        other_branch = o->branch1;
                        mode = b_mode;
                        oid = b_oid;
                        conf = _("directory/file");
                }
                if (dir_in_way(path,
                               !o->call_depth && !S_ISGITLINK(a_mode),
                               0)) {
                        char *new_path = unique_path(o, path, add_branch);
                        clean_merge = 0;
                        output(o, 1, _("CONFLICT (%s): There is a directory with name %s in %s. "
                               "Adding %s as %s"),
                               conf, path, other_branch, path, new_path);
                        if (update_file(o, 0, oid, mode, new_path))
                                clean_merge = -1;
                        else if (o->call_depth)
                                remove_file_from_cache(path);
                        free(new_path);
                } else {
                        output(o, 2, _("Adding %s"), path);
                        /* do not overwrite file if already present */
                        if (update_file_flags(o, oid, mode, path, 1, !a_oid))
                                clean_merge = -1;
                }
        } else if (a_oid && b_oid) {
                /* Case C: Added in both (check for same permissions) and */
                /* case D: Modified in both, but differently. */
                int is_dirty = 0; /* unpack_trees would have bailed if dirty */
                clean_merge = merge_content(o, path, is_dirty,
                                            o_oid, o_mode, a_oid, a_mode, b_oid, b_mode,
                                            NULL);
        } else if (!o_oid && !a_oid && !b_oid) {
                /*
                 * this entry was deleted altogether. a_mode == 0 means
                 * we had that path and want to actively remove it.
                 */
                remove_file(o, 1, path, !a_mode);
        } else
                die("BUG: fatal merge failure, shouldn't happen.");

        return clean_merge;
}

int merge_trees(struct merge_options *o,
                struct tree *head,
                struct tree *merge,
                struct tree *common,
                struct tree **result)
{
        int code, clean;

        if (o->subtree_shift) {
                merge = shift_tree_object(head, merge, o->subtree_shift);
                common = shift_tree_object(head, common, o->subtree_shift);
        }

        if (oid_eq(&common->object.oid, &merge->object.oid)) {
                struct strbuf sb = STRBUF_INIT;

                if (!o->call_depth && index_has_changes(&sb)) {
                        err(o, _("Dirty index: cannot merge (dirty: %s)"),
                            sb.buf);
                        return 0;
                }
                output(o, 0, _("Already up to date!"));
                *result = head;
                return 1;
        }

        code = git_merge_trees(o, common, head, merge);

        if (code != 0) {
                if (show(o, 4) || o->call_depth)
                        err(o, _("merging of trees %s and %s failed"),
                            oid_to_hex(&head->object.oid),
                            oid_to_hex(&merge->object.oid));
                return -1;
        }

        if (unmerged_cache()) {
                struct string_list *entries;
                struct rename_info re_info;
                int i;
                /*
                 * Only need the hashmap while processing entries, so
                 * initialize it here and free it when we are done running
                 * through the entries. Keeping it in the merge_options as
                 * opposed to decaring a local hashmap is for convenience
                 * so that we don't have to pass it to around.
                 */
                hashmap_init(&o->current_file_dir_set, path_hashmap_cmp, NULL, 512);
                get_files_dirs(o, head);
                get_files_dirs(o, merge);

                entries = get_unmerged();
                clean = handle_renames(o, common, head, merge, entries,
                                       &re_info);
                record_df_conflict_files(o, entries);
                if (clean < 0)
                        goto cleanup;
                for (i = entries->nr-1; 0 <= i; i--) {
                        const char *path = entries->items[i].string;
                        struct stage_data *e = entries->items[i].util;
                        if (!e->processed) {
                                int ret = process_entry(o, path, e);
                                if (!ret)
                                        clean = 0;
                                else if (ret < 0) {
                                        clean = ret;
                                        goto cleanup;
                                }
                        }
                }
                for (i = 0; i < entries->nr; i++) {
                        struct stage_data *e = entries->items[i].util;
                        if (!e->processed)
                                die("BUG: unprocessed path??? %s",
                                    entries->items[i].string);
                }

cleanup:
                final_cleanup_renames(&re_info);

                string_list_clear(entries, 1);
                free(entries);

                hashmap_free(&o->current_file_dir_set, 1);

                if (clean < 0)
                        return clean;
        }
        else
                clean = 1;

        /* Free the extra index left from git_merge_trees() */
        /*
         * FIXME: Need to also free data allocated by
         * setup_unpack_trees_porcelain() tucked away in o->unpack_opts.msgs,
         * but the problem is that only half of it refers to dynamically
         * allocated data, while the other half points at static strings.
         */
        discard_index(&o->orig_index);

        if (o->call_depth && !(*result = write_tree_from_memory(o)))
                return -1;

        return clean;
}

static struct commit_list *reverse_commit_list(struct commit_list *list)
{
        struct commit_list *next = NULL, *current, *backup;
        for (current = list; current; current = backup) {
                backup = current->next;
                current->next = next;
                next = current;
        }
        return next;
}

/*
 * Merge the commits h1 and h2, return the resulting virtual
 * commit object and a flag indicating the cleanness of the merge.
 */
int merge_recursive(struct merge_options *o,
                    struct commit *h1,
                    struct commit *h2,
                    struct commit_list *ca,
                    struct commit **result)
{
        struct commit_list *iter;
        struct commit *merged_common_ancestors;
        struct tree *mrtree;
        int clean;

        if (show(o, 4)) {
                output(o, 4, _("Merging:"));
                output_commit_title(o, h1);
                output_commit_title(o, h2);
        }

        if (!ca) {
                ca = get_merge_bases(h1, h2);
                ca = reverse_commit_list(ca);
        }

        if (show(o, 5)) {
                unsigned cnt = commit_list_count(ca);

                output(o, 5, Q_("found %u common ancestor:",
                                "found %u common ancestors:", cnt), cnt);
                for (iter = ca; iter; iter = iter->next)
                        output_commit_title(o, iter->item);
        }

        merged_common_ancestors = pop_commit(&ca);
        if (merged_common_ancestors == NULL) {
                /* if there is no common ancestor, use an empty tree */
                struct tree *tree;

                tree = lookup_tree(the_hash_algo->empty_tree);
                merged_common_ancestors = make_virtual_commit(tree, "ancestor");
        }

        for (iter = ca; iter; iter = iter->next) {
                const char *saved_b1, *saved_b2;
                o->call_depth++;
                /*
                 * When the merge fails, the result contains files
                 * with conflict markers. The cleanness flag is
                 * ignored (unless indicating an error), it was never
                 * actually used, as result of merge_trees has always
                 * overwritten it: the committed "conflicts" were
                 * already resolved.
                 */
                discard_cache();
                saved_b1 = o->branch1;
                saved_b2 = o->branch2;
                o->branch1 = "Temporary merge branch 1";
                o->branch2 = "Temporary merge branch 2";
                if (merge_recursive(o, merged_common_ancestors, iter->item,
                                    NULL, &merged_common_ancestors) < 0)
                        return -1;
                o->branch1 = saved_b1;
                o->branch2 = saved_b2;
                o->call_depth--;

                if (!merged_common_ancestors)
                        return err(o, _("merge returned no commit"));
        }

        discard_cache();
        if (!o->call_depth)
                read_cache();

        o->ancestor = "merged common ancestors";
        clean = merge_trees(o, get_commit_tree(h1), get_commit_tree(h2),
                            get_commit_tree(merged_common_ancestors),
                            &mrtree);
        if (clean < 0) {
                flush_output(o);
                return clean;
        }

        if (o->call_depth) {
                *result = make_virtual_commit(mrtree, "merged tree");
                commit_list_insert(h1, &(*result)->parents);
                commit_list_insert(h2, &(*result)->parents->next);
        }
        flush_output(o);
        if (!o->call_depth && o->buffer_output < 2)
                strbuf_release(&o->obuf);
        if (show(o, 2))
                diff_warn_rename_limit("merge.renamelimit",
                                       o->needed_rename_limit, 0);
        return clean;
}

static struct commit *get_ref(const struct object_id *oid, const char *name)
{
        struct object *object;

        object = deref_tag(parse_object(oid), name, strlen(name));
        if (!object)
                return NULL;
        if (object->type == OBJ_TREE)
                return make_virtual_commit((struct tree*)object, name);
        if (object->type != OBJ_COMMIT)
                return NULL;
        if (parse_commit((struct commit *)object))
                return NULL;
        return (struct commit *)object;
}

int merge_recursive_generic(struct merge_options *o,
                            const struct object_id *head,
                            const struct object_id *merge,
                            int num_base_list,
                            const struct object_id **base_list,
                            struct commit **result)
{
        int clean;
        struct lock_file lock = LOCK_INIT;
        struct commit *head_commit = get_ref(head, o->branch1);
        struct commit *next_commit = get_ref(merge, o->branch2);
        struct commit_list *ca = NULL;

        if (base_list) {
                int i;
                for (i = 0; i < num_base_list; ++i) {
                        struct commit *base;
                        if (!(base = get_ref(base_list[i], oid_to_hex(base_list[i]))))
                                return err(o, _("Could not parse object '%s'"),
                                        oid_to_hex(base_list[i]));
                        commit_list_insert(base, &ca);
                }
        }

        hold_locked_index(&lock, LOCK_DIE_ON_ERROR);
        clean = merge_recursive(o, head_commit, next_commit, ca,
                        result);
        if (clean < 0) {
                rollback_lock_file(&lock);
                return clean;
        }

        if (write_locked_index(&the_index, &lock,
                               COMMIT_LOCK | SKIP_IF_UNCHANGED))
                return err(o, _("Unable to write index."));

        return clean ? 0 : 1;
}

static void merge_recursive_config(struct merge_options *o)
{
        char *value = NULL;
        git_config_get_int("merge.verbosity", &o->verbosity);
        git_config_get_int("diff.renamelimit", &o->diff_rename_limit);
        git_config_get_int("merge.renamelimit", &o->merge_rename_limit);
        if (!git_config_get_string("diff.renames", &value)) {
                o->diff_detect_rename = git_config_rename("diff.renames", value);
                free(value);
        }
        if (!git_config_get_string("merge.renames", &value)) {
                o->merge_detect_rename = git_config_rename("merge.renames", value);
                free(value);
        }
        git_config(git_xmerge_config, NULL);
}

void init_merge_options(struct merge_options *o)
{
        const char *merge_verbosity;
        memset(o, 0, sizeof(struct merge_options));
        o->verbosity = 2;
        o->buffer_output = 1;
        o->diff_rename_limit = -1;
        o->merge_rename_limit = -1;
        o->renormalize = 0;
        o->diff_detect_rename = -1;
        o->merge_detect_rename = -1;
        merge_recursive_config(o);
        merge_verbosity = getenv("GIT_MERGE_VERBOSITY");
        if (merge_verbosity)
                o->verbosity = strtol(merge_verbosity, NULL, 10);
        if (o->verbosity >= 5)
                o->buffer_output = 0;
        strbuf_init(&o->obuf, 0);
        string_list_init(&o->df_conflict_file_set, 1);
}

int parse_merge_opt(struct merge_options *o, const char *s)
{
        const char *arg;

        if (!s || !*s)
                return -1;
        if (!strcmp(s, "ours"))
                o->recursive_variant = MERGE_RECURSIVE_OURS;
        else if (!strcmp(s, "theirs"))
                o->recursive_variant = MERGE_RECURSIVE_THEIRS;
        else if (!strcmp(s, "subtree"))
                o->subtree_shift = "";
        else if (skip_prefix(s, "subtree=", &arg))
                o->subtree_shift = arg;
        else if (!strcmp(s, "patience"))
                o->xdl_opts = DIFF_WITH_ALG(o, PATIENCE_DIFF);
        else if (!strcmp(s, "histogram"))
                o->xdl_opts = DIFF_WITH_ALG(o, HISTOGRAM_DIFF);
        else if (skip_prefix(s, "diff-algorithm=", &arg)) {
                long value = parse_algorithm_value(arg);
                if (value < 0)
                        return -1;
                /* clear out previous settings */
                DIFF_XDL_CLR(o, NEED_MINIMAL);
                o->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
                o->xdl_opts |= value;
        }
        else if (!strcmp(s, "ignore-space-change"))
                DIFF_XDL_SET(o, IGNORE_WHITESPACE_CHANGE);
        else if (!strcmp(s, "ignore-all-space"))
                DIFF_XDL_SET(o, IGNORE_WHITESPACE);
        else if (!strcmp(s, "ignore-space-at-eol"))
                DIFF_XDL_SET(o, IGNORE_WHITESPACE_AT_EOL);
        else if (!strcmp(s, "ignore-cr-at-eol"))
                DIFF_XDL_SET(o, IGNORE_CR_AT_EOL);
        else if (!strcmp(s, "renormalize"))
                o->renormalize = 1;
        else if (!strcmp(s, "no-renormalize"))
                o->renormalize = 0;
        else if (!strcmp(s, "no-renames"))
                o->merge_detect_rename = 0;
        else if (!strcmp(s, "find-renames")) {
                o->merge_detect_rename = 1;
                o->rename_score = 0;
        }
        else if (skip_prefix(s, "find-renames=", &arg) ||
                 skip_prefix(s, "rename-threshold=", &arg)) {
                if ((o->rename_score = parse_rename_score(&arg)) == -1 || *arg != 0)
                        return -1;
                o->merge_detect_rename = 1;
        }
        else
                return -1;
        return 0;
}