#define NO_THE_INDEX_COMPATIBILITY_MACROS
#include "cache.h"
#include "dir.h"
#include "tree.h"
#include "tree-walk.h"
#include "cache-tree.h"
#include "unpack-trees.h"
#include "progress.h"
#include "refs.h"
#include "attr.h"

/*
 * Error messages expected by scripts out of plumbing commands such as
 * read-tree.  Non-scripted Porcelain is not required to use these messages
 * and in fact are encouraged to reword them to better suit their particular
 * situation better.  See how "git checkout" replaces not_uptodate_file to
 * explain why it does not allow switching between branches when you have
 * local changes, for example.
 */
static struct unpack_trees_error_msgs unpack_plumbing_errors = {
        /* would_overwrite */
        "Entry '%s' would be overwritten by merge. Cannot merge.",

        /* not_uptodate_file */
        "Entry '%s' not uptodate. Cannot merge.",

        /* not_uptodate_dir */
        "Updating '%s' would lose untracked files in it",

        /* would_lose_untracked */
        "Untracked working tree file '%s' would be %s by merge.",

        /* bind_overlap */
        "Entry '%s' overlaps with '%s'.  Cannot bind.",
};

#define ERRORMSG(o,fld) \
        ( ((o) && (o)->msgs.fld) \
        ? ((o)->msgs.fld) \
        : (unpack_plumbing_errors.fld) )

static void add_entry(struct unpack_trees_options *o, struct cache_entry *ce,
        unsigned int set, unsigned int clear)
{
        unsigned int size = ce_size(ce);
        struct cache_entry *new = xmalloc(size);

        clear |= CE_HASHED | CE_UNHASHED;

        memcpy(new, ce, size);
        new->next = NULL;
        new->ce_flags = (new->ce_flags & ~clear) | set;
        add_index_entry(&o->result, new, ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE);
}

/*
 * Unlink the last component and schedule the leading directories for
 * removal, such that empty directories get removed.
 */
static void unlink_entry(struct cache_entry *ce)
{
        if (has_symlink_or_noent_leading_path(ce->name, ce_namelen(ce)))
                return;
        if (unlink_or_warn(ce->name))
                return;
        schedule_dir_for_removal(ce->name, ce_namelen(ce));
}

static struct checkout state;
static int check_updates(struct unpack_trees_options *o)
{
        unsigned cnt = 0, total = 0;
        struct progress *progress = NULL;
        struct index_state *index = &o->result;
        int i;
        int errs = 0;

        if (o->update && o->verbose_update) {
                for (total = cnt = 0; cnt < index->cache_nr; cnt++) {
                        struct cache_entry *ce = index->cache[cnt];
                        if (ce->ce_flags & (CE_UPDATE | CE_REMOVE))
                                total++;
                }

                progress = start_progress_delay("Checking out files",
                                                total, 50, 1);
                cnt = 0;
        }

        if (o->update)
                git_attr_set_direction(GIT_ATTR_CHECKOUT, &o->result);
        for (i = 0; i < index->cache_nr; i++) {
                struct cache_entry *ce = index->cache[i];

                if (ce->ce_flags & CE_REMOVE) {
                        display_progress(progress, ++cnt);
                        if (o->update)
                                unlink_entry(ce);
                }
        }
        remove_marked_cache_entries(&o->result);
        remove_scheduled_dirs();

        for (i = 0; i < index->cache_nr; i++) {
                struct cache_entry *ce = index->cache[i];

                if (ce->ce_flags & CE_UPDATE) {
                        display_progress(progress, ++cnt);
                        ce->ce_flags &= ~CE_UPDATE;
                        if (o->update) {
                                errs |= checkout_entry(ce, &state, NULL);
                        }
                }
        }
        stop_progress(&progress);
        if (o->update)
                git_attr_set_direction(GIT_ATTR_CHECKIN, NULL);
        return errs != 0;
}

static inline int call_unpack_fn(struct cache_entry **src, struct unpack_trees_options *o)
{
        int ret = o->fn(src, o);
        if (ret > 0)
                ret = 0;
        return ret;
}

static int unpack_index_entry(struct cache_entry *ce, struct unpack_trees_options *o)
{
        struct cache_entry *src[5] = { ce, NULL, };

        o->pos++;
        if (ce_stage(ce)) {
                if (o->skip_unmerged) {
                        add_entry(o, ce, 0, 0);
                        return 0;
                }
        }
        return call_unpack_fn(src, o);
}

static int traverse_trees_recursive(int n, unsigned long dirmask, unsigned long df_conflicts, struct name_entry *names, struct traverse_info *info)
{
        int i;
        struct tree_desc t[MAX_UNPACK_TREES];
        struct traverse_info newinfo;
        struct name_entry *p;

        p = names;
        while (!p->mode)
                p++;

        newinfo = *info;
        newinfo.prev = info;
        newinfo.name = *p;
        newinfo.pathlen += tree_entry_len(p->path, p->sha1) + 1;
        newinfo.conflicts |= df_conflicts;

        for (i = 0; i < n; i++, dirmask >>= 1) {
                const unsigned char *sha1 = NULL;
                if (dirmask & 1)
                        sha1 = names[i].sha1;
                fill_tree_descriptor(t+i, sha1);
        }
        return traverse_trees(n, t, &newinfo);
}

/*
 * Compare the traverse-path to the cache entry without actually
 * having to generate the textual representation of the traverse
 * path.
 *
 * NOTE! This *only* compares up to the size of the traverse path
 * itself - the caller needs to do the final check for the cache
 * entry having more data at the end!
 */
static int do_compare_entry(const struct cache_entry *ce, const struct traverse_info *info, const struct name_entry *n)
{
        int len, pathlen, ce_len;
        const char *ce_name;

        if (info->prev) {
                int cmp = do_compare_entry(ce, info->prev, &info->name);
                if (cmp)
                        return cmp;
        }
        pathlen = info->pathlen;
        ce_len = ce_namelen(ce);

        /* If ce_len < pathlen then we must have previously hit "name == directory" entry */
        if (ce_len < pathlen)
                return -1;

        ce_len -= pathlen;
        ce_name = ce->name + pathlen;

        len = tree_entry_len(n->path, n->sha1);
        return df_name_compare(ce_name, ce_len, S_IFREG, n->path, len, n->mode);
}

static int compare_entry(const struct cache_entry *ce, const struct traverse_info *info, const struct name_entry *n)
{
        int cmp = do_compare_entry(ce, info, n);
        if (cmp)
                return cmp;

        /*
         * Even if the beginning compared identically, the ce should
         * compare as bigger than a directory leading up to it!
         */
        return ce_namelen(ce) > traverse_path_len(info, n);
}

static struct cache_entry *create_ce_entry(const struct traverse_info *info, const struct name_entry *n, int stage)
{
        int len = traverse_path_len(info, n);
        struct cache_entry *ce = xcalloc(1, cache_entry_size(len));

        ce->ce_mode = create_ce_mode(n->mode);
        ce->ce_flags = create_ce_flags(len, stage);
        hashcpy(ce->sha1, n->sha1);
        make_traverse_path(ce->name, info, n);

        return ce;
}

static int unpack_nondirectories(int n, unsigned long mask,
                                 unsigned long dirmask,
                                 struct cache_entry **src,
                                 const struct name_entry *names,
                                 const struct traverse_info *info)
{
        int i;
        struct unpack_trees_options *o = info->data;
        unsigned long conflicts;

        /* Do we have *only* directories? Nothing to do */
        if (mask == dirmask && !src[0])
                return 0;

        conflicts = info->conflicts;
        if (o->merge)
                conflicts >>= 1;
        conflicts |= dirmask;

        /*
         * Ok, we've filled in up to any potential index entry in src[0],
         * now do the rest.
         */
        for (i = 0; i < n; i++) {
                int stage;
                unsigned int bit = 1ul << i;
                if (conflicts & bit) {
                        src[i + o->merge] = o->df_conflict_entry;
                        continue;
                }
                if (!(mask & bit))
                        continue;
                if (!o->merge)
                        stage = 0;
                else if (i + 1 < o->head_idx)
                        stage = 1;
                else if (i + 1 > o->head_idx)
                        stage = 3;
                else
                        stage = 2;
                src[i + o->merge] = create_ce_entry(info, names + i, stage);
        }

        if (o->merge)
                return call_unpack_fn(src, o);

        for (i = 0; i < n; i++)
                if (src[i] && src[i] != o->df_conflict_entry)
                        add_entry(o, src[i], 0, 0);
        return 0;
}

static int unpack_failed(struct unpack_trees_options *o, const char *message)
{
        discard_index(&o->result);
        if (!o->gently) {
                if (message)
                        return error("%s", message);
                return -1;
        }
        return -1;
}

static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
{
        struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
        struct unpack_trees_options *o = info->data;
        const struct name_entry *p = names;

        /* Find first entry with a real name (we could use "mask" too) */
        while (!p->mode)
                p++;

        /* Are we supposed to look at the index too? */
        if (o->merge) {
                while (o->pos < o->src_index->cache_nr) {
                        struct cache_entry *ce = o->src_index->cache[o->pos];
                        int cmp = compare_entry(ce, info, p);
                        if (cmp < 0) {
                                if (unpack_index_entry(ce, o) < 0)
                                        return unpack_failed(o, NULL);
                                continue;
                        }
                        if (!cmp) {
                                o->pos++;
                                if (ce_stage(ce)) {
                                        /*
                                         * If we skip unmerged index entries, we'll skip this
                                         * entry *and* the tree entries associated with it!
                                         */
                                        if (o->skip_unmerged) {
                                                add_entry(o, ce, 0, 0);
                                                return mask;
                                        }
                                }
                                src[0] = ce;
                        }
                        break;
                }
        }

        if (unpack_nondirectories(n, mask, dirmask, src, names, info) < 0)
                return -1;

        /* Now handle any directories.. */
        if (dirmask) {
                unsigned long conflicts = mask & ~dirmask;
                if (o->merge) {
                        conflicts <<= 1;
                        if (src[0])
                                conflicts |= 1;
                }

                /* special case: "diff-index --cached" looking at a tree */
                if (o->diff_index_cached &&
                    n == 1 && dirmask == 1 && S_ISDIR(names->mode)) {
                        int matches;
                        matches = cache_tree_matches_traversal(o->src_index->cache_tree,
                                                               names, info);
                        /*
                         * Everything under the name matches.  Adjust o->pos to
                         * skip the entire hierarchy.
                         */
                        if (matches) {
                                o->pos += matches;
                                return mask;
                        }
                }

                if (traverse_trees_recursive(n, dirmask, conflicts,
                                             names, info) < 0)
                        return -1;
                return mask;
        }

        return mask;
}

/*
 * N-way merge "len" trees.  Returns 0 on success, -1 on failure to manipulate the
 * resulting index, -2 on failure to reflect the changes to the work tree.
 */
int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options *o)
{
        int ret;
        static struct cache_entry *dfc;

        if (len > MAX_UNPACK_TREES)
                die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);
        memset(&state, 0, sizeof(state));
        state.base_dir = "";
        state.force = 1;
        state.quiet = 1;
        state.refresh_cache = 1;

        memset(&o->result, 0, sizeof(o->result));
        o->result.initialized = 1;
        if (o->src_index) {
                o->result.timestamp.sec = o->src_index->timestamp.sec;
                o->result.timestamp.nsec = o->src_index->timestamp.nsec;
        }
        o->merge_size = len;

        if (!dfc)
                dfc = xcalloc(1, cache_entry_size(0));
        o->df_conflict_entry = dfc;

        if (len) {
                const char *prefix = o->prefix ? o->prefix : "";
                struct traverse_info info;

                setup_traverse_info(&info, prefix);
                info.fn = unpack_callback;
                info.data = o;

                if (traverse_trees(len, t, &info) < 0)
                        return unpack_failed(o, NULL);
        }

        /* Any left-over entries in the index? */
        if (o->merge) {
                while (o->pos < o->src_index->cache_nr) {
                        struct cache_entry *ce = o->src_index->cache[o->pos];
                        if (unpack_index_entry(ce, o) < 0)
                                return unpack_failed(o, NULL);
                }
        }

        if (o->trivial_merges_only && o->nontrivial_merge)
                return unpack_failed(o, "Merge requires file-level merging");

        o->src_index = NULL;
        ret = check_updates(o) ? (-2) : 0;
        if (o->dst_index)
                *o->dst_index = o->result;
        return ret;
}

/* Here come the merge functions */

static int reject_merge(struct cache_entry *ce, struct unpack_trees_options *o)
{
        return error(ERRORMSG(o, would_overwrite), ce->name);
}

static int same(struct cache_entry *a, struct cache_entry *b)
{
        if (!!a != !!b)
                return 0;
        if (!a && !b)
                return 1;
        return a->ce_mode == b->ce_mode &&
               !hashcmp(a->sha1, b->sha1);
}


/*
 * When a CE gets turned into an unmerged entry, we
 * want it to be up-to-date
 */
static int verify_uptodate(struct cache_entry *ce,
                struct unpack_trees_options *o)
{
        struct stat st;

        if (o->index_only || o->reset || ce_uptodate(ce))
                return 0;

        if (!lstat(ce->name, &st)) {
                unsigned changed = ie_match_stat(o->src_index, ce, &st, CE_MATCH_IGNORE_VALID);
                if (!changed)
                        return 0;
                /*
                 * NEEDSWORK: the current default policy is to allow
                 * submodule to be out of sync wrt the supermodule
                 * index.  This needs to be tightened later for
                 * submodules that are marked to be automatically
                 * checked out.
                 */
                if (S_ISGITLINK(ce->ce_mode))
                        return 0;
                errno = 0;
        }
        if (errno == ENOENT)
                return 0;
        return o->gently ? -1 :
                error(ERRORMSG(o, not_uptodate_file), ce->name);
}

static void invalidate_ce_path(struct cache_entry *ce, struct unpack_trees_options *o)
{
        if (ce)
                cache_tree_invalidate_path(o->src_index->cache_tree, ce->name);
}

/*
 * Check that checking out ce->sha1 in subdir ce->name is not
 * going to overwrite any working files.
 *
 * Currently, git does not checkout subprojects during a superproject
 * checkout, so it is not going to overwrite anything.
 */
static int verify_clean_submodule(struct cache_entry *ce, const char *action,
                                      struct unpack_trees_options *o)
{
        return 0;
}

static int verify_clean_subdirectory(struct cache_entry *ce, const char *action,
                                      struct unpack_trees_options *o)
{
        /*
         * we are about to extract "ce->name"; we would not want to lose
         * anything in the existing directory there.
         */
        int namelen;
        int i;
        struct dir_struct d;
        char *pathbuf;
        int cnt = 0;
        unsigned char sha1[20];

        if (S_ISGITLINK(ce->ce_mode) &&
            resolve_gitlink_ref(ce->name, "HEAD", sha1) == 0) {
                /* If we are not going to update the submodule, then
                 * we don't care.
                 */
                if (!hashcmp(sha1, ce->sha1))
                        return 0;
                return verify_clean_submodule(ce, action, o);
        }

        /*
         * First let's make sure we do not have a local modification
         * in that directory.
         */
        namelen = strlen(ce->name);
        for (i = o->pos; i < o->src_index->cache_nr; i++) {
                struct cache_entry *ce2 = o->src_index->cache[i];
                int len = ce_namelen(ce2);
                if (len < namelen ||
                    strncmp(ce->name, ce2->name, namelen) ||
                    ce2->name[namelen] != '/')
                        break;
                /*
                 * ce2->name is an entry in the subdirectory.
                 */
                if (!ce_stage(ce2)) {
                        if (verify_uptodate(ce2, o))
                                return -1;
                        add_entry(o, ce2, CE_REMOVE, 0);
                }
                cnt++;
        }

        /*
         * Then we need to make sure that we do not lose a locally
         * present file that is not ignored.
         */
        pathbuf = xmalloc(namelen + 2);
        memcpy(pathbuf, ce->name, namelen);
        strcpy(pathbuf+namelen, "/");

        memset(&d, 0, sizeof(d));
        if (o->dir)
                d.exclude_per_dir = o->dir->exclude_per_dir;
        i = read_directory(&d, pathbuf, namelen+1, NULL);
        if (i)
                return o->gently ? -1 :
                        error(ERRORMSG(o, not_uptodate_dir), ce->name);
        free(pathbuf);
        return cnt;
}

/*
 * This gets called when there was no index entry for the tree entry 'dst',
 * but we found a file in the working tree that 'lstat()' said was fine,
 * and we're on a case-insensitive filesystem.
 *
 * See if we can find a case-insensitive match in the index that also
 * matches the stat information, and assume it's that other file!
 */
static int icase_exists(struct unpack_trees_options *o, struct cache_entry *dst, struct stat *st)
{
        struct cache_entry *src;

        src = index_name_exists(o->src_index, dst->name, ce_namelen(dst), 1);
        return src && !ie_match_stat(o->src_index, src, st, CE_MATCH_IGNORE_VALID);
}

/*
 * We do not want to remove or overwrite a working tree file that
 * is not tracked, unless it is ignored.
 */
static int verify_absent(struct cache_entry *ce, const char *action,
                         struct unpack_trees_options *o)
{
        struct stat st;

        if (o->index_only || o->reset || !o->update)
                return 0;

        if (has_symlink_or_noent_leading_path(ce->name, ce_namelen(ce)))
                return 0;

        if (!lstat(ce->name, &st)) {
                int ret;
                int dtype = ce_to_dtype(ce);
                struct cache_entry *result;

                /*
                 * It may be that the 'lstat()' succeeded even though
                 * target 'ce' was absent, because there is an old
                 * entry that is different only in case..
                 *
                 * Ignore that lstat() if it matches.
                 */
                if (ignore_case && icase_exists(o, ce, &st))
                        return 0;

                if (o->dir && excluded(o->dir, ce->name, &dtype))
                        /*
                         * ce->name is explicitly excluded, so it is Ok to
                         * overwrite it.
                         */
                        return 0;
                if (S_ISDIR(st.st_mode)) {
                        /*
                         * We are checking out path "foo" and
                         * found "foo/." in the working tree.
                         * This is tricky -- if we have modified
                         * files that are in "foo/" we would lose
                         * them.
                         */
                        ret = verify_clean_subdirectory(ce, action, o);
                        if (ret < 0)
                                return ret;

                        /*
                         * If this removed entries from the index,
                         * what that means is:
                         *
                         * (1) the caller unpack_callback() saw path/foo
                         * in the index, and it has not removed it because
                         * it thinks it is handling 'path' as blob with
                         * D/F conflict;
                         * (2) we will return "ok, we placed a merged entry
                         * in the index" which would cause o->pos to be
                         * incremented by one;
                         * (3) however, original o->pos now has 'path/foo'
                         * marked with "to be removed".
                         *
                         * We need to increment it by the number of
                         * deleted entries here.
                         */
                        o->pos += ret;
                        return 0;
                }

                /*
                 * The previous round may already have decided to
                 * delete this path, which is in a subdirectory that
                 * is being replaced with a blob.
                 */
                result = index_name_exists(&o->result, ce->name, ce_namelen(ce), 0);
                if (result) {
                        if (result->ce_flags & CE_REMOVE)
                                return 0;
                }

                return o->gently ? -1 :
                        error(ERRORMSG(o, would_lose_untracked), ce->name, action);
        }
        return 0;
}

static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
                struct unpack_trees_options *o)
{
        int update = CE_UPDATE;

        if (old) {
                /*
                 * See if we can re-use the old CE directly?
                 * That way we get the uptodate stat info.
                 *
                 * This also removes the UPDATE flag on a match; otherwise
                 * we will end up overwriting local changes in the work tree.
                 */
                if (same(old, merge)) {
                        copy_cache_entry(merge, old);
                        update = 0;
                } else {
                        if (verify_uptodate(old, o))
                                return -1;
                        invalidate_ce_path(old, o);
                }
        }
        else {
                if (verify_absent(merge, "overwritten", o))
                        return -1;
                invalidate_ce_path(merge, o);
        }

        add_entry(o, merge, update, CE_STAGEMASK);
        return 1;
}

static int deleted_entry(struct cache_entry *ce, struct cache_entry *old,
                struct unpack_trees_options *o)
{
        /* Did it exist in the index? */
        if (!old) {
                if (verify_absent(ce, "removed", o))
                        return -1;
                return 0;
        }
        if (verify_uptodate(old, o))
                return -1;
        add_entry(o, ce, CE_REMOVE, 0);
        invalidate_ce_path(ce, o);
        return 1;
}

static int keep_entry(struct cache_entry *ce, struct unpack_trees_options *o)
{
        add_entry(o, ce, 0, 0);
        return 1;
}

#if DBRT_DEBUG
static void show_stage_entry(FILE *o,
                             const char *label, const struct cache_entry *ce)
{
        if (!ce)
                fprintf(o, "%s (missing)\n", label);
        else
                fprintf(o, "%s%06o %s %d\t%s\n",
                        label,
                        ce->ce_mode,
                        sha1_to_hex(ce->sha1),
                        ce_stage(ce),
                        ce->name);
}
#endif

int threeway_merge(struct cache_entry **stages, struct unpack_trees_options *o)
{
        struct cache_entry *index;
        struct cache_entry *head;
        struct cache_entry *remote = stages[o->head_idx + 1];
        int count;
        int head_match = 0;
        int remote_match = 0;

        int df_conflict_head = 0;
        int df_conflict_remote = 0;

        int any_anc_missing = 0;
        int no_anc_exists = 1;
        int i;

        for (i = 1; i < o->head_idx; i++) {
                if (!stages[i] || stages[i] == o->df_conflict_entry)
                        any_anc_missing = 1;
                else
                        no_anc_exists = 0;
        }

        index = stages[0];
        head = stages[o->head_idx];

        if (head == o->df_conflict_entry) {
                df_conflict_head = 1;
                head = NULL;
        }

        if (remote == o->df_conflict_entry) {
                df_conflict_remote = 1;
                remote = NULL;
        }

        /* First, if there's a #16 situation, note that to prevent #13
         * and #14.
         */
        if (!same(remote, head)) {
                for (i = 1; i < o->head_idx; i++) {
                        if (same(stages[i], head)) {
                                head_match = i;
                        }
                        if (same(stages[i], remote)) {
                                remote_match = i;
                        }
                }
        }

        /* We start with cases where the index is allowed to match
         * something other than the head: #14(ALT) and #2ALT, where it
         * is permitted to match the result instead.
         */
        /* #14, #14ALT, #2ALT */
        if (remote && !df_conflict_head && head_match && !remote_match) {
                if (index && !same(index, remote) && !same(index, head))
                        return o->gently ? -1 : reject_merge(index, o);
                return merged_entry(remote, index, o);
        }
        /*
         * If we have an entry in the index cache, then we want to
         * make sure that it matches head.
         */
        if (index && !same(index, head))
                return o->gently ? -1 : reject_merge(index, o);

        if (head) {
                /* #5ALT, #15 */
                if (same(head, remote))
                        return merged_entry(head, index, o);
                /* #13, #3ALT */
                if (!df_conflict_remote && remote_match && !head_match)
                        return merged_entry(head, index, o);
        }

        /* #1 */
        if (!head && !remote && any_anc_missing)
                return 0;

        /* Under the new "aggressive" rule, we resolve mostly trivial
         * cases that we historically had git-merge-one-file resolve.
         */
        if (o->aggressive) {
                int head_deleted = !head && !df_conflict_head;
                int remote_deleted = !remote && !df_conflict_remote;
                struct cache_entry *ce = NULL;

                if (index)
                        ce = index;
                else if (head)
                        ce = head;
                else if (remote)
                        ce = remote;
                else {
                        for (i = 1; i < o->head_idx; i++) {
                                if (stages[i] && stages[i] != o->df_conflict_entry) {
                                        ce = stages[i];
                                        break;
                                }
                        }
                }

                /*
                 * Deleted in both.
                 * Deleted in one and unchanged in the other.
                 */
                if ((head_deleted && remote_deleted) ||
                    (head_deleted && remote && remote_match) ||
                    (remote_deleted && head && head_match)) {
                        if (index)
                                return deleted_entry(index, index, o);
                        if (ce && !head_deleted) {
                                if (verify_absent(ce, "removed", o))
                                        return -1;
                        }
                        return 0;
                }
                /*
                 * Added in both, identically.
                 */
                if (no_anc_exists && head && remote && same(head, remote))
                        return merged_entry(head, index, o);

        }

        /* Below are "no merge" cases, which require that the index be
         * up-to-date to avoid the files getting overwritten with
         * conflict resolution files.
         */
        if (index) {
                if (verify_uptodate(index, o))
                        return -1;
        }

        o->nontrivial_merge = 1;

        /* #2, #3, #4, #6, #7, #9, #10, #11. */
        count = 0;
        if (!head_match || !remote_match) {
                for (i = 1; i < o->head_idx; i++) {
                        if (stages[i] && stages[i] != o->df_conflict_entry) {
                                keep_entry(stages[i], o);
                                count++;
                                break;
                        }
                }
        }
#if DBRT_DEBUG
        else {
                fprintf(stderr, "read-tree: warning #16 detected\n");
                show_stage_entry(stderr, "head   ", stages[head_match]);
                show_stage_entry(stderr, "remote ", stages[remote_match]);
        }
#endif
        if (head) { count += keep_entry(head, o); }
        if (remote) { count += keep_entry(remote, o); }
        return count;
}

/*
 * Two-way merge.
 *
 * The rule is to "carry forward" what is in the index without losing
 * information across a "fast-forward", favoring a successful merge
 * over a merge failure when it makes sense.  For details of the
 * "carry forward" rule, please see <Documentation/git-read-tree.txt>.
 *
 */
int twoway_merge(struct cache_entry **src, struct unpack_trees_options *o)
{
        struct cache_entry *current = src[0];
        struct cache_entry *oldtree = src[1];
        struct cache_entry *newtree = src[2];

        if (o->merge_size != 2)
                return error("Cannot do a twoway merge of %d trees",
                             o->merge_size);

        if (oldtree == o->df_conflict_entry)
                oldtree = NULL;
        if (newtree == o->df_conflict_entry)
                newtree = NULL;

        if (current) {
                if ((!oldtree && !newtree) || /* 4 and 5 */
                    (!oldtree && newtree &&
                     same(current, newtree)) || /* 6 and 7 */
                    (oldtree && newtree &&
                     same(oldtree, newtree)) || /* 14 and 15 */
                    (oldtree && newtree &&
                     !same(oldtree, newtree) && /* 18 and 19 */
                     same(current, newtree))) {
                        return keep_entry(current, o);
                }
                else if (oldtree && !newtree && same(current, oldtree)) {
                        /* 10 or 11 */
                        return deleted_entry(oldtree, current, o);
                }
                else if (oldtree && newtree &&
                         same(current, oldtree) && !same(current, newtree)) {
                        /* 20 or 21 */
                        return merged_entry(newtree, current, o);
                }
                else {
                        /* all other failures */
                        if (oldtree)
                                return o->gently ? -1 : reject_merge(oldtree, o);
                        if (current)
                                return o->gently ? -1 : reject_merge(current, o);
                        if (newtree)
                                return o->gently ? -1 : reject_merge(newtree, o);
                        return -1;
                }
        }
        else if (newtree) {
                if (oldtree && !o->initial_checkout) {
                        /*
                         * deletion of the path was staged;
                         */
                        if (same(oldtree, newtree))
                                return 1;
                        return reject_merge(oldtree, o);
                }
                return merged_entry(newtree, current, o);
        }
        return deleted_entry(oldtree, current, o);
}

/*
 * Bind merge.
 *
 * Keep the index entries at stage0, collapse stage1 but make sure
 * stage0 does not have anything there.
 */
int bind_merge(struct cache_entry **src,
                struct unpack_trees_options *o)
{
        struct cache_entry *old = src[0];
        struct cache_entry *a = src[1];

        if (o->merge_size != 1)
                return error("Cannot do a bind merge of %d trees\n",
                             o->merge_size);
        if (a && old)
                return o->gently ? -1 :
                        error(ERRORMSG(o, bind_overlap), a->name, old->name);
        if (!a)
                return keep_entry(old, o);
        else
                return merged_entry(a, NULL, o);
}

/*
 * One-way merge.
 *
 * The rule is:
 * - take the stat information from stage0, take the data from stage1
 */
int oneway_merge(struct cache_entry **src, struct unpack_trees_options *o)
{
        struct cache_entry *old = src[0];
        struct cache_entry *a = src[1];

        if (o->merge_size != 1)
                return error("Cannot do a oneway merge of %d trees",
                             o->merge_size);

        if (!a || a == o->df_conflict_entry)
                return deleted_entry(old, old, o);

        if (old && same(old, a)) {
                int update = 0;
                if (o->reset && !ce_uptodate(old)) {
                        struct stat st;
                        if (lstat(old->name, &st) ||
                            ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID))
                                update |= CE_UPDATE;
                }
                add_entry(o, old, update, 0);
                return 0;
        }
        return merged_entry(a, old, o);
}