/*
 * Builtin "git merge"
 *
 * Copyright (c) 2008 Miklos Vajna <vmiklos@frugalware.org>
 *
 * Based on git-merge.sh by Junio C Hamano.
 */

#include "cache.h"
#include "parse-options.h"
#include "builtin.h"
#include "run-command.h"
#include "diff.h"
#include "refs.h"
#include "commit.h"
#include "diffcore.h"
#include "revision.h"
#include "unpack-trees.h"
#include "cache-tree.h"
#include "dir.h"
#include "utf8.h"
#include "log-tree.h"
#include "color.h"
#include "rerere.h"
#include "help.h"
#include "merge-recursive.h"
#include "resolve-undo.h"
#include "remote.h"

#define DEFAULT_TWOHEAD (1<<0)
#define DEFAULT_OCTOPUS (1<<1)
#define NO_FAST_FORWARD (1<<2)
#define NO_TRIVIAL      (1<<3)

struct strategy {
        const char *name;
        unsigned attr;
};

static const char * const builtin_merge_usage[] = {
        "git merge [options] [<commit>...]",
        "git merge [options] <msg> HEAD <commit>",
        "git merge --abort",
        NULL
};

static int show_diffstat = 1, shortlog_len, squash;
static int option_commit = 1, allow_fast_forward = 1;
static int fast_forward_only, option_edit;
static int allow_trivial = 1, have_message;
static struct strbuf merge_msg;
static struct commit_list *remoteheads;
static unsigned char head[20], stash[20];
static struct strategy **use_strategies;
static size_t use_strategies_nr, use_strategies_alloc;
static const char **xopts;
static size_t xopts_nr, xopts_alloc;
static const char *branch;
static char *branch_mergeoptions;
static int option_renormalize;
static int verbosity;
static int allow_rerere_auto;
static int abort_current_merge;
static int show_progress = -1;
static int default_to_upstream;

static struct strategy all_strategy[] = {
        { "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
        { "octopus",    DEFAULT_OCTOPUS },
        { "resolve",    0 },
        { "ours",       NO_FAST_FORWARD | NO_TRIVIAL },
        { "subtree",    NO_FAST_FORWARD | NO_TRIVIAL },
};

static const char *pull_twohead, *pull_octopus;

static int option_parse_message(const struct option *opt,
                                const char *arg, int unset)
{
        struct strbuf *buf = opt->value;

        if (unset)
                strbuf_setlen(buf, 0);
        else if (arg) {
                strbuf_addf(buf, "%s%s", buf->len ? "\n\n" : "", arg);
                have_message = 1;
        } else
                return error(_("switch `m' requires a value"));
        return 0;
}

static struct strategy *get_strategy(const char *name)
{
        int i;
        struct strategy *ret;
        static struct cmdnames main_cmds, other_cmds;
        static int loaded;

        if (!name)
                return NULL;

        for (i = 0; i < ARRAY_SIZE(all_strategy); i++)
                if (!strcmp(name, all_strategy[i].name))
                        return &all_strategy[i];

        if (!loaded) {
                struct cmdnames not_strategies;
                loaded = 1;

                memset(&not_strategies, 0, sizeof(struct cmdnames));
                load_command_list("git-merge-", &main_cmds, &other_cmds);
                for (i = 0; i < main_cmds.cnt; i++) {
                        int j, found = 0;
                        struct cmdname *ent = main_cmds.names[i];
                        for (j = 0; j < ARRAY_SIZE(all_strategy); j++)
                                if (!strncmp(ent->name, all_strategy[j].name, ent->len)
                                                && !all_strategy[j].name[ent->len])
                                        found = 1;
                        if (!found)
                                add_cmdname(&not_strategies, ent->name, ent->len);
                }
                exclude_cmds(&main_cmds, &not_strategies);
        }
        if (!is_in_cmdlist(&main_cmds, name) && !is_in_cmdlist(&other_cmds, name)) {
                fprintf(stderr, _("Could not find merge strategy '%s'.\n"), name);
                fprintf(stderr, _("Available strategies are:"));
                for (i = 0; i < main_cmds.cnt; i++)
                        fprintf(stderr, " %s", main_cmds.names[i]->name);
                fprintf(stderr, ".\n");
                if (other_cmds.cnt) {
                        fprintf(stderr, _("Available custom strategies are:"));
                        for (i = 0; i < other_cmds.cnt; i++)
                                fprintf(stderr, " %s", other_cmds.names[i]->name);
                        fprintf(stderr, ".\n");
                }
                exit(1);
        }

        ret = xcalloc(1, sizeof(struct strategy));
        ret->name = xstrdup(name);
        ret->attr = NO_TRIVIAL;
        return ret;
}

static void append_strategy(struct strategy *s)
{
        ALLOC_GROW(use_strategies, use_strategies_nr + 1, use_strategies_alloc);
        use_strategies[use_strategies_nr++] = s;
}

static int option_parse_strategy(const struct option *opt,
                                 const char *name, int unset)
{
        if (unset)
                return 0;

        append_strategy(get_strategy(name));
        return 0;
}

static int option_parse_x(const struct option *opt,
                          const char *arg, int unset)
{
        if (unset)
                return 0;

        ALLOC_GROW(xopts, xopts_nr + 1, xopts_alloc);
        xopts[xopts_nr++] = xstrdup(arg);
        return 0;
}

static int option_parse_n(const struct option *opt,
                          const char *arg, int unset)
{
        show_diffstat = unset;
        return 0;
}

static struct option builtin_merge_options[] = {
        { OPTION_CALLBACK, 'n', NULL, NULL, NULL,
                "do not show a diffstat at the end of the merge",
                PARSE_OPT_NOARG, option_parse_n },
        OPT_BOOLEAN(0, "stat", &show_diffstat,
                "show a diffstat at the end of the merge"),
        OPT_BOOLEAN(0, "summary", &show_diffstat, "(synonym to --stat)"),
        { OPTION_INTEGER, 0, "log", &shortlog_len, "n",
          "add (at most <n>) entries from shortlog to merge commit message",
          PARSE_OPT_OPTARG, NULL, DEFAULT_MERGE_LOG_LEN },
        OPT_BOOLEAN(0, "squash", &squash,
                "create a single commit instead of doing a merge"),
        OPT_BOOLEAN(0, "commit", &option_commit,
                "perform a commit if the merge succeeds (default)"),
        OPT_BOOLEAN('e', "edit", &option_edit,
                "edit message before committing"),
        OPT_BOOLEAN(0, "ff", &allow_fast_forward,
                "allow fast-forward (default)"),
        OPT_BOOLEAN(0, "ff-only", &fast_forward_only,
                "abort if fast-forward is not possible"),
        OPT_RERERE_AUTOUPDATE(&allow_rerere_auto),
        OPT_CALLBACK('s', "strategy", &use_strategies, "strategy",
                "merge strategy to use", option_parse_strategy),
        OPT_CALLBACK('X', "strategy-option", &xopts, "option=value",
                "option for selected merge strategy", option_parse_x),
        OPT_CALLBACK('m', "message", &merge_msg, "message",
                "merge commit message (for a non-fast-forward merge)",
                option_parse_message),
        OPT__VERBOSITY(&verbosity),
        OPT_BOOLEAN(0, "abort", &abort_current_merge,
                "abort the current in-progress merge"),
        OPT_SET_INT(0, "progress", &show_progress, "force progress reporting", 1),
        OPT_END()
};

/* Cleans up metadata that is uninteresting after a succeeded merge. */
static void drop_save(void)
{
        unlink(git_path("MERGE_HEAD"));
        unlink(git_path("MERGE_MSG"));
        unlink(git_path("MERGE_MODE"));
}

static void save_state(void)
{
        int len;
        struct child_process cp;
        struct strbuf buffer = STRBUF_INIT;
        const char *argv[] = {"stash", "create", NULL};

        memset(&cp, 0, sizeof(cp));
        cp.argv = argv;
        cp.out = -1;
        cp.git_cmd = 1;

        if (start_command(&cp))
                die(_("could not run stash."));
        len = strbuf_read(&buffer, cp.out, 1024);
        close(cp.out);

        if (finish_command(&cp) || len < 0)
                die(_("stash failed"));
        else if (!len)
                return;
        strbuf_setlen(&buffer, buffer.len-1);
        if (get_sha1(buffer.buf, stash))
                die(_("not a valid object: %s"), buffer.buf);
}

static void read_empty(unsigned const char *sha1, int verbose)
{
        int i = 0;
        const char *args[7];

        args[i++] = "read-tree";
        if (verbose)
                args[i++] = "-v";
        args[i++] = "-m";
        args[i++] = "-u";
        args[i++] = EMPTY_TREE_SHA1_HEX;
        args[i++] = sha1_to_hex(sha1);
        args[i] = NULL;

        if (run_command_v_opt(args, RUN_GIT_CMD))
                die(_("read-tree failed"));
}

static void reset_hard(unsigned const char *sha1, int verbose)
{
        int i = 0;
        const char *args[6];

        args[i++] = "read-tree";
        if (verbose)
                args[i++] = "-v";
        args[i++] = "--reset";
        args[i++] = "-u";
        args[i++] = sha1_to_hex(sha1);
        args[i] = NULL;

        if (run_command_v_opt(args, RUN_GIT_CMD))
                die(_("read-tree failed"));
}

static void restore_state(void)
{
        struct strbuf sb = STRBUF_INIT;
        const char *args[] = { "stash", "apply", NULL, NULL };

        if (is_null_sha1(stash))
                return;

        reset_hard(head, 1);

        args[2] = sha1_to_hex(stash);

        /*
         * It is OK to ignore error here, for example when there was
         * nothing to restore.
         */
        run_command_v_opt(args, RUN_GIT_CMD);

        strbuf_release(&sb);
        refresh_cache(REFRESH_QUIET);
}

/* This is called when no merge was necessary. */
static void finish_up_to_date(const char *msg)
{
        if (verbosity >= 0)
                printf("%s%s\n", squash ? _(" (nothing to squash)") : "", msg);
        drop_save();
}

static void squash_message(void)
{
        struct rev_info rev;
        struct commit *commit;
        struct strbuf out = STRBUF_INIT;
        struct commit_list *j;
        int fd;
        struct pretty_print_context ctx = {0};

        printf(_("Squash commit -- not updating HEAD\n"));
        fd = open(git_path("SQUASH_MSG"), O_WRONLY | O_CREAT, 0666);
        if (fd < 0)
                die_errno(_("Could not write to '%s'"), git_path("SQUASH_MSG"));

        init_revisions(&rev, NULL);
        rev.ignore_merges = 1;
        rev.commit_format = CMIT_FMT_MEDIUM;

        commit = lookup_commit(head);
        commit->object.flags |= UNINTERESTING;
        add_pending_object(&rev, &commit->object, NULL);

        for (j = remoteheads; j; j = j->next)
                add_pending_object(&rev, &j->item->object, NULL);

        setup_revisions(0, NULL, &rev, NULL);
        if (prepare_revision_walk(&rev))
                die(_("revision walk setup failed"));

        ctx.abbrev = rev.abbrev;
        ctx.date_mode = rev.date_mode;
        ctx.fmt = rev.commit_format;

        strbuf_addstr(&out, "Squashed commit of the following:\n");
        while ((commit = get_revision(&rev)) != NULL) {
                strbuf_addch(&out, '\n');
                strbuf_addf(&out, "commit %s\n",
                        sha1_to_hex(commit->object.sha1));
                pretty_print_commit(&ctx, commit, &out);
        }
        if (write(fd, out.buf, out.len) < 0)
                die_errno(_("Writing SQUASH_MSG"));
        if (close(fd))
                die_errno(_("Finishing SQUASH_MSG"));
        strbuf_release(&out);
}

static void finish(const unsigned char *new_head, const char *msg)
{
        struct strbuf reflog_message = STRBUF_INIT;

        if (!msg)
                strbuf_addstr(&reflog_message, getenv("GIT_REFLOG_ACTION"));
        else {
                if (verbosity >= 0)
                        printf("%s\n", msg);
                strbuf_addf(&reflog_message, "%s: %s",
                        getenv("GIT_REFLOG_ACTION"), msg);
        }
        if (squash) {
                squash_message();
        } else {
                if (verbosity >= 0 && !merge_msg.len)
                        printf(_("No merge message -- not updating HEAD\n"));
                else {
                        const char *argv_gc_auto[] = { "gc", "--auto", NULL };
                        update_ref(reflog_message.buf, "HEAD",
                                new_head, head, 0,
                                DIE_ON_ERR);
                        /*
                         * We ignore errors in 'gc --auto', since the
                         * user should see them.
                         */
                        run_command_v_opt(argv_gc_auto, RUN_GIT_CMD);
                }
        }
        if (new_head && show_diffstat) {
                struct diff_options opts;
                diff_setup(&opts);
                opts.output_format |=
                        DIFF_FORMAT_SUMMARY | DIFF_FORMAT_DIFFSTAT;
                opts.detect_rename = DIFF_DETECT_RENAME;
                if (diff_setup_done(&opts) < 0)
                        die(_("diff_setup_done failed"));
                diff_tree_sha1(head, new_head, "", &opts);
                diffcore_std(&opts);
                diff_flush(&opts);
        }

        /* Run a post-merge hook */
        run_hook(NULL, "post-merge", squash ? "1" : "0", NULL);

        strbuf_release(&reflog_message);
}

/* Get the name for the merge commit's message. */
static void merge_name(const char *remote, struct strbuf *msg)
{
        struct object *remote_head;
        unsigned char branch_head[20], buf_sha[20];
        struct strbuf buf = STRBUF_INIT;
        struct strbuf bname = STRBUF_INIT;
        const char *ptr;
        char *found_ref;
        int len, early;

        strbuf_branchname(&bname, remote);
        remote = bname.buf;

        memset(branch_head, 0, sizeof(branch_head));
        remote_head = peel_to_type(remote, 0, NULL, OBJ_COMMIT);
        if (!remote_head)
                die(_("'%s' does not point to a commit"), remote);

        if (dwim_ref(remote, strlen(remote), branch_head, &found_ref) > 0) {
                if (!prefixcmp(found_ref, "refs/heads/")) {
                        strbuf_addf(msg, "%s\t\tbranch '%s' of .\n",
                                    sha1_to_hex(branch_head), remote);
                        goto cleanup;
                }
                if (!prefixcmp(found_ref, "refs/remotes/")) {
                        strbuf_addf(msg, "%s\t\tremote-tracking branch '%s' of .\n",
                                    sha1_to_hex(branch_head), remote);
                        goto cleanup;
                }
        }

        /* See if remote matches <name>^^^.. or <name>~<number> */
        for (len = 0, ptr = remote + strlen(remote);
             remote < ptr && ptr[-1] == '^';
             ptr--)
                len++;
        if (len)
                early = 1;
        else {
                early = 0;
                ptr = strrchr(remote, '~');
                if (ptr) {
                        int seen_nonzero = 0;

                        len++; /* count ~ */
                        while (*++ptr && isdigit(*ptr)) {
                                seen_nonzero |= (*ptr != '0');
                                len++;
                        }
                        if (*ptr)
                                len = 0; /* not ...~<number> */
                        else if (seen_nonzero)
                                early = 1;
                        else if (len == 1)
                                early = 1; /* "name~" is "name~1"! */
                }
        }
        if (len) {
                struct strbuf truname = STRBUF_INIT;
                strbuf_addstr(&truname, "refs/heads/");
                strbuf_addstr(&truname, remote);
                strbuf_setlen(&truname, truname.len - len);
                if (resolve_ref(truname.buf, buf_sha, 1, NULL)) {
                        strbuf_addf(msg,
                                    "%s\t\tbranch '%s'%s of .\n",
                                    sha1_to_hex(remote_head->sha1),
                                    truname.buf + 11,
                                    (early ? " (early part)" : ""));
                        strbuf_release(&truname);
                        goto cleanup;
                }
        }

        if (!strcmp(remote, "FETCH_HEAD") &&
                        !access(git_path("FETCH_HEAD"), R_OK)) {
                FILE *fp;
                struct strbuf line = STRBUF_INIT;
                char *ptr;

                fp = fopen(git_path("FETCH_HEAD"), "r");
                if (!fp)
                        die_errno(_("could not open '%s' for reading"),
                                  git_path("FETCH_HEAD"));
                strbuf_getline(&line, fp, '\n');
                fclose(fp);
                ptr = strstr(line.buf, "\tnot-for-merge\t");
                if (ptr)
                        strbuf_remove(&line, ptr-line.buf+1, 13);
                strbuf_addbuf(msg, &line);
                strbuf_release(&line);
                goto cleanup;
        }
        strbuf_addf(msg, "%s\t\tcommit '%s'\n",
                sha1_to_hex(remote_head->sha1), remote);
cleanup:
        strbuf_release(&buf);
        strbuf_release(&bname);
}

static void parse_branch_merge_options(char *bmo)
{
        const char **argv;
        int argc;

        if (!bmo)
                return;
        argc = split_cmdline(bmo, &argv);
        if (argc < 0)
                die(_("Bad branch.%s.mergeoptions string: %s"), branch,
                    split_cmdline_strerror(argc));
        argv = xrealloc(argv, sizeof(*argv) * (argc + 2));
        memmove(argv + 1, argv, sizeof(*argv) * (argc + 1));
        argc++;
        argv[0] = "branch.*.mergeoptions";
        parse_options(argc, argv, NULL, builtin_merge_options,
                      builtin_merge_usage, 0);
        free(argv);
}

static int git_merge_config(const char *k, const char *v, void *cb)
{
        if (branch && !prefixcmp(k, "branch.") &&
                !prefixcmp(k + 7, branch) &&
                !strcmp(k + 7 + strlen(branch), ".mergeoptions")) {
                free(branch_mergeoptions);
                branch_mergeoptions = xstrdup(v);
                return 0;
        }

        if (!strcmp(k, "merge.diffstat") || !strcmp(k, "merge.stat"))
                show_diffstat = git_config_bool(k, v);
        else if (!strcmp(k, "pull.twohead"))
                return git_config_string(&pull_twohead, k, v);
        else if (!strcmp(k, "pull.octopus"))
                return git_config_string(&pull_octopus, k, v);
        else if (!strcmp(k, "merge.renormalize"))
                option_renormalize = git_config_bool(k, v);
        else if (!strcmp(k, "merge.log") || !strcmp(k, "merge.summary")) {
                int is_bool;
                shortlog_len = git_config_bool_or_int(k, v, &is_bool);
                if (!is_bool && shortlog_len < 0)
                        return error(_("%s: negative length %s"), k, v);
                if (is_bool && shortlog_len)
                        shortlog_len = DEFAULT_MERGE_LOG_LEN;
                return 0;
        } else if (!strcmp(k, "merge.ff")) {
                int boolval = git_config_maybe_bool(k, v);
                if (0 <= boolval) {
                        allow_fast_forward = boolval;
                } else if (v && !strcmp(v, "only")) {
                        allow_fast_forward = 1;
                        fast_forward_only = 1;
                } /* do not barf on values from future versions of git */
                return 0;
        } else if (!strcmp(k, "merge.defaulttoupstream")) {
                default_to_upstream = git_config_bool(k, v);
                return 0;
        }
        return git_diff_ui_config(k, v, cb);
}

static int read_tree_trivial(unsigned char *common, unsigned char *head,
                             unsigned char *one)
{
        int i, nr_trees = 0;
        struct tree *trees[MAX_UNPACK_TREES];
        struct tree_desc t[MAX_UNPACK_TREES];
        struct unpack_trees_options opts;

        memset(&opts, 0, sizeof(opts));
        opts.head_idx = 2;
        opts.src_index = &the_index;
        opts.dst_index = &the_index;
        opts.update = 1;
        opts.verbose_update = 1;
        opts.trivial_merges_only = 1;
        opts.merge = 1;
        trees[nr_trees] = parse_tree_indirect(common);
        if (!trees[nr_trees++])
                return -1;
        trees[nr_trees] = parse_tree_indirect(head);
        if (!trees[nr_trees++])
                return -1;
        trees[nr_trees] = parse_tree_indirect(one);
        if (!trees[nr_trees++])
                return -1;
        opts.fn = threeway_merge;
        cache_tree_free(&active_cache_tree);
        for (i = 0; i < nr_trees; i++) {
                parse_tree(trees[i]);
                init_tree_desc(t+i, trees[i]->buffer, trees[i]->size);
        }
        if (unpack_trees(nr_trees, t, &opts))
                return -1;
        return 0;
}

static void write_tree_trivial(unsigned char *sha1)
{
        if (write_cache_as_tree(sha1, 0, NULL))
                die(_("git write-tree failed to write a tree"));
}

static const char *merge_argument(struct commit *commit)
{
        if (commit)
                return sha1_to_hex(commit->object.sha1);
        else
                return EMPTY_TREE_SHA1_HEX;
}

int try_merge_command(const char *strategy, size_t xopts_nr,
                      const char **xopts, struct commit_list *common,
                      const char *head_arg, struct commit_list *remotes)
{
        const char **args;
        int i = 0, x = 0, ret;
        struct commit_list *j;
        struct strbuf buf = STRBUF_INIT;

        args = xmalloc((4 + xopts_nr + commit_list_count(common) +
                        commit_list_count(remotes)) * sizeof(char *));
        strbuf_addf(&buf, "merge-%s", strategy);
        args[i++] = buf.buf;
        for (x = 0; x < xopts_nr; x++) {
                char *s = xmalloc(strlen(xopts[x])+2+1);
                strcpy(s, "--");
                strcpy(s+2, xopts[x]);
                args[i++] = s;
        }
        for (j = common; j; j = j->next)
                args[i++] = xstrdup(merge_argument(j->item));
        args[i++] = "--";
        args[i++] = head_arg;
        for (j = remotes; j; j = j->next)
                args[i++] = xstrdup(merge_argument(j->item));
        args[i] = NULL;
        ret = run_command_v_opt(args, RUN_GIT_CMD);
        strbuf_release(&buf);
        i = 1;
        for (x = 0; x < xopts_nr; x++)
                free((void *)args[i++]);
        for (j = common; j; j = j->next)
                free((void *)args[i++]);
        i += 2;
        for (j = remotes; j; j = j->next)
                free((void *)args[i++]);
        free(args);
        discard_cache();
        if (read_cache() < 0)
                die(_("failed to read the cache"));
        resolve_undo_clear();

        return ret;
}

static int try_merge_strategy(const char *strategy, struct commit_list *common,
                              const char *head_arg)
{
        int index_fd;
        struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));

        index_fd = hold_locked_index(lock, 1);
        refresh_cache(REFRESH_QUIET);
        if (active_cache_changed &&
                        (write_cache(index_fd, active_cache, active_nr) ||
                         commit_locked_index(lock)))
                return error(_("Unable to write index."));
        rollback_lock_file(lock);

        if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree")) {
                int clean, x;
                struct commit *result;
                struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
                int index_fd;
                struct commit_list *reversed = NULL;
                struct merge_options o;
                struct commit_list *j;

                if (remoteheads->next) {
                        error(_("Not handling anything other than two heads merge."));
                        return 2;
                }

                init_merge_options(&o);
                if (!strcmp(strategy, "subtree"))
                        o.subtree_shift = "";

                o.renormalize = option_renormalize;
                o.show_rename_progress =
                        show_progress == -1 ? isatty(2) : show_progress;

                for (x = 0; x < xopts_nr; x++)
                        if (parse_merge_opt(&o, xopts[x]))
                                die(_("Unknown option for merge-recursive: -X%s"), xopts[x]);

                o.branch1 = head_arg;
                o.branch2 = remoteheads->item->util;

                for (j = common; j; j = j->next)
                        commit_list_insert(j->item, &reversed);

                index_fd = hold_locked_index(lock, 1);
                clean = merge_recursive(&o, lookup_commit(head),
                                remoteheads->item, reversed, &result);
                if (active_cache_changed &&
                                (write_cache(index_fd, active_cache, active_nr) ||
                                 commit_locked_index(lock)))
                        die (_("unable to write %s"), get_index_file());
                rollback_lock_file(lock);
                return clean ? 0 : 1;
        } else {
                return try_merge_command(strategy, xopts_nr, xopts,
                                                common, head_arg, remoteheads);
        }
}

static void count_diff_files(struct diff_queue_struct *q,
                             struct diff_options *opt, void *data)
{
        int *count = data;

        (*count) += q->nr;
}

static int count_unmerged_entries(void)
{
        int i, ret = 0;

        for (i = 0; i < active_nr; i++)
                if (ce_stage(active_cache[i]))
                        ret++;

        return ret;
}

int checkout_fast_forward(const unsigned char *head, const unsigned char *remote)
{
        struct tree *trees[MAX_UNPACK_TREES];
        struct unpack_trees_options opts;
        struct tree_desc t[MAX_UNPACK_TREES];
        int i, fd, nr_trees = 0;
        struct dir_struct dir;
        struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));

        refresh_cache(REFRESH_QUIET);

        fd = hold_locked_index(lock_file, 1);

        memset(&trees, 0, sizeof(trees));
        memset(&opts, 0, sizeof(opts));
        memset(&t, 0, sizeof(t));
        memset(&dir, 0, sizeof(dir));
        dir.flags |= DIR_SHOW_IGNORED;
        dir.exclude_per_dir = ".gitignore";
        opts.dir = &dir;

        opts.head_idx = 1;
        opts.src_index = &the_index;
        opts.dst_index = &the_index;
        opts.update = 1;
        opts.verbose_update = 1;
        opts.merge = 1;
        opts.fn = twoway_merge;
        setup_unpack_trees_porcelain(&opts, "merge");

        trees[nr_trees] = parse_tree_indirect(head);
        if (!trees[nr_trees++])
                return -1;
        trees[nr_trees] = parse_tree_indirect(remote);
        if (!trees[nr_trees++])
                return -1;
        for (i = 0; i < nr_trees; i++) {
                parse_tree(trees[i]);
                init_tree_desc(t+i, trees[i]->buffer, trees[i]->size);
        }
        if (unpack_trees(nr_trees, t, &opts))
                return -1;
        if (write_cache(fd, active_cache, active_nr) ||
                commit_locked_index(lock_file))
                die(_("unable to write new index file"));
        return 0;
}

static void split_merge_strategies(const char *string, struct strategy **list,
                                   int *nr, int *alloc)
{
        char *p, *q, *buf;

        if (!string)
                return;

        buf = xstrdup(string);
        q = buf;
        for (;;) {
                p = strchr(q, ' ');
                if (!p) {
                        ALLOC_GROW(*list, *nr + 1, *alloc);
                        (*list)[(*nr)++].name = xstrdup(q);
                        free(buf);
                        return;
                } else {
                        *p = '\0';
                        ALLOC_GROW(*list, *nr + 1, *alloc);
                        (*list)[(*nr)++].name = xstrdup(q);
                        q = ++p;
                }
        }
}

static void add_strategies(const char *string, unsigned attr)
{
        struct strategy *list = NULL;
        int list_alloc = 0, list_nr = 0, i;

        memset(&list, 0, sizeof(list));
        split_merge_strategies(string, &list, &list_nr, &list_alloc);
        if (list) {
                for (i = 0; i < list_nr; i++)
                        append_strategy(get_strategy(list[i].name));
                return;
        }
        for (i = 0; i < ARRAY_SIZE(all_strategy); i++)
                if (all_strategy[i].attr & attr)
                        append_strategy(&all_strategy[i]);

}

static void write_merge_msg(struct strbuf *msg)
{
        int fd = open(git_path("MERGE_MSG"), O_WRONLY | O_CREAT, 0666);
        if (fd < 0)
                die_errno(_("Could not open '%s' for writing"),
                          git_path("MERGE_MSG"));
        if (write_in_full(fd, msg->buf, msg->len) != msg->len)
                die_errno(_("Could not write to '%s'"), git_path("MERGE_MSG"));
        close(fd);
}

static void read_merge_msg(struct strbuf *msg)
{
        strbuf_reset(msg);
        if (strbuf_read_file(msg, git_path("MERGE_MSG"), 0) < 0)
                die_errno(_("Could not read from '%s'"), git_path("MERGE_MSG"));
}

static void write_merge_state(void);
static void abort_commit(const char *err_msg)
{
        if (err_msg)
                error("%s", err_msg);
        fprintf(stderr,
                _("Not committing merge; use 'git commit' to complete the merge.\n"));
        write_merge_state();
        exit(1);
}

static void prepare_to_commit(void)
{
        struct strbuf msg = STRBUF_INIT;
        strbuf_addbuf(&msg, &merge_msg);
        strbuf_addch(&msg, '\n');
        write_merge_msg(&msg);
        run_hook(get_index_file(), "prepare-commit-msg",
                 git_path("MERGE_MSG"), "merge", NULL, NULL);
        if (option_edit) {
                if (launch_editor(git_path("MERGE_MSG"), NULL, NULL))
                        abort_commit(NULL);
        }
        read_merge_msg(&msg);
        stripspace(&msg, option_edit);
        if (!msg.len)
                abort_commit(_("Empty commit message."));
        strbuf_release(&merge_msg);
        strbuf_addbuf(&merge_msg, &msg);
        strbuf_release(&msg);
}

static int merge_trivial(void)
{
        unsigned char result_tree[20], result_commit[20];
        struct commit_list *parent = xmalloc(sizeof(*parent));

        write_tree_trivial(result_tree);
        printf(_("Wonderful.\n"));
        parent->item = lookup_commit(head);
        parent->next = xmalloc(sizeof(*parent->next));
        parent->next->item = remoteheads->item;
        parent->next->next = NULL;
        prepare_to_commit();
        commit_tree(merge_msg.buf, result_tree, parent, result_commit, NULL);
        finish(result_commit, "In-index merge");
        drop_save();
        return 0;
}

static int finish_automerge(struct commit_list *common,
                            unsigned char *result_tree,
                            const char *wt_strategy)
{
        struct commit_list *parents = NULL, *j;
        struct strbuf buf = STRBUF_INIT;
        unsigned char result_commit[20];

        free_commit_list(common);
        if (allow_fast_forward) {
                parents = remoteheads;
                commit_list_insert(lookup_commit(head), &parents);
                parents = reduce_heads(parents);
        } else {
                struct commit_list **pptr = &parents;

                pptr = &commit_list_insert(lookup_commit(head),
                                pptr)->next;
                for (j = remoteheads; j; j = j->next)
                        pptr = &commit_list_insert(j->item, pptr)->next;
        }
        strbuf_addch(&merge_msg, '\n');
        prepare_to_commit();
        free_commit_list(remoteheads);
        commit_tree(merge_msg.buf, result_tree, parents, result_commit, NULL);
        strbuf_addf(&buf, "Merge made by the '%s' strategy.", wt_strategy);
        finish(result_commit, buf.buf);
        strbuf_release(&buf);
        drop_save();
        return 0;
}

static int suggest_conflicts(int renormalizing)
{
        FILE *fp;
        int pos;

        fp = fopen(git_path("MERGE_MSG"), "a");
        if (!fp)
                die_errno(_("Could not open '%s' for writing"),
                          git_path("MERGE_MSG"));
        fprintf(fp, "\nConflicts:\n");
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];

                if (ce_stage(ce)) {
                        fprintf(fp, "\t%s\n", ce->name);
                        while (pos + 1 < active_nr &&
                                        !strcmp(ce->name,
                                                active_cache[pos + 1]->name))
                                pos++;
                }
        }
        fclose(fp);
        rerere(allow_rerere_auto);
        printf(_("Automatic merge failed; "
                        "fix conflicts and then commit the result.\n"));
        return 1;
}

static struct commit *is_old_style_invocation(int argc, const char **argv)
{
        struct commit *second_token = NULL;
        if (argc > 2) {
                unsigned char second_sha1[20];

                if (get_sha1(argv[1], second_sha1))
                        return NULL;
                second_token = lookup_commit_reference_gently(second_sha1, 0);
                if (!second_token)
                        die(_("'%s' is not a commit"), argv[1]);
                if (hashcmp(second_token->object.sha1, head))
                        return NULL;
        }
        return second_token;
}

static int evaluate_result(void)
{
        int cnt = 0;
        struct rev_info rev;

        /* Check how many files differ. */
        init_revisions(&rev, "");
        setup_revisions(0, NULL, &rev, NULL);
        rev.diffopt.output_format |=
                DIFF_FORMAT_CALLBACK;
        rev.diffopt.format_callback = count_diff_files;
        rev.diffopt.format_callback_data = &cnt;
        run_diff_files(&rev, 0);

        /*
         * Check how many unmerged entries are
         * there.
         */
        cnt += count_unmerged_entries();

        return cnt;
}

/*
 * Pretend as if the user told us to merge with the tracking
 * branch we have for the upstream of the current branch
 */
static int setup_with_upstream(const char ***argv)
{
        struct branch *branch = branch_get(NULL);
        int i;
        const char **args;

        if (!branch)
                die(_("No current branch."));
        if (!branch->remote)
                die(_("No remote for the current branch."));
        if (!branch->merge_nr)
                die(_("No default upstream defined for the current branch."));

        args = xcalloc(branch->merge_nr + 1, sizeof(char *));
        for (i = 0; i < branch->merge_nr; i++) {
                if (!branch->merge[i]->dst)
                        die(_("No remote tracking branch for %s from %s"),
                            branch->merge[i]->src, branch->remote_name);
                args[i] = branch->merge[i]->dst;
        }
        args[i] = NULL;
        *argv = args;
        return i;
}

static void write_merge_state(void)
{
        int fd;
        struct commit_list *j;
        struct strbuf buf = STRBUF_INIT;

        for (j = remoteheads; j; j = j->next)
                strbuf_addf(&buf, "%s\n",
                        sha1_to_hex(j->item->object.sha1));
        fd = open(git_path("MERGE_HEAD"), O_WRONLY | O_CREAT, 0666);
        if (fd < 0)
                die_errno(_("Could not open '%s' for writing"),
                          git_path("MERGE_HEAD"));
        if (write_in_full(fd, buf.buf, buf.len) != buf.len)
                die_errno(_("Could not write to '%s'"), git_path("MERGE_HEAD"));
        close(fd);
        strbuf_addch(&merge_msg, '\n');
        write_merge_msg(&merge_msg);
        fd = open(git_path("MERGE_MODE"), O_WRONLY | O_CREAT | O_TRUNC, 0666);
        if (fd < 0)
                die_errno(_("Could not open '%s' for writing"),
                          git_path("MERGE_MODE"));
        strbuf_reset(&buf);
        if (!allow_fast_forward)
                strbuf_addf(&buf, "no-ff");
        if (write_in_full(fd, buf.buf, buf.len) != buf.len)
                die_errno(_("Could not write to '%s'"), git_path("MERGE_MODE"));
        close(fd);
}

int cmd_merge(int argc, const char **argv, const char *prefix)
{
        unsigned char result_tree[20];
        struct strbuf buf = STRBUF_INIT;
        const char *head_arg;
        int flag, head_invalid = 0, i;
        int best_cnt = -1, merge_was_ok = 0, automerge_was_ok = 0;
        struct commit_list *common = NULL;
        const char *best_strategy = NULL, *wt_strategy = NULL;
        struct commit_list **remotes = &remoteheads;

        if (argc == 2 && !strcmp(argv[1], "-h"))
                usage_with_options(builtin_merge_usage, builtin_merge_options);

        /*
         * Check if we are _not_ on a detached HEAD, i.e. if there is a
         * current branch.
         */
        branch = resolve_ref("HEAD", head, 0, &flag);
        if (branch && !prefixcmp(branch, "refs/heads/"))
                branch += 11;
        if (is_null_sha1(head))
                head_invalid = 1;

        git_config(git_merge_config, NULL);

        if (branch_mergeoptions)
                parse_branch_merge_options(branch_mergeoptions);
        argc = parse_options(argc, argv, prefix, builtin_merge_options,
                        builtin_merge_usage, 0);

        if (verbosity < 0 && show_progress == -1)
                show_progress = 0;

        if (abort_current_merge) {
                int nargc = 2;
                const char *nargv[] = {"reset", "--merge", NULL};

                if (!file_exists(git_path("MERGE_HEAD")))
                        die(_("There is no merge to abort (MERGE_HEAD missing)."));

                /* Invoke 'git reset --merge' */
                return cmd_reset(nargc, nargv, prefix);
        }

        if (read_cache_unmerged())
                die_resolve_conflict("merge");

        if (file_exists(git_path("MERGE_HEAD"))) {
                /*
                 * There is no unmerged entry, don't advise 'git
                 * add/rm <file>', just 'git commit'.
                 */
                if (advice_resolve_conflict)
                        die(_("You have not concluded your merge (MERGE_HEAD exists).\n"
                                  "Please, commit your changes before you can merge."));
                else
                        die(_("You have not concluded your merge (MERGE_HEAD exists)."));
        }
        if (file_exists(git_path("CHERRY_PICK_HEAD"))) {
                if (advice_resolve_conflict)
                        die(_("You have not concluded your cherry-pick (CHERRY_PICK_HEAD exists).\n"
                            "Please, commit your changes before you can merge."));
                else
                        die(_("You have not concluded your cherry-pick (CHERRY_PICK_HEAD exists)."));
        }
        resolve_undo_clear();

        if (verbosity < 0)
                show_diffstat = 0;

        if (squash) {
                if (!allow_fast_forward)
                        die(_("You cannot combine --squash with --no-ff."));
                option_commit = 0;
        }

        if (!allow_fast_forward && fast_forward_only)
                die(_("You cannot combine --no-ff with --ff-only."));

        if (!abort_current_merge) {
                if (!argc && default_to_upstream)
                        argc = setup_with_upstream(&argv);
                else if (argc == 1 && !strcmp(argv[0], "-"))
                        argv[0] = "@{-1}";
        }
        if (!argc)
                usage_with_options(builtin_merge_usage,
                        builtin_merge_options);

        /*
         * This could be traditional "merge <msg> HEAD <commit>..."  and
         * the way we can tell it is to see if the second token is HEAD,
         * but some people might have misused the interface and used a
         * committish that is the same as HEAD there instead.
         * Traditional format never would have "-m" so it is an
         * additional safety measure to check for it.
         */

        if (!have_message && is_old_style_invocation(argc, argv)) {
                strbuf_addstr(&merge_msg, argv[0]);
                head_arg = argv[1];
                argv += 2;
                argc -= 2;
        } else if (head_invalid) {
                struct object *remote_head;
                /*
                 * If the merged head is a valid one there is no reason
                 * to forbid "git merge" into a branch yet to be born.
                 * We do the same for "git pull".
                 */
                if (argc != 1)
                        die(_("Can merge only exactly one commit into "
                                "empty head"));
                if (squash)
                        die(_("Squash commit into empty head not supported yet"));
                if (!allow_fast_forward)
                        die(_("Non-fast-forward commit does not make sense into "
                            "an empty head"));
                remote_head = peel_to_type(argv[0], 0, NULL, OBJ_COMMIT);
                if (!remote_head)
                        die(_("%s - not something we can merge"), argv[0]);
                read_empty(remote_head->sha1, 0);
                update_ref("initial pull", "HEAD", remote_head->sha1, NULL, 0,
                                DIE_ON_ERR);
                return 0;
        } else {
                struct strbuf merge_names = STRBUF_INIT;

                /* We are invoked directly as the first-class UI. */
                head_arg = "HEAD";

                /*
                 * All the rest are the commits being merged;
                 * prepare the standard merge summary message to
                 * be appended to the given message.  If remote
                 * is invalid we will die later in the common
                 * codepath so we discard the error in this
                 * loop.
                 */
                for (i = 0; i < argc; i++)
                        merge_name(argv[i], &merge_names);

                if (!have_message || shortlog_len) {
                        fmt_merge_msg(&merge_names, &merge_msg, !have_message,
                                      shortlog_len);
                        if (merge_msg.len)
                                strbuf_setlen(&merge_msg, merge_msg.len - 1);
                }
        }

        if (head_invalid || !argc)
                usage_with_options(builtin_merge_usage,
                        builtin_merge_options);

        strbuf_addstr(&buf, "merge");
        for (i = 0; i < argc; i++)
                strbuf_addf(&buf, " %s", argv[i]);
        setenv("GIT_REFLOG_ACTION", buf.buf, 0);
        strbuf_reset(&buf);

        for (i = 0; i < argc; i++) {
                struct object *o;
                struct commit *commit;

                o = peel_to_type(argv[i], 0, NULL, OBJ_COMMIT);
                if (!o)
                        die(_("%s - not something we can merge"), argv[i]);
                commit = lookup_commit(o->sha1);
                commit->util = (void *)argv[i];
                remotes = &commit_list_insert(commit, remotes)->next;

                strbuf_addf(&buf, "GITHEAD_%s", sha1_to_hex(o->sha1));
                setenv(buf.buf, argv[i], 1);
                strbuf_reset(&buf);
        }

        if (!use_strategies) {
                if (!remoteheads->next)
                        add_strategies(pull_twohead, DEFAULT_TWOHEAD);
                else
                        add_strategies(pull_octopus, DEFAULT_OCTOPUS);
        }

        for (i = 0; i < use_strategies_nr; i++) {
                if (use_strategies[i]->attr & NO_FAST_FORWARD)
                        allow_fast_forward = 0;
                if (use_strategies[i]->attr & NO_TRIVIAL)
                        allow_trivial = 0;
        }

        if (!remoteheads->next)
                common = get_merge_bases(lookup_commit(head),
                                remoteheads->item, 1);
        else {
                struct commit_list *list = remoteheads;
                commit_list_insert(lookup_commit(head), &list);
                common = get_octopus_merge_bases(list);
                free(list);
        }

        update_ref("updating ORIG_HEAD", "ORIG_HEAD", head, NULL, 0,
                DIE_ON_ERR);

        if (!common)
                ; /* No common ancestors found. We need a real merge. */
        else if (!remoteheads->next && !common->next &&
                        common->item == remoteheads->item) {
                /*
                 * If head can reach all the merge then we are up to date.
                 * but first the most common case of merging one remote.
                 */
                finish_up_to_date("Already up-to-date.");
                return 0;
        } else if (allow_fast_forward && !remoteheads->next &&
                        !common->next &&
                        !hashcmp(common->item->object.sha1, head)) {
                /* Again the most common case of merging one remote. */
                struct strbuf msg = STRBUF_INIT;
                struct object *o;
                char hex[41];

                strcpy(hex, find_unique_abbrev(head, DEFAULT_ABBREV));

                if (verbosity >= 0)
                        printf(_("Updating %s..%s\n"),
                                hex,
                                find_unique_abbrev(remoteheads->item->object.sha1,
                                DEFAULT_ABBREV));
                strbuf_addstr(&msg, "Fast-forward");
                if (have_message)
                        strbuf_addstr(&msg,
                                " (no commit created; -m option ignored)");
                o = peel_to_type(sha1_to_hex(remoteheads->item->object.sha1),
                        0, NULL, OBJ_COMMIT);
                if (!o)
                        return 1;

                if (checkout_fast_forward(head, remoteheads->item->object.sha1))
                        return 1;

                finish(o->sha1, msg.buf);
                drop_save();
                return 0;
        } else if (!remoteheads->next && common->next)
                ;
                /*
                 * We are not doing octopus and not fast-forward.  Need
                 * a real merge.
                 */
        else if (!remoteheads->next && !common->next && option_commit) {
                /*
                 * We are not doing octopus, not fast-forward, and have
                 * only one common.
                 */
                refresh_cache(REFRESH_QUIET);
                if (allow_trivial && !fast_forward_only) {
                        /* See if it is really trivial. */
                        git_committer_info(IDENT_ERROR_ON_NO_NAME);
                        printf(_("Trying really trivial in-index merge...\n"));
                        if (!read_tree_trivial(common->item->object.sha1,
                                        head, remoteheads->item->object.sha1))
                                return merge_trivial();
                        printf(_("Nope.\n"));
                }
        } else {
                /*
                 * An octopus.  If we can reach all the remote we are up
                 * to date.
                 */
                int up_to_date = 1;
                struct commit_list *j;

                for (j = remoteheads; j; j = j->next) {
                        struct commit_list *common_one;

                        /*
                         * Here we *have* to calculate the individual
                         * merge_bases again, otherwise "git merge HEAD^
                         * HEAD^^" would be missed.
                         */
                        common_one = get_merge_bases(lookup_commit(head),
                                j->item, 1);
                        if (hashcmp(common_one->item->object.sha1,
                                j->item->object.sha1)) {
                                up_to_date = 0;
                                break;
                        }
                }
                if (up_to_date) {
                        finish_up_to_date("Already up-to-date. Yeeah!");
                        return 0;
                }
        }

        if (fast_forward_only)
                die(_("Not possible to fast-forward, aborting."));

        /* We are going to make a new commit. */
        git_committer_info(IDENT_ERROR_ON_NO_NAME);

        /*
         * At this point, we need a real merge.  No matter what strategy
         * we use, it would operate on the index, possibly affecting the
         * working tree, and when resolved cleanly, have the desired
         * tree in the index -- this means that the index must be in
         * sync with the head commit.  The strategies are responsible
         * to ensure this.
         */
        if (use_strategies_nr != 1) {
                /*
                 * Stash away the local changes so that we can try more
                 * than one.
                 */
                save_state();
        } else {
                memcpy(stash, null_sha1, 20);
        }

        for (i = 0; i < use_strategies_nr; i++) {
                int ret;
                if (i) {
                        printf(_("Rewinding the tree to pristine...\n"));
                        restore_state();
                }
                if (use_strategies_nr != 1)
                        printf(_("Trying merge strategy %s...\n"),
                                use_strategies[i]->name);
                /*
                 * Remember which strategy left the state in the working
                 * tree.
                 */
                wt_strategy = use_strategies[i]->name;

                ret = try_merge_strategy(use_strategies[i]->name,
                        common, head_arg);
                if (!option_commit && !ret) {
                        merge_was_ok = 1;
                        /*
                         * This is necessary here just to avoid writing
                         * the tree, but later we will *not* exit with
                         * status code 1 because merge_was_ok is set.
                         */
                        ret = 1;
                }

                if (ret) {
                        /*
                         * The backend exits with 1 when conflicts are
                         * left to be resolved, with 2 when it does not
                         * handle the given merge at all.
                         */
                        if (ret == 1) {
                                int cnt = evaluate_result();

                                if (best_cnt <= 0 || cnt <= best_cnt) {
                                        best_strategy = use_strategies[i]->name;
                                        best_cnt = cnt;
                                }
                        }
                        if (merge_was_ok)
                                break;
                        else
                                continue;
                }

                /* Automerge succeeded. */
                write_tree_trivial(result_tree);
                automerge_was_ok = 1;
                break;
        }

        /*
         * If we have a resulting tree, that means the strategy module
         * auto resolved the merge cleanly.
         */
        if (automerge_was_ok)
                return finish_automerge(common, result_tree, wt_strategy);

        /*
         * Pick the result from the best strategy and have the user fix
         * it up.
         */
        if (!best_strategy) {
                restore_state();
                if (use_strategies_nr > 1)
                        fprintf(stderr,
                                _("No merge strategy handled the merge.\n"));
                else
                        fprintf(stderr, _("Merge with strategy %s failed.\n"),
                                use_strategies[0]->name);
                return 2;
        } else if (best_strategy == wt_strategy)
                ; /* We already have its result in the working tree. */
        else {
                printf(_("Rewinding the tree to pristine...\n"));
                restore_state();
                printf(_("Using the %s to prepare resolving by hand.\n"),
                        best_strategy);
                try_merge_strategy(best_strategy, common, head_arg);
        }

        if (squash)
                finish(NULL, NULL);
        else
                write_merge_state();

        if (merge_was_ok) {
                fprintf(stderr, _("Automatic merge went well; "
                        "stopped before committing as requested\n"));
                return 0;
        } else
                return suggest_conflicts(option_renormalize);
}