/*
 * 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"

#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] <remote>...",
        "git-merge [options] <msg> HEAD <remote>",
        NULL
};

static int show_diffstat = 1, option_log, squash;
static int option_commit = 1, allow_fast_forward = 1;
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 *branch;

static struct strategy all_strategy[] = {
        { "recur",      NO_TRIVIAL },
        { "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
        { "octopus",    DEFAULT_OCTOPUS },
        { "resolve",    0 },
        { "stupid",     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 {
                strbuf_addf(buf, "%s\n\n", arg);
                have_message = 1;
        }
        return 0;
}

static struct strategy *get_strategy(const char *name)
{
        int i;

        if (!name)
                return NULL;

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

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)
{
        int i;
        struct strategy *s;

        if (unset)
                return 0;

        s = get_strategy(name);

        if (s)
                append_strategy(s);
        else {
                struct strbuf err;
                strbuf_init(&err, 0);
                for (i = 0; i < ARRAY_SIZE(all_strategy); i++)
                        strbuf_addf(&err, " %s", all_strategy[i].name);
                fprintf(stderr, "Could not find merge strategy '%s'.\n", name);
                fprintf(stderr, "Available strategies are:%s.\n", err.buf);
                exit(1);
        }
        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)"),
        OPT_BOOLEAN(0, "log", &option_log,
                "add list of one-line log to merge commit message"),
        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(0, "ff", &allow_fast_forward,
                "allow fast forward (default)"),
        OPT_CALLBACK('s', "strategy", &use_strategies, "strategy",
                "merge strategy to use", option_parse_strategy),
        OPT_CALLBACK('m', "message", &merge_msg, "message",
                "message to be used for the merge commit (if any)",
                option_parse_message),
        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"));
}

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 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;
        const char *args[] = { "stash", "apply", NULL, NULL };

        if (is_null_sha1(stash))
                return;

        reset_hard(head, 1);

        strbuf_init(&sb, 0);
        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)
{
        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;
        struct commit_list *j;
        int fd;

        printf("Squash commit -- not updating HEAD\n");
        fd = open(git_path("SQUASH_MSG"), O_WRONLY | O_CREAT, 0666);
        if (fd < 0)
                die("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");

        strbuf_init(&out, 0);
        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(rev.commit_format, commit, &out, rev.abbrev,
                        NULL, NULL, rev.date_mode, 0);
        }
        write(fd, out.buf, out.len);
        close(fd);
        strbuf_release(&out);
}

static int run_hook(const char *name)
{
        struct child_process hook;
        const char *argv[3], *env[2];
        char index[PATH_MAX];

        argv[0] = git_path("hooks/%s", name);
        if (access(argv[0], X_OK) < 0)
                return 0;

        snprintf(index, sizeof(index), "GIT_INDEX_FILE=%s", get_index_file());
        env[0] = index;
        env[1] = NULL;

        if (squash)
                argv[1] = "1";
        else
                argv[1] = "0";
        argv[2] = NULL;

        memset(&hook, 0, sizeof(hook));
        hook.argv = argv;
        hook.no_stdin = 1;
        hook.stdout_to_stderr = 1;
        hook.env = env;

        return run_command(&hook);
}

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

        strbuf_init(&reflog_message, 0);
        if (!msg)
                strbuf_addstr(&reflog_message, getenv("GIT_REFLOG_ACTION"));
        else {
                printf("%s\n", msg);
                strbuf_addf(&reflog_message, "%s: %s",
                        getenv("GIT_REFLOG_ACTION"), msg);
        }
        if (squash) {
                squash_message();
        } else {
                if (!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_use_color_default > 0)
                        DIFF_OPT_SET(&opts, COLOR_DIFF);
                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("post-merge");

        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;
        const char *ptr;
        int len, early;

        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);

        strbuf_init(&buf, 0);
        strbuf_addstr(&buf, "refs/heads/");
        strbuf_addstr(&buf, remote);
        resolve_ref(buf.buf, branch_head, 0, 0);

        if (!hashcmp(remote_head->sha1, branch_head)) {
                strbuf_addf(msg, "%s\t\tbranch '%s' of .\n",
                        sha1_to_hex(branch_head), remote);
                return;
        }

        /* 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, len+11);
                if (resolve_ref(truname.buf, buf_sha, 0, 0)) {
                        strbuf_addf(msg,
                                    "%s\t\tbranch '%s'%s of .\n",
                                    sha1_to_hex(remote_head->sha1),
                                    truname.buf,
                                    (early ? " (early part)" : ""));
                        return;
                }
        }

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

                strbuf_init(&line, 0);
                fp = fopen(git_path("FETCH_HEAD"), "r");
                if (!fp)
                        die("could not open %s for reading: %s",
                                git_path("FETCH_HEAD"), strerror(errno));
                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);
                return;
        }
        strbuf_addf(msg, "%s\t\tcommit '%s'\n",
                sha1_to_hex(remote_head->sha1), remote);
}

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")) {
                const char **argv;
                int argc;
                char *buf;

                buf = xstrdup(v);
                argc = split_cmdline(buf, &argv);
                argv = xrealloc(argv, sizeof(*argv) * (argc + 2));
                memmove(argv + 1, argv, sizeof(*argv) * (argc + 1));
                argc++;
                parse_options(argc, argv, builtin_merge_options,
                              builtin_merge_usage, 0);
                free(buf);
        }

        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);
        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 int try_merge_strategy(const char *strategy, struct commit_list *common,
                              const char *head_arg)
{
        const char **args;
        int i = 0, ret;
        struct commit_list *j;
        struct strbuf buf;

        args = xmalloc((4 + commit_list_count(common) +
                        commit_list_count(remoteheads)) * sizeof(char *));
        strbuf_init(&buf, 0);
        strbuf_addf(&buf, "merge-%s", strategy);
        args[i++] = buf.buf;
        for (j = common; j; j = j->next)
                args[i++] = xstrdup(sha1_to_hex(j->item->object.sha1));
        args[i++] = "--";
        args[i++] = head_arg;
        for (j = remoteheads; j; j = j->next)
                args[i++] = xstrdup(sha1_to_hex(j->item->object.sha1));
        args[i] = NULL;
        ret = run_command_v_opt(args, RUN_GIT_CMD);
        strbuf_release(&buf);
        i = 1;
        for (j = common; j; j = j->next)
                free((void *)args[i++]);
        i += 2;
        for (j = remoteheads; j; j = j->next)
                free((void *)args[i++]);
        free(args);
        return -ret;
}

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)
{
        const struct index_state *state = &the_index;
        int i, ret = 0;

        for (i = 0; i < state->cache_nr; i++)
                if (ce_stage(state->cache[i]))
                        ret++;

        return ret;
}

static int checkout_fast_forward(unsigned char *head, 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));

        if (read_cache_unmerged())
                die("you need to resolve your current index first");

        fd = hold_locked_index(lock_file, 1);

        memset(&trees, 0, sizeof(trees));
        memset(&opts, 0, sizeof(opts));
        memset(&t, 0, sizeof(t));
        dir.show_ignored = 1;
        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;

        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 != NULL) {
                for (i = 0; i < list_nr; i++) {
                        struct strategy *s;

                        s = get_strategy(list[i].name);
                        if (s)
                                append_strategy(s);
                }
                return;
        }
        for (i = 0; i < ARRAY_SIZE(all_strategy); i++)
                if (all_strategy[i].attr & attr)
                        append_strategy(&all_strategy[i]);

}

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

        write_tree_trivial(result_tree);
        printf("Wonderful.\n");
        parent.item = remoteheads->item;
        parent.next = NULL;
        commit_tree(merge_msg.buf, result_tree, &parent, result_commit);
        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;
        }
        free_commit_list(remoteheads);
        strbuf_addch(&merge_msg, '\n');
        commit_tree(merge_msg.buf, result_tree, parents, result_commit);
        strbuf_addf(&buf, "Merge made by %s.", wt_strategy);
        finish(result_commit, buf.buf);
        strbuf_release(&buf);
        drop_save();
        return 0;
}

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

        fp = fopen(git_path("MERGE_MSG"), "a");
        if (!fp)
                die("Could 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();
        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 > 1) {
                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;

        if (read_cache() < 0)
                die("failed to read the cache");

        /* 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;
}

int cmd_merge(int argc, const char **argv, const char *prefix)
{
        unsigned char result_tree[20];
        struct strbuf buf;
        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;

        setup_work_tree();
        if (unmerged_cache())
                die("You are in the middle of a conflicted merge.");

        /*
         * 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);

        /* for color.ui */
        if (diff_use_color_default == -1)
                diff_use_color_default = git_use_color_default;

        argc = parse_options(argc, argv, builtin_merge_options,
                        builtin_merge_usage, 0);

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

        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.
         */
        strbuf_init(&buf, 0);

        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");
                remote_head = peel_to_type(argv[0], 0, NULL, OBJ_COMMIT);
                if (!remote_head)
                        die("%s - not something we can merge", argv[0]);
                update_ref("initial pull", "HEAD", remote_head->sha1, NULL, 0,
                                DIE_ON_ERR);
                reset_hard(remote_head->sha1, 0);
                return 0;
        } else {
                struct strbuf msg;

                /* 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.
                 */
                strbuf_init(&msg, 0);
                for (i = 0; i < argc; i++)
                        merge_name(argv[i], &msg);
                fmt_merge_msg(option_log, &msg, &merge_msg);
                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;

                o = peel_to_type(argv[i], 0, NULL, OBJ_COMMIT);
                if (!o)
                        die("%s - not something we can merge", argv[i]);
                remotes = &commit_list_insert(lookup_commit(o->sha1),
                        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;
                struct object *o;
                char hex[41];

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

                printf("Updating %s..%s\n",
                        hex,
                        find_unique_abbrev(remoteheads->item->object.sha1,
                        DEFAULT_ABBREV));
                refresh_cache(REFRESH_QUIET);
                strbuf_init(&msg, 0);
                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) {
                        /* 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;
                }
        }

        /* 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 {
                int fd;
                struct commit_list *j;

                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("Could open %s for writing",
                                git_path("MERGE_HEAD"));
                if (write_in_full(fd, buf.buf, buf.len) != buf.len)
                        die("Could not write to %s", git_path("MERGE_HEAD"));
                close(fd);
                strbuf_addch(&merge_msg, '\n');
                fd = open(git_path("MERGE_MSG"), O_WRONLY | O_CREAT, 0666);
                if (fd < 0)
                        die("Could open %s for writing", git_path("MERGE_MSG"));
                if (write_in_full(fd, merge_msg.buf, merge_msg.len) !=
                        merge_msg.len)
                        die("Could not write to %s", git_path("MERGE_MSG"));
                close(fd);
        }

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