#include "cache.h"
#include "builtin.h"
#include "parse-options.h"
#include "refs.h"
#include "commit.h"
#include "tree.h"
#include "tree-walk.h"
#include "cache-tree.h"
#include "unpack-trees.h"
#include "dir.h"
#include "run-command.h"
#include "merge-recursive.h"
#include "branch.h"
#include "diff.h"
#include "revision.h"
#include "remote.h"
#include "blob.h"
#include "xdiff-interface.h"
#include "ll-merge.h"
#include "resolve-undo.h"

static const char * const checkout_usage[] = {
        "git checkout [options] <branch>",
        "git checkout [options] [<branch>] -- <file>...",
        NULL,
};

struct checkout_opts {
        int quiet;
        int merge;
        int force;
        int writeout_stage;
        int writeout_error;

        const char *new_branch;
        int new_branch_log;
        enum branch_track track;
};

static int post_checkout_hook(struct commit *old, struct commit *new,
                              int changed)
{
        return run_hook(NULL, "post-checkout",
                        sha1_to_hex(old ? old->object.sha1 : null_sha1),
                        sha1_to_hex(new ? new->object.sha1 : null_sha1),
                        changed ? "1" : "0", NULL);
        /* "new" can be NULL when checking out from the index before
           a commit exists. */

}

static int update_some(const unsigned char *sha1, const char *base, int baselen,
                const char *pathname, unsigned mode, int stage, void *context)
{
        int len;
        struct cache_entry *ce;

        if (S_ISDIR(mode))
                return READ_TREE_RECURSIVE;

        len = baselen + strlen(pathname);
        ce = xcalloc(1, cache_entry_size(len));
        hashcpy(ce->sha1, sha1);
        memcpy(ce->name, base, baselen);
        memcpy(ce->name + baselen, pathname, len - baselen);
        ce->ce_flags = create_ce_flags(len, 0);
        ce->ce_mode = create_ce_mode(mode);
        add_cache_entry(ce, ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
        return 0;
}

static int read_tree_some(struct tree *tree, const char **pathspec)
{
        read_tree_recursive(tree, "", 0, 0, pathspec, update_some, NULL);

        /* update the index with the given tree's info
         * for all args, expanding wildcards, and exit
         * with any non-zero return code.
         */
        return 0;
}

static int skip_same_name(struct cache_entry *ce, int pos)
{
        while (++pos < active_nr &&
               !strcmp(active_cache[pos]->name, ce->name))
                ; /* skip */
        return pos;
}

static int check_stage(int stage, struct cache_entry *ce, int pos)
{
        while (pos < active_nr &&
               !strcmp(active_cache[pos]->name, ce->name)) {
                if (ce_stage(active_cache[pos]) == stage)
                        return 0;
                pos++;
        }
        return error("path '%s' does not have %s version",
                     ce->name,
                     (stage == 2) ? "our" : "their");
}

static int check_all_stages(struct cache_entry *ce, int pos)
{
        if (ce_stage(ce) != 1 ||
            active_nr <= pos + 2 ||
            strcmp(active_cache[pos+1]->name, ce->name) ||
            ce_stage(active_cache[pos+1]) != 2 ||
            strcmp(active_cache[pos+2]->name, ce->name) ||
            ce_stage(active_cache[pos+2]) != 3)
                return error("path '%s' does not have all three versions",
                             ce->name);
        return 0;
}

static int checkout_stage(int stage, struct cache_entry *ce, int pos,
                          struct checkout *state)
{
        while (pos < active_nr &&
               !strcmp(active_cache[pos]->name, ce->name)) {
                if (ce_stage(active_cache[pos]) == stage)
                        return checkout_entry(active_cache[pos], state, NULL);
                pos++;
        }
        return error("path '%s' does not have %s version",
                     ce->name,
                     (stage == 2) ? "our" : "their");
}

/* NEEDSWORK: share with merge-recursive */
static void fill_mm(const unsigned char *sha1, mmfile_t *mm)
{
        unsigned long size;
        enum object_type type;

        if (!hashcmp(sha1, null_sha1)) {
                mm->ptr = xstrdup("");
                mm->size = 0;
                return;
        }

        mm->ptr = read_sha1_file(sha1, &type, &size);
        if (!mm->ptr || type != OBJ_BLOB)
                die("unable to read blob object %s", sha1_to_hex(sha1));
        mm->size = size;
}

static int checkout_merged(int pos, struct checkout *state)
{
        struct cache_entry *ce = active_cache[pos];
        const char *path = ce->name;
        mmfile_t ancestor, ours, theirs;
        int status;
        unsigned char sha1[20];
        mmbuffer_t result_buf;

        if (ce_stage(ce) != 1 ||
            active_nr <= pos + 2 ||
            strcmp(active_cache[pos+1]->name, path) ||
            ce_stage(active_cache[pos+1]) != 2 ||
            strcmp(active_cache[pos+2]->name, path) ||
            ce_stage(active_cache[pos+2]) != 3)
                return error("path '%s' does not have all 3 versions", path);

        fill_mm(active_cache[pos]->sha1, &ancestor);
        fill_mm(active_cache[pos+1]->sha1, &ours);
        fill_mm(active_cache[pos+2]->sha1, &theirs);

        status = ll_merge(&result_buf, path, &ancestor,
                          &ours, "ours", &theirs, "theirs", 1);
        free(ancestor.ptr);
        free(ours.ptr);
        free(theirs.ptr);
        if (status < 0 || !result_buf.ptr) {
                free(result_buf.ptr);
                return error("path '%s': cannot merge", path);
        }

        /*
         * NEEDSWORK:
         * There is absolutely no reason to write this as a blob object
         * and create a phony cache entry just to leak.  This hack is
         * primarily to get to the write_entry() machinery that massages
         * the contents to work-tree format and writes out which only
         * allows it for a cache entry.  The code in write_entry() needs
         * to be refactored to allow us to feed a <buffer, size, mode>
         * instead of a cache entry.  Such a refactoring would help
         * merge_recursive as well (it also writes the merge result to the
         * object database even when it may contain conflicts).
         */
        if (write_sha1_file(result_buf.ptr, result_buf.size,
                            blob_type, sha1))
                die("Unable to add merge result for '%s'", path);
        ce = make_cache_entry(create_ce_mode(active_cache[pos+1]->ce_mode),
                              sha1,
                              path, 2, 0);
        if (!ce)
                die("make_cache_entry failed for path '%s'", path);
        status = checkout_entry(ce, state, NULL);
        return status;
}

static int checkout_paths(struct tree *source_tree, const char **pathspec,
                          struct checkout_opts *opts)
{
        int pos;
        struct checkout state;
        static char *ps_matched;
        unsigned char rev[20];
        int flag;
        struct commit *head;
        int errs = 0;
        int stage = opts->writeout_stage;
        int merge = opts->merge;
        int newfd;
        struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));

        newfd = hold_locked_index(lock_file, 1);
        if (read_cache_preload(pathspec) < 0)
                return error("corrupt index file");

        if (source_tree)
                read_tree_some(source_tree, pathspec);

        for (pos = 0; pathspec[pos]; pos++)
                ;
        ps_matched = xcalloc(1, pos);

        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
                match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, ps_matched);
        }

        if (report_path_error(ps_matched, pathspec, 0))
                return 1;

        /* "checkout -m path" to recreate conflicted state */
        if (opts->merge)
                unmerge_cache(pathspec);

        /* Any unmerged paths? */
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
                if (match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, NULL)) {
                        if (!ce_stage(ce))
                                continue;
                        if (opts->force) {
                                warning("path '%s' is unmerged", ce->name);
                        } else if (stage) {
                                errs |= check_stage(stage, ce, pos);
                        } else if (opts->merge) {
                                errs |= check_all_stages(ce, pos);
                        } else {
                                errs = 1;
                                error("path '%s' is unmerged", ce->name);
                        }
                        pos = skip_same_name(ce, pos) - 1;
                }
        }
        if (errs)
                return 1;

        /* Now we are committed to check them out */
        memset(&state, 0, sizeof(state));
        state.force = 1;
        state.refresh_cache = 1;
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
                if (match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, NULL)) {
                        if (!ce_stage(ce)) {
                                errs |= checkout_entry(ce, &state, NULL);
                                continue;
                        }
                        if (stage)
                                errs |= checkout_stage(stage, ce, pos, &state);
                        else if (merge)
                                errs |= checkout_merged(pos, &state);
                        pos = skip_same_name(ce, pos) - 1;
                }
        }

        if (write_cache(newfd, active_cache, active_nr) ||
            commit_locked_index(lock_file))
                die("unable to write new index file");

        resolve_ref("HEAD", rev, 0, &flag);
        head = lookup_commit_reference_gently(rev, 1);

        errs |= post_checkout_hook(head, head, 0);
        return errs;
}

static void show_local_changes(struct object *head)
{
        struct rev_info rev;
        /* I think we want full paths, even if we're in a subdirectory. */
        init_revisions(&rev, NULL);
        rev.abbrev = 0;
        rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
        if (diff_setup_done(&rev.diffopt) < 0)
                die("diff_setup_done failed");
        add_pending_object(&rev, head, NULL);
        run_diff_index(&rev, 0);
}

static void describe_detached_head(char *msg, struct commit *commit)
{
        struct strbuf sb = STRBUF_INIT;
        struct pretty_print_context ctx = {0};
        parse_commit(commit);
        pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, &ctx);
        fprintf(stderr, "%s %s... %s\n", msg,
                find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV), sb.buf);
        strbuf_release(&sb);
}

static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
{
        struct unpack_trees_options opts;
        struct tree_desc tree_desc;

        memset(&opts, 0, sizeof(opts));
        opts.head_idx = -1;
        opts.update = worktree;
        opts.skip_unmerged = !worktree;
        opts.reset = 1;
        opts.merge = 1;
        opts.fn = oneway_merge;
        opts.verbose_update = !o->quiet;
        opts.src_index = &the_index;
        opts.dst_index = &the_index;
        parse_tree(tree);
        init_tree_desc(&tree_desc, tree->buffer, tree->size);
        switch (unpack_trees(1, &tree_desc, &opts)) {
        case -2:
                o->writeout_error = 1;
                /*
                 * We return 0 nevertheless, as the index is all right
                 * and more importantly we have made best efforts to
                 * update paths in the work tree, and we cannot revert
                 * them.
                 */
        case 0:
                return 0;
        default:
                return 128;
        }
}

struct branch_info {
        const char *name; /* The short name used */
        const char *path; /* The full name of a real branch */
        struct commit *commit; /* The named commit */
};

static void setup_branch_path(struct branch_info *branch)
{
        struct strbuf buf = STRBUF_INIT;

        strbuf_branchname(&buf, branch->name);
        if (strcmp(buf.buf, branch->name))
                branch->name = xstrdup(buf.buf);
        strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
        branch->path = strbuf_detach(&buf, NULL);
}

static int merge_working_tree(struct checkout_opts *opts,
                              struct branch_info *old, struct branch_info *new)
{
        int ret;
        struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
        int newfd = hold_locked_index(lock_file, 1);

        if (read_cache_preload(NULL) < 0)
                return error("corrupt index file");

        resolve_undo_clear();
        if (opts->force) {
                ret = reset_tree(new->commit->tree, opts, 1);
                if (ret)
                        return ret;
        } else {
                struct tree_desc trees[2];
                struct tree *tree;
                struct unpack_trees_options topts;

                memset(&topts, 0, sizeof(topts));
                topts.head_idx = -1;
                topts.src_index = &the_index;
                topts.dst_index = &the_index;

                topts.msgs.not_uptodate_file = "You have local changes to '%s'; cannot switch branches.";

                refresh_cache(REFRESH_QUIET);

                if (unmerged_cache()) {
                        error("you need to resolve your current index first");
                        return 1;
                }

                /* 2-way merge to the new branch */
                topts.initial_checkout = is_cache_unborn();
                topts.update = 1;
                topts.merge = 1;
                topts.gently = opts->merge;
                topts.verbose_update = !opts->quiet;
                topts.fn = twoway_merge;
                topts.dir = xcalloc(1, sizeof(*topts.dir));
                topts.dir->flags |= DIR_SHOW_IGNORED;
                topts.dir->exclude_per_dir = ".gitignore";
                tree = parse_tree_indirect(old->commit ?
                                           old->commit->object.sha1 :
                                           (unsigned char *)EMPTY_TREE_SHA1_BIN);
                init_tree_desc(&trees[0], tree->buffer, tree->size);
                tree = parse_tree_indirect(new->commit->object.sha1);
                init_tree_desc(&trees[1], tree->buffer, tree->size);

                ret = unpack_trees(2, trees, &topts);
                if (ret == -1) {
                        /*
                         * Unpack couldn't do a trivial merge; either
                         * give up or do a real merge, depending on
                         * whether the merge flag was used.
                         */
                        struct tree *result;
                        struct tree *work;
                        struct merge_options o;
                        if (!opts->merge)
                                return 1;
                        parse_commit(old->commit);

                        /* Do more real merge */

                        /*
                         * We update the index fully, then write the
                         * tree from the index, then merge the new
                         * branch with the current tree, with the old
                         * branch as the base. Then we reset the index
                         * (but not the working tree) to the new
                         * branch, leaving the working tree as the
                         * merged version, but skipping unmerged
                         * entries in the index.
                         */

                        add_files_to_cache(NULL, NULL, 0);
                        init_merge_options(&o);
                        o.verbosity = 0;
                        work = write_tree_from_memory(&o);

                        ret = reset_tree(new->commit->tree, opts, 1);
                        if (ret)
                                return ret;
                        o.branch1 = new->name;
                        o.branch2 = "local";
                        merge_trees(&o, new->commit->tree, work,
                                old->commit->tree, &result);
                        ret = reset_tree(new->commit->tree, opts, 0);
                        if (ret)
                                return ret;
                }
        }

        if (write_cache(newfd, active_cache, active_nr) ||
            commit_locked_index(lock_file))
                die("unable to write new index file");

        if (!opts->force && !opts->quiet)
                show_local_changes(&new->commit->object);

        return 0;
}

static void report_tracking(struct branch_info *new)
{
        struct strbuf sb = STRBUF_INIT;
        struct branch *branch = branch_get(new->name);

        if (!format_tracking_info(branch, &sb))
                return;
        fputs(sb.buf, stdout);
        strbuf_release(&sb);
}

static void update_refs_for_switch(struct checkout_opts *opts,
                                   struct branch_info *old,
                                   struct branch_info *new)
{
        struct strbuf msg = STRBUF_INIT;
        const char *old_desc;
        if (opts->new_branch) {
                create_branch(old->name, opts->new_branch, new->name, 0,
                              opts->new_branch_log, opts->track);
                new->name = opts->new_branch;
                setup_branch_path(new);
        }

        old_desc = old->name;
        if (!old_desc && old->commit)
                old_desc = sha1_to_hex(old->commit->object.sha1);
        strbuf_addf(&msg, "checkout: moving from %s to %s",
                    old_desc ? old_desc : "(invalid)", new->name);

        if (new->path) {
                create_symref("HEAD", new->path, msg.buf);
                if (!opts->quiet) {
                        if (old->path && !strcmp(new->path, old->path))
                                fprintf(stderr, "Already on '%s'\n",
                                        new->name);
                        else
                                fprintf(stderr, "Switched to%s branch '%s'\n",
                                        opts->new_branch ? " a new" : "",
                                        new->name);
                }
        } else if (strcmp(new->name, "HEAD")) {
                update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
                           REF_NODEREF, DIE_ON_ERR);
                if (!opts->quiet) {
                        if (old->path)
                                fprintf(stderr, "Note: moving to '%s' which isn't a local branch\nIf you want to create a new branch from this checkout, you may do so\n(now or later) by using -b with the checkout command again. Example:\n  git checkout -b <new_branch_name>\n", new->name);
                        describe_detached_head("HEAD is now at", new->commit);
                }
        }
        remove_branch_state();
        strbuf_release(&msg);
        if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
                report_tracking(new);
}

static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
{
        int ret = 0;
        struct branch_info old;
        unsigned char rev[20];
        int flag;
        memset(&old, 0, sizeof(old));
        old.path = resolve_ref("HEAD", rev, 0, &flag);
        old.commit = lookup_commit_reference_gently(rev, 1);
        if (!(flag & REF_ISSYMREF))
                old.path = NULL;

        if (old.path && !prefixcmp(old.path, "refs/heads/"))
                old.name = old.path + strlen("refs/heads/");

        if (!new->name) {
                new->name = "HEAD";
                new->commit = old.commit;
                if (!new->commit)
                        die("You are on a branch yet to be born");
                parse_commit(new->commit);
        }

        ret = merge_working_tree(opts, &old, new);
        if (ret)
                return ret;

        /*
         * If we were on a detached HEAD, but have now moved to
         * a new commit, we want to mention the old commit once more
         * to remind the user that it might be lost.
         */
        if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
                describe_detached_head("Previous HEAD position was", old.commit);

        update_refs_for_switch(opts, &old, new);

        ret = post_checkout_hook(old.commit, new->commit, 1);
        return ret || opts->writeout_error;
}

static int git_checkout_config(const char *var, const char *value, void *cb)
{
        return git_xmerge_config(var, value, cb);
}

static int interactive_checkout(const char *revision, const char **pathspec,
                                struct checkout_opts *opts)
{
        return run_add_interactive(revision, "--patch=checkout", pathspec);
}

struct tracking_name_data {
        const char *name;
        char *remote;
        int unique;
};

static int check_tracking_name(const char *refname, const unsigned char *sha1,
                               int flags, void *cb_data)
{
        struct tracking_name_data *cb = cb_data;
        const char *slash;

        if (prefixcmp(refname, "refs/remotes/"))
                return 0;
        slash = strchr(refname + 13, '/');
        if (!slash || strcmp(slash + 1, cb->name))
                return 0;
        if (cb->remote) {
                cb->unique = 0;
                return 0;
        }
        cb->remote = xstrdup(refname);
        return 0;
}

static const char *unique_tracking_name(const char *name)
{
        struct tracking_name_data cb_data = { name, NULL, 1 };
        for_each_ref(check_tracking_name, &cb_data);
        if (cb_data.unique)
                return cb_data.remote;
        free(cb_data.remote);
        return NULL;
}

int cmd_checkout(int argc, const char **argv, const char *prefix)
{
        struct checkout_opts opts;
        unsigned char rev[20];
        const char *arg;
        struct branch_info new;
        struct tree *source_tree = NULL;
        char *conflict_style = NULL;
        int patch_mode = 0;
        int dwim_new_local_branch = 1;
        struct option options[] = {
                OPT__QUIET(&opts.quiet),
                OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
                OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
                OPT_SET_INT('t', "track",  &opts.track, "track",
                        BRANCH_TRACK_EXPLICIT),
                OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
                            2),
                OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
                            3),
                OPT_BOOLEAN('f', "force", &opts.force, "force"),
                OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
                OPT_STRING(0, "conflict", &conflict_style, "style",
                           "conflict style (merge or diff3)"),
                OPT_BOOLEAN('p', "patch", &patch_mode, "select hunks interactively"),
                { OPTION_BOOLEAN, 0, "guess", &dwim_new_local_branch, NULL,
                  "second guess 'git checkout no-such-branch'",
                  PARSE_OPT_NOARG | PARSE_OPT_HIDDEN },
                OPT_END(),
        };
        int has_dash_dash;

        memset(&opts, 0, sizeof(opts));
        memset(&new, 0, sizeof(new));

        git_config(git_checkout_config, NULL);

        opts.track = BRANCH_TRACK_UNSPECIFIED;

        argc = parse_options(argc, argv, prefix, options, checkout_usage,
                             PARSE_OPT_KEEP_DASHDASH);

        if (patch_mode && (opts.track > 0 || opts.new_branch
                           || opts.new_branch_log || opts.merge || opts.force))
                die ("--patch is incompatible with all other options");

        /* --track without -b should DWIM */
        if (0 < opts.track && !opts.new_branch) {
                const char *argv0 = argv[0];
                if (!argc || !strcmp(argv0, "--"))
                        die ("--track needs a branch name");
                if (!prefixcmp(argv0, "refs/"))
                        argv0 += 5;
                if (!prefixcmp(argv0, "remotes/"))
                        argv0 += 8;
                argv0 = strchr(argv0, '/');
                if (!argv0 || !argv0[1])
                        die ("Missing branch name; try -b");
                opts.new_branch = argv0 + 1;
        }

        if (conflict_style) {
                opts.merge = 1; /* implied */
                git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
        }

        if (opts.force && opts.merge)
                die("git checkout: -f and -m are incompatible");

        /*
         * case 1: git checkout <ref> -- [<paths>]
         *
         *   <ref> must be a valid tree, everything after the '--' must be
         *   a path.
         *
         * case 2: git checkout -- [<paths>]
         *
         *   everything after the '--' must be paths.
         *
         * case 3: git checkout <something> [<paths>]
         *
         *   With no paths, if <something> is a commit, that is to
         *   switch to the branch or detach HEAD at it.
         *
         *   With no paths, if <something> is _not_ a commit, no -t nor -b
         *   was given, and there is a tracking branch whose name is
         *   <something> in one and only one remote, then this is a short-hand
         *   to fork local <something> from that remote tracking branch.
         *
         *   Otherwise <something> shall not be ambiguous.
         *   - If it's *only* a reference, treat it like case (1).
         *   - If it's only a path, treat it like case (2).
         *   - else: fail.
         *
         */
        if (argc) {
                if (!strcmp(argv[0], "--")) {       /* case (2) */
                        argv++;
                        argc--;
                        goto no_reference;
                }

                arg = argv[0];
                has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");

                if (!strcmp(arg, "-"))
                        arg = "@{-1}";

                if (get_sha1(arg, rev)) {
                        if (has_dash_dash)          /* case (1) */
                                die("invalid reference: %s", arg);
                        if (!patch_mode &&
                            dwim_new_local_branch &&
                            opts.track == BRANCH_TRACK_UNSPECIFIED &&
                            !opts.new_branch &&
                            !check_filename(NULL, arg) &&
                            argc == 1) {
                                const char *remote = unique_tracking_name(arg);
                                if (!remote || get_sha1(remote, rev))
                                        goto no_reference;
                                opts.new_branch = arg;
                                arg = remote;
                                /* DWIMmed to create local branch */
                        }
                        else
                                goto no_reference;
                }

                /* we can't end up being in (2) anymore, eat the argument */
                argv++;
                argc--;

                new.name = arg;
                if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
                        setup_branch_path(&new);
                        if (resolve_ref(new.path, rev, 1, NULL))
                                new.commit = lookup_commit_reference(rev);
                        else
                                new.path = NULL;
                        parse_commit(new.commit);
                        source_tree = new.commit->tree;
                } else
                        source_tree = parse_tree_indirect(rev);

                if (!source_tree)                   /* case (1): want a tree */
                        die("reference is not a tree: %s", arg);
                if (!has_dash_dash) {/* case (3 -> 1) */
                        /*
                         * Do not complain the most common case
                         *      git checkout branch
                         * even if there happen to be a file called 'branch';
                         * it would be extremely annoying.
                         */
                        if (argc)
                                verify_non_filename(NULL, arg);
                }
                else {
                        argv++;
                        argc--;
                }
        }

no_reference:

        if (opts.track == BRANCH_TRACK_UNSPECIFIED)
                opts.track = git_branch_track;

        if (argc) {
                const char **pathspec = get_pathspec(prefix, argv);

                if (!pathspec)
                        die("invalid path specification");

                if (patch_mode)
                        return interactive_checkout(new.name, pathspec, &opts);

                /* Checkout paths */
                if (opts.new_branch) {
                        if (argc == 1) {
                                die("git checkout: updating paths is incompatible with switching branches.\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
                        } else {
                                die("git checkout: updating paths is incompatible with switching branches.");
                        }
                }

                if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
                        die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");

                return checkout_paths(source_tree, pathspec, &opts);
        }

        if (patch_mode)
                return interactive_checkout(new.name, NULL, &opts);

        if (opts.new_branch) {
                struct strbuf buf = STRBUF_INIT;
                if (strbuf_check_branch_ref(&buf, opts.new_branch))
                        die("git checkout: we do not like '%s' as a branch name.",
                            opts.new_branch);
                if (!get_sha1(buf.buf, rev))
                        die("git checkout: branch %s already exists", opts.new_branch);
                strbuf_release(&buf);
        }

        if (new.name && !new.commit) {
                die("Cannot switch branch to a non-commit.");
        }
        if (opts.writeout_stage)
                die("--ours/--theirs is incompatible with switching branches.");

        return switch_branches(&opts, &new);
}