#include "builtin.h"
#include "lockfile.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"
#include "submodule.h"

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

struct checkout_opts {
        int patch_mode;
        int quiet;
        int merge;
        int force;
        int force_detach;
        int writeout_stage;
        int overwrite_ignore;
        int ignore_skipworktree;
        int ignore_other_worktrees;

        const char *new_branch;
        const char *new_branch_force;
        const char *new_orphan_branch;
        int new_branch_log;
        enum branch_track track;
        struct diff_options diff_options;

        int branch_exists;
        const char *prefix;
        struct pathspec pathspec;
        struct tree *source_tree;

        int new_worktree_mode;
};

static int post_checkout_hook(struct commit *old, struct commit *new,
                              int changed)
{
        return run_hook_le(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, struct strbuf *base,
                const char *pathname, unsigned mode, int stage, void *context)
{
        int len;
        struct cache_entry *ce;
        int pos;

        if (S_ISDIR(mode))
                return READ_TREE_RECURSIVE;

        len = base->len + strlen(pathname);
        ce = xcalloc(1, cache_entry_size(len));
        hashcpy(ce->sha1, sha1);
        memcpy(ce->name, base->buf, base->len);
        memcpy(ce->name + base->len, pathname, len - base->len);
        ce->ce_flags = create_ce_flags(0) | CE_UPDATE;
        ce->ce_namelen = len;
        ce->ce_mode = create_ce_mode(mode);

        /*
         * If the entry is the same as the current index, we can leave the old
         * entry in place. Whether it is UPTODATE or not, checkout_entry will
         * do the right thing.
         */
        pos = cache_name_pos(ce->name, ce->ce_namelen);
        if (pos >= 0) {
                struct cache_entry *old = active_cache[pos];
                if (ce->ce_mode == old->ce_mode &&
                    !hashcmp(ce->sha1, old->sha1)) {
                        old->ce_flags |= CE_UPDATE;
                        free(ce);
                        return 0;
                }
        }

        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 struct pathspec *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(const 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, const 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++;
        }
        if (stage == 2)
                return error(_("path '%s' does not have our version"), ce->name);
        else
                return error(_("path '%s' does not have their version"), ce->name);
}

static int check_stages(unsigned stages, const struct cache_entry *ce, int pos)
{
        unsigned seen = 0;
        const char *name = ce->name;

        while (pos < active_nr) {
                ce = active_cache[pos];
                if (strcmp(name, ce->name))
                        break;
                seen |= (1 << ce_stage(ce));
                pos++;
        }
        if ((stages & seen) != stages)
                return error(_("path '%s' does not have all necessary versions"),
                             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++;
        }
        if (stage == 2)
                return error(_("path '%s' does not have our version"), ce->name);
        else
                return error(_("path '%s' does not have their version"), ce->name);
}

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;
        unsigned char threeway[3][20];
        unsigned mode = 0;

        memset(threeway, 0, sizeof(threeway));
        while (pos < active_nr) {
                int stage;
                stage = ce_stage(ce);
                if (!stage || strcmp(path, ce->name))
                        break;
                hashcpy(threeway[stage - 1], ce->sha1);
                if (stage == 2)
                        mode = create_ce_mode(ce->ce_mode);
                pos++;
                ce = active_cache[pos];
        }
        if (is_null_sha1(threeway[1]) || is_null_sha1(threeway[2]))
                return error(_("path '%s' does not have necessary versions"), path);

        read_mmblob(&ancestor, threeway[0]);
        read_mmblob(&ours, threeway[1]);
        read_mmblob(&theirs, threeway[2]);

        /*
         * NEEDSWORK: re-create conflicts from merges with
         * merge.renormalize set, too
         */
        status = ll_merge(&result_buf, path, &ancestor, "base",
                          &ours, "ours", &theirs, "theirs", NULL);
        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(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(const struct checkout_opts *opts,
                          const char *revision)
{
        int pos;
        struct checkout state;
        static char *ps_matched;
        unsigned char rev[20];
        int flag;
        struct commit *head;
        int errs = 0;
        struct lock_file *lock_file;

        if (opts->track != BRANCH_TRACK_UNSPECIFIED)
                die(_("'%s' cannot be used with updating paths"), "--track");

        if (opts->new_branch_log)
                die(_("'%s' cannot be used with updating paths"), "-l");

        if (opts->force && opts->patch_mode)
                die(_("'%s' cannot be used with updating paths"), "-f");

        if (opts->force_detach)
                die(_("'%s' cannot be used with updating paths"), "--detach");

        if (opts->merge && opts->patch_mode)
                die(_("'%s' cannot be used with %s"), "--merge", "--patch");

        if (opts->force && opts->merge)
                die(_("'%s' cannot be used with %s"), "-f", "-m");

        if (opts->new_branch)
                die(_("Cannot update paths and switch to branch '%s' at the same time."),
                    opts->new_branch);

        if (opts->patch_mode)
                return run_add_interactive(revision, "--patch=checkout",
                                           &opts->pathspec);

        lock_file = xcalloc(1, sizeof(struct lock_file));

        hold_locked_index(lock_file, 1);
        if (read_cache_preload(&opts->pathspec) < 0)
                return error(_("corrupt index file"));

        if (opts->source_tree)
                read_tree_some(opts->source_tree, &opts->pathspec);

        ps_matched = xcalloc(1, opts->pathspec.nr);

        /*
         * Make sure all pathspecs participated in locating the paths
         * to be checked out.
         */
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
                ce->ce_flags &= ~CE_MATCHED;
                if (!opts->ignore_skipworktree && ce_skip_worktree(ce))
                        continue;
                if (opts->source_tree && !(ce->ce_flags & CE_UPDATE))
                        /*
                         * "git checkout tree-ish -- path", but this entry
                         * is in the original index; it will not be checked
                         * out to the working tree and it does not matter
                         * if pathspec matched this entry.  We will not do
                         * anything to this entry at all.
                         */
                        continue;
                /*
                 * Either this entry came from the tree-ish we are
                 * checking the paths out of, or we are checking out
                 * of the index.
                 *
                 * If it comes from the tree-ish, we already know it
                 * matches the pathspec and could just stamp
                 * CE_MATCHED to it from update_some(). But we still
                 * need ps_matched and read_tree_recursive (and
                 * eventually tree_entry_interesting) cannot fill
                 * ps_matched yet. Once it can, we can avoid calling
                 * match_pathspec() for _all_ entries when
                 * opts->source_tree != NULL.
                 */
                if (ce_path_match(ce, &opts->pathspec, ps_matched))
                        ce->ce_flags |= CE_MATCHED;
        }

        if (report_path_error(ps_matched, &opts->pathspec, opts->prefix)) {
                free(ps_matched);
                return 1;
        }
        free(ps_matched);

        /* "checkout -m path" to recreate conflicted state */
        if (opts->merge)
                unmerge_marked_index(&the_index);

        /* Any unmerged paths? */
        for (pos = 0; pos < active_nr; pos++) {
                const struct cache_entry *ce = active_cache[pos];
                if (ce->ce_flags & CE_MATCHED) {
                        if (!ce_stage(ce))
                                continue;
                        if (opts->force) {
                                warning(_("path '%s' is unmerged"), ce->name);
                        } else if (opts->writeout_stage) {
                                errs |= check_stage(opts->writeout_stage, ce, pos);
                        } else if (opts->merge) {
                                errs |= check_stages((1<<2) | (1<<3), 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;
        state.istate = &the_index;
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
                if (ce->ce_flags & CE_MATCHED) {
                        if (!ce_stage(ce)) {
                                errs |= checkout_entry(ce, &state, NULL);
                                continue;
                        }
                        if (opts->writeout_stage)
                                errs |= checkout_stage(opts->writeout_stage, ce, pos, &state);
                        else if (opts->merge)
                                errs |= checkout_merged(pos, &state);
                        pos = skip_same_name(ce, pos) - 1;
                }
        }

        if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
                die(_("unable to write new index file"));

        read_ref_full("HEAD", 0, rev, &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,
                               const struct diff_options *opts)
{
        struct rev_info rev;
        /* I think we want full paths, even if we're in a subdirectory. */
        init_revisions(&rev, NULL);
        rev.diffopt.flags = opts->flags;
        rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
        diff_setup_done(&rev.diffopt);
        add_pending_object(&rev, head, NULL);
        run_diff_index(&rev, 0);
}

static void describe_detached_head(const char *msg, struct commit *commit)
{
        struct strbuf sb = STRBUF_INIT;
        if (!parse_commit(commit))
                pp_commit_easy(CMIT_FMT_ONELINE, commit, &sb);
        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, const struct checkout_opts *o,
                      int worktree, int *writeout_error)
{
        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 && isatty(2);
        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:
                *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 */
        /*
         * if not null the branch is detached because it's already
         * checked out in this checkout
         */
        char *checkout;
};

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(const struct checkout_opts *opts,
                              struct branch_info *old,
                              struct branch_info *new,
                              int *writeout_error)
{
        int ret;
        struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));

        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, writeout_error);
                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;

                setup_unpack_trees_porcelain(&topts, "checkout");

                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 && old->commit;
                topts.verbose_update = !opts->quiet && isatty(2);
                topts.fn = twoway_merge;
                if (opts->overwrite_ignore) {
                        topts.dir = xcalloc(1, sizeof(*topts.dir));
                        topts.dir->flags |= DIR_SHOW_IGNORED;
                        setup_standard_excludes(topts.dir);
                }
                tree = parse_tree_indirect(old->commit && !opts->new_worktree_mode ?
                                           old->commit->object.sha1 :
                                           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;

                        /*
                         * Without old->commit, the below is the same as
                         * the two-tree unpack we already tried and failed.
                         */
                        if (!old->commit)
                                return 1;

                        /* 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);
                        /*
                         * NEEDSWORK: carrying over local changes
                         * when branches have different end-of-line
                         * normalization (or clean+smudge rules) is
                         * a pain; plumb in an option to set
                         * o.renormalize?
                         */
                        init_merge_options(&o);
                        o.verbosity = 0;
                        work = write_tree_from_memory(&o);

                        ret = reset_tree(new->commit->tree, opts, 1,
                                         writeout_error);
                        if (ret)
                                return ret;
                        o.ancestor = old->name;
                        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,
                                         writeout_error);
                        if (ret)
                                return ret;
                }
        }

        if (!active_cache_tree)
                active_cache_tree = cache_tree();

        if (!cache_tree_fully_valid(active_cache_tree))
                cache_tree_update(&the_index, WRITE_TREE_SILENT | WRITE_TREE_REPAIR);

        if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
                die(_("unable to write new index file"));

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

        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(const struct checkout_opts *opts,
                                   struct branch_info *old,
                                   struct branch_info *new)
{
        struct strbuf msg = STRBUF_INIT;
        const char *old_desc, *reflog_msg;
        if (opts->new_branch) {
                if (opts->new_orphan_branch) {
                        if (opts->new_branch_log && !log_all_ref_updates) {
                                int temp;
                                struct strbuf log_file = STRBUF_INIT;
                                int ret;
                                const char *ref_name;

                                ref_name = mkpath("refs/heads/%s", opts->new_orphan_branch);
                                temp = log_all_ref_updates;
                                log_all_ref_updates = 1;
                                ret = log_ref_setup(ref_name, &log_file);
                                log_all_ref_updates = temp;
                                strbuf_release(&log_file);
                                if (ret) {
                                        fprintf(stderr, _("Can not do reflog for '%s'\n"),
                                            opts->new_orphan_branch);
                                        return;
                                }
                        }
                }
                else
                        create_branch(old->name, opts->new_branch, new->name,
                                      opts->new_branch_force ? 1 : 0,
                                      opts->new_branch_log,
                                      opts->new_branch_force ? 1 : 0,
                                      opts->quiet,
                                      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);

        reflog_msg = getenv("GIT_REFLOG_ACTION");
        if (!reflog_msg)
                strbuf_addf(&msg, "checkout: moving from %s to %s",
                        old_desc ? old_desc : "(invalid)", new->name);
        else
                strbuf_insert(&msg, 0, reflog_msg, strlen(reflog_msg));

        if (!strcmp(new->name, "HEAD") && !new->path && !opts->force_detach) {
                /* Nothing to do. */
        } else if (opts->force_detach || !new->path) {  /* No longer on any branch. */
                update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
                           REF_NODEREF, UPDATE_REFS_DIE_ON_ERR);
                if (!opts->quiet) {
                        if (old->path && advice_detached_head)
                                detach_advice(new->name);
                        describe_detached_head(_("HEAD is now at"), new->commit);
                }
        } else if (new->path) { /* Switch branches. */
                create_symref("HEAD", new->path, msg.buf);
                if (!opts->quiet) {
                        if (old->path && !strcmp(new->path, old->path)) {
                                if (opts->new_branch_force)
                                        fprintf(stderr, _("Reset branch '%s'\n"),
                                                new->name);
                                else
                                        fprintf(stderr, _("Already on '%s'\n"),
                                                new->name);
                        } else if (opts->new_branch) {
                                if (opts->branch_exists)
                                        fprintf(stderr, _("Switched to and reset branch '%s'\n"), new->name);
                                else
                                        fprintf(stderr, _("Switched to a new branch '%s'\n"), new->name);
                        } else {
                                fprintf(stderr, _("Switched to branch '%s'\n"),
                                        new->name);
                        }
                }
                if (old->path && old->name) {
                        if (!ref_exists(old->path) && reflog_exists(old->path))
                                delete_reflog(old->path);
                }
        }
        remove_branch_state();
        strbuf_release(&msg);
        if (!opts->quiet &&
            (new->path || (!opts->force_detach && !strcmp(new->name, "HEAD"))))
                report_tracking(new);
}

static int add_pending_uninteresting_ref(const char *refname,
                                         const struct object_id *oid,
                                         int flags, void *cb_data)
{
        add_pending_sha1(cb_data, refname, oid->hash, UNINTERESTING);
        return 0;
}

static void describe_one_orphan(struct strbuf *sb, struct commit *commit)
{
        strbuf_addstr(sb, "  ");
        strbuf_addstr(sb,
                find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
        strbuf_addch(sb, ' ');
        if (!parse_commit(commit))
                pp_commit_easy(CMIT_FMT_ONELINE, commit, sb);
        strbuf_addch(sb, '\n');
}

#define ORPHAN_CUTOFF 4
static void suggest_reattach(struct commit *commit, struct rev_info *revs)
{
        struct commit *c, *last = NULL;
        struct strbuf sb = STRBUF_INIT;
        int lost = 0;
        while ((c = get_revision(revs)) != NULL) {
                if (lost < ORPHAN_CUTOFF)
                        describe_one_orphan(&sb, c);
                last = c;
                lost++;
        }
        if (ORPHAN_CUTOFF < lost) {
                int more = lost - ORPHAN_CUTOFF;
                if (more == 1)
                        describe_one_orphan(&sb, last);
                else
                        strbuf_addf(&sb, _(" ... and %d more.\n"), more);
        }

        fprintf(stderr,
                Q_(
                /* The singular version */
                "Warning: you are leaving %d commit behind, "
                "not connected to\n"
                "any of your branches:\n\n"
                "%s\n",
                /* The plural version */
                "Warning: you are leaving %d commits behind, "
                "not connected to\n"
                "any of your branches:\n\n"
                "%s\n",
                /* Give ngettext() the count */
                lost),
                lost,
                sb.buf);
        strbuf_release(&sb);

        if (advice_detached_head)
                fprintf(stderr,
                        Q_(
                        /* The singular version */
                        "If you want to keep it by creating a new branch, "
                        "this may be a good time\nto do so with:\n\n"
                        " git branch <new-branch-name> %s\n\n",
                        /* The plural version */
                        "If you want to keep them by creating a new branch, "
                        "this may be a good time\nto do so with:\n\n"
                        " git branch <new-branch-name> %s\n\n",
                        /* Give ngettext() the count */
                        lost),
                        find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
}

/*
 * We are about to leave commit that was at the tip of a detached
 * HEAD.  If it is not reachable from any ref, this is the last chance
 * for the user to do so without resorting to reflog.
 */
static void orphaned_commit_warning(struct commit *old, struct commit *new)
{
        struct rev_info revs;
        struct object *object = &old->object;
        struct object_array refs;

        init_revisions(&revs, NULL);
        setup_revisions(0, NULL, &revs, NULL);

        object->flags &= ~UNINTERESTING;
        add_pending_object(&revs, object, sha1_to_hex(object->sha1));

        for_each_ref(add_pending_uninteresting_ref, &revs);
        add_pending_sha1(&revs, "HEAD", new->object.sha1, UNINTERESTING);

        refs = revs.pending;
        revs.leak_pending = 1;

        if (prepare_revision_walk(&revs))
                die(_("internal error in revision walk"));
        if (!(old->object.flags & UNINTERESTING))
                suggest_reattach(old, &revs);
        else
                describe_detached_head(_("Previous HEAD position was"), old);

        clear_commit_marks_for_object_array(&refs, ALL_REV_FLAGS);
        free(refs.objects);
}

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

        if (old.path)
                skip_prefix(old.path, "refs/heads/", &old.name);

        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_or_die(new->commit);
        }

        ret = merge_working_tree(opts, &old, new, &writeout_error);
        if (ret) {
                free(path_to_free);
                return ret;
        }

        if (!opts->quiet && !old.path && old.commit &&
            new->commit != old.commit && !opts->new_worktree_mode)
                orphaned_commit_warning(old.commit, new->commit);

        update_refs_for_switch(opts, &old, new);

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

static int git_checkout_config(const char *var, const char *value, void *cb)
{
        if (!strcmp(var, "diff.ignoresubmodules")) {
                struct checkout_opts *opts = cb;
                handle_ignore_submodules_arg(&opts->diff_options, value);
                return 0;
        }

        if (starts_with(var, "submodule."))
                return parse_submodule_config_option(var, value);

        return git_xmerge_config(var, value, NULL);
}

struct tracking_name_data {
        /* const */ char *src_ref;
        char *dst_ref;
        unsigned char *dst_sha1;
        int unique;
};

static int check_tracking_name(struct remote *remote, void *cb_data)
{
        struct tracking_name_data *cb = cb_data;
        struct refspec query;
        memset(&query, 0, sizeof(struct refspec));
        query.src = cb->src_ref;
        if (remote_find_tracking(remote, &query) ||
            get_sha1(query.dst, cb->dst_sha1)) {
                free(query.dst);
                return 0;
        }
        if (cb->dst_ref) {
                free(query.dst);
                cb->unique = 0;
                return 0;
        }
        cb->dst_ref = query.dst;
        return 0;
}

static const char *unique_tracking_name(const char *name, unsigned char *sha1)
{
        struct tracking_name_data cb_data = { NULL, NULL, NULL, 1 };
        char src_ref[PATH_MAX];
        snprintf(src_ref, PATH_MAX, "refs/heads/%s", name);
        cb_data.src_ref = src_ref;
        cb_data.dst_sha1 = sha1;
        for_each_remote(check_tracking_name, &cb_data);
        if (cb_data.unique)
                return cb_data.dst_ref;
        free(cb_data.dst_ref);
        return NULL;
}

static void check_linked_checkout(struct branch_info *new, const char *id)
{
        struct strbuf sb = STRBUF_INIT;
        struct strbuf path = STRBUF_INIT;
        struct strbuf gitdir = STRBUF_INIT;
        const char *start, *end;

        if (id)
                strbuf_addf(&path, "%s/worktrees/%s/HEAD", get_git_common_dir(), id);
        else
                strbuf_addf(&path, "%s/HEAD", get_git_common_dir());

        if (strbuf_read_file(&sb, path.buf, 0) < 0 ||
            !skip_prefix(sb.buf, "ref:", &start))
                goto done;
        while (isspace(*start))
                start++;
        end = start;
        while (*end && !isspace(*end))
                end++;
        if (strncmp(start, new->path, end - start) || new->path[end - start] != '\0')
                goto done;
        if (id) {
                strbuf_reset(&path);
                strbuf_addf(&path, "%s/worktrees/%s/gitdir", get_git_common_dir(), id);
                if (strbuf_read_file(&gitdir, path.buf, 0) <= 0)
                        goto done;
                strbuf_rtrim(&gitdir);
        } else
                strbuf_addstr(&gitdir, get_git_common_dir());
        die(_("'%s' is already checked out at '%s'"), new->name, gitdir.buf);
done:
        strbuf_release(&path);
        strbuf_release(&sb);
        strbuf_release(&gitdir);
}

static void check_linked_checkouts(struct branch_info *new)
{
        struct strbuf path = STRBUF_INIT;
        DIR *dir;
        struct dirent *d;

        strbuf_addf(&path, "%s/worktrees", get_git_common_dir());
        if ((dir = opendir(path.buf)) == NULL) {
                strbuf_release(&path);
                return;
        }

        /*
         * $GIT_COMMON_DIR/HEAD is practically outside
         * $GIT_DIR so resolve_ref_unsafe() won't work (it
         * uses git_path). Parse the ref ourselves.
         */
        check_linked_checkout(new, NULL);

        while ((d = readdir(dir)) != NULL) {
                if (!strcmp(d->d_name, ".") || !strcmp(d->d_name, ".."))
                        continue;
                check_linked_checkout(new, d->d_name);
        }
        strbuf_release(&path);
        closedir(dir);
}

static int parse_branchname_arg(int argc, const char **argv,
                                int dwim_new_local_branch_ok,
                                struct branch_info *new,
                                struct checkout_opts *opts,
                                unsigned char rev[20])
{
        struct tree **source_tree = &opts->source_tree;
        const char **new_branch = &opts->new_branch;
        int argcount = 0;
        unsigned char branch_rev[20];
        const char *arg;
        int dash_dash_pos;
        int has_dash_dash = 0;
        int i;

        /*
         * 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> [--]
         *
         *   (a) If <something> is a commit, that is to
         *       switch to the branch or detach HEAD at it.  As a special case,
         *       if <something> is A...B (missing A or B means HEAD but you can
         *       omit at most one side), and if there is a unique merge base
         *       between A and B, A...B names that merge base.
         *
         *   (b) If <something> is _not_ a commit, either "--" is present
         *       or <something> is not a path, no -t or -b was given, and
         *       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.
         *
         *   (c) Otherwise, if "--" is present, treat it like case (1).
         *
         *   (d) Otherwise :
         *       - if it's a reference, treat it like case (1)
         *       - else if it's a path, treat it like case (2)
         *       - else: fail.
         *
         * case 4: git checkout <something> <paths>
         *
         *   The first argument must 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)
                return 0;

        arg = argv[0];
        dash_dash_pos = -1;
        for (i = 0; i < argc; i++) {
                if (!strcmp(argv[i], "--")) {
                        dash_dash_pos = i;
                        break;
                }
        }
        if (dash_dash_pos == 0)
                return 1; /* case (2) */
        else if (dash_dash_pos == 1)
                has_dash_dash = 1; /* case (3) or (1) */
        else if (dash_dash_pos >= 2)
                die(_("only one reference expected, %d given."), dash_dash_pos);

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

        if (get_sha1_mb(arg, rev)) {
                /*
                 * Either case (3) or (4), with <something> not being
                 * a commit, or an attempt to use case (1) with an
                 * invalid ref.
                 *
                 * It's likely an error, but we need to find out if
                 * we should auto-create the branch, case (3).(b).
                 */
                int recover_with_dwim = dwim_new_local_branch_ok;

                if (check_filename(NULL, arg) && !has_dash_dash)
                        recover_with_dwim = 0;
                /*
                 * Accept "git checkout foo" and "git checkout foo --"
                 * as candidates for dwim.
                 */
                if (!(argc == 1 && !has_dash_dash) &&
                    !(argc == 2 && has_dash_dash))
                        recover_with_dwim = 0;

                if (recover_with_dwim) {
                        const char *remote = unique_tracking_name(arg, rev);
                        if (remote) {
                                *new_branch = arg;
                                arg = remote;
                                /* DWIMmed to create local branch, case (3).(b) */
                        } else {
                                recover_with_dwim = 0;
                        }
                }

                if (!recover_with_dwim) {
                        if (has_dash_dash)
                                die(_("invalid reference: %s"), arg);
                        return argcount;
                }
        }

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

        new->name = arg;
        setup_branch_path(new);

        if (!check_refname_format(new->path, 0) &&
            !read_ref(new->path, branch_rev))
                hashcpy(rev, branch_rev);
        else
                new->path = NULL; /* not an existing branch */

        new->commit = lookup_commit_reference_gently(rev, 1);
        if (!new->commit) {
                /* not a commit */
                *source_tree = parse_tree_indirect(rev);
        } else {
                parse_commit_or_die(new->commit);
                *source_tree = new->commit->tree;
        }

        if (!*source_tree)                   /* case (1): want a tree */
                die(_("reference is not a tree: %s"), arg);
        if (!has_dash_dash) {/* case (3).(d) -> (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 {
                argcount++;
                argv++;
                argc--;
        }

        return argcount;
}

static int switch_unborn_to_new_branch(const struct checkout_opts *opts)
{
        int status;
        struct strbuf branch_ref = STRBUF_INIT;

        if (!opts->new_branch)
                die(_("You are on a branch yet to be born"));
        strbuf_addf(&branch_ref, "refs/heads/%s", opts->new_branch);
        status = create_symref("HEAD", branch_ref.buf, "checkout -b");
        strbuf_release(&branch_ref);
        if (!opts->quiet)
                fprintf(stderr, _("Switched to a new branch '%s'\n"),
                        opts->new_branch);
        return status;
}

static int checkout_branch(struct checkout_opts *opts,
                           struct branch_info *new)
{
        if (opts->pathspec.nr)
                die(_("paths cannot be used with switching branches"));

        if (opts->patch_mode)
                die(_("'%s' cannot be used with switching branches"),
                    "--patch");

        if (opts->writeout_stage)
                die(_("'%s' cannot be used with switching branches"),
                    "--ours/--theirs");

        if (opts->force && opts->merge)
                die(_("'%s' cannot be used with '%s'"), "-f", "-m");

        if (opts->force_detach && opts->new_branch)
                die(_("'%s' cannot be used with '%s'"),
                    "--detach", "-b/-B/--orphan");

        if (opts->new_orphan_branch) {
                if (opts->track != BRANCH_TRACK_UNSPECIFIED)
                        die(_("'%s' cannot be used with '%s'"), "--orphan", "-t");
        } else if (opts->force_detach) {
                if (opts->track != BRANCH_TRACK_UNSPECIFIED)
                        die(_("'%s' cannot be used with '%s'"), "--detach", "-t");
        } else if (opts->track == BRANCH_TRACK_UNSPECIFIED)
                opts->track = git_branch_track;

        if (new->name && !new->commit)
                die(_("Cannot switch branch to a non-commit '%s'"),
                    new->name);

        if (new->path && !opts->force_detach && !opts->new_branch) {
                unsigned char sha1[20];
                int flag;
                char *head_ref = resolve_refdup("HEAD", 0, sha1, &flag);
                if (head_ref &&
                    (!(flag & REF_ISSYMREF) || strcmp(head_ref, new->path)) &&
                    !opts->ignore_other_worktrees)
                        check_linked_checkouts(new);
                free(head_ref);
        }

        if (!new->commit && opts->new_branch) {
                unsigned char rev[20];
                int flag;

                if (!read_ref_full("HEAD", 0, rev, &flag) &&
                    (flag & REF_ISSYMREF) && is_null_sha1(rev))
                        return switch_unborn_to_new_branch(opts);
        }
        return switch_branches(opts, new);
}

int cmd_checkout(int argc, const char **argv, const char *prefix)
{
        struct checkout_opts opts;
        struct branch_info new;
        char *conflict_style = NULL;
        int dwim_new_local_branch = 1;
        struct option options[] = {
                OPT__QUIET(&opts.quiet, N_("suppress progress reporting")),
                OPT_STRING('b', NULL, &opts.new_branch, N_("branch"),
                           N_("create and checkout a new branch")),
                OPT_STRING('B', NULL, &opts.new_branch_force, N_("branch"),
                           N_("create/reset and checkout a branch")),
                OPT_BOOL('l', NULL, &opts.new_branch_log, N_("create reflog for new branch")),
                OPT_BOOL(0, "detach", &opts.force_detach, N_("detach the HEAD at named commit")),
                OPT_SET_INT('t', "track",  &opts.track, N_("set upstream info for new branch"),
                        BRANCH_TRACK_EXPLICIT),
                OPT_STRING(0, "orphan", &opts.new_orphan_branch, N_("new-branch"), N_("new unparented branch")),
                OPT_SET_INT('2', "ours", &opts.writeout_stage, N_("checkout our version for unmerged files"),
                            2),
                OPT_SET_INT('3', "theirs", &opts.writeout_stage, N_("checkout their version for unmerged files"),
                            3),
                OPT__FORCE(&opts.force, N_("force checkout (throw away local modifications)")),
                OPT_BOOL('m', "merge", &opts.merge, N_("perform a 3-way merge with the new branch")),
                OPT_BOOL(0, "overwrite-ignore", &opts.overwrite_ignore, N_("update ignored files (default)")),
                OPT_STRING(0, "conflict", &conflict_style, N_("style"),
                           N_("conflict style (merge or diff3)")),
                OPT_BOOL('p', "patch", &opts.patch_mode, N_("select hunks interactively")),
                OPT_BOOL(0, "ignore-skip-worktree-bits", &opts.ignore_skipworktree,
                         N_("do not limit pathspecs to sparse entries only")),
                OPT_HIDDEN_BOOL(0, "guess", &dwim_new_local_branch,
                                N_("second guess 'git checkout <no-such-branch>'")),
                OPT_BOOL(0, "ignore-other-worktrees", &opts.ignore_other_worktrees,
                         N_("do not check if another worktree is holding the given ref")),
                OPT_END(),
        };

        memset(&opts, 0, sizeof(opts));
        memset(&new, 0, sizeof(new));
        opts.overwrite_ignore = 1;
        opts.prefix = prefix;

        gitmodules_config();
        git_config(git_checkout_config, &opts);

        opts.track = BRANCH_TRACK_UNSPECIFIED;

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

        opts.new_worktree_mode = getenv("GIT_CHECKOUT_NEW_WORKTREE") != NULL;

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

        if ((!!opts.new_branch + !!opts.new_branch_force + !!opts.new_orphan_branch) > 1)
                die(_("-b, -B and --orphan are mutually exclusive"));

        /*
         * From here on, new_branch will contain the branch to be checked out,
         * and new_branch_force and new_orphan_branch will tell us which one of
         * -b/-B/--orphan is being used.
         */
        if (opts.new_branch_force)
                opts.new_branch = opts.new_branch_force;

        if (opts.new_orphan_branch)
                opts.new_branch = opts.new_orphan_branch;

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

        /*
         * Extract branch name from command line arguments, so
         * all that is left is pathspecs.
         *
         * Handle
         *
         *  1) git checkout <tree> -- [<paths>]
         *  2) git checkout -- [<paths>]
         *  3) git checkout <something> [<paths>]
         *
         * including "last branch" syntax and DWIM-ery for names of
         * remote branches, erroring out for invalid or ambiguous cases.
         */
        if (argc) {
                unsigned char rev[20];
                int dwim_ok =
                        !opts.patch_mode &&
                        dwim_new_local_branch &&
                        opts.track == BRANCH_TRACK_UNSPECIFIED &&
                        !opts.new_branch;
                int n = parse_branchname_arg(argc, argv, dwim_ok,
                                             &new, &opts, rev);
                argv += n;
                argc -= n;
        }

        if (argc) {
                parse_pathspec(&opts.pathspec, 0,
                               opts.patch_mode ? PATHSPEC_PREFIX_ORIGIN : 0,
                               prefix, argv);

                if (!opts.pathspec.nr)
                        die(_("invalid path specification"));

                /*
                 * Try to give more helpful suggestion.
                 * new_branch && argc > 1 will be caught later.
                 */
                if (opts.new_branch && argc == 1)
                        die(_("Cannot update paths and switch to branch '%s' at the same time.\n"
                              "Did you intend to checkout '%s' which can not be resolved as commit?"),
                            opts.new_branch, argv[0]);

                if (opts.force_detach)
                        die(_("git checkout: --detach does not take a path argument '%s'"),
                            argv[0]);

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

        if (opts.new_branch) {
                struct strbuf buf = STRBUF_INIT;

                opts.branch_exists =
                        validate_new_branchname(opts.new_branch, &buf,
                                                !!opts.new_branch_force,
                                                !!opts.new_branch_force);

                strbuf_release(&buf);
        }

        if (opts.patch_mode || opts.pathspec.nr)
                return checkout_paths(&opts, new.name);
        else
                return checkout_branch(&opts, &new);
}