Merge branch 'cn/commit-amend-doc'

[git] / unpack-trees.c

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

/*
 * Error messages expected by scripts out of plumbing commands such as
 * read-tree.  Non-scripted Porcelain is not required to use these messages
 * and in fact are encouraged to reword them to better suit their particular
 * situation better.  See how "git checkout" and "git merge" replaces
 * them using setup_unpack_trees_porcelain(), for example.
 */
static const char *unpack_plumbing_errors[NB_UNPACK_TREES_ERROR_TYPES] = {
        /* ERROR_WOULD_OVERWRITE */
        "Entry '%s' would be overwritten by merge. Cannot merge.",

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

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

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

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

        /* ERROR_BIND_OVERLAP */
        "Entry '%s' overlaps with '%s'.  Cannot bind.",

        /* ERROR_SPARSE_NOT_UPTODATE_FILE */
        "Entry '%s' not uptodate. Cannot update sparse checkout.",

        /* ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN */
        "Working tree file '%s' would be overwritten by sparse checkout update.",

        /* ERROR_WOULD_LOSE_ORPHANED_REMOVED */
        "Working tree file '%s' would be removed by sparse checkout update.",
};

#define ERRORMSG(o,type) \
        ( ((o) && (o)->msgs[(type)]) \
          ? ((o)->msgs[(type)])      \
          : (unpack_plumbing_errors[(type)]) )

void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
                                  const char *cmd)
{
        int i;
        const char **msgs = opts->msgs;
        const char *msg;
        char *tmp;
        const char *cmd2 = strcmp(cmd, "checkout") ? cmd : "switch branches";
        if (advice_commit_before_merge)
                msg = "Your local changes to the following files would be overwritten by %s:\n%%s"
                        "Please, commit your changes or stash them before you can %s.";
        else
                msg = "Your local changes to the following files would be overwritten by %s:\n%%s";
        tmp = xmalloc(strlen(msg) + strlen(cmd) + strlen(cmd2) - 2);
        sprintf(tmp, msg, cmd, cmd2);
        msgs[ERROR_WOULD_OVERWRITE] = tmp;
        msgs[ERROR_NOT_UPTODATE_FILE] = tmp;

        msgs[ERROR_NOT_UPTODATE_DIR] =
                "Updating the following directories would lose untracked files in it:\n%s";

        if (advice_commit_before_merge)
                msg = "The following untracked working tree files would be %s by %s:\n%%s"
                        "Please move or remove them before you can %s.";
        else
                msg = "The following untracked working tree files would be %s by %s:\n%%s";
        tmp = xmalloc(strlen(msg) + strlen(cmd) + strlen("removed") + strlen(cmd2) - 4);
        sprintf(tmp, msg, "removed", cmd, cmd2);
        msgs[ERROR_WOULD_LOSE_UNTRACKED_REMOVED] = tmp;
        tmp = xmalloc(strlen(msg) + strlen(cmd) + strlen("overwritten") + strlen(cmd2) - 4);
        sprintf(tmp, msg, "overwritten", cmd, cmd2);
        msgs[ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN] = tmp;

        /*
         * Special case: ERROR_BIND_OVERLAP refers to a pair of paths, we
         * cannot easily display it as a list.
         */
        msgs[ERROR_BIND_OVERLAP] = "Entry '%s' overlaps with '%s'.  Cannot bind.";

        msgs[ERROR_SPARSE_NOT_UPTODATE_FILE] =
                "Cannot update sparse checkout: the following entries are not up-to-date:\n%s";
        msgs[ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN] =
                "The following Working tree files would be overwritten by sparse checkout update:\n%s";
        msgs[ERROR_WOULD_LOSE_ORPHANED_REMOVED] =
                "The following Working tree files would be removed by sparse checkout update:\n%s";

        opts->show_all_errors = 1;
        /* rejected paths may not have a static buffer */
        for (i = 0; i < ARRAY_SIZE(opts->unpack_rejects); i++)
                opts->unpack_rejects[i].strdup_strings = 1;
}

static void do_add_entry(struct unpack_trees_options *o, struct cache_entry *ce,
                         unsigned int set, unsigned int clear)
{
        clear |= CE_HASHED | CE_UNHASHED;

        if (set & CE_REMOVE)
                set |= CE_WT_REMOVE;

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

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

        memcpy(new, ce, size);
        do_add_entry(o, new, set, clear);
}

/*
 * add error messages on path <path>
 * corresponding to the type <e> with the message <msg>
 * indicating if it should be display in porcelain or not
 */
static int add_rejected_path(struct unpack_trees_options *o,
                             enum unpack_trees_error_types e,
                             const char *path)
{
        if (!o->show_all_errors)
                return error(ERRORMSG(o, e), path);

        /*
         * Otherwise, insert in a list for future display by
         * display_error_msgs()
         */
        string_list_append(&o->unpack_rejects[e], path);
        return -1;
}

/*
 * display all the error messages stored in a nice way
 */
static void display_error_msgs(struct unpack_trees_options *o)
{
        int e, i;
        int something_displayed = 0;
        for (e = 0; e < NB_UNPACK_TREES_ERROR_TYPES; e++) {
                struct string_list *rejects = &o->unpack_rejects[e];
                if (rejects->nr > 0) {
                        struct strbuf path = STRBUF_INIT;
                        something_displayed = 1;
                        for (i = 0; i < rejects->nr; i++)
                                strbuf_addf(&path, "\t%s\n", rejects->items[i].string);
                        error(ERRORMSG(o, e), path.buf);
                        strbuf_release(&path);
                }
                string_list_clear(rejects, 0);
        }
        if (something_displayed)
                fprintf(stderr, "Aborting\n");
}

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

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

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

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

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

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

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

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

static int verify_uptodate_sparse(struct cache_entry *ce, struct unpack_trees_options *o);
static int verify_absent_sparse(struct cache_entry *ce, enum unpack_trees_error_types, struct unpack_trees_options *o);

static int apply_sparse_checkout(struct cache_entry *ce, struct unpack_trees_options *o)
{
        int was_skip_worktree = ce_skip_worktree(ce);

        if (ce->ce_flags & CE_NEW_SKIP_WORKTREE)
                ce->ce_flags |= CE_SKIP_WORKTREE;
        else
                ce->ce_flags &= ~CE_SKIP_WORKTREE;

        /*
         * if (!was_skip_worktree && !ce_skip_worktree()) {
         *      This is perfectly normal. Move on;
         * }
         */

        /*
         * Merge strategies may set CE_UPDATE|CE_REMOVE outside checkout
         * area as a result of ce_skip_worktree() shortcuts in
         * verify_absent() and verify_uptodate().
         * Make sure they don't modify worktree if they are already
         * outside checkout area
         */
        if (was_skip_worktree && ce_skip_worktree(ce)) {
                ce->ce_flags &= ~CE_UPDATE;

                /*
                 * By default, when CE_REMOVE is on, CE_WT_REMOVE is also
                 * on to get that file removed from both index and worktree.
                 * If that file is already outside worktree area, don't
                 * bother remove it.
                 */
                if (ce->ce_flags & CE_REMOVE)
                        ce->ce_flags &= ~CE_WT_REMOVE;
        }

        if (!was_skip_worktree && ce_skip_worktree(ce)) {
                /*
                 * If CE_UPDATE is set, verify_uptodate() must be called already
                 * also stat info may have lost after merged_entry() so calling
                 * verify_uptodate() again may fail
                 */
                if (!(ce->ce_flags & CE_UPDATE) && verify_uptodate_sparse(ce, o))
                        return -1;
                ce->ce_flags |= CE_WT_REMOVE;
        }
        if (was_skip_worktree && !ce_skip_worktree(ce)) {
                if (verify_absent_sparse(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o))
                        return -1;
                ce->ce_flags |= CE_UPDATE;
        }
        return 0;
}

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

static void mark_ce_used(struct cache_entry *ce, struct unpack_trees_options *o)
{
        ce->ce_flags |= CE_UNPACKED;

        if (o->cache_bottom < o->src_index->cache_nr &&
            o->src_index->cache[o->cache_bottom] == ce) {
                int bottom = o->cache_bottom;
                while (bottom < o->src_index->cache_nr &&
                       o->src_index->cache[bottom]->ce_flags & CE_UNPACKED)
                        bottom++;
                o->cache_bottom = bottom;
        }
}

static void mark_all_ce_unused(struct index_state *index)
{
        int i;
        for (i = 0; i < index->cache_nr; i++)
                index->cache[i]->ce_flags &= ~(CE_UNPACKED | CE_ADDED | CE_NEW_SKIP_WORKTREE);
}

static int locate_in_src_index(struct cache_entry *ce,
                               struct unpack_trees_options *o)
{
        struct index_state *index = o->src_index;
        int len = ce_namelen(ce);
        int pos = index_name_pos(index, ce->name, len);
        if (pos < 0)
                pos = -1 - pos;
        return pos;
}

/*
 * We call unpack_index_entry() with an unmerged cache entry
 * only in diff-index, and it wants a single callback.  Skip
 * the other unmerged entry with the same name.
 */
static void mark_ce_used_same_name(struct cache_entry *ce,
                                   struct unpack_trees_options *o)
{
        struct index_state *index = o->src_index;
        int len = ce_namelen(ce);
        int pos;

        for (pos = locate_in_src_index(ce, o); pos < index->cache_nr; pos++) {
                struct cache_entry *next = index->cache[pos];
                if (len != ce_namelen(next) ||
                    memcmp(ce->name, next->name, len))
                        break;
                mark_ce_used(next, o);
        }
}

static struct cache_entry *next_cache_entry(struct unpack_trees_options *o)
{
        const struct index_state *index = o->src_index;
        int pos = o->cache_bottom;

        while (pos < index->cache_nr) {
                struct cache_entry *ce = index->cache[pos];
                if (!(ce->ce_flags & CE_UNPACKED))
                        return ce;
                pos++;
        }
        return NULL;
}

static void add_same_unmerged(struct cache_entry *ce,
                              struct unpack_trees_options *o)
{
        struct index_state *index = o->src_index;
        int len = ce_namelen(ce);
        int pos = index_name_pos(index, ce->name, len);

        if (0 <= pos)
                die("programming error in a caller of mark_ce_used_same_name");
        for (pos = -pos - 1; pos < index->cache_nr; pos++) {
                struct cache_entry *next = index->cache[pos];
                if (len != ce_namelen(next) ||
                    memcmp(ce->name, next->name, len))
                        break;
                add_entry(o, next, 0, 0);
                mark_ce_used(next, o);
        }
}

static int unpack_index_entry(struct cache_entry *ce,
                              struct unpack_trees_options *o)
{
        struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
        int ret;

        src[0] = ce;

        mark_ce_used(ce, o);
        if (ce_stage(ce)) {
                if (o->skip_unmerged) {
                        add_entry(o, ce, 0, 0);
                        return 0;
                }
        }
        ret = call_unpack_fn(src, o);
        if (ce_stage(ce))
                mark_ce_used_same_name(ce, o);
        return ret;
}

static int find_cache_pos(struct traverse_info *, const struct name_entry *);

static void restore_cache_bottom(struct traverse_info *info, int bottom)
{
        struct unpack_trees_options *o = info->data;

        if (o->diff_index_cached)
                return;
        o->cache_bottom = bottom;
}

static int switch_cache_bottom(struct traverse_info *info)
{
        struct unpack_trees_options *o = info->data;
        int ret, pos;

        if (o->diff_index_cached)
                return 0;
        ret = o->cache_bottom;
        pos = find_cache_pos(info->prev, &info->name);

        if (pos < -1)
                o->cache_bottom = -2 - pos;
        else if (pos < 0)
                o->cache_bottom = o->src_index->cache_nr;
        return ret;
}

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

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

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

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

        bottom = switch_cache_bottom(&newinfo);
        ret = traverse_trees(n, t, &newinfo);
        restore_cache_bottom(&newinfo, bottom);

        for (i = 0; i < n; i++)
                free(buf[i]);

        return ret;
}

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

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

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

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

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

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

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

static int ce_in_traverse_path(const struct cache_entry *ce,
                               const struct traverse_info *info)
{
        if (!info->prev)
                return 1;
        if (do_compare_entry(ce, info->prev, &info->name))
                return 0;
        /*
         * If ce (blob) is the same name as the path (which is a tree
         * we will be descending into), it won't be inside it.
         */
        return (info->pathlen < ce_namelen(ce));
}

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

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

        return ce;
}

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

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

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

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

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

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

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

/* NEEDSWORK: give this a better name and share with tree-walk.c */
static int name_compare(const char *a, int a_len,
                        const char *b, int b_len)
{
        int len = (a_len < b_len) ? a_len : b_len;
        int cmp = memcmp(a, b, len);
        if (cmp)
                return cmp;
        return (a_len - b_len);
}

/*
 * The tree traversal is looking at name p.  If we have a matching entry,
 * return it.  If name p is a directory in the index, do not return
 * anything, as we will want to match it when the traversal descends into
 * the directory.
 */
static int find_cache_pos(struct traverse_info *info,
                          const struct name_entry *p)
{
        int pos;
        struct unpack_trees_options *o = info->data;
        struct index_state *index = o->src_index;
        int pfxlen = info->pathlen;
        int p_len = tree_entry_len(p);

        for (pos = o->cache_bottom; pos < index->cache_nr; pos++) {
                struct cache_entry *ce = index->cache[pos];
                const char *ce_name, *ce_slash;
                int cmp, ce_len;

                if (ce->ce_flags & CE_UNPACKED) {
                        /*
                         * cache_bottom entry is already unpacked, so
                         * we can never match it; don't check it
                         * again.
                         */
                        if (pos == o->cache_bottom)
                                ++o->cache_bottom;
                        continue;
                }
                if (!ce_in_traverse_path(ce, info))
                        continue;
                ce_name = ce->name + pfxlen;
                ce_slash = strchr(ce_name, '/');
                if (ce_slash)
                        ce_len = ce_slash - ce_name;
                else
                        ce_len = ce_namelen(ce) - pfxlen;
                cmp = name_compare(p->path, p_len, ce_name, ce_len);
                /*
                 * Exact match; if we have a directory we need to
                 * delay returning it.
                 */
                if (!cmp)
                        return ce_slash ? -2 - pos : pos;
                if (0 < cmp)
                        continue; /* keep looking */
                /*
                 * ce_name sorts after p->path; could it be that we
                 * have files under p->path directory in the index?
                 * E.g.  ce_name == "t-i", and p->path == "t"; we may
                 * have "t/a" in the index.
                 */
                if (p_len < ce_len && !memcmp(ce_name, p->path, p_len) &&
                    ce_name[p_len] < '/')
                        continue; /* keep looking */
                break;
        }
        return -1;
}

static struct cache_entry *find_cache_entry(struct traverse_info *info,
                                            const struct name_entry *p)
{
        int pos = find_cache_pos(info, p);
        struct unpack_trees_options *o = info->data;

        if (0 <= pos)
                return o->src_index->cache[pos];
        else
                return NULL;
}

static void debug_path(struct traverse_info *info)
{
        if (info->prev) {
                debug_path(info->prev);
                if (*info->prev->name.path)
                        putchar('/');
        }
        printf("%s", info->name.path);
}

static void debug_name_entry(int i, struct name_entry *n)
{
        printf("ent#%d %06o %s\n", i,
               n->path ? n->mode : 0,
               n->path ? n->path : "(missing)");
}

static void debug_unpack_callback(int n,
                                  unsigned long mask,
                                  unsigned long dirmask,
                                  struct name_entry *names,
                                  struct traverse_info *info)
{
        int i;
        printf("* unpack mask %lu, dirmask %lu, cnt %d ",
               mask, dirmask, n);
        debug_path(info);
        putchar('\n');
        for (i = 0; i < n; i++)
                debug_name_entry(i, names + i);
}

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

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

        if (o->debug_unpack)
                debug_unpack_callback(n, mask, dirmask, names, info);

        /* Are we supposed to look at the index too? */
        if (o->merge) {
                while (1) {
                        int cmp;
                        struct cache_entry *ce;

                        if (o->diff_index_cached)
                                ce = next_cache_entry(o);
                        else
                                ce = find_cache_entry(info, p);

                        if (!ce)
                                break;
                        cmp = compare_entry(ce, info, p);
                        if (cmp < 0) {
                                if (unpack_index_entry(ce, o) < 0)
                                        return unpack_failed(o, NULL);
                                continue;
                        }
                        if (!cmp) {
                                if (ce_stage(ce)) {
                                        /*
                                         * If we skip unmerged index
                                         * entries, we'll skip this
                                         * entry *and* the tree
                                         * entries associated with it!
                                         */
                                        if (o->skip_unmerged) {
                                                add_same_unmerged(ce, o);
                                                return mask;
                                        }
                                }
                                src[0] = ce;
                        }
                        break;
                }
        }

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

        if (o->merge && src[0]) {
                if (ce_stage(src[0]))
                        mark_ce_used_same_name(src[0], o);
                else
                        mark_ce_used(src[0], o);
        }

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

                /* special case: "diff-index --cached" looking at a tree */
                if (o->diff_index_cached &&
                    n == 1 && dirmask == 1 && S_ISDIR(names->mode)) {
                        int matches;
                        matches = cache_tree_matches_traversal(o->src_index->cache_tree,
                                                               names, info);
                        /*
                         * Everything under the name matches; skip the
                         * entire hierarchy.  diff_index_cached codepath
                         * special cases D/F conflicts in such a way that
                         * it does not do any look-ahead, so this is safe.
                         */
                        if (matches) {
                                o->cache_bottom += matches;
                                return mask;
                        }
                }

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

        return mask;
}

static int clear_ce_flags_1(struct cache_entry **cache, int nr,
                            char *prefix, int prefix_len,
                            int select_mask, int clear_mask,
                            struct exclude_list *el, int defval);

/* Whole directory matching */
static int clear_ce_flags_dir(struct cache_entry **cache, int nr,
                              char *prefix, int prefix_len,
                              char *basename,
                              int select_mask, int clear_mask,
                              struct exclude_list *el, int defval)
{
        struct cache_entry **cache_end;
        int dtype = DT_DIR;
        int ret = is_excluded_from_list(prefix, prefix_len,
                                        basename, &dtype, el);

        prefix[prefix_len++] = '/';

        /* If undecided, use matching result of parent dir in defval */
        if (ret < 0)
                ret = defval;

        for (cache_end = cache; cache_end != cache + nr; cache_end++) {
                struct cache_entry *ce = *cache_end;
                if (strncmp(ce->name, prefix, prefix_len))
                        break;
        }

        /*
         * TODO: check el, if there are no patterns that may conflict
         * with ret (iow, we know in advance the incl/excl
         * decision for the entire directory), clear flag here without
         * calling clear_ce_flags_1(). That function will call
         * the expensive is_excluded_from_list() on every entry.
         */
        return clear_ce_flags_1(cache, cache_end - cache,
                                prefix, prefix_len,
                                select_mask, clear_mask,
                                el, ret);
}

/*
 * Traverse the index, find every entry that matches according to
 * o->el. Do "ce_flags &= ~clear_mask" on those entries. Return the
 * number of traversed entries.
 *
 * If select_mask is non-zero, only entries whose ce_flags has on of
 * those bits enabled are traversed.
 *
 * cache        : pointer to an index entry
 * prefix_len   : an offset to its path
 *
 * The current path ("prefix") including the trailing '/' is
 *   cache[0]->name[0..(prefix_len-1)]
 * Top level path has prefix_len zero.
 */
static int clear_ce_flags_1(struct cache_entry **cache, int nr,
                            char *prefix, int prefix_len,
                            int select_mask, int clear_mask,
                            struct exclude_list *el, int defval)
{
        struct cache_entry **cache_end = cache + nr;

        /*
         * Process all entries that have the given prefix and meet
         * select_mask condition
         */
        while(cache != cache_end) {
                struct cache_entry *ce = *cache;
                const char *name, *slash;
                int len, dtype, ret;

                if (select_mask && !(ce->ce_flags & select_mask)) {
                        cache++;
                        continue;
                }

                if (prefix_len && strncmp(ce->name, prefix, prefix_len))
                        break;

                name = ce->name + prefix_len;
                slash = strchr(name, '/');

                /* If it's a directory, try whole directory match first */
                if (slash) {
                        int processed;

                        len = slash - name;
                        memcpy(prefix + prefix_len, name, len);

                        /*
                         * terminate the string (no trailing slash),
                         * clear_c_f_dir needs it
                         */
                        prefix[prefix_len + len] = '\0';
                        processed = clear_ce_flags_dir(cache, cache_end - cache,
                                                       prefix, prefix_len + len,
                                                       prefix + prefix_len,
                                                       select_mask, clear_mask,
                                                       el, defval);

                        /* clear_c_f_dir eats a whole dir already? */
                        if (processed) {
                                cache += processed;
                                continue;
                        }

                        prefix[prefix_len + len++] = '/';
                        cache += clear_ce_flags_1(cache, cache_end - cache,
                                                  prefix, prefix_len + len,
                                                  select_mask, clear_mask, el, defval);
                        continue;
                }

                /* Non-directory */
                dtype = ce_to_dtype(ce);
                ret = is_excluded_from_list(ce->name, ce_namelen(ce),
                                            name, &dtype, el);
                if (ret < 0)
                        ret = defval;
                if (ret > 0)
                        ce->ce_flags &= ~clear_mask;
                cache++;
        }
        return nr - (cache_end - cache);
}

static int clear_ce_flags(struct cache_entry **cache, int nr,
                            int select_mask, int clear_mask,
                            struct exclude_list *el)
{
        char prefix[PATH_MAX];
        return clear_ce_flags_1(cache, nr,
                                prefix, 0,
                                select_mask, clear_mask,
                                el, 0);
}

/*
 * Set/Clear CE_NEW_SKIP_WORKTREE according to $GIT_DIR/info/sparse-checkout
 */
static void mark_new_skip_worktree(struct exclude_list *el,
                                   struct index_state *the_index,
                                   int select_flag, int skip_wt_flag)
{
        int i;

        /*
         * 1. Pretend the narrowest worktree: only unmerged entries
         * are checked out
         */
        for (i = 0; i < the_index->cache_nr; i++) {
                struct cache_entry *ce = the_index->cache[i];

                if (select_flag && !(ce->ce_flags & select_flag))
                        continue;

                if (!ce_stage(ce))
                        ce->ce_flags |= skip_wt_flag;
                else
                        ce->ce_flags &= ~skip_wt_flag;
        }

        /*
         * 2. Widen worktree according to sparse-checkout file.
         * Matched entries will have skip_wt_flag cleared (i.e. "in")
         */
        clear_ce_flags(the_index->cache, the_index->cache_nr,
                       select_flag, skip_wt_flag, el);
}

static int verify_absent(struct cache_entry *, enum unpack_trees_error_types, struct unpack_trees_options *);
/*
 * N-way merge "len" trees.  Returns 0 on success, -1 on failure to manipulate the
 * resulting index, -2 on failure to reflect the changes to the work tree.
 *
 * CE_ADDED, CE_UNPACKED and CE_NEW_SKIP_WORKTREE are used internally
 */
int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options *o)
{
        int i, ret;
        static struct cache_entry *dfc;
        struct exclude_list el;

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

        memset(&el, 0, sizeof(el));
        if (!core_apply_sparse_checkout || !o->update)
                o->skip_sparse_checkout = 1;
        if (!o->skip_sparse_checkout) {
                if (add_excludes_from_file_to_list(git_path("info/sparse-checkout"), "", 0, &el, 0) < 0)
                        o->skip_sparse_checkout = 1;
                else
                        o->el = &el;
        }

        if (o->dir) {
                o->path_exclude_check = xmalloc(sizeof(struct path_exclude_check));
                path_exclude_check_init(o->path_exclude_check, o->dir);
        }
        memset(&o->result, 0, sizeof(o->result));
        o->result.initialized = 1;
        o->result.timestamp.sec = o->src_index->timestamp.sec;
        o->result.timestamp.nsec = o->src_index->timestamp.nsec;
        o->result.version = o->src_index->version;
        o->merge_size = len;
        mark_all_ce_unused(o->src_index);

        /*
         * Sparse checkout loop #1: set NEW_SKIP_WORKTREE on existing entries
         */
        if (!o->skip_sparse_checkout)
                mark_new_skip_worktree(o->el, o->src_index, 0, CE_NEW_SKIP_WORKTREE);

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

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

                setup_traverse_info(&info, prefix);
                info.fn = unpack_callback;
                info.data = o;
                info.show_all_errors = o->show_all_errors;
                info.pathspec = o->pathspec;

                if (o->prefix) {
                        /*
                         * Unpack existing index entries that sort before the
                         * prefix the tree is spliced into.  Note that o->merge
                         * is always true in this case.
                         */
                        while (1) {
                                struct cache_entry *ce = next_cache_entry(o);
                                if (!ce)
                                        break;
                                if (ce_in_traverse_path(ce, &info))
                                        break;
                                if (unpack_index_entry(ce, o) < 0)
                                        goto return_failed;
                        }
                }

                if (traverse_trees(len, t, &info) < 0)
                        goto return_failed;
        }

        /* Any left-over entries in the index? */
        if (o->merge) {
                while (1) {
                        struct cache_entry *ce = next_cache_entry(o);
                        if (!ce)
                                break;
                        if (unpack_index_entry(ce, o) < 0)
                                goto return_failed;
                }
        }
        mark_all_ce_unused(o->src_index);

        if (o->trivial_merges_only && o->nontrivial_merge) {
                ret = unpack_failed(o, "Merge requires file-level merging");
                goto done;
        }

        if (!o->skip_sparse_checkout) {
                int empty_worktree = 1;

                /*
                 * Sparse checkout loop #2: set NEW_SKIP_WORKTREE on entries not in loop #1
                 * If the will have NEW_SKIP_WORKTREE, also set CE_SKIP_WORKTREE
                 * so apply_sparse_checkout() won't attempt to remove it from worktree
                 */
                mark_new_skip_worktree(o->el, &o->result, CE_ADDED, CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);

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

                        /*
                         * Entries marked with CE_ADDED in merged_entry() do not have
                         * verify_absent() check (the check is effectively disabled
                         * because CE_NEW_SKIP_WORKTREE is set unconditionally).
                         *
                         * Do the real check now because we have had
                         * correct CE_NEW_SKIP_WORKTREE
                         */
                        if (ce->ce_flags & CE_ADDED &&
                            verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
                                if (!o->show_all_errors)
                                        goto return_failed;
                                ret = -1;
                        }

                        if (apply_sparse_checkout(ce, o)) {
                                if (!o->show_all_errors)
                                        goto return_failed;
                                ret = -1;
                        }
                        if (!ce_skip_worktree(ce))
                                empty_worktree = 0;

                }
                if (ret < 0)
                        goto return_failed;
                /*
                 * Sparse checkout is meant to narrow down checkout area
                 * but it does not make sense to narrow down to empty working
                 * tree. This is usually a mistake in sparse checkout rules.
                 * Do not allow users to do that.
                 */
                if (o->result.cache_nr && empty_worktree) {
                        ret = unpack_failed(o, "Sparse checkout leaves no entry on working directory");
                        goto done;
                }
        }

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

done:
        clear_exclude_list(&el);
        if (o->path_exclude_check) {
                path_exclude_check_clear(o->path_exclude_check);
                free(o->path_exclude_check);
        }
        return ret;

return_failed:
        if (o->show_all_errors)
                display_error_msgs(o);
        mark_all_ce_unused(o->src_index);
        ret = unpack_failed(o, NULL);
        if (o->exiting_early)
                ret = 0;
        goto done;
}

/* Here come the merge functions */

static int reject_merge(struct cache_entry *ce, struct unpack_trees_options *o)
{
        return add_rejected_path(o, ERROR_WOULD_OVERWRITE, ce->name);
}

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


/*
 * When a CE gets turned into an unmerged entry, we
 * want it to be up-to-date
 */
static int verify_uptodate_1(struct cache_entry *ce,
                                   struct unpack_trees_options *o,
                                   enum unpack_trees_error_types error_type)
{
        struct stat st;

        if (o->index_only)
                return 0;

        /*
         * CE_VALID and CE_SKIP_WORKTREE cheat, we better check again
         * if this entry is truly up-to-date because this file may be
         * overwritten.
         */
        if ((ce->ce_flags & CE_VALID) || ce_skip_worktree(ce))
                ; /* keep checking */
        else if (o->reset || ce_uptodate(ce))
                return 0;

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

static int verify_uptodate(struct cache_entry *ce,
                           struct unpack_trees_options *o)
{
        if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
                return 0;
        return verify_uptodate_1(ce, o, ERROR_NOT_UPTODATE_FILE);
}

static int verify_uptodate_sparse(struct cache_entry *ce,
                                  struct unpack_trees_options *o)
{
        return verify_uptodate_1(ce, o, ERROR_SPARSE_NOT_UPTODATE_FILE);
}

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

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

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

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

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

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

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

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

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

static int check_ok_to_remove(const char *name, int len, int dtype,
                              struct cache_entry *ce, struct stat *st,
                              enum unpack_trees_error_types error_type,
                              struct unpack_trees_options *o)
{
        struct cache_entry *result;

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

        if (o->dir &&
            is_path_excluded(o->path_exclude_check, name, -1, &dtype))
                /*
                 * ce->name is explicitly excluded, so it is Ok to
                 * overwrite it.
                 */
                return 0;
        if (S_ISDIR(st->st_mode)) {
                /*
                 * We are checking out path "foo" and
                 * found "foo/." in the working tree.
                 * This is tricky -- if we have modified
                 * files that are in "foo/" we would lose
                 * them.
                 */
                if (verify_clean_subdirectory(ce, error_type, o) < 0)
                        return -1;
                return 0;
        }

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

        return o->gently ? -1 :
                add_rejected_path(o, error_type, name);
}

/*
 * We do not want to remove or overwrite a working tree file that
 * is not tracked, unless it is ignored.
 */
static int verify_absent_1(struct cache_entry *ce,
                                 enum unpack_trees_error_types error_type,
                                 struct unpack_trees_options *o)
{
        int len;
        struct stat st;

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

        len = check_leading_path(ce->name, ce_namelen(ce));
        if (!len)
                return 0;
        else if (len > 0) {
                char path[PATH_MAX + 1];
                memcpy(path, ce->name, len);
                path[len] = 0;
                if (lstat(path, &st))
                        return error("cannot stat '%s': %s", path,
                                        strerror(errno));

                return check_ok_to_remove(path, len, DT_UNKNOWN, NULL, &st,
                                error_type, o);
        } else if (lstat(ce->name, &st)) {
                if (errno != ENOENT)
                        return error("cannot stat '%s': %s", ce->name,
                                     strerror(errno));
                return 0;
        } else {
                return check_ok_to_remove(ce->name, ce_namelen(ce),
                                          ce_to_dtype(ce), ce, &st,
                                          error_type, o);
        }
}

static int verify_absent(struct cache_entry *ce,
                         enum unpack_trees_error_types error_type,
                         struct unpack_trees_options *o)
{
        if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
                return 0;
        return verify_absent_1(ce, error_type, o);
}

static int verify_absent_sparse(struct cache_entry *ce,
                         enum unpack_trees_error_types error_type,
                         struct unpack_trees_options *o)
{
        enum unpack_trees_error_types orphaned_error = error_type;
        if (orphaned_error == ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN)
                orphaned_error = ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN;

        return verify_absent_1(ce, orphaned_error, o);
}

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

        if (!old) {
                /*
                 * New index entries. In sparse checkout, the following
                 * verify_absent() will be delayed until after
                 * traverse_trees() finishes in unpack_trees(), then:
                 *
                 *  - CE_NEW_SKIP_WORKTREE will be computed correctly
                 *  - verify_absent() be called again, this time with
                 *    correct CE_NEW_SKIP_WORKTREE
                 *
                 * verify_absent() call here does nothing in sparse
                 * checkout (i.e. o->skip_sparse_checkout == 0)
                 */
                update |= CE_ADDED;
                merge->ce_flags |= CE_NEW_SKIP_WORKTREE;

                if (verify_absent(merge, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o))
                        return -1;
                invalidate_ce_path(merge, o);
        } else if (!(old->ce_flags & CE_CONFLICTED)) {
                /*
                 * See if we can re-use the old CE directly?
                 * That way we get the uptodate stat info.
                 *
                 * This also removes the UPDATE flag on a match; otherwise
                 * we will end up overwriting local changes in the work tree.
                 */
                if (same(old, merge)) {
                        copy_cache_entry(merge, old);
                        update = 0;
                } else {
                        if (verify_uptodate(old, o))
                                return -1;
                        /* Migrate old flags over */
                        update |= old->ce_flags & (CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);
                        invalidate_ce_path(old, o);
                }
        } else {
                /*
                 * Previously unmerged entry left as an existence
                 * marker by read_index_unmerged();
                 */
                invalidate_ce_path(old, o);
        }

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

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

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

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

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

        int df_conflict_head = 0;
        int df_conflict_remote = 0;

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

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

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

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

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

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

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

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

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

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

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

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

        }

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

        o->nontrivial_merge = 1;

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

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

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

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

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

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

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

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

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

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

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