root/pack-bitmap.c

DEFINITIONS

1. lookup_stored_bitmap
2. read_bitmap_1
3. load_bitmap_header
4. store_bitmap
5. read_be32
6. read_u8
7. load_bitmap_entries_v1
8. pack_bitmap_filename
9. open_pack_bitmap_1
10. load_pack_bitmap
11. open_pack_bitmap
12. prepare_bitmap_git
13. bitmap_position_extended
14. bitmap_position_packfile
15. bitmap_position
16. ext_index_add_object
17. show_object
18. show_commit
19. add_to_include_set
20. should_include
21. find_objects
22. show_extended_objects
23. show_objects_for_type
24. in_bitmapped_pack
25. prepare_bitmap_walk
26. reuse_partial_packfile_from_bitmap
27. traverse_bitmap_commit_list
28. count_object_type
29. count_bitmap_commit_list
30. test_show_object
31. test_show_commit
32. test_bitmap_walk
33. rebuild_bitmap
34. rebuild_existing_bitmaps

#include "cache.h"
#include "commit.h"
#include "tag.h"
#include "diff.h"
#include "revision.h"
#include "progress.h"
#include "list-objects.h"
#include "pack.h"
#include "pack-bitmap.h"
#include "pack-revindex.h"
#include "pack-objects.h"

/*
 * An entry on the bitmap index, representing the bitmap for a given
 * commit.
 */
struct stored_bitmap {
        unsigned char sha1[20];
        struct ewah_bitmap *root;
        struct stored_bitmap *xor;
        int flags;
};

/*
 * The currently active bitmap index. By design, repositories only have
 * a single bitmap index available (the index for the biggest packfile in
 * the repository), since bitmap indexes need full closure.
 *
 * If there is more than one bitmap index available (e.g. because of alternates),
 * the active bitmap index is the largest one.
 */
static struct bitmap_index {
        /* Packfile to which this bitmap index belongs to */
        struct packed_git *pack;

        /* reverse index for the packfile */
        struct pack_revindex *reverse_index;

        /*
         * Mark the first `reuse_objects` in the packfile as reused:
         * they will be sent as-is without using them for repacking
         * calculations
         */
        uint32_t reuse_objects;

        /* mmapped buffer of the whole bitmap index */
        unsigned char *map;
        size_t map_size; /* size of the mmaped buffer */
        size_t map_pos; /* current position when loading the index */

        /*
         * Type indexes.
         *
         * Each bitmap marks which objects in the packfile  are of the given
         * type. This provides type information when yielding the objects from
         * the packfile during a walk, which allows for better delta bases.
         */
        struct ewah_bitmap *commits;
        struct ewah_bitmap *trees;
        struct ewah_bitmap *blobs;
        struct ewah_bitmap *tags;

        /* Map from SHA1 -> `stored_bitmap` for all the bitmapped commits */
        khash_sha1 *bitmaps;

        /* Number of bitmapped commits */
        uint32_t entry_count;

        /* Name-hash cache (or NULL if not present). */
        uint32_t *hashes;

        /*
         * Extended index.
         *
         * When trying to perform bitmap operations with objects that are not
         * packed in `pack`, these objects are added to this "fake index" and
         * are assumed to appear at the end of the packfile for all operations
         */
        struct eindex {
                struct object **objects;
                uint32_t *hashes;
                uint32_t count, alloc;
                khash_sha1_pos *positions;
        } ext_index;

        /* Bitmap result of the last performed walk */
        struct bitmap *result;

        /* Version of the bitmap index */
        unsigned int version;

        unsigned loaded : 1;

} bitmap_git;

static struct ewah_bitmap *lookup_stored_bitmap(struct stored_bitmap *st)
{
        struct ewah_bitmap *parent;
        struct ewah_bitmap *composed;

        if (st->xor == NULL)
                return st->root;

        composed = ewah_pool_new();
        parent = lookup_stored_bitmap(st->xor);
        ewah_xor(st->root, parent, composed);

        ewah_pool_free(st->root);
        st->root = composed;
        st->xor = NULL;

        return composed;
}

/*
 * Read a bitmap from the current read position on the mmaped
 * index, and increase the read position accordingly
 */
static struct ewah_bitmap *read_bitmap_1(struct bitmap_index *index)
{
        struct ewah_bitmap *b = ewah_pool_new();

        int bitmap_size = ewah_read_mmap(b,
                index->map + index->map_pos,
                index->map_size - index->map_pos);

        if (bitmap_size < 0) {
                error("Failed to load bitmap index (corrupted?)");
                ewah_pool_free(b);
                return NULL;
        }

        index->map_pos += bitmap_size;
        return b;
}

static int load_bitmap_header(struct bitmap_index *index)
{
        struct bitmap_disk_header *header = (void *)index->map;

        if (index->map_size < sizeof(*header) + 20)
                return error("Corrupted bitmap index (missing header data)");

        if (memcmp(header->magic, BITMAP_IDX_SIGNATURE, sizeof(BITMAP_IDX_SIGNATURE)) != 0)
                return error("Corrupted bitmap index file (wrong header)");

        index->version = ntohs(header->version);
        if (index->version != 1)
                return error("Unsupported version for bitmap index file (%d)", index->version);

        /* Parse known bitmap format options */
        {
                uint32_t flags = ntohs(header->options);

                if ((flags & BITMAP_OPT_FULL_DAG) == 0)
                        return error("Unsupported options for bitmap index file "
                                "(Git requires BITMAP_OPT_FULL_DAG)");

                if (flags & BITMAP_OPT_HASH_CACHE) {
                        unsigned char *end = index->map + index->map_size - 20;
                        index->hashes = ((uint32_t *)end) - index->pack->num_objects;
                }
        }

        index->entry_count = ntohl(header->entry_count);
        index->map_pos += sizeof(*header);
        return 0;
}

static struct stored_bitmap *store_bitmap(struct bitmap_index *index,
                                          struct ewah_bitmap *root,
                                          const unsigned char *sha1,
                                          struct stored_bitmap *xor_with,
                                          int flags)
{
        struct stored_bitmap *stored;
        khiter_t hash_pos;
        int ret;

        stored = xmalloc(sizeof(struct stored_bitmap));
        stored->root = root;
        stored->xor = xor_with;
        stored->flags = flags;
        hashcpy(stored->sha1, sha1);

        hash_pos = kh_put_sha1(index->bitmaps, stored->sha1, &ret);

        /* a 0 return code means the insertion succeeded with no changes,
         * because the SHA1 already existed on the map. this is bad, there
         * shouldn't be duplicated commits in the index */
        if (ret == 0) {
                error("Duplicate entry in bitmap index: %s", sha1_to_hex(sha1));
                return NULL;
        }

        kh_value(index->bitmaps, hash_pos) = stored;
        return stored;
}

static inline uint32_t read_be32(const unsigned char *buffer, size_t *pos)
{
        uint32_t result = get_be32(buffer + *pos);
        (*pos) += sizeof(result);
        return result;
}

static inline uint8_t read_u8(const unsigned char *buffer, size_t *pos)
{
        return buffer[(*pos)++];
}

#define MAX_XOR_OFFSET 160

static int load_bitmap_entries_v1(struct bitmap_index *index)
{
        uint32_t i;
        struct stored_bitmap *recent_bitmaps[MAX_XOR_OFFSET] = { NULL };

        for (i = 0; i < index->entry_count; ++i) {
                int xor_offset, flags;
                struct ewah_bitmap *bitmap = NULL;
                struct stored_bitmap *xor_bitmap = NULL;
                uint32_t commit_idx_pos;
                const unsigned char *sha1;

                commit_idx_pos = read_be32(index->map, &index->map_pos);
                xor_offset = read_u8(index->map, &index->map_pos);
                flags = read_u8(index->map, &index->map_pos);

                sha1 = nth_packed_object_sha1(index->pack, commit_idx_pos);

                bitmap = read_bitmap_1(index);
                if (!bitmap)
                        return -1;

                if (xor_offset > MAX_XOR_OFFSET || xor_offset > i)
                        return error("Corrupted bitmap pack index");

                if (xor_offset > 0) {
                        xor_bitmap = recent_bitmaps[(i - xor_offset) % MAX_XOR_OFFSET];

                        if (xor_bitmap == NULL)
                                return error("Invalid XOR offset in bitmap pack index");
                }

                recent_bitmaps[i % MAX_XOR_OFFSET] = store_bitmap(
                        index, bitmap, sha1, xor_bitmap, flags);
        }

        return 0;
}

static char *pack_bitmap_filename(struct packed_git *p)
{
        char *idx_name;
        int len;

        len = strlen(p->pack_name) - strlen(".pack");
        idx_name = xmalloc(len + strlen(".bitmap") + 1);

        memcpy(idx_name, p->pack_name, len);
        memcpy(idx_name + len, ".bitmap", strlen(".bitmap") + 1);

        return idx_name;
}

static int open_pack_bitmap_1(struct packed_git *packfile)
{
        int fd;
        struct stat st;
        char *idx_name;

        if (open_pack_index(packfile))
                return -1;

        idx_name = pack_bitmap_filename(packfile);
        fd = git_open_noatime(idx_name);
        free(idx_name);

        if (fd < 0)
                return -1;

        if (fstat(fd, &st)) {
                close(fd);
                return -1;
        }

        if (bitmap_git.pack) {
                warning("ignoring extra bitmap file: %s", packfile->pack_name);
                close(fd);
                return -1;
        }

        bitmap_git.pack = packfile;
        bitmap_git.map_size = xsize_t(st.st_size);
        bitmap_git.map = xmmap(NULL, bitmap_git.map_size, PROT_READ, MAP_PRIVATE, fd, 0);
        bitmap_git.map_pos = 0;
        close(fd);

        if (load_bitmap_header(&bitmap_git) < 0) {
                munmap(bitmap_git.map, bitmap_git.map_size);
                bitmap_git.map = NULL;
                bitmap_git.map_size = 0;
                return -1;
        }

        return 0;
}

static int load_pack_bitmap(void)
{
        assert(bitmap_git.map && !bitmap_git.loaded);

        bitmap_git.bitmaps = kh_init_sha1();
        bitmap_git.ext_index.positions = kh_init_sha1_pos();
        bitmap_git.reverse_index = revindex_for_pack(bitmap_git.pack);

        if (!(bitmap_git.commits = read_bitmap_1(&bitmap_git)) ||
                !(bitmap_git.trees = read_bitmap_1(&bitmap_git)) ||
                !(bitmap_git.blobs = read_bitmap_1(&bitmap_git)) ||
                !(bitmap_git.tags = read_bitmap_1(&bitmap_git)))
                goto failed;

        if (load_bitmap_entries_v1(&bitmap_git) < 0)
                goto failed;

        bitmap_git.loaded = 1;
        return 0;

failed:
        munmap(bitmap_git.map, bitmap_git.map_size);
        bitmap_git.map = NULL;
        bitmap_git.map_size = 0;
        return -1;
}

static int open_pack_bitmap(void)
{
        struct packed_git *p;
        int ret = -1;

        assert(!bitmap_git.map && !bitmap_git.loaded);

        prepare_packed_git();
        for (p = packed_git; p; p = p->next) {
                if (open_pack_bitmap_1(p) == 0)
                        ret = 0;
        }

        return ret;
}

int prepare_bitmap_git(void)
{
        if (bitmap_git.loaded)
                return 0;

        if (!open_pack_bitmap())
                return load_pack_bitmap();

        return -1;
}

struct include_data {
        struct bitmap *base;
        struct bitmap *seen;
};

static inline int bitmap_position_extended(const unsigned char *sha1)
{
        khash_sha1_pos *positions = bitmap_git.ext_index.positions;
        khiter_t pos = kh_get_sha1_pos(positions, sha1);

        if (pos < kh_end(positions)) {
                int bitmap_pos = kh_value(positions, pos);
                return bitmap_pos + bitmap_git.pack->num_objects;
        }

        return -1;
}

static inline int bitmap_position_packfile(const unsigned char *sha1)
{
        off_t offset = find_pack_entry_one(sha1, bitmap_git.pack);
        if (!offset)
                return -1;

        return find_revindex_position(bitmap_git.reverse_index, offset);
}

static int bitmap_position(const unsigned char *sha1)
{
        int pos = bitmap_position_packfile(sha1);
        return (pos >= 0) ? pos : bitmap_position_extended(sha1);
}

static int ext_index_add_object(struct object *object, const char *name)
{
        struct eindex *eindex = &bitmap_git.ext_index;

        khiter_t hash_pos;
        int hash_ret;
        int bitmap_pos;

        hash_pos = kh_put_sha1_pos(eindex->positions, object->sha1, &hash_ret);
        if (hash_ret > 0) {
                if (eindex->count >= eindex->alloc) {
                        eindex->alloc = (eindex->alloc + 16) * 3 / 2;
                        REALLOC_ARRAY(eindex->objects, eindex->alloc);
                        REALLOC_ARRAY(eindex->hashes, eindex->alloc);
                }

                bitmap_pos = eindex->count;
                eindex->objects[eindex->count] = object;
                eindex->hashes[eindex->count] = pack_name_hash(name);
                kh_value(eindex->positions, hash_pos) = bitmap_pos;
                eindex->count++;
        } else {
                bitmap_pos = kh_value(eindex->positions, hash_pos);
        }

        return bitmap_pos + bitmap_git.pack->num_objects;
}

static void show_object(struct object *object, const struct name_path *path,
                        const char *last, void *data)
{
        struct bitmap *base = data;
        int bitmap_pos;

        bitmap_pos = bitmap_position(object->sha1);

        if (bitmap_pos < 0) {
                char *name = path_name(path, last);
                bitmap_pos = ext_index_add_object(object, name);
                free(name);
        }

        bitmap_set(base, bitmap_pos);
}

static void show_commit(struct commit *commit, void *data)
{
}

static int add_to_include_set(struct include_data *data,
                              const unsigned char *sha1,
                              int bitmap_pos)
{
        khiter_t hash_pos;

        if (data->seen && bitmap_get(data->seen, bitmap_pos))
                return 0;

        if (bitmap_get(data->base, bitmap_pos))
                return 0;

        hash_pos = kh_get_sha1(bitmap_git.bitmaps, sha1);
        if (hash_pos < kh_end(bitmap_git.bitmaps)) {
                struct stored_bitmap *st = kh_value(bitmap_git.bitmaps, hash_pos);
                bitmap_or_ewah(data->base, lookup_stored_bitmap(st));
                return 0;
        }

        bitmap_set(data->base, bitmap_pos);
        return 1;
}

static int should_include(struct commit *commit, void *_data)
{
        struct include_data *data = _data;
        int bitmap_pos;

        bitmap_pos = bitmap_position(commit->object.sha1);
        if (bitmap_pos < 0)
                bitmap_pos = ext_index_add_object((struct object *)commit, NULL);

        if (!add_to_include_set(data, commit->object.sha1, bitmap_pos)) {
                struct commit_list *parent = commit->parents;

                while (parent) {
                        parent->item->object.flags |= SEEN;
                        parent = parent->next;
                }

                return 0;
        }

        return 1;
}

static struct bitmap *find_objects(struct rev_info *revs,
                                   struct object_list *roots,
                                   struct bitmap *seen)
{
        struct bitmap *base = NULL;
        int needs_walk = 0;

        struct object_list *not_mapped = NULL;

        /*
         * Go through all the roots for the walk. The ones that have bitmaps
         * on the bitmap index will be `or`ed together to form an initial
         * global reachability analysis.
         *
         * The ones without bitmaps in the index will be stored in the
         * `not_mapped_list` for further processing.
         */
        while (roots) {
                struct object *object = roots->item;
                roots = roots->next;

                if (object->type == OBJ_COMMIT) {
                        khiter_t pos = kh_get_sha1(bitmap_git.bitmaps, object->sha1);

                        if (pos < kh_end(bitmap_git.bitmaps)) {
                                struct stored_bitmap *st = kh_value(bitmap_git.bitmaps, pos);
                                struct ewah_bitmap *or_with = lookup_stored_bitmap(st);

                                if (base == NULL)
                                        base = ewah_to_bitmap(or_with);
                                else
                                        bitmap_or_ewah(base, or_with);

                                object->flags |= SEEN;
                                continue;
                        }
                }

                object_list_insert(object, &not_mapped);
        }

        /*
         * Best case scenario: We found bitmaps for all the roots,
         * so the resulting `or` bitmap has the full reachability analysis
         */
        if (not_mapped == NULL)
                return base;

        roots = not_mapped;

        /*
         * Let's iterate through all the roots that don't have bitmaps to
         * check if we can determine them to be reachable from the existing
         * global bitmap.
         *
         * If we cannot find them in the existing global bitmap, we'll need
         * to push them to an actual walk and run it until we can confirm
         * they are reachable
         */
        while (roots) {
                struct object *object = roots->item;
                int pos;

                roots = roots->next;
                pos = bitmap_position(object->sha1);

                if (pos < 0 || base == NULL || !bitmap_get(base, pos)) {
                        object->flags &= ~UNINTERESTING;
                        add_pending_object(revs, object, "");
                        needs_walk = 1;
                } else {
                        object->flags |= SEEN;
                }
        }

        if (needs_walk) {
                struct include_data incdata;

                if (base == NULL)
                        base = bitmap_new();

                incdata.base = base;
                incdata.seen = seen;

                revs->include_check = should_include;
                revs->include_check_data = &incdata;

                if (prepare_revision_walk(revs))
                        die("revision walk setup failed");

                traverse_commit_list(revs, show_commit, show_object, base);
        }

        return base;
}

static void show_extended_objects(struct bitmap *objects,
                                  show_reachable_fn show_reach)
{
        struct eindex *eindex = &bitmap_git.ext_index;
        uint32_t i;

        for (i = 0; i < eindex->count; ++i) {
                struct object *obj;

                if (!bitmap_get(objects, bitmap_git.pack->num_objects + i))
                        continue;

                obj = eindex->objects[i];
                show_reach(obj->sha1, obj->type, 0, eindex->hashes[i], NULL, 0);
        }
}

static void show_objects_for_type(
        struct bitmap *objects,
        struct ewah_bitmap *type_filter,
        enum object_type object_type,
        show_reachable_fn show_reach)
{
        size_t pos = 0, i = 0;
        uint32_t offset;

        struct ewah_iterator it;
        eword_t filter;

        if (bitmap_git.reuse_objects == bitmap_git.pack->num_objects)
                return;

        ewah_iterator_init(&it, type_filter);

        while (i < objects->word_alloc && ewah_iterator_next(&filter, &it)) {
                eword_t word = objects->words[i] & filter;

                for (offset = 0; offset < BITS_IN_EWORD; ++offset) {
                        const unsigned char *sha1;
                        struct revindex_entry *entry;
                        uint32_t hash = 0;

                        if ((word >> offset) == 0)
                                break;

                        offset += ewah_bit_ctz64(word >> offset);

                        if (pos + offset < bitmap_git.reuse_objects)
                                continue;

                        entry = &bitmap_git.reverse_index->revindex[pos + offset];
                        sha1 = nth_packed_object_sha1(bitmap_git.pack, entry->nr);

                        if (bitmap_git.hashes)
                                hash = ntohl(bitmap_git.hashes[entry->nr]);

                        show_reach(sha1, object_type, 0, hash, bitmap_git.pack, entry->offset);
                }

                pos += BITS_IN_EWORD;
                i++;
        }
}

static int in_bitmapped_pack(struct object_list *roots)
{
        while (roots) {
                struct object *object = roots->item;
                roots = roots->next;

                if (find_pack_entry_one(object->sha1, bitmap_git.pack) > 0)
                        return 1;
        }

        return 0;
}

int prepare_bitmap_walk(struct rev_info *revs)
{
        unsigned int i;
        unsigned int pending_nr = revs->pending.nr;
        struct object_array_entry *pending_e = revs->pending.objects;

        struct object_list *wants = NULL;
        struct object_list *haves = NULL;

        struct bitmap *wants_bitmap = NULL;
        struct bitmap *haves_bitmap = NULL;

        if (!bitmap_git.loaded) {
                /* try to open a bitmapped pack, but don't parse it yet
                 * because we may not need to use it */
                if (open_pack_bitmap() < 0)
                        return -1;
        }

        for (i = 0; i < pending_nr; ++i) {
                struct object *object = pending_e[i].item;

                if (object->type == OBJ_NONE)
                        parse_object_or_die(object->sha1, NULL);

                while (object->type == OBJ_TAG) {
                        struct tag *tag = (struct tag *) object;

                        if (object->flags & UNINTERESTING)
                                object_list_insert(object, &haves);
                        else
                                object_list_insert(object, &wants);

                        if (!tag->tagged)
                                die("bad tag");
                        object = parse_object_or_die(tag->tagged->sha1, NULL);
                }

                if (object->flags & UNINTERESTING)
                        object_list_insert(object, &haves);
                else
                        object_list_insert(object, &wants);
        }

        /*
         * if we have a HAVES list, but none of those haves is contained
         * in the packfile that has a bitmap, we don't have anything to
         * optimize here
         */
        if (haves && !in_bitmapped_pack(haves))
                return -1;

        /* if we don't want anything, we're done here */
        if (!wants)
                return -1;

        /*
         * now we're going to use bitmaps, so load the actual bitmap entries
         * from disk. this is the point of no return; after this the rev_list
         * becomes invalidated and we must perform the revwalk through bitmaps
         */
        if (!bitmap_git.loaded && load_pack_bitmap() < 0)
                return -1;

        revs->pending.nr = 0;
        revs->pending.alloc = 0;
        revs->pending.objects = NULL;

        if (haves) {
                revs->ignore_missing_links = 1;
                haves_bitmap = find_objects(revs, haves, NULL);
                reset_revision_walk();
                revs->ignore_missing_links = 0;

                if (haves_bitmap == NULL)
                        die("BUG: failed to perform bitmap walk");
        }

        wants_bitmap = find_objects(revs, wants, haves_bitmap);

        if (!wants_bitmap)
                die("BUG: failed to perform bitmap walk");

        if (haves_bitmap)
                bitmap_and_not(wants_bitmap, haves_bitmap);

        bitmap_git.result = wants_bitmap;

        bitmap_free(haves_bitmap);
        return 0;
}

int reuse_partial_packfile_from_bitmap(struct packed_git **packfile,
                                       uint32_t *entries,
                                       off_t *up_to)
{
        /*
         * Reuse the packfile content if we need more than
         * 90% of its objects
         */
        static const double REUSE_PERCENT = 0.9;

        struct bitmap *result = bitmap_git.result;
        uint32_t reuse_threshold;
        uint32_t i, reuse_objects = 0;

        assert(result);

        for (i = 0; i < result->word_alloc; ++i) {
                if (result->words[i] != (eword_t)~0) {
                        reuse_objects += ewah_bit_ctz64(~result->words[i]);
                        break;
                }

                reuse_objects += BITS_IN_EWORD;
        }

#ifdef GIT_BITMAP_DEBUG
        {
                const unsigned char *sha1;
                struct revindex_entry *entry;

                entry = &bitmap_git.reverse_index->revindex[reuse_objects];
                sha1 = nth_packed_object_sha1(bitmap_git.pack, entry->nr);

                fprintf(stderr, "Failed to reuse at %d (%016llx)\n",
                        reuse_objects, result->words[i]);
                fprintf(stderr, " %s\n", sha1_to_hex(sha1));
        }
#endif

        if (!reuse_objects)
                return -1;

        if (reuse_objects >= bitmap_git.pack->num_objects) {
                bitmap_git.reuse_objects = *entries = bitmap_git.pack->num_objects;
                *up_to = -1; /* reuse the full pack */
                *packfile = bitmap_git.pack;
                return 0;
        }

        reuse_threshold = bitmap_popcount(bitmap_git.result) * REUSE_PERCENT;

        if (reuse_objects < reuse_threshold)
                return -1;

        bitmap_git.reuse_objects = *entries = reuse_objects;
        *up_to = bitmap_git.reverse_index->revindex[reuse_objects].offset;
        *packfile = bitmap_git.pack;

        return 0;
}

void traverse_bitmap_commit_list(show_reachable_fn show_reachable)
{
        assert(bitmap_git.result);

        show_objects_for_type(bitmap_git.result, bitmap_git.commits,
                OBJ_COMMIT, show_reachable);
        show_objects_for_type(bitmap_git.result, bitmap_git.trees,
                OBJ_TREE, show_reachable);
        show_objects_for_type(bitmap_git.result, bitmap_git.blobs,
                OBJ_BLOB, show_reachable);
        show_objects_for_type(bitmap_git.result, bitmap_git.tags,
                OBJ_TAG, show_reachable);

        show_extended_objects(bitmap_git.result, show_reachable);

        bitmap_free(bitmap_git.result);
        bitmap_git.result = NULL;
}

static uint32_t count_object_type(struct bitmap *objects,
                                  enum object_type type)
{
        struct eindex *eindex = &bitmap_git.ext_index;

        uint32_t i = 0, count = 0;
        struct ewah_iterator it;
        eword_t filter;

        switch (type) {
        case OBJ_COMMIT:
                ewah_iterator_init(&it, bitmap_git.commits);
                break;

        case OBJ_TREE:
                ewah_iterator_init(&it, bitmap_git.trees);
                break;

        case OBJ_BLOB:
                ewah_iterator_init(&it, bitmap_git.blobs);
                break;

        case OBJ_TAG:
                ewah_iterator_init(&it, bitmap_git.tags);
                break;

        default:
                return 0;
        }

        while (i < objects->word_alloc && ewah_iterator_next(&filter, &it)) {
                eword_t word = objects->words[i++] & filter;
                count += ewah_bit_popcount64(word);
        }

        for (i = 0; i < eindex->count; ++i) {
                if (eindex->objects[i]->type == type &&
                        bitmap_get(objects, bitmap_git.pack->num_objects + i))
                        count++;
        }

        return count;
}

void count_bitmap_commit_list(uint32_t *commits, uint32_t *trees,
                              uint32_t *blobs, uint32_t *tags)
{
        assert(bitmap_git.result);

        if (commits)
                *commits = count_object_type(bitmap_git.result, OBJ_COMMIT);

        if (trees)
                *trees = count_object_type(bitmap_git.result, OBJ_TREE);

        if (blobs)
                *blobs = count_object_type(bitmap_git.result, OBJ_BLOB);

        if (tags)
                *tags = count_object_type(bitmap_git.result, OBJ_TAG);
}

struct bitmap_test_data {
        struct bitmap *base;
        struct progress *prg;
        size_t seen;
};

static void test_show_object(struct object *object,
                             const struct name_path *path,
                             const char *last, void *data)
{
        struct bitmap_test_data *tdata = data;
        int bitmap_pos;

        bitmap_pos = bitmap_position(object->sha1);
        if (bitmap_pos < 0)
                die("Object not in bitmap: %s\n", sha1_to_hex(object->sha1));

        bitmap_set(tdata->base, bitmap_pos);
        display_progress(tdata->prg, ++tdata->seen);
}

static void test_show_commit(struct commit *commit, void *data)
{
        struct bitmap_test_data *tdata = data;
        int bitmap_pos;

        bitmap_pos = bitmap_position(commit->object.sha1);
        if (bitmap_pos < 0)
                die("Object not in bitmap: %s\n", sha1_to_hex(commit->object.sha1));

        bitmap_set(tdata->base, bitmap_pos);
        display_progress(tdata->prg, ++tdata->seen);
}

void test_bitmap_walk(struct rev_info *revs)
{
        struct object *root;
        struct bitmap *result = NULL;
        khiter_t pos;
        size_t result_popcnt;
        struct bitmap_test_data tdata;

        if (prepare_bitmap_git())
                die("failed to load bitmap indexes");

        if (revs->pending.nr != 1)
                die("you must specify exactly one commit to test");

        fprintf(stderr, "Bitmap v%d test (%d entries loaded)\n",
                bitmap_git.version, bitmap_git.entry_count);

        root = revs->pending.objects[0].item;
        pos = kh_get_sha1(bitmap_git.bitmaps, root->sha1);

        if (pos < kh_end(bitmap_git.bitmaps)) {
                struct stored_bitmap *st = kh_value(bitmap_git.bitmaps, pos);
                struct ewah_bitmap *bm = lookup_stored_bitmap(st);

                fprintf(stderr, "Found bitmap for %s. %d bits / %08x checksum\n",
                        sha1_to_hex(root->sha1), (int)bm->bit_size, ewah_checksum(bm));

                result = ewah_to_bitmap(bm);
        }

        if (result == NULL)
                die("Commit %s doesn't have an indexed bitmap", sha1_to_hex(root->sha1));

        revs->tag_objects = 1;
        revs->tree_objects = 1;
        revs->blob_objects = 1;

        result_popcnt = bitmap_popcount(result);

        if (prepare_revision_walk(revs))
                die("revision walk setup failed");

        tdata.base = bitmap_new();
        tdata.prg = start_progress("Verifying bitmap entries", result_popcnt);
        tdata.seen = 0;

        traverse_commit_list(revs, &test_show_commit, &test_show_object, &tdata);

        stop_progress(&tdata.prg);

        if (bitmap_equals(result, tdata.base))
                fprintf(stderr, "OK!\n");
        else
                fprintf(stderr, "Mismatch!\n");

        bitmap_free(result);
}

static int rebuild_bitmap(uint32_t *reposition,
                          struct ewah_bitmap *source,
                          struct bitmap *dest)
{
        uint32_t pos = 0;
        struct ewah_iterator it;
        eword_t word;

        ewah_iterator_init(&it, source);

        while (ewah_iterator_next(&word, &it)) {
                uint32_t offset, bit_pos;

                for (offset = 0; offset < BITS_IN_EWORD; ++offset) {
                        if ((word >> offset) == 0)
                                break;

                        offset += ewah_bit_ctz64(word >> offset);

                        bit_pos = reposition[pos + offset];
                        if (bit_pos > 0)
                                bitmap_set(dest, bit_pos - 1);
                        else /* can't reuse, we don't have the object */
                                return -1;
                }

                pos += BITS_IN_EWORD;
        }
        return 0;
}

int rebuild_existing_bitmaps(struct packing_data *mapping,
                             khash_sha1 *reused_bitmaps,
                             int show_progress)
{
        uint32_t i, num_objects;
        uint32_t *reposition;
        struct bitmap *rebuild;
        struct stored_bitmap *stored;
        struct progress *progress = NULL;

        khiter_t hash_pos;
        int hash_ret;

        if (prepare_bitmap_git() < 0)
                return -1;

        num_objects = bitmap_git.pack->num_objects;
        reposition = xcalloc(num_objects, sizeof(uint32_t));

        for (i = 0; i < num_objects; ++i) {
                const unsigned char *sha1;
                struct revindex_entry *entry;
                struct object_entry *oe;

                entry = &bitmap_git.reverse_index->revindex[i];
                sha1 = nth_packed_object_sha1(bitmap_git.pack, entry->nr);
                oe = packlist_find(mapping, sha1, NULL);

                if (oe)
                        reposition[i] = oe->in_pack_pos + 1;
        }

        rebuild = bitmap_new();
        i = 0;

        if (show_progress)
                progress = start_progress("Reusing bitmaps", 0);

        kh_foreach_value(bitmap_git.bitmaps, stored, {
                if (stored->flags & BITMAP_FLAG_REUSE) {
                        if (!rebuild_bitmap(reposition,
                                            lookup_stored_bitmap(stored),
                                            rebuild)) {
                                hash_pos = kh_put_sha1(reused_bitmaps,
                                                       stored->sha1,
                                                       &hash_ret);
                                kh_value(reused_bitmaps, hash_pos) =
                                        bitmap_to_ewah(rebuild);
                        }
                        bitmap_reset(rebuild);
                        display_progress(progress, ++i);
                }
        });

        stop_progress(&progress);

        free(reposition);
        bitmap_free(rebuild);
        return 0;
}