commit-reach: implement ahead_behind() logic

[alt-git.git] / pack-bitmap.c

#include "cache.h"
#include "commit.h"
#include "strbuf.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"
#include "packfile.h"
#include "repository.h"
#include "object-store.h"
#include "list-objects-filter-options.h"
#include "midx.h"
#include "config.h"

/*
 * An entry on the bitmap index, representing the bitmap for a given
 * commit.
 */
struct stored_bitmap {
        struct object_id oid;
        struct ewah_bitmap *root;
        struct stored_bitmap *xor;
        int flags;
};

/*
 * The active bitmap index for a repository. 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.
 */
struct bitmap_index {
        /*
         * The pack or multi-pack index (MIDX) that this bitmap index belongs
         * to.
         *
         * Exactly one of these must be non-NULL; this specifies the object
         * order used to interpret this bitmap.
         */
        struct packed_git *pack;
        struct multi_pack_index *midx;

        /*
         * 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 object ID -> `stored_bitmap` for all the bitmapped commits */
        kh_oid_map_t *bitmaps;

        /* Number of bitmapped commits */
        uint32_t entry_count;

        /* If not NULL, this is a name-hash cache pointing into map. */
        uint32_t *hashes;

        /* The checksum of the packfile or MIDX; points into map. */
        const unsigned char *checksum;

        /*
         * If not NULL, this point into the commit table extension
         * (within the memory mapped region `map`).
         */
        unsigned char *table_lookup;

        /*
         * 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;
                kh_oid_pos_t *positions;
        } ext_index;

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

        /* "have" bitmap from the last performed walk */
        struct bitmap *haves;

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

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

        if (!st->xor)
                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();

        ssize_t 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 uint32_t bitmap_num_objects(struct bitmap_index *index)
{
        if (index->midx)
                return index->midx->num_objects;
        return index->pack->num_objects;
}

static int load_bitmap_header(struct bitmap_index *index)
{
        struct bitmap_disk_header *header = (void *)index->map;
        size_t header_size = sizeof(*header) - GIT_MAX_RAWSZ + the_hash_algo->rawsz;

        if (index->map_size < header_size + the_hash_algo->rawsz)
                return error(_("corrupted bitmap index (too small)"));

        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 '%d' for bitmap index file"), index->version);

        /* Parse known bitmap format options */
        {
                uint32_t flags = ntohs(header->options);
                size_t cache_size = st_mult(bitmap_num_objects(index), sizeof(uint32_t));
                unsigned char *index_end = index->map + index->map_size - the_hash_algo->rawsz;

                if ((flags & BITMAP_OPT_FULL_DAG) == 0)
                        BUG("unsupported options for bitmap index file "
                                "(Git requires BITMAP_OPT_FULL_DAG)");

                if (flags & BITMAP_OPT_HASH_CACHE) {
                        if (cache_size > index_end - index->map - header_size)
                                return error(_("corrupted bitmap index file (too short to fit hash cache)"));
                        index->hashes = (void *)(index_end - cache_size);
                        index_end -= cache_size;
                }

                if (flags & BITMAP_OPT_LOOKUP_TABLE) {
                        size_t table_size = st_mult(ntohl(header->entry_count),
                                                    BITMAP_LOOKUP_TABLE_TRIPLET_WIDTH);
                        if (table_size > index_end - index->map - header_size)
                                return error(_("corrupted bitmap index file (too short to fit lookup table)"));
                        if (git_env_bool("GIT_TEST_READ_COMMIT_TABLE", 1))
                                index->table_lookup = (void *)(index_end - table_size);
                        index_end -= table_size;
                }
        }

        index->entry_count = ntohl(header->entry_count);
        index->checksum = header->checksum;
        index->map_pos += header_size;
        return 0;
}

static struct stored_bitmap *store_bitmap(struct bitmap_index *index,
                                          struct ewah_bitmap *root,
                                          const struct object_id *oid,
                                          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;
        oidcpy(&stored->oid, oid);

        hash_pos = kh_put_oid_map(index->bitmaps, stored->oid, &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'"), oid_to_hex(oid));
                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 nth_bitmap_object_oid(struct bitmap_index *index,
                                 struct object_id *oid,
                                 uint32_t n)
{
        if (index->midx)
                return nth_midxed_object_oid(oid, index->midx, n) ? 0 : -1;
        return nth_packed_object_id(oid, index->pack, n);
}

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;
                struct object_id oid;

                if (index->map_size - index->map_pos < 6)
                        return error(_("corrupt ewah bitmap: truncated header for entry %d"), i);

                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);

                if (nth_bitmap_object_oid(index, &oid, commit_idx_pos) < 0)
                        return error(_("corrupt ewah bitmap: commit index %u out of range"),
                                     (unsigned)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)
                                return error(_("invalid XOR offset in bitmap pack index"));
                }

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

        return 0;
}

char *midx_bitmap_filename(struct multi_pack_index *midx)
{
        struct strbuf buf = STRBUF_INIT;

        get_midx_filename(&buf, midx->object_dir);
        strbuf_addf(&buf, "-%s.bitmap", hash_to_hex(get_midx_checksum(midx)));

        return strbuf_detach(&buf, NULL);
}

char *pack_bitmap_filename(struct packed_git *p)
{
        size_t len;

        if (!strip_suffix(p->pack_name, ".pack", &len))
                BUG("pack_name does not end in .pack");
        return xstrfmt("%.*s.bitmap", (int)len, p->pack_name);
}

static int open_midx_bitmap_1(struct bitmap_index *bitmap_git,
                              struct multi_pack_index *midx)
{
        struct stat st;
        char *bitmap_name = midx_bitmap_filename(midx);
        int fd = git_open(bitmap_name);
        uint32_t i;
        struct packed_git *preferred;

        if (fd < 0) {
                if (errno != ENOENT)
                        warning_errno("cannot open '%s'", bitmap_name);
                free(bitmap_name);
                return -1;
        }
        free(bitmap_name);

        if (fstat(fd, &st)) {
                error_errno(_("cannot fstat bitmap file"));
                close(fd);
                return -1;
        }

        if (bitmap_git->pack || bitmap_git->midx) {
                struct strbuf buf = STRBUF_INIT;
                get_midx_filename(&buf, midx->object_dir);
                trace2_data_string("bitmap", the_repository,
                                   "ignoring extra midx bitmap file", buf.buf);
                close(fd);
                strbuf_release(&buf);
                return -1;
        }

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

        if (load_bitmap_header(bitmap_git) < 0)
                goto cleanup;

        if (!hasheq(get_midx_checksum(bitmap_git->midx), bitmap_git->checksum)) {
                error(_("checksum doesn't match in MIDX and bitmap"));
                goto cleanup;
        }

        if (load_midx_revindex(bitmap_git->midx) < 0) {
                warning(_("multi-pack bitmap is missing required reverse index"));
                goto cleanup;
        }

        for (i = 0; i < bitmap_git->midx->num_packs; i++) {
                if (prepare_midx_pack(the_repository, bitmap_git->midx, i))
                        die(_("could not open pack %s"),
                            bitmap_git->midx->pack_names[i]);
        }

        preferred = bitmap_git->midx->packs[midx_preferred_pack(bitmap_git)];
        if (!is_pack_valid(preferred)) {
                warning(_("preferred pack (%s) is invalid"),
                        preferred->pack_name);
                goto cleanup;
        }

        return 0;

cleanup:
        munmap(bitmap_git->map, bitmap_git->map_size);
        bitmap_git->map_size = 0;
        bitmap_git->map_pos = 0;
        bitmap_git->map = NULL;
        bitmap_git->midx = NULL;
        return -1;
}

static int open_pack_bitmap_1(struct bitmap_index *bitmap_git, struct packed_git *packfile)
{
        int fd;
        struct stat st;
        char *bitmap_name;

        bitmap_name = pack_bitmap_filename(packfile);
        fd = git_open(bitmap_name);

        if (fd < 0) {
                if (errno != ENOENT)
                        warning_errno("cannot open '%s'", bitmap_name);
                free(bitmap_name);
                return -1;
        }
        free(bitmap_name);

        if (fstat(fd, &st)) {
                error_errno(_("cannot fstat bitmap file"));
                close(fd);
                return -1;
        }

        if (bitmap_git->pack || bitmap_git->midx) {
                trace2_data_string("bitmap", the_repository,
                                   "ignoring extra bitmap file", packfile->pack_name);
                close(fd);
                return -1;
        }

        if (!is_pack_valid(packfile)) {
                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;
                bitmap_git->map_pos = 0;
                bitmap_git->pack = NULL;
                return -1;
        }

        trace2_data_string("bitmap", the_repository, "opened bitmap file",
                           packfile->pack_name);
        return 0;
}

static int load_reverse_index(struct bitmap_index *bitmap_git)
{
        if (bitmap_is_midx(bitmap_git)) {
                uint32_t i;
                int ret;

                /*
                 * The multi-pack-index's .rev file is already loaded via
                 * open_pack_bitmap_1().
                 *
                 * But we still need to open the individual pack .rev files,
                 * since we will need to make use of them in pack-objects.
                 */
                for (i = 0; i < bitmap_git->midx->num_packs; i++) {
                        ret = load_pack_revindex(bitmap_git->midx->packs[i]);
                        if (ret)
                                return ret;
                }
                return 0;
        }
        return load_pack_revindex(bitmap_git->pack);
}

static int load_bitmap(struct bitmap_index *bitmap_git)
{
        assert(bitmap_git->map);

        bitmap_git->bitmaps = kh_init_oid_map();
        bitmap_git->ext_index.positions = kh_init_oid_pos();

        if (load_reverse_index(bitmap_git))
                goto failed;

        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 (!bitmap_git->table_lookup && load_bitmap_entries_v1(bitmap_git) < 0)
                goto failed;

        return 0;

failed:
        munmap(bitmap_git->map, bitmap_git->map_size);
        bitmap_git->map = NULL;
        bitmap_git->map_size = 0;

        kh_destroy_oid_map(bitmap_git->bitmaps);
        bitmap_git->bitmaps = NULL;

        kh_destroy_oid_pos(bitmap_git->ext_index.positions);
        bitmap_git->ext_index.positions = NULL;

        return -1;
}

static int open_pack_bitmap(struct repository *r,
                            struct bitmap_index *bitmap_git)
{
        struct packed_git *p;
        int ret = -1;

        for (p = get_all_packs(r); p; p = p->next) {
                if (open_pack_bitmap_1(bitmap_git, p) == 0) {
                        ret = 0;
                        /*
                         * The only reason to keep looking is to report
                         * duplicates.
                         */
                        if (!trace2_is_enabled())
                                break;
                }
        }

        return ret;
}

static int open_midx_bitmap(struct repository *r,
                            struct bitmap_index *bitmap_git)
{
        int ret = -1;
        struct multi_pack_index *midx;

        assert(!bitmap_git->map);

        for (midx = get_multi_pack_index(r); midx; midx = midx->next) {
                if (!open_midx_bitmap_1(bitmap_git, midx))
                        ret = 0;
        }
        return ret;
}

static int open_bitmap(struct repository *r,
                       struct bitmap_index *bitmap_git)
{
        int found;

        assert(!bitmap_git->map);

        found = !open_midx_bitmap(r, bitmap_git);

        /*
         * these will all be skipped if we opened a midx bitmap; but run it
         * anyway if tracing is enabled to report the duplicates
         */
        if (!found || trace2_is_enabled())
                found |= !open_pack_bitmap(r, bitmap_git);

        return found ? 0 : -1;
}

struct bitmap_index *prepare_bitmap_git(struct repository *r)
{
        struct bitmap_index *bitmap_git = xcalloc(1, sizeof(*bitmap_git));

        if (!open_bitmap(r, bitmap_git) && !load_bitmap(bitmap_git))
                return bitmap_git;

        free_bitmap_index(bitmap_git);
        return NULL;
}

struct bitmap_index *prepare_midx_bitmap_git(struct multi_pack_index *midx)
{
        struct bitmap_index *bitmap_git = xcalloc(1, sizeof(*bitmap_git));

        if (!open_midx_bitmap_1(bitmap_git, midx) && !load_bitmap(bitmap_git))
                return bitmap_git;

        free_bitmap_index(bitmap_git);
        return NULL;
}

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

struct bitmap_lookup_table_triplet {
        uint32_t commit_pos;
        uint64_t offset;
        uint32_t xor_row;
};

struct bitmap_lookup_table_xor_item {
        struct object_id oid;
        uint64_t offset;
};

/*
 * Given a `triplet` struct pointer and pointer `p`, this
 * function reads the triplet beginning at `p` into the struct.
 * Note that this function assumes that there is enough memory
 * left for filling the `triplet` struct from `p`.
 */
static int bitmap_lookup_table_get_triplet_by_pointer(struct bitmap_lookup_table_triplet *triplet,
                                                      const unsigned char *p)
{
        if (!triplet)
                return -1;

        triplet->commit_pos = get_be32(p);
        p += sizeof(uint32_t);
        triplet->offset = get_be64(p);
        p += sizeof(uint64_t);
        triplet->xor_row = get_be32(p);
        return 0;
}

/*
 * This function gets the raw triplet from `row`'th row in the
 * lookup table and fills that data to the `triplet`.
 */
static int bitmap_lookup_table_get_triplet(struct bitmap_index *bitmap_git,
                                           uint32_t pos,
                                           struct bitmap_lookup_table_triplet *triplet)
{
        unsigned char *p = NULL;
        if (pos >= bitmap_git->entry_count)
                return error(_("corrupt bitmap lookup table: triplet position out of index"));

        p = bitmap_git->table_lookup + st_mult(pos, BITMAP_LOOKUP_TABLE_TRIPLET_WIDTH);

        return bitmap_lookup_table_get_triplet_by_pointer(triplet, p);
}

/*
 * Searches for a matching triplet. `commit_pos` is a pointer
 * to the wanted commit position value. `table_entry` points to
 * a triplet in lookup table. The first 4 bytes of each
 * triplet (pointed by `table_entry`) are compared with `*commit_pos`.
 */
static int triplet_cmp(const void *commit_pos, const void *table_entry)
{

        uint32_t a = *(uint32_t *)commit_pos;
        uint32_t b = get_be32(table_entry);
        if (a > b)
                return 1;
        else if (a < b)
                return -1;

        return 0;
}

static uint32_t bitmap_bsearch_pos(struct bitmap_index *bitmap_git,
                            struct object_id *oid,
                            uint32_t *result)
{
        int found;

        if (bitmap_is_midx(bitmap_git))
                found = bsearch_midx(oid, bitmap_git->midx, result);
        else
                found = bsearch_pack(oid, bitmap_git->pack, result);

        return found;
}

/*
 * `bsearch_triplet_by_pos` function searches for the raw triplet
 * having commit position same as `commit_pos` and fills `triplet`
 * object from the raw triplet. Returns 1 on success and 0 on
 * failure.
 */
static int bitmap_bsearch_triplet_by_pos(uint32_t commit_pos,
                                  struct bitmap_index *bitmap_git,
                                  struct bitmap_lookup_table_triplet *triplet)
{
        unsigned char *p = bsearch(&commit_pos, bitmap_git->table_lookup, bitmap_git->entry_count,
                                   BITMAP_LOOKUP_TABLE_TRIPLET_WIDTH, triplet_cmp);

        if (!p)
                return -1;

        return bitmap_lookup_table_get_triplet_by_pointer(triplet, p);
}

static struct stored_bitmap *lazy_bitmap_for_commit(struct bitmap_index *bitmap_git,
                                                    struct commit *commit)
{
        uint32_t commit_pos, xor_row;
        uint64_t offset;
        int flags;
        struct bitmap_lookup_table_triplet triplet;
        struct object_id *oid = &commit->object.oid;
        struct ewah_bitmap *bitmap;
        struct stored_bitmap *xor_bitmap = NULL;
        const int bitmap_header_size = 6;
        static struct bitmap_lookup_table_xor_item *xor_items = NULL;
        static size_t xor_items_nr = 0, xor_items_alloc = 0;
        static int is_corrupt = 0;
        int xor_flags;
        khiter_t hash_pos;
        struct bitmap_lookup_table_xor_item *xor_item;

        if (is_corrupt)
                return NULL;

        if (!bitmap_bsearch_pos(bitmap_git, oid, &commit_pos))
                return NULL;

        if (bitmap_bsearch_triplet_by_pos(commit_pos, bitmap_git, &triplet) < 0)
                return NULL;

        xor_items_nr = 0;
        offset = triplet.offset;
        xor_row = triplet.xor_row;

        while (xor_row != 0xffffffff) {
                ALLOC_GROW(xor_items, xor_items_nr + 1, xor_items_alloc);

                if (xor_items_nr + 1 >= bitmap_git->entry_count) {
                        error(_("corrupt bitmap lookup table: xor chain exceeds entry count"));
                        goto corrupt;
                }

                if (bitmap_lookup_table_get_triplet(bitmap_git, xor_row, &triplet) < 0)
                        goto corrupt;

                xor_item = &xor_items[xor_items_nr];
                xor_item->offset = triplet.offset;

                if (nth_bitmap_object_oid(bitmap_git, &xor_item->oid, triplet.commit_pos) < 0) {
                        error(_("corrupt bitmap lookup table: commit index %u out of range"),
                                triplet.commit_pos);
                        goto corrupt;
                }

                hash_pos = kh_get_oid_map(bitmap_git->bitmaps, xor_item->oid);

                /*
                 * If desired bitmap is already stored, we don't need
                 * to iterate further. Because we know that bitmaps
                 * that are needed to be parsed to parse this bitmap
                 * has already been stored. So, assign this stored bitmap
                 * to the xor_bitmap.
                 */
                if (hash_pos < kh_end(bitmap_git->bitmaps) &&
                        (xor_bitmap = kh_value(bitmap_git->bitmaps, hash_pos)))
                        break;
                xor_items_nr++;
                xor_row = triplet.xor_row;
        }

        while (xor_items_nr) {
                xor_item = &xor_items[xor_items_nr - 1];
                bitmap_git->map_pos = xor_item->offset;
                if (bitmap_git->map_size - bitmap_git->map_pos < bitmap_header_size) {
                        error(_("corrupt ewah bitmap: truncated header for bitmap of commit \"%s\""),
                                oid_to_hex(&xor_item->oid));
                        goto corrupt;
                }

                bitmap_git->map_pos += sizeof(uint32_t) + sizeof(uint8_t);
                xor_flags = read_u8(bitmap_git->map, &bitmap_git->map_pos);
                bitmap = read_bitmap_1(bitmap_git);

                if (!bitmap)
                        goto corrupt;

                xor_bitmap = store_bitmap(bitmap_git, bitmap, &xor_item->oid, xor_bitmap, xor_flags);
                xor_items_nr--;
        }

        bitmap_git->map_pos = offset;
        if (bitmap_git->map_size - bitmap_git->map_pos < bitmap_header_size) {
                error(_("corrupt ewah bitmap: truncated header for bitmap of commit \"%s\""),
                        oid_to_hex(oid));
                goto corrupt;
        }

        /*
         * Don't bother reading the commit's index position or its xor
         * offset:
         *
         *   - The commit's index position is irrelevant to us, since
         *     load_bitmap_entries_v1 only uses it to learn the object
         *     id which is used to compute the hashmap's key. We already
         *     have an object id, so no need to look it up again.
         *
         *   - The xor_offset is unusable for us, since it specifies how
         *     many entries previous to ours we should look at. This
         *     makes sense when reading the bitmaps sequentially (as in
         *     load_bitmap_entries_v1()), since we can keep track of
         *     each bitmap as we read them.
         *
         *     But it can't work for us, since the bitmap's don't have a
         *     fixed size. So we learn the position of the xor'd bitmap
         *     from the commit table (and resolve it to a bitmap in the
         *     above if-statement).
         *
         * Instead, we can skip ahead and immediately read the flags and
         * ewah bitmap.
         */
        bitmap_git->map_pos += sizeof(uint32_t) + sizeof(uint8_t);
        flags = read_u8(bitmap_git->map, &bitmap_git->map_pos);
        bitmap = read_bitmap_1(bitmap_git);

        if (!bitmap)
                goto corrupt;

        return store_bitmap(bitmap_git, bitmap, oid, xor_bitmap, flags);

corrupt:
        free(xor_items);
        is_corrupt = 1;
        return NULL;
}

struct ewah_bitmap *bitmap_for_commit(struct bitmap_index *bitmap_git,
                                      struct commit *commit)
{
        khiter_t hash_pos = kh_get_oid_map(bitmap_git->bitmaps,
                                           commit->object.oid);
        if (hash_pos >= kh_end(bitmap_git->bitmaps)) {
                struct stored_bitmap *bitmap = NULL;
                if (!bitmap_git->table_lookup)
                        return NULL;

                /* this is a fairly hot codepath - no trace2_region please */
                /* NEEDSWORK: cache misses aren't recorded */
                bitmap = lazy_bitmap_for_commit(bitmap_git, commit);
                if (!bitmap)
                        return NULL;
                return lookup_stored_bitmap(bitmap);
        }
        return lookup_stored_bitmap(kh_value(bitmap_git->bitmaps, hash_pos));
}

static inline int bitmap_position_extended(struct bitmap_index *bitmap_git,
                                           const struct object_id *oid)
{
        kh_oid_pos_t *positions = bitmap_git->ext_index.positions;
        khiter_t pos = kh_get_oid_pos(positions, *oid);

        if (pos < kh_end(positions)) {
                int bitmap_pos = kh_value(positions, pos);
                return bitmap_pos + bitmap_num_objects(bitmap_git);
        }

        return -1;
}

static inline int bitmap_position_packfile(struct bitmap_index *bitmap_git,
                                           const struct object_id *oid)
{
        uint32_t pos;
        off_t offset = find_pack_entry_one(oid->hash, bitmap_git->pack);
        if (!offset)
                return -1;

        if (offset_to_pack_pos(bitmap_git->pack, offset, &pos) < 0)
                return -1;
        return pos;
}

static int bitmap_position_midx(struct bitmap_index *bitmap_git,
                                const struct object_id *oid)
{
        uint32_t want, got;
        if (!bsearch_midx(oid, bitmap_git->midx, &want))
                return -1;

        if (midx_to_pack_pos(bitmap_git->midx, want, &got) < 0)
                return -1;
        return got;
}

static int bitmap_position(struct bitmap_index *bitmap_git,
                           const struct object_id *oid)
{
        int pos;
        if (bitmap_is_midx(bitmap_git))
                pos = bitmap_position_midx(bitmap_git, oid);
        else
                pos = bitmap_position_packfile(bitmap_git, oid);
        return (pos >= 0) ? pos : bitmap_position_extended(bitmap_git, oid);
}

static int ext_index_add_object(struct bitmap_index *bitmap_git,
                                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_oid_pos(eindex->positions, object->oid, &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_num_objects(bitmap_git);
}

struct bitmap_show_data {
        struct bitmap_index *bitmap_git;
        struct bitmap *base;
};

static void show_object(struct object *object, const char *name, void *data_)
{
        struct bitmap_show_data *data = data_;
        int bitmap_pos;

        bitmap_pos = bitmap_position(data->bitmap_git, &object->oid);

        if (bitmap_pos < 0)
                bitmap_pos = ext_index_add_object(data->bitmap_git, object,
                                                  name);

        bitmap_set(data->base, bitmap_pos);
}

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

static int add_to_include_set(struct bitmap_index *bitmap_git,
                              struct include_data *data,
                              struct commit *commit,
                              int bitmap_pos)
{
        struct ewah_bitmap *partial;

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

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

        partial = bitmap_for_commit(bitmap_git, commit);
        if (partial) {
                bitmap_or_ewah(data->base, partial);
                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(data->bitmap_git, &commit->object.oid);
        if (bitmap_pos < 0)
                bitmap_pos = ext_index_add_object(data->bitmap_git,
                                                  (struct object *)commit,
                                                  NULL);

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

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

                return 0;
        }

        return 1;
}

static int should_include_obj(struct object *obj, void *_data)
{
        struct include_data *data = _data;
        int bitmap_pos;

        bitmap_pos = bitmap_position(data->bitmap_git, &obj->oid);
        if (bitmap_pos < 0)
                return 1;
        if ((data->seen && bitmap_get(data->seen, bitmap_pos)) ||
             bitmap_get(data->base, bitmap_pos)) {
                obj->flags |= SEEN;
                return 0;
        }
        return 1;
}

static int add_commit_to_bitmap(struct bitmap_index *bitmap_git,
                                struct bitmap **base,
                                struct commit *commit)
{
        struct ewah_bitmap *or_with = bitmap_for_commit(bitmap_git, commit);

        if (!or_with)
                return 0;

        if (!*base)
                *base = ewah_to_bitmap(or_with);
        else
                bitmap_or_ewah(*base, or_with);

        return 1;
}

static struct bitmap *find_objects(struct bitmap_index *bitmap_git,
                                   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 &&
                    add_commit_to_bitmap(bitmap_git, &base, (struct commit *)object)) {
                        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)
                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(bitmap_git, &object->oid);

                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;
                struct bitmap_show_data show_data;

                if (!base)
                        base = bitmap_new();

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

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

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

                show_data.bitmap_git = bitmap_git;
                show_data.base = base;

                traverse_commit_list(revs,
                                     show_commit, show_object,
                                     &show_data);

                revs->include_check = NULL;
                revs->include_check_obj = NULL;
                revs->include_check_data = NULL;
        }

        return base;
}

static void show_extended_objects(struct bitmap_index *bitmap_git,
                                  struct rev_info *revs,
                                  show_reachable_fn show_reach)
{
        struct bitmap *objects = bitmap_git->result;
        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_num_objects(bitmap_git) + i))
                        continue;

                obj = eindex->objects[i];
                if ((obj->type == OBJ_BLOB && !revs->blob_objects) ||
                    (obj->type == OBJ_TREE && !revs->tree_objects) ||
                    (obj->type == OBJ_TAG && !revs->tag_objects))
                        continue;

                show_reach(&obj->oid, obj->type, 0, eindex->hashes[i], NULL, 0);
        }
}

static void init_type_iterator(struct ewah_iterator *it,
                               struct bitmap_index *bitmap_git,
                               enum object_type type)
{
        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:
                BUG("object type %d not stored by bitmap type index", type);
                break;
        }
}

static void show_objects_for_type(
        struct bitmap_index *bitmap_git,
        enum object_type object_type,
        show_reachable_fn show_reach)
{
        size_t i = 0;
        uint32_t offset;

        struct ewah_iterator it;
        eword_t filter;

        struct bitmap *objects = bitmap_git->result;

        init_type_iterator(&it, bitmap_git, object_type);

        for (i = 0; i < objects->word_alloc &&
                        ewah_iterator_next(&filter, &it); i++) {
                eword_t word = objects->words[i] & filter;
                size_t pos = (i * BITS_IN_EWORD);

                if (!word)
                        continue;

                for (offset = 0; offset < BITS_IN_EWORD; ++offset) {
                        struct packed_git *pack;
                        struct object_id oid;
                        uint32_t hash = 0, index_pos;
                        off_t ofs;

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

                        offset += ewah_bit_ctz64(word >> offset);

                        if (bitmap_is_midx(bitmap_git)) {
                                struct multi_pack_index *m = bitmap_git->midx;
                                uint32_t pack_id;

                                index_pos = pack_pos_to_midx(m, pos + offset);
                                ofs = nth_midxed_offset(m, index_pos);
                                nth_midxed_object_oid(&oid, m, index_pos);

                                pack_id = nth_midxed_pack_int_id(m, index_pos);
                                pack = bitmap_git->midx->packs[pack_id];
                        } else {
                                index_pos = pack_pos_to_index(bitmap_git->pack, pos + offset);
                                ofs = pack_pos_to_offset(bitmap_git->pack, pos + offset);
                                nth_bitmap_object_oid(bitmap_git, &oid, index_pos);

                                pack = bitmap_git->pack;
                        }

                        if (bitmap_git->hashes)
                                hash = get_be32(bitmap_git->hashes + index_pos);

                        show_reach(&oid, object_type, 0, hash, pack, ofs);
                }
        }
}

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

                if (bitmap_is_midx(bitmap_git)) {
                        if (bsearch_midx(&object->oid, bitmap_git->midx, NULL))
                                return 1;
                } else {
                        if (find_pack_entry_one(object->oid.hash, bitmap_git->pack) > 0)
                                return 1;
                }
        }

        return 0;
}

static struct bitmap *find_tip_objects(struct bitmap_index *bitmap_git,
                                       struct object_list *tip_objects,
                                       enum object_type type)
{
        struct bitmap *result = bitmap_new();
        struct object_list *p;

        for (p = tip_objects; p; p = p->next) {
                int pos;

                if (p->item->type != type)
                        continue;

                pos = bitmap_position(bitmap_git, &p->item->oid);
                if (pos < 0)
                        continue;

                bitmap_set(result, pos);
        }

        return result;
}

static void filter_bitmap_exclude_type(struct bitmap_index *bitmap_git,
                                       struct object_list *tip_objects,
                                       struct bitmap *to_filter,
                                       enum object_type type)
{
        struct eindex *eindex = &bitmap_git->ext_index;
        struct bitmap *tips;
        struct ewah_iterator it;
        eword_t mask;
        uint32_t i;

        /*
         * The non-bitmap version of this filter never removes
         * objects which the other side specifically asked for,
         * so we must match that behavior.
         */
        tips = find_tip_objects(bitmap_git, tip_objects, type);

        /*
         * We can use the type-level bitmap for 'type' to work in whole
         * words for the objects that are actually in the bitmapped
         * packfile.
         */
        for (i = 0, init_type_iterator(&it, bitmap_git, type);
             i < to_filter->word_alloc && ewah_iterator_next(&mask, &it);
             i++) {
                if (i < tips->word_alloc)
                        mask &= ~tips->words[i];
                to_filter->words[i] &= ~mask;
        }

        /*
         * Clear any objects that weren't in the packfile (and so would
         * not have been caught by the loop above. We'll have to check
         * them individually.
         */
        for (i = 0; i < eindex->count; i++) {
                uint32_t pos = i + bitmap_num_objects(bitmap_git);
                if (eindex->objects[i]->type == type &&
                    bitmap_get(to_filter, pos) &&
                    !bitmap_get(tips, pos))
                        bitmap_unset(to_filter, pos);
        }

        bitmap_free(tips);
}

static void filter_bitmap_blob_none(struct bitmap_index *bitmap_git,
                                    struct object_list *tip_objects,
                                    struct bitmap *to_filter)
{
        filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter,
                                   OBJ_BLOB);
}

static unsigned long get_size_by_pos(struct bitmap_index *bitmap_git,
                                     uint32_t pos)
{
        unsigned long size;
        struct object_info oi = OBJECT_INFO_INIT;

        oi.sizep = &size;

        if (pos < bitmap_num_objects(bitmap_git)) {
                struct packed_git *pack;
                off_t ofs;

                if (bitmap_is_midx(bitmap_git)) {
                        uint32_t midx_pos = pack_pos_to_midx(bitmap_git->midx, pos);
                        uint32_t pack_id = nth_midxed_pack_int_id(bitmap_git->midx, midx_pos);

                        pack = bitmap_git->midx->packs[pack_id];
                        ofs = nth_midxed_offset(bitmap_git->midx, midx_pos);
                } else {
                        pack = bitmap_git->pack;
                        ofs = pack_pos_to_offset(pack, pos);
                }

                if (packed_object_info(the_repository, pack, ofs, &oi) < 0) {
                        struct object_id oid;
                        nth_bitmap_object_oid(bitmap_git, &oid,
                                              pack_pos_to_index(pack, pos));
                        die(_("unable to get size of %s"), oid_to_hex(&oid));
                }
        } else {
                struct eindex *eindex = &bitmap_git->ext_index;
                struct object *obj = eindex->objects[pos - bitmap_num_objects(bitmap_git)];
                if (oid_object_info_extended(the_repository, &obj->oid, &oi, 0) < 0)
                        die(_("unable to get size of %s"), oid_to_hex(&obj->oid));
        }

        return size;
}

static void filter_bitmap_blob_limit(struct bitmap_index *bitmap_git,
                                     struct object_list *tip_objects,
                                     struct bitmap *to_filter,
                                     unsigned long limit)
{
        struct eindex *eindex = &bitmap_git->ext_index;
        struct bitmap *tips;
        struct ewah_iterator it;
        eword_t mask;
        uint32_t i;

        tips = find_tip_objects(bitmap_git, tip_objects, OBJ_BLOB);

        for (i = 0, init_type_iterator(&it, bitmap_git, OBJ_BLOB);
             i < to_filter->word_alloc && ewah_iterator_next(&mask, &it);
             i++) {
                eword_t word = to_filter->words[i] & mask;
                unsigned offset;

                for (offset = 0; offset < BITS_IN_EWORD; offset++) {
                        uint32_t pos;

                        if ((word >> offset) == 0)
                                break;
                        offset += ewah_bit_ctz64(word >> offset);
                        pos = i * BITS_IN_EWORD + offset;

                        if (!bitmap_get(tips, pos) &&
                            get_size_by_pos(bitmap_git, pos) >= limit)
                                bitmap_unset(to_filter, pos);
                }
        }

        for (i = 0; i < eindex->count; i++) {
                uint32_t pos = i + bitmap_num_objects(bitmap_git);
                if (eindex->objects[i]->type == OBJ_BLOB &&
                    bitmap_get(to_filter, pos) &&
                    !bitmap_get(tips, pos) &&
                    get_size_by_pos(bitmap_git, pos) >= limit)
                        bitmap_unset(to_filter, pos);
        }

        bitmap_free(tips);
}

static void filter_bitmap_tree_depth(struct bitmap_index *bitmap_git,
                                     struct object_list *tip_objects,
                                     struct bitmap *to_filter,
                                     unsigned long limit)
{
        if (limit)
                BUG("filter_bitmap_tree_depth given non-zero limit");

        filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter,
                                   OBJ_TREE);
        filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter,
                                   OBJ_BLOB);
}

static void filter_bitmap_object_type(struct bitmap_index *bitmap_git,
                                      struct object_list *tip_objects,
                                      struct bitmap *to_filter,
                                      enum object_type object_type)
{
        if (object_type < OBJ_COMMIT || object_type > OBJ_TAG)
                BUG("filter_bitmap_object_type given invalid object");

        if (object_type != OBJ_TAG)
                filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter, OBJ_TAG);
        if (object_type != OBJ_COMMIT)
                filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter, OBJ_COMMIT);
        if (object_type != OBJ_TREE)
                filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter, OBJ_TREE);
        if (object_type != OBJ_BLOB)
                filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter, OBJ_BLOB);
}

static int filter_bitmap(struct bitmap_index *bitmap_git,
                         struct object_list *tip_objects,
                         struct bitmap *to_filter,
                         struct list_objects_filter_options *filter)
{
        if (!filter || filter->choice == LOFC_DISABLED)
                return 0;

        if (filter->choice == LOFC_BLOB_NONE) {
                if (bitmap_git)
                        filter_bitmap_blob_none(bitmap_git, tip_objects,
                                                to_filter);
                return 0;
        }

        if (filter->choice == LOFC_BLOB_LIMIT) {
                if (bitmap_git)
                        filter_bitmap_blob_limit(bitmap_git, tip_objects,
                                                 to_filter,
                                                 filter->blob_limit_value);
                return 0;
        }

        if (filter->choice == LOFC_TREE_DEPTH &&
            filter->tree_exclude_depth == 0) {
                if (bitmap_git)
                        filter_bitmap_tree_depth(bitmap_git, tip_objects,
                                                 to_filter,
                                                 filter->tree_exclude_depth);
                return 0;
        }

        if (filter->choice == LOFC_OBJECT_TYPE) {
                if (bitmap_git)
                        filter_bitmap_object_type(bitmap_git, tip_objects,
                                                  to_filter,
                                                  filter->object_type);
                return 0;
        }

        if (filter->choice == LOFC_COMBINE) {
                int i;
                for (i = 0; i < filter->sub_nr; i++) {
                        if (filter_bitmap(bitmap_git, tip_objects, to_filter,
                                          &filter->sub[i]) < 0)
                                return -1;
                }
                return 0;
        }

        /* filter choice not handled */
        return -1;
}

static int can_filter_bitmap(struct list_objects_filter_options *filter)
{
        return !filter_bitmap(NULL, NULL, NULL, filter);
}

struct bitmap_index *prepare_bitmap_walk(struct rev_info *revs,
                                         int filter_provided_objects)
{
        unsigned int i;

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

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

        struct bitmap_index *bitmap_git;

        /*
         * We can't do pathspec limiting with bitmaps, because we don't know
         * which commits are associated with which object changes (let alone
         * even which objects are associated with which paths).
         */
        if (revs->prune)
                return NULL;

        if (!can_filter_bitmap(&revs->filter))
                return NULL;

        /* try to open a bitmapped pack, but don't parse it yet
         * because we may not need to use it */
        CALLOC_ARRAY(bitmap_git, 1);
        if (open_bitmap(revs->repo, bitmap_git) < 0)
                goto cleanup;

        for (i = 0; i < revs->pending.nr; ++i) {
                struct object *object = revs->pending.objects[i].item;

                if (object->type == OBJ_NONE)
                        parse_object_or_die(&object->oid, 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);

                        object = parse_object_or_die(get_tagged_oid(tag), NULL);
                        object->flags |= (tag->object.flags & UNINTERESTING);
                }

                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(bitmap_git, haves))
                goto cleanup;

        /* if we don't want anything, we're done here */
        if (!wants)
                goto cleanup;

        /*
         * 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 (load_bitmap(bitmap_git) < 0)
                goto cleanup;

        object_array_clear(&revs->pending);

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

                if (!haves_bitmap)
                        BUG("failed to perform bitmap walk");
        }

        wants_bitmap = find_objects(bitmap_git, revs, wants, haves_bitmap);

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

        if (haves_bitmap)
                bitmap_and_not(wants_bitmap, haves_bitmap);

        filter_bitmap(bitmap_git,
                      (revs->filter.choice && filter_provided_objects) ? NULL : wants,
                      wants_bitmap,
                      &revs->filter);

        bitmap_git->result = wants_bitmap;
        bitmap_git->haves = haves_bitmap;

        object_list_free(&wants);
        object_list_free(&haves);

        return bitmap_git;

cleanup:
        free_bitmap_index(bitmap_git);
        object_list_free(&wants);
        object_list_free(&haves);
        return NULL;
}

/*
 * -1 means "stop trying further objects"; 0 means we may or may not have
 * reused, but you can keep feeding bits.
 */
static int try_partial_reuse(struct packed_git *pack,
                             size_t pos,
                             struct bitmap *reuse,
                             struct pack_window **w_curs)
{
        off_t offset, delta_obj_offset;
        enum object_type type;
        unsigned long size;

        /*
         * try_partial_reuse() is called either on (a) objects in the
         * bitmapped pack (in the case of a single-pack bitmap) or (b)
         * objects in the preferred pack of a multi-pack bitmap.
         * Importantly, the latter can pretend as if only a single pack
         * exists because:
         *
         *   - The first pack->num_objects bits of a MIDX bitmap are
         *     reserved for the preferred pack, and
         *
         *   - Ties due to duplicate objects are always resolved in
         *     favor of the preferred pack.
         *
         * Therefore we do not need to ever ask the MIDX for its copy of
         * an object by OID, since it will always select it from the
         * preferred pack. Likewise, the selected copy of the base
         * object for any deltas will reside in the same pack.
         *
         * This means that we can reuse pos when looking up the bit in
         * the reuse bitmap, too, since bits corresponding to the
         * preferred pack precede all bits from other packs.
         */

        if (pos >= pack->num_objects)
                return -1; /* not actually in the pack or MIDX preferred pack */

        offset = delta_obj_offset = pack_pos_to_offset(pack, pos);
        type = unpack_object_header(pack, w_curs, &offset, &size);
        if (type < 0)
                return -1; /* broken packfile, punt */

        if (type == OBJ_REF_DELTA || type == OBJ_OFS_DELTA) {
                off_t base_offset;
                uint32_t base_pos;

                /*
                 * Find the position of the base object so we can look it up
                 * in our bitmaps. If we can't come up with an offset, or if
                 * that offset is not in the revidx, the pack is corrupt.
                 * There's nothing we can do, so just punt on this object,
                 * and the normal slow path will complain about it in
                 * more detail.
                 */
                base_offset = get_delta_base(pack, w_curs, &offset, type,
                                             delta_obj_offset);
                if (!base_offset)
                        return 0;
                if (offset_to_pack_pos(pack, base_offset, &base_pos) < 0)
                        return 0;

                /*
                 * We assume delta dependencies always point backwards. This
                 * lets us do a single pass, and is basically always true
                 * due to the way OFS_DELTAs work. You would not typically
                 * find REF_DELTA in a bitmapped pack, since we only bitmap
                 * packs we write fresh, and OFS_DELTA is the default). But
                 * let's double check to make sure the pack wasn't written with
                 * odd parameters.
                 */
                if (base_pos >= pos)
                        return 0;

                /*
                 * And finally, if we're not sending the base as part of our
                 * reuse chunk, then don't send this object either. The base
                 * would come after us, along with other objects not
                 * necessarily in the pack, which means we'd need to convert
                 * to REF_DELTA on the fly. Better to just let the normal
                 * object_entry code path handle it.
                 */
                if (!bitmap_get(reuse, base_pos))
                        return 0;
        }

        /*
         * If we got here, then the object is OK to reuse. Mark it.
         */
        bitmap_set(reuse, pos);
        return 0;
}

uint32_t midx_preferred_pack(struct bitmap_index *bitmap_git)
{
        struct multi_pack_index *m = bitmap_git->midx;
        if (!m)
                BUG("midx_preferred_pack: requires non-empty MIDX");
        return nth_midxed_pack_int_id(m, pack_pos_to_midx(bitmap_git->midx, 0));
}

int reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
                                       struct packed_git **packfile_out,
                                       uint32_t *entries,
                                       struct bitmap **reuse_out)
{
        struct packed_git *pack;
        struct bitmap *result = bitmap_git->result;
        struct bitmap *reuse;
        struct pack_window *w_curs = NULL;
        size_t i = 0;
        uint32_t offset;
        uint32_t objects_nr;

        assert(result);

        load_reverse_index(bitmap_git);

        if (bitmap_is_midx(bitmap_git))
                pack = bitmap_git->midx->packs[midx_preferred_pack(bitmap_git)];
        else
                pack = bitmap_git->pack;
        objects_nr = pack->num_objects;

        while (i < result->word_alloc && result->words[i] == (eword_t)~0)
                i++;

        /*
         * Don't mark objects not in the packfile or preferred pack. This bitmap
         * marks objects eligible for reuse, but the pack-reuse code only
         * understands how to reuse a single pack. Since the preferred pack is
         * guaranteed to have all bases for its deltas (in a multi-pack bitmap),
         * we use it instead of another pack. In single-pack bitmaps, the choice
         * is made for us.
         */
        if (i > objects_nr / BITS_IN_EWORD)
                i = objects_nr / BITS_IN_EWORD;

        reuse = bitmap_word_alloc(i);
        memset(reuse->words, 0xFF, i * sizeof(eword_t));

        for (; i < result->word_alloc; ++i) {
                eword_t word = result->words[i];
                size_t pos = (i * BITS_IN_EWORD);

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

                        offset += ewah_bit_ctz64(word >> offset);
                        if (try_partial_reuse(pack, pos + offset,
                                              reuse, &w_curs) < 0) {
                                /*
                                 * try_partial_reuse indicated we couldn't reuse
                                 * any bits, so there is no point in trying more
                                 * bits in the current word, or any other words
                                 * in result.
                                 *
                                 * Jump out of both loops to avoid future
                                 * unnecessary calls to try_partial_reuse.
                                 */
                                goto done;
                        }
                }
        }

done:
        unuse_pack(&w_curs);

        *entries = bitmap_popcount(reuse);
        if (!*entries) {
                bitmap_free(reuse);
                return -1;
        }

        /*
         * Drop any reused objects from the result, since they will not
         * need to be handled separately.
         */
        bitmap_and_not(result, reuse);
        *packfile_out = pack;
        *reuse_out = reuse;
        return 0;
}

int bitmap_walk_contains(struct bitmap_index *bitmap_git,
                         struct bitmap *bitmap, const struct object_id *oid)
{
        int idx;

        if (!bitmap)
                return 0;

        idx = bitmap_position(bitmap_git, oid);
        return idx >= 0 && bitmap_get(bitmap, idx);
}

void traverse_bitmap_commit_list(struct bitmap_index *bitmap_git,
                                 struct rev_info *revs,
                                 show_reachable_fn show_reachable)
{
        assert(bitmap_git->result);

        show_objects_for_type(bitmap_git, OBJ_COMMIT, show_reachable);
        if (revs->tree_objects)
                show_objects_for_type(bitmap_git, OBJ_TREE, show_reachable);
        if (revs->blob_objects)
                show_objects_for_type(bitmap_git, OBJ_BLOB, show_reachable);
        if (revs->tag_objects)
                show_objects_for_type(bitmap_git, OBJ_TAG, show_reachable);

        show_extended_objects(bitmap_git, revs, show_reachable);
}

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

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

        init_type_iterator(&it, bitmap_git, type);

        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_num_objects(bitmap_git) + i))
                        count++;
        }

        return count;
}

void count_bitmap_commit_list(struct bitmap_index *bitmap_git,
                              uint32_t *commits, uint32_t *trees,
                              uint32_t *blobs, uint32_t *tags)
{
        assert(bitmap_git->result);

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

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

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

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

struct bitmap_test_data {
        struct bitmap_index *bitmap_git;
        struct bitmap *base;
        struct bitmap *commits;
        struct bitmap *trees;
        struct bitmap *blobs;
        struct bitmap *tags;
        struct progress *prg;
        size_t seen;
};

static void test_bitmap_type(struct bitmap_test_data *tdata,
                             struct object *obj, int pos)
{
        enum object_type bitmap_type = OBJ_NONE;
        int bitmaps_nr = 0;

        if (bitmap_get(tdata->commits, pos)) {
                bitmap_type = OBJ_COMMIT;
                bitmaps_nr++;
        }
        if (bitmap_get(tdata->trees, pos)) {
                bitmap_type = OBJ_TREE;
                bitmaps_nr++;
        }
        if (bitmap_get(tdata->blobs, pos)) {
                bitmap_type = OBJ_BLOB;
                bitmaps_nr++;
        }
        if (bitmap_get(tdata->tags, pos)) {
                bitmap_type = OBJ_TAG;
                bitmaps_nr++;
        }

        if (bitmap_type == OBJ_NONE)
                die(_("object '%s' not found in type bitmaps"),
                    oid_to_hex(&obj->oid));

        if (bitmaps_nr > 1)
                die(_("object '%s' does not have a unique type"),
                    oid_to_hex(&obj->oid));

        if (bitmap_type != obj->type)
                die(_("object '%s': real type '%s', expected: '%s'"),
                    oid_to_hex(&obj->oid),
                    type_name(obj->type),
                    type_name(bitmap_type));
}

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

        bitmap_pos = bitmap_position(tdata->bitmap_git, &object->oid);
        if (bitmap_pos < 0)
                die(_("object not in bitmap: '%s'"), oid_to_hex(&object->oid));
        test_bitmap_type(tdata, object, bitmap_pos);

        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(tdata->bitmap_git,
                                     &commit->object.oid);
        if (bitmap_pos < 0)
                die(_("object not in bitmap: '%s'"), oid_to_hex(&commit->object.oid));
        test_bitmap_type(tdata, &commit->object, bitmap_pos);

        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;
        size_t result_popcnt;
        struct bitmap_test_data tdata;
        struct bitmap_index *bitmap_git;
        struct ewah_bitmap *bm;

        if (!(bitmap_git = prepare_bitmap_git(revs->repo)))
                die(_("failed to load bitmap indexes"));

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

        fprintf_ln(stderr, "Bitmap v%d test (%d entries%s)",
                bitmap_git->version,
                bitmap_git->entry_count,
                bitmap_git->table_lookup ? "" : " loaded");

        root = revs->pending.objects[0].item;
        bm = bitmap_for_commit(bitmap_git, (struct commit *)root);

        if (bm) {
                fprintf_ln(stderr, "Found bitmap for '%s'. %d bits / %08x checksum",
                        oid_to_hex(&root->oid), (int)bm->bit_size, ewah_checksum(bm));

                result = ewah_to_bitmap(bm);
        }

        if (!result)
                die(_("commit '%s' doesn't have an indexed bitmap"), oid_to_hex(&root->oid));

        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.bitmap_git = bitmap_git;
        tdata.base = bitmap_new();
        tdata.commits = ewah_to_bitmap(bitmap_git->commits);
        tdata.trees = ewah_to_bitmap(bitmap_git->trees);
        tdata.blobs = ewah_to_bitmap(bitmap_git->blobs);
        tdata.tags = ewah_to_bitmap(bitmap_git->tags);
        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_ln(stderr, "OK!");
        else
                die(_("mismatch in bitmap results"));

        bitmap_free(result);
        bitmap_free(tdata.base);
        bitmap_free(tdata.commits);
        bitmap_free(tdata.trees);
        bitmap_free(tdata.blobs);
        bitmap_free(tdata.tags);
        free_bitmap_index(bitmap_git);
}

int test_bitmap_commits(struct repository *r)
{
        struct object_id oid;
        MAYBE_UNUSED void *value;
        struct bitmap_index *bitmap_git = prepare_bitmap_git(r);

        if (!bitmap_git)
                die(_("failed to load bitmap indexes"));

        /*
         * As this function is only used to print bitmap selected
         * commits, we don't have to read the commit table.
         */
        if (bitmap_git->table_lookup) {
                if (load_bitmap_entries_v1(bitmap_git) < 0)
                        die(_("failed to load bitmap indexes"));
        }

        kh_foreach(bitmap_git->bitmaps, oid, value, {
                printf_ln("%s", oid_to_hex(&oid));
        });

        free_bitmap_index(bitmap_git);

        return 0;
}

int test_bitmap_hashes(struct repository *r)
{
        struct bitmap_index *bitmap_git = prepare_bitmap_git(r);
        struct object_id oid;
        uint32_t i, index_pos;

        if (!bitmap_git || !bitmap_git->hashes)
                goto cleanup;

        for (i = 0; i < bitmap_num_objects(bitmap_git); i++) {
                if (bitmap_is_midx(bitmap_git))
                        index_pos = pack_pos_to_midx(bitmap_git->midx, i);
                else
                        index_pos = pack_pos_to_index(bitmap_git->pack, i);

                nth_bitmap_object_oid(bitmap_git, &oid, index_pos);

                printf_ln("%s %"PRIu32"",
                       oid_to_hex(&oid), get_be32(bitmap_git->hashes + index_pos));
        }

cleanup:
        free_bitmap_index(bitmap_git);

        return 0;
}

int rebuild_bitmap(const 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;
}

uint32_t *create_bitmap_mapping(struct bitmap_index *bitmap_git,
                                struct packing_data *mapping)
{
        uint32_t i, num_objects;
        uint32_t *reposition;

        if (!bitmap_is_midx(bitmap_git))
                load_reverse_index(bitmap_git);
        else if (load_midx_revindex(bitmap_git->midx) < 0)
                BUG("rebuild_existing_bitmaps: missing required rev-cache "
                    "extension");

        num_objects = bitmap_num_objects(bitmap_git);
        CALLOC_ARRAY(reposition, num_objects);

        for (i = 0; i < num_objects; ++i) {
                struct object_id oid;
                struct object_entry *oe;
                uint32_t index_pos;

                if (bitmap_is_midx(bitmap_git))
                        index_pos = pack_pos_to_midx(bitmap_git->midx, i);
                else
                        index_pos = pack_pos_to_index(bitmap_git->pack, i);
                nth_bitmap_object_oid(bitmap_git, &oid, index_pos);
                oe = packlist_find(mapping, &oid);

                if (oe) {
                        reposition[i] = oe_in_pack_pos(mapping, oe) + 1;
                        if (bitmap_git->hashes && !oe->hash)
                                oe->hash = get_be32(bitmap_git->hashes + index_pos);
                }
        }

        return reposition;
}

void free_bitmap_index(struct bitmap_index *b)
{
        if (!b)
                return;

        if (b->map)
                munmap(b->map, b->map_size);
        ewah_pool_free(b->commits);
        ewah_pool_free(b->trees);
        ewah_pool_free(b->blobs);
        ewah_pool_free(b->tags);
        if (b->bitmaps) {
                struct stored_bitmap *sb;
                kh_foreach_value(b->bitmaps, sb, {
                        ewah_pool_free(sb->root);
                        free(sb);
                });
        }
        kh_destroy_oid_map(b->bitmaps);
        free(b->ext_index.objects);
        free(b->ext_index.hashes);
        kh_destroy_oid_pos(b->ext_index.positions);
        bitmap_free(b->result);
        bitmap_free(b->haves);
        if (bitmap_is_midx(b)) {
                /*
                 * Multi-pack bitmaps need to have resources associated with
                 * their on-disk reverse indexes unmapped so that stale .rev and
                 * .bitmap files can be removed.
                 *
                 * Unlike pack-based bitmaps, multi-pack bitmaps can be read and
                 * written in the same 'git multi-pack-index write --bitmap'
                 * process. Close resources so they can be removed safely on
                 * platforms like Windows.
                 */
                close_midx_revindex(b->midx);
        }
        free(b);
}

int bitmap_has_oid_in_uninteresting(struct bitmap_index *bitmap_git,
                                    const struct object_id *oid)
{
        return bitmap_git &&
                bitmap_walk_contains(bitmap_git, bitmap_git->haves, oid);
}

static off_t get_disk_usage_for_type(struct bitmap_index *bitmap_git,
                                     enum object_type object_type)
{
        struct bitmap *result = bitmap_git->result;
        off_t total = 0;
        struct ewah_iterator it;
        eword_t filter;
        size_t i;

        init_type_iterator(&it, bitmap_git, object_type);
        for (i = 0; i < result->word_alloc &&
                        ewah_iterator_next(&filter, &it); i++) {
                eword_t word = result->words[i] & filter;
                size_t base = (i * BITS_IN_EWORD);
                unsigned offset;

                if (!word)
                        continue;

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

                        offset += ewah_bit_ctz64(word >> offset);

                        if (bitmap_is_midx(bitmap_git)) {
                                uint32_t pack_pos;
                                uint32_t midx_pos = pack_pos_to_midx(bitmap_git->midx, base + offset);
                                off_t offset = nth_midxed_offset(bitmap_git->midx, midx_pos);

                                uint32_t pack_id = nth_midxed_pack_int_id(bitmap_git->midx, midx_pos);
                                struct packed_git *pack = bitmap_git->midx->packs[pack_id];

                                if (offset_to_pack_pos(pack, offset, &pack_pos) < 0) {
                                        struct object_id oid;
                                        nth_midxed_object_oid(&oid, bitmap_git->midx, midx_pos);

                                        die(_("could not find '%s' in pack '%s' at offset %"PRIuMAX),
                                            oid_to_hex(&oid),
                                            pack->pack_name,
                                            (uintmax_t)offset);
                                }

                                total += pack_pos_to_offset(pack, pack_pos + 1) - offset;
                        } else {
                                size_t pos = base + offset;
                                total += pack_pos_to_offset(bitmap_git->pack, pos + 1) -
                                         pack_pos_to_offset(bitmap_git->pack, pos);
                        }
                }
        }

        return total;
}

static off_t get_disk_usage_for_extended(struct bitmap_index *bitmap_git)
{
        struct bitmap *result = bitmap_git->result;
        struct eindex *eindex = &bitmap_git->ext_index;
        off_t total = 0;
        struct object_info oi = OBJECT_INFO_INIT;
        off_t object_size;
        size_t i;

        oi.disk_sizep = &object_size;

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

                if (!bitmap_get(result, bitmap_num_objects(bitmap_git) + i))
                        continue;

                if (oid_object_info_extended(the_repository, &obj->oid, &oi, 0) < 0)
                        die(_("unable to get disk usage of '%s'"),
                            oid_to_hex(&obj->oid));

                total += object_size;
        }
        return total;
}

off_t get_disk_usage_from_bitmap(struct bitmap_index *bitmap_git,
                                 struct rev_info *revs)
{
        off_t total = 0;

        total += get_disk_usage_for_type(bitmap_git, OBJ_COMMIT);
        if (revs->tree_objects)
                total += get_disk_usage_for_type(bitmap_git, OBJ_TREE);
        if (revs->blob_objects)
                total += get_disk_usage_for_type(bitmap_git, OBJ_BLOB);
        if (revs->tag_objects)
                total += get_disk_usage_for_type(bitmap_git, OBJ_TAG);

        total += get_disk_usage_for_extended(bitmap_git);

        return total;
}

int bitmap_is_midx(struct bitmap_index *bitmap_git)
{
        return !!bitmap_git->midx;
}

const struct string_list *bitmap_preferred_tips(struct repository *r)
{
        return repo_config_get_value_multi(r, "pack.preferbitmaptips");
}

int bitmap_is_preferred_refname(struct repository *r, const char *refname)
{
        const struct string_list *preferred_tips = bitmap_preferred_tips(r);
        struct string_list_item *item;

        if (!preferred_tips)
                return 0;

        for_each_string_list_item(item, preferred_tips) {
                if (starts_with(refname, item->string))
                        return 1;
        }

        return 0;
}