#include "cache.h"
#include "config.h"
#include "dir.h"
#include "git-compat-util.h"
#include "lockfile.h"
#include "pack.h"
#include "packfile.h"
#include "commit.h"
#include "object.h"
#include "refs.h"
#include "revision.h"
#include "sha1-lookup.h"
#include "commit-graph.h"
#include "object-store.h"
#include "alloc.h"

#define GRAPH_SIGNATURE 0x43475048 /* "CGPH" */
#define GRAPH_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
#define GRAPH_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
#define GRAPH_CHUNKID_DATA 0x43444154 /* "CDAT" */
#define GRAPH_CHUNKID_LARGEEDGES 0x45444745 /* "EDGE" */

#define GRAPH_DATA_WIDTH 36

#define GRAPH_VERSION_1 0x1
#define GRAPH_VERSION GRAPH_VERSION_1

#define GRAPH_OID_VERSION_SHA1 1
#define GRAPH_OID_LEN_SHA1 GIT_SHA1_RAWSZ
#define GRAPH_OID_VERSION GRAPH_OID_VERSION_SHA1
#define GRAPH_OID_LEN GRAPH_OID_LEN_SHA1

#define GRAPH_OCTOPUS_EDGES_NEEDED 0x80000000
#define GRAPH_PARENT_MISSING 0x7fffffff
#define GRAPH_EDGE_LAST_MASK 0x7fffffff
#define GRAPH_PARENT_NONE 0x70000000

#define GRAPH_LAST_EDGE 0x80000000

#define GRAPH_HEADER_SIZE 8
#define GRAPH_FANOUT_SIZE (4 * 256)
#define GRAPH_CHUNKLOOKUP_WIDTH 12
#define GRAPH_MIN_SIZE (GRAPH_HEADER_SIZE + 4 * GRAPH_CHUNKLOOKUP_WIDTH \
                        + GRAPH_FANOUT_SIZE + GRAPH_OID_LEN)

char *get_commit_graph_filename(const char *obj_dir)
{
        return xstrfmt("%s/info/commit-graph", obj_dir);
}

static struct commit_graph *alloc_commit_graph(void)
{
        struct commit_graph *g = xcalloc(1, sizeof(*g));
        g->graph_fd = -1;

        return g;
}

struct commit_graph *load_commit_graph_one(const char *graph_file)
{
        void *graph_map;
        const unsigned char *data, *chunk_lookup;
        size_t graph_size;
        struct stat st;
        uint32_t i;
        struct commit_graph *graph;
        int fd = git_open(graph_file);
        uint64_t last_chunk_offset;
        uint32_t last_chunk_id;
        uint32_t graph_signature;
        unsigned char graph_version, hash_version;

        if (fd < 0)
                return NULL;
        if (fstat(fd, &st)) {
                close(fd);
                return NULL;
        }
        graph_size = xsize_t(st.st_size);

        if (graph_size < GRAPH_MIN_SIZE) {
                close(fd);
                die(_("graph file %s is too small"), graph_file);
        }
        graph_map = xmmap(NULL, graph_size, PROT_READ, MAP_PRIVATE, fd, 0);
        data = (const unsigned char *)graph_map;

        graph_signature = get_be32(data);
        if (graph_signature != GRAPH_SIGNATURE) {
                error(_("graph signature %X does not match signature %X"),
                      graph_signature, GRAPH_SIGNATURE);
                goto cleanup_fail;
        }

        graph_version = *(unsigned char*)(data + 4);
        if (graph_version != GRAPH_VERSION) {
                error(_("graph version %X does not match version %X"),
                      graph_version, GRAPH_VERSION);
                goto cleanup_fail;
        }

        hash_version = *(unsigned char*)(data + 5);
        if (hash_version != GRAPH_OID_VERSION) {
                error(_("hash version %X does not match version %X"),
                      hash_version, GRAPH_OID_VERSION);
                goto cleanup_fail;
        }

        graph = alloc_commit_graph();

        graph->hash_len = GRAPH_OID_LEN;
        graph->num_chunks = *(unsigned char*)(data + 6);
        graph->graph_fd = fd;
        graph->data = graph_map;
        graph->data_len = graph_size;

        last_chunk_id = 0;
        last_chunk_offset = 8;
        chunk_lookup = data + 8;
        for (i = 0; i < graph->num_chunks; i++) {
                uint32_t chunk_id = get_be32(chunk_lookup + 0);
                uint64_t chunk_offset = get_be64(chunk_lookup + 4);
                int chunk_repeated = 0;

                chunk_lookup += GRAPH_CHUNKLOOKUP_WIDTH;

                if (chunk_offset > graph_size - GIT_MAX_RAWSZ) {
                        error(_("improper chunk offset %08x%08x"), (uint32_t)(chunk_offset >> 32),
                              (uint32_t)chunk_offset);
                        goto cleanup_fail;
                }

                switch (chunk_id) {
                case GRAPH_CHUNKID_OIDFANOUT:
                        if (graph->chunk_oid_fanout)
                                chunk_repeated = 1;
                        else
                                graph->chunk_oid_fanout = (uint32_t*)(data + chunk_offset);
                        break;

                case GRAPH_CHUNKID_OIDLOOKUP:
                        if (graph->chunk_oid_lookup)
                                chunk_repeated = 1;
                        else
                                graph->chunk_oid_lookup = data + chunk_offset;
                        break;

                case GRAPH_CHUNKID_DATA:
                        if (graph->chunk_commit_data)
                                chunk_repeated = 1;
                        else
                                graph->chunk_commit_data = data + chunk_offset;
                        break;

                case GRAPH_CHUNKID_LARGEEDGES:
                        if (graph->chunk_large_edges)
                                chunk_repeated = 1;
                        else
                                graph->chunk_large_edges = data + chunk_offset;
                        break;
                }

                if (chunk_repeated) {
                        error(_("chunk id %08x appears multiple times"), chunk_id);
                        goto cleanup_fail;
                }

                if (last_chunk_id == GRAPH_CHUNKID_OIDLOOKUP)
                {
                        graph->num_commits = (chunk_offset - last_chunk_offset)
                                             / graph->hash_len;
                }

                last_chunk_id = chunk_id;
                last_chunk_offset = chunk_offset;
        }

        return graph;

cleanup_fail:
        munmap(graph_map, graph_size);
        close(fd);
        exit(1);
}

static void prepare_commit_graph_one(struct repository *r, const char *obj_dir)
{
        char *graph_name;

        if (r->objects->commit_graph)
                return;

        graph_name = get_commit_graph_filename(obj_dir);
        r->objects->commit_graph =
                load_commit_graph_one(graph_name);

        FREE_AND_NULL(graph_name);
}

/*
 * Return 1 if commit_graph is non-NULL, and 0 otherwise.
 *
 * On the first invocation, this function attemps to load the commit
 * graph if the_repository is configured to have one.
 */
static int prepare_commit_graph(struct repository *r)
{
        struct alternate_object_database *alt;
        char *obj_dir;
        int config_value;

        if (r->objects->commit_graph_attempted)
                return !!r->objects->commit_graph;
        r->objects->commit_graph_attempted = 1;

        if (!git_env_bool(GIT_TEST_COMMIT_GRAPH, 0) &&
            (repo_config_get_bool(r, "core.commitgraph", &config_value) ||
            !config_value))
                /*
                 * This repository is not configured to use commit graphs, so
                 * do not load one. (But report commit_graph_attempted anyway
                 * so that commit graph loading is not attempted again for this
                 * repository.)
                 */
                return 0;

        obj_dir = r->objects->objectdir;
        prepare_commit_graph_one(r, obj_dir);
        prepare_alt_odb(r);
        for (alt = r->objects->alt_odb_list;
             !r->objects->commit_graph && alt;
             alt = alt->next)
                prepare_commit_graph_one(r, alt->path);
        return !!r->objects->commit_graph;
}

static void close_commit_graph(void)
{
        free_commit_graph(the_repository->objects->commit_graph);
        the_repository->objects->commit_graph = NULL;
}

static int bsearch_graph(struct commit_graph *g, struct object_id *oid, uint32_t *pos)
{
        return bsearch_hash(oid->hash, g->chunk_oid_fanout,
                            g->chunk_oid_lookup, g->hash_len, pos);
}

static struct commit_list **insert_parent_or_die(struct commit_graph *g,
                                                 uint64_t pos,
                                                 struct commit_list **pptr)
{
        struct commit *c;
        struct object_id oid;

        if (pos >= g->num_commits)
                die("invalid parent position %"PRIu64, pos);

        hashcpy(oid.hash, g->chunk_oid_lookup + g->hash_len * pos);
        c = lookup_commit(the_repository, &oid);
        if (!c)
                die(_("could not find commit %s"), oid_to_hex(&oid));
        c->graph_pos = pos;
        return &commit_list_insert(c, pptr)->next;
}

static void fill_commit_graph_info(struct commit *item, struct commit_graph *g, uint32_t pos)
{
        const unsigned char *commit_data = g->chunk_commit_data + GRAPH_DATA_WIDTH * pos;
        item->graph_pos = pos;
        item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
}

static int fill_commit_in_graph(struct commit *item, struct commit_graph *g, uint32_t pos)
{
        uint32_t edge_value;
        uint32_t *parent_data_ptr;
        uint64_t date_low, date_high;
        struct commit_list **pptr;
        const unsigned char *commit_data = g->chunk_commit_data + (g->hash_len + 16) * pos;

        item->object.parsed = 1;
        item->graph_pos = pos;

        item->maybe_tree = NULL;

        date_high = get_be32(commit_data + g->hash_len + 8) & 0x3;
        date_low = get_be32(commit_data + g->hash_len + 12);
        item->date = (timestamp_t)((date_high << 32) | date_low);

        item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;

        pptr = &item->parents;

        edge_value = get_be32(commit_data + g->hash_len);
        if (edge_value == GRAPH_PARENT_NONE)
                return 1;
        pptr = insert_parent_or_die(g, edge_value, pptr);

        edge_value = get_be32(commit_data + g->hash_len + 4);
        if (edge_value == GRAPH_PARENT_NONE)
                return 1;
        if (!(edge_value & GRAPH_OCTOPUS_EDGES_NEEDED)) {
                pptr = insert_parent_or_die(g, edge_value, pptr);
                return 1;
        }

        parent_data_ptr = (uint32_t*)(g->chunk_large_edges +
                          4 * (uint64_t)(edge_value & GRAPH_EDGE_LAST_MASK));
        do {
                edge_value = get_be32(parent_data_ptr);
                pptr = insert_parent_or_die(g,
                                            edge_value & GRAPH_EDGE_LAST_MASK,
                                            pptr);
                parent_data_ptr++;
        } while (!(edge_value & GRAPH_LAST_EDGE));

        return 1;
}

static int find_commit_in_graph(struct commit *item, struct commit_graph *g, uint32_t *pos)
{
        if (item->graph_pos != COMMIT_NOT_FROM_GRAPH) {
                *pos = item->graph_pos;
                return 1;
        } else {
                return bsearch_graph(g, &(item->object.oid), pos);
        }
}

static int parse_commit_in_graph_one(struct commit_graph *g, struct commit *item)
{
        uint32_t pos;

        if (item->object.parsed)
                return 1;

        if (find_commit_in_graph(item, g, &pos))
                return fill_commit_in_graph(item, g, pos);

        return 0;
}

int parse_commit_in_graph(struct repository *r, struct commit *item)
{
        if (!prepare_commit_graph(r))
                return 0;
        return parse_commit_in_graph_one(r->objects->commit_graph, item);
}

void load_commit_graph_info(struct repository *r, struct commit *item)
{
        uint32_t pos;
        if (!prepare_commit_graph(r))
                return;
        if (find_commit_in_graph(item, r->objects->commit_graph, &pos))
                fill_commit_graph_info(item, r->objects->commit_graph, pos);
}

static struct tree *load_tree_for_commit(struct commit_graph *g, struct commit *c)
{
        struct object_id oid;
        const unsigned char *commit_data = g->chunk_commit_data +
                                           GRAPH_DATA_WIDTH * (c->graph_pos);

        hashcpy(oid.hash, commit_data);
        c->maybe_tree = lookup_tree(the_repository, &oid);

        return c->maybe_tree;
}

static struct tree *get_commit_tree_in_graph_one(struct commit_graph *g,
                                                 const struct commit *c)
{
        if (c->maybe_tree)
                return c->maybe_tree;
        if (c->graph_pos == COMMIT_NOT_FROM_GRAPH)
                BUG("get_commit_tree_in_graph_one called from non-commit-graph commit");

        return load_tree_for_commit(g, (struct commit *)c);
}

struct tree *get_commit_tree_in_graph(struct repository *r, const struct commit *c)
{
        return get_commit_tree_in_graph_one(r->objects->commit_graph, c);
}

static void write_graph_chunk_fanout(struct hashfile *f,
                                     struct commit **commits,
                                     int nr_commits)
{
        int i, count = 0;
        struct commit **list = commits;

        /*
         * Write the first-level table (the list is sorted,
         * but we use a 256-entry lookup to be able to avoid
         * having to do eight extra binary search iterations).
         */
        for (i = 0; i < 256; i++) {
                while (count < nr_commits) {
                        if ((*list)->object.oid.hash[0] != i)
                                break;
                        count++;
                        list++;
                }

                hashwrite_be32(f, count);
        }
}

static void write_graph_chunk_oids(struct hashfile *f, int hash_len,
                                   struct commit **commits, int nr_commits)
{
        struct commit **list = commits;
        int count;
        for (count = 0; count < nr_commits; count++, list++)
                hashwrite(f, (*list)->object.oid.hash, (int)hash_len);
}

static const unsigned char *commit_to_sha1(size_t index, void *table)
{
        struct commit **commits = table;
        return commits[index]->object.oid.hash;
}

static void write_graph_chunk_data(struct hashfile *f, int hash_len,
                                   struct commit **commits, int nr_commits)
{
        struct commit **list = commits;
        struct commit **last = commits + nr_commits;
        uint32_t num_extra_edges = 0;

        while (list < last) {
                struct commit_list *parent;
                int edge_value;
                uint32_t packedDate[2];

                parse_commit(*list);
                hashwrite(f, get_commit_tree_oid(*list)->hash, hash_len);

                parent = (*list)->parents;

                if (!parent)
                        edge_value = GRAPH_PARENT_NONE;
                else {
                        edge_value = sha1_pos(parent->item->object.oid.hash,
                                              commits,
                                              nr_commits,
                                              commit_to_sha1);

                        if (edge_value < 0)
                                edge_value = GRAPH_PARENT_MISSING;
                }

                hashwrite_be32(f, edge_value);

                if (parent)
                        parent = parent->next;

                if (!parent)
                        edge_value = GRAPH_PARENT_NONE;
                else if (parent->next)
                        edge_value = GRAPH_OCTOPUS_EDGES_NEEDED | num_extra_edges;
                else {
                        edge_value = sha1_pos(parent->item->object.oid.hash,
                                              commits,
                                              nr_commits,
                                              commit_to_sha1);
                        if (edge_value < 0)
                                edge_value = GRAPH_PARENT_MISSING;
                }

                hashwrite_be32(f, edge_value);

                if (edge_value & GRAPH_OCTOPUS_EDGES_NEEDED) {
                        do {
                                num_extra_edges++;
                                parent = parent->next;
                        } while (parent);
                }

                if (sizeof((*list)->date) > 4)
                        packedDate[0] = htonl(((*list)->date >> 32) & 0x3);
                else
                        packedDate[0] = 0;

                packedDate[0] |= htonl((*list)->generation << 2);

                packedDate[1] = htonl((*list)->date);
                hashwrite(f, packedDate, 8);

                list++;
        }
}

static void write_graph_chunk_large_edges(struct hashfile *f,
                                          struct commit **commits,
                                          int nr_commits)
{
        struct commit **list = commits;
        struct commit **last = commits + nr_commits;
        struct commit_list *parent;

        while (list < last) {
                int num_parents = 0;
                for (parent = (*list)->parents; num_parents < 3 && parent;
                     parent = parent->next)
                        num_parents++;

                if (num_parents <= 2) {
                        list++;
                        continue;
                }

                /* Since num_parents > 2, this initializer is safe. */
                for (parent = (*list)->parents->next; parent; parent = parent->next) {
                        int edge_value = sha1_pos(parent->item->object.oid.hash,
                                                  commits,
                                                  nr_commits,
                                                  commit_to_sha1);

                        if (edge_value < 0)
                                edge_value = GRAPH_PARENT_MISSING;
                        else if (!parent->next)
                                edge_value |= GRAPH_LAST_EDGE;

                        hashwrite_be32(f, edge_value);
                }

                list++;
        }
}

static int commit_compare(const void *_a, const void *_b)
{
        const struct object_id *a = (const struct object_id *)_a;
        const struct object_id *b = (const struct object_id *)_b;
        return oidcmp(a, b);
}

struct packed_commit_list {
        struct commit **list;
        int nr;
        int alloc;
};

struct packed_oid_list {
        struct object_id *list;
        int nr;
        int alloc;
};

static int add_packed_commits(const struct object_id *oid,
                              struct packed_git *pack,
                              uint32_t pos,
                              void *data)
{
        struct packed_oid_list *list = (struct packed_oid_list*)data;
        enum object_type type;
        off_t offset = nth_packed_object_offset(pack, pos);
        struct object_info oi = OBJECT_INFO_INIT;

        oi.typep = &type;
        if (packed_object_info(the_repository, pack, offset, &oi) < 0)
                die(_("unable to get type of object %s"), oid_to_hex(oid));

        if (type != OBJ_COMMIT)
                return 0;

        ALLOC_GROW(list->list, list->nr + 1, list->alloc);
        oidcpy(&(list->list[list->nr]), oid);
        list->nr++;

        return 0;
}

static void add_missing_parents(struct packed_oid_list *oids, struct commit *commit)
{
        struct commit_list *parent;
        for (parent = commit->parents; parent; parent = parent->next) {
                if (!(parent->item->object.flags & UNINTERESTING)) {
                        ALLOC_GROW(oids->list, oids->nr + 1, oids->alloc);
                        oidcpy(&oids->list[oids->nr], &(parent->item->object.oid));
                        oids->nr++;
                        parent->item->object.flags |= UNINTERESTING;
                }
        }
}

static void close_reachable(struct packed_oid_list *oids)
{
        int i;
        struct commit *commit;

        for (i = 0; i < oids->nr; i++) {
                commit = lookup_commit(the_repository, &oids->list[i]);
                if (commit)
                        commit->object.flags |= UNINTERESTING;
        }

        /*
         * As this loop runs, oids->nr may grow, but not more
         * than the number of missing commits in the reachable
         * closure.
         */
        for (i = 0; i < oids->nr; i++) {
                commit = lookup_commit(the_repository, &oids->list[i]);

                if (commit && !parse_commit(commit))
                        add_missing_parents(oids, commit);
        }

        for (i = 0; i < oids->nr; i++) {
                commit = lookup_commit(the_repository, &oids->list[i]);

                if (commit)
                        commit->object.flags &= ~UNINTERESTING;
        }
}

static void compute_generation_numbers(struct packed_commit_list* commits)
{
        int i;
        struct commit_list *list = NULL;

        for (i = 0; i < commits->nr; i++) {
                if (commits->list[i]->generation != GENERATION_NUMBER_INFINITY &&
                    commits->list[i]->generation != GENERATION_NUMBER_ZERO)
                        continue;

                commit_list_insert(commits->list[i], &list);
                while (list) {
                        struct commit *current = list->item;
                        struct commit_list *parent;
                        int all_parents_computed = 1;
                        uint32_t max_generation = 0;

                        for (parent = current->parents; parent; parent = parent->next) {
                                if (parent->item->generation == GENERATION_NUMBER_INFINITY ||
                                    parent->item->generation == GENERATION_NUMBER_ZERO) {
                                        all_parents_computed = 0;
                                        commit_list_insert(parent->item, &list);
                                        break;
                                } else if (parent->item->generation > max_generation) {
                                        max_generation = parent->item->generation;
                                }
                        }

                        if (all_parents_computed) {
                                current->generation = max_generation + 1;
                                pop_commit(&list);

                                if (current->generation > GENERATION_NUMBER_MAX)
                                        current->generation = GENERATION_NUMBER_MAX;
                        }
                }
        }
}

static int add_ref_to_list(const char *refname,
                           const struct object_id *oid,
                           int flags, void *cb_data)
{
        struct string_list *list = (struct string_list *)cb_data;

        string_list_append(list, oid_to_hex(oid));
        return 0;
}

void write_commit_graph_reachable(const char *obj_dir, int append)
{
        struct string_list list;

        string_list_init(&list, 1);
        for_each_ref(add_ref_to_list, &list);
        write_commit_graph(obj_dir, NULL, &list, append);
}

void write_commit_graph(const char *obj_dir,
                        struct string_list *pack_indexes,
                        struct string_list *commit_hex,
                        int append)
{
        struct packed_oid_list oids;
        struct packed_commit_list commits;
        struct hashfile *f;
        uint32_t i, count_distinct = 0;
        char *graph_name;
        struct lock_file lk = LOCK_INIT;
        uint32_t chunk_ids[5];
        uint64_t chunk_offsets[5];
        int num_chunks;
        int num_extra_edges;
        struct commit_list *parent;

        oids.nr = 0;
        oids.alloc = approximate_object_count() / 4;

        if (append) {
                prepare_commit_graph_one(the_repository, obj_dir);
                if (the_repository->objects->commit_graph)
                        oids.alloc += the_repository->objects->commit_graph->num_commits;
        }

        if (oids.alloc < 1024)
                oids.alloc = 1024;
        ALLOC_ARRAY(oids.list, oids.alloc);

        if (append && the_repository->objects->commit_graph) {
                struct commit_graph *commit_graph =
                        the_repository->objects->commit_graph;
                for (i = 0; i < commit_graph->num_commits; i++) {
                        const unsigned char *hash = commit_graph->chunk_oid_lookup +
                                commit_graph->hash_len * i;
                        hashcpy(oids.list[oids.nr++].hash, hash);
                }
        }

        if (pack_indexes) {
                struct strbuf packname = STRBUF_INIT;
                int dirlen;
                strbuf_addf(&packname, "%s/pack/", obj_dir);
                dirlen = packname.len;
                for (i = 0; i < pack_indexes->nr; i++) {
                        struct packed_git *p;
                        strbuf_setlen(&packname, dirlen);
                        strbuf_addstr(&packname, pack_indexes->items[i].string);
                        p = add_packed_git(packname.buf, packname.len, 1);
                        if (!p)
                                die(_("error adding pack %s"), packname.buf);
                        if (open_pack_index(p))
                                die(_("error opening index for %s"), packname.buf);
                        for_each_object_in_pack(p, add_packed_commits, &oids, 0);
                        close_pack(p);
                }
                strbuf_release(&packname);
        }

        if (commit_hex) {
                for (i = 0; i < commit_hex->nr; i++) {
                        const char *end;
                        struct object_id oid;
                        struct commit *result;

                        if (commit_hex->items[i].string &&
                            parse_oid_hex(commit_hex->items[i].string, &oid, &end))
                                continue;

                        result = lookup_commit_reference_gently(the_repository, &oid, 1);

                        if (result) {
                                ALLOC_GROW(oids.list, oids.nr + 1, oids.alloc);
                                oidcpy(&oids.list[oids.nr], &(result->object.oid));
                                oids.nr++;
                        }
                }
        }

        if (!pack_indexes && !commit_hex)
                for_each_packed_object(add_packed_commits, &oids, 0);

        close_reachable(&oids);

        QSORT(oids.list, oids.nr, commit_compare);

        count_distinct = 1;
        for (i = 1; i < oids.nr; i++) {
                if (oidcmp(&oids.list[i-1], &oids.list[i]))
                        count_distinct++;
        }

        if (count_distinct >= GRAPH_PARENT_MISSING)
                die(_("the commit graph format cannot write %d commits"), count_distinct);

        commits.nr = 0;
        commits.alloc = count_distinct;
        ALLOC_ARRAY(commits.list, commits.alloc);

        num_extra_edges = 0;
        for (i = 0; i < oids.nr; i++) {
                int num_parents = 0;
                if (i > 0 && !oidcmp(&oids.list[i-1], &oids.list[i]))
                        continue;

                commits.list[commits.nr] = lookup_commit(the_repository, &oids.list[i]);
                parse_commit(commits.list[commits.nr]);

                for (parent = commits.list[commits.nr]->parents;
                     parent; parent = parent->next)
                        num_parents++;

                if (num_parents > 2)
                        num_extra_edges += num_parents - 1;

                commits.nr++;
        }
        num_chunks = num_extra_edges ? 4 : 3;

        if (commits.nr >= GRAPH_PARENT_MISSING)
                die(_("too many commits to write graph"));

        compute_generation_numbers(&commits);

        graph_name = get_commit_graph_filename(obj_dir);
        if (safe_create_leading_directories(graph_name))
                die_errno(_("unable to create leading directories of %s"),
                          graph_name);

        hold_lock_file_for_update(&lk, graph_name, LOCK_DIE_ON_ERROR);
        f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);

        hashwrite_be32(f, GRAPH_SIGNATURE);

        hashwrite_u8(f, GRAPH_VERSION);
        hashwrite_u8(f, GRAPH_OID_VERSION);
        hashwrite_u8(f, num_chunks);
        hashwrite_u8(f, 0); /* unused padding byte */

        chunk_ids[0] = GRAPH_CHUNKID_OIDFANOUT;
        chunk_ids[1] = GRAPH_CHUNKID_OIDLOOKUP;
        chunk_ids[2] = GRAPH_CHUNKID_DATA;
        if (num_extra_edges)
                chunk_ids[3] = GRAPH_CHUNKID_LARGEEDGES;
        else
                chunk_ids[3] = 0;
        chunk_ids[4] = 0;

        chunk_offsets[0] = 8 + (num_chunks + 1) * GRAPH_CHUNKLOOKUP_WIDTH;
        chunk_offsets[1] = chunk_offsets[0] + GRAPH_FANOUT_SIZE;
        chunk_offsets[2] = chunk_offsets[1] + GRAPH_OID_LEN * commits.nr;
        chunk_offsets[3] = chunk_offsets[2] + (GRAPH_OID_LEN + 16) * commits.nr;
        chunk_offsets[4] = chunk_offsets[3] + 4 * num_extra_edges;

        for (i = 0; i <= num_chunks; i++) {
                uint32_t chunk_write[3];

                chunk_write[0] = htonl(chunk_ids[i]);
                chunk_write[1] = htonl(chunk_offsets[i] >> 32);
                chunk_write[2] = htonl(chunk_offsets[i] & 0xffffffff);
                hashwrite(f, chunk_write, 12);
        }

        write_graph_chunk_fanout(f, commits.list, commits.nr);
        write_graph_chunk_oids(f, GRAPH_OID_LEN, commits.list, commits.nr);
        write_graph_chunk_data(f, GRAPH_OID_LEN, commits.list, commits.nr);
        write_graph_chunk_large_edges(f, commits.list, commits.nr);

        close_commit_graph();
        finalize_hashfile(f, NULL, CSUM_HASH_IN_STREAM | CSUM_FSYNC);
        commit_lock_file(&lk);

        free(oids.list);
        oids.alloc = 0;
        oids.nr = 0;
}

#define VERIFY_COMMIT_GRAPH_ERROR_HASH 2
static int verify_commit_graph_error;

static void graph_report(const char *fmt, ...)
{
        va_list ap;

        verify_commit_graph_error = 1;
        va_start(ap, fmt);
        vfprintf(stderr, fmt, ap);
        fprintf(stderr, "\n");
        va_end(ap);
}

#define GENERATION_ZERO_EXISTS 1
#define GENERATION_NUMBER_EXISTS 2

int verify_commit_graph(struct repository *r, struct commit_graph *g)
{
        uint32_t i, cur_fanout_pos = 0;
        struct object_id prev_oid, cur_oid, checksum;
        int generation_zero = 0;
        struct hashfile *f;
        int devnull;

        if (!g) {
                graph_report("no commit-graph file loaded");
                return 1;
        }

        verify_commit_graph_error = 0;

        if (!g->chunk_oid_fanout)
                graph_report("commit-graph is missing the OID Fanout chunk");
        if (!g->chunk_oid_lookup)
                graph_report("commit-graph is missing the OID Lookup chunk");
        if (!g->chunk_commit_data)
                graph_report("commit-graph is missing the Commit Data chunk");

        if (verify_commit_graph_error)
                return verify_commit_graph_error;

        devnull = open("/dev/null", O_WRONLY);
        f = hashfd(devnull, NULL);
        hashwrite(f, g->data, g->data_len - g->hash_len);
        finalize_hashfile(f, checksum.hash, CSUM_CLOSE);
        if (hashcmp(checksum.hash, g->data + g->data_len - g->hash_len)) {
                graph_report(_("the commit-graph file has incorrect checksum and is likely corrupt"));
                verify_commit_graph_error = VERIFY_COMMIT_GRAPH_ERROR_HASH;
        }

        for (i = 0; i < g->num_commits; i++) {
                struct commit *graph_commit;

                hashcpy(cur_oid.hash, g->chunk_oid_lookup + g->hash_len * i);

                if (i && oidcmp(&prev_oid, &cur_oid) >= 0)
                        graph_report("commit-graph has incorrect OID order: %s then %s",
                                     oid_to_hex(&prev_oid),
                                     oid_to_hex(&cur_oid));

                oidcpy(&prev_oid, &cur_oid);

                while (cur_oid.hash[0] > cur_fanout_pos) {
                        uint32_t fanout_value = get_be32(g->chunk_oid_fanout + cur_fanout_pos);

                        if (i != fanout_value)
                                graph_report("commit-graph has incorrect fanout value: fanout[%d] = %u != %u",
                                             cur_fanout_pos, fanout_value, i);
                        cur_fanout_pos++;
                }

                graph_commit = lookup_commit(r, &cur_oid);
                if (!parse_commit_in_graph_one(g, graph_commit))
                        graph_report("failed to parse %s from commit-graph",
                                     oid_to_hex(&cur_oid));
        }

        while (cur_fanout_pos < 256) {
                uint32_t fanout_value = get_be32(g->chunk_oid_fanout + cur_fanout_pos);

                if (g->num_commits != fanout_value)
                        graph_report("commit-graph has incorrect fanout value: fanout[%d] = %u != %u",
                                     cur_fanout_pos, fanout_value, i);

                cur_fanout_pos++;
        }

        if (verify_commit_graph_error & ~VERIFY_COMMIT_GRAPH_ERROR_HASH)
                return verify_commit_graph_error;

        for (i = 0; i < g->num_commits; i++) {
                struct commit *graph_commit, *odb_commit;
                struct commit_list *graph_parents, *odb_parents;
                uint32_t max_generation = 0;

                hashcpy(cur_oid.hash, g->chunk_oid_lookup + g->hash_len * i);

                graph_commit = lookup_commit(r, &cur_oid);
                odb_commit = (struct commit *)create_object(r, cur_oid.hash, alloc_commit_node(r));
                if (parse_commit_internal(odb_commit, 0, 0)) {
                        graph_report("failed to parse %s from object database",
                                     oid_to_hex(&cur_oid));
                        continue;
                }

                if (oidcmp(&get_commit_tree_in_graph_one(g, graph_commit)->object.oid,
                           get_commit_tree_oid(odb_commit)))
                        graph_report("root tree OID for commit %s in commit-graph is %s != %s",
                                     oid_to_hex(&cur_oid),
                                     oid_to_hex(get_commit_tree_oid(graph_commit)),
                                     oid_to_hex(get_commit_tree_oid(odb_commit)));

                graph_parents = graph_commit->parents;
                odb_parents = odb_commit->parents;

                while (graph_parents) {
                        if (odb_parents == NULL) {
                                graph_report("commit-graph parent list for commit %s is too long",
                                             oid_to_hex(&cur_oid));
                                break;
                        }

                        if (oidcmp(&graph_parents->item->object.oid, &odb_parents->item->object.oid))
                                graph_report("commit-graph parent for %s is %s != %s",
                                             oid_to_hex(&cur_oid),
                                             oid_to_hex(&graph_parents->item->object.oid),
                                             oid_to_hex(&odb_parents->item->object.oid));

                        if (graph_parents->item->generation > max_generation)
                                max_generation = graph_parents->item->generation;

                        graph_parents = graph_parents->next;
                        odb_parents = odb_parents->next;
                }

                if (odb_parents != NULL)
                        graph_report("commit-graph parent list for commit %s terminates early",
                                     oid_to_hex(&cur_oid));

                if (!graph_commit->generation) {
                        if (generation_zero == GENERATION_NUMBER_EXISTS)
                                graph_report("commit-graph has generation number zero for commit %s, but non-zero elsewhere",
                                             oid_to_hex(&cur_oid));
                        generation_zero = GENERATION_ZERO_EXISTS;
                } else if (generation_zero == GENERATION_ZERO_EXISTS)
                        graph_report("commit-graph has non-zero generation number for commit %s, but zero elsewhere",
                                     oid_to_hex(&cur_oid));

                if (generation_zero == GENERATION_ZERO_EXISTS)
                        continue;

                /*
                 * If one of our parents has generation GENERATION_NUMBER_MAX, then
                 * our generation is also GENERATION_NUMBER_MAX. Decrement to avoid
                 * extra logic in the following condition.
                 */
                if (max_generation == GENERATION_NUMBER_MAX)
                        max_generation--;

                if (graph_commit->generation != max_generation + 1)
                        graph_report("commit-graph generation for commit %s is %u != %u",
                                     oid_to_hex(&cur_oid),
                                     graph_commit->generation,
                                     max_generation + 1);

                if (graph_commit->date != odb_commit->date)
                        graph_report("commit date for commit %s in commit-graph is %"PRItime" != %"PRItime,
                                     oid_to_hex(&cur_oid),
                                     graph_commit->date,
                                     odb_commit->date);
        }

        return verify_commit_graph_error;
}

void free_commit_graph(struct commit_graph *g)
{
        if (!g)
                return;
        if (g->graph_fd >= 0) {
                munmap((void *)g->data, g->data_len);
                g->data = NULL;
                close(g->graph_fd);
        }
        free(g);
}