const char *commit_type = "commit";
 
-struct commit *lookup_commit_reference_gently(const unsigned char *sha1,
+struct commit *lookup_commit_reference_gently(const struct object_id *oid,
                                              int quiet)
 {
-       struct object *obj = deref_tag(parse_object(sha1), NULL, 0);
+       struct object *obj = deref_tag(parse_object(oid), NULL, 0);
 
        if (!obj)
                return NULL;
        return object_as_type(obj, OBJ_COMMIT, quiet);
 }
 
-struct commit *lookup_commit_reference(const unsigned char *sha1)
+struct commit *lookup_commit_reference(const struct object_id *oid)
 {
-       return lookup_commit_reference_gently(sha1, 0);
+       return lookup_commit_reference_gently(oid, 0);
 }
 
-struct commit *lookup_commit_or_die(const unsigned char *sha1, const char *ref_name)
+struct commit *lookup_commit_or_die(const struct object_id *oid, const char *ref_name)
 {
-       struct commit *c = lookup_commit_reference(sha1);
+       struct commit *c = lookup_commit_reference(oid);
        if (!c)
                die(_("could not parse %s"), ref_name);
-       if (hashcmp(sha1, c->object.oid.hash)) {
+       if (oidcmp(oid, &c->object.oid)) {
                warning(_("%s %s is not a commit!"),
-                       ref_name, sha1_to_hex(sha1));
+                       ref_name, oid_to_hex(oid));
        }
        return c;
 }
 
-struct commit *lookup_commit(const unsigned char *sha1)
+struct commit *lookup_commit(const struct object_id *oid)
 {
-       struct object *obj = lookup_object(sha1);
+       struct object *obj = lookup_object(oid->hash);
        if (!obj)
-               return create_object(sha1, alloc_commit_node());
+               return create_object(oid->hash, alloc_commit_node());
        return object_as_type(obj, OBJ_COMMIT, 0);
 }
 
 
        if (get_sha1_committish(name, oid.hash))
                return NULL;
-       commit = lookup_commit_reference(oid.hash);
+       commit = lookup_commit_reference(&oid);
        if (parse_commit(commit))
                return NULL;
        return commit;
 }
 
-static unsigned long parse_commit_date(const char *buf, const char *tail)
+static timestamp_t parse_commit_date(const char *buf, const char *tail)
 {
        const char *dateptr;
 
                /* nada */;
        if (buf >= tail)
                return 0;
-       /* dateptr < buf && buf[-1] == '\n', so strtoul will stop at buf-1 */
-       return strtoul(dateptr, NULL, 10);
+       /* dateptr < buf && buf[-1] == '\n', so parsing will stop at buf-1 */
+       return parse_timestamp(dateptr, NULL, 10);
 }
 
 static struct commit_graft **commit_graft;
 
 static int read_graft_file(const char *graft_file)
 {
-       FILE *fp = fopen(graft_file, "r");
+       FILE *fp = fopen_or_warn(graft_file, "r");
        struct strbuf buf = STRBUF_INIT;
        if (!fp)
                return -1;
        return ret;
 }
 
-int unregister_shallow(const unsigned char *sha1)
+int unregister_shallow(const struct object_id *oid)
 {
-       int pos = commit_graft_pos(sha1);
+       int pos = commit_graft_pos(oid->hash);
        if (pos < 0)
                return -1;
        if (pos + 1 < commit_graft_nr)
-               memmove(commit_graft + pos, commit_graft + pos + 1,
-                               sizeof(struct commit_graft *)
-                               * (commit_graft_nr - pos - 1));
+               MOVE_ARRAY(commit_graft + pos, commit_graft + pos + 1,
+                          commit_graft_nr - pos - 1);
        commit_graft_nr--;
        return 0;
 }
 {
        struct commit_buffer *v = buffer_slab_peek(&buffer_slab, commit);
        if (v) {
-               free(v->buffer);
-               v->buffer = NULL;
+               FREE_AND_NULL(v->buffer);
                v->size = 0;
        }
 }
        if (get_sha1_hex(bufptr + 5, parent.hash) < 0)
                return error("bad tree pointer in commit %s",
                             oid_to_hex(&item->object.oid));
-       item->tree = lookup_tree(parent.hash);
+       item->tree = lookup_tree(&parent);
        bufptr += tree_entry_len + 1; /* "tree " + "hex sha1" + "\n" */
        pptr = &item->parents;
 
                 */
                if (graft && (graft->nr_parent < 0 || grafts_replace_parents))
                        continue;
-               new_parent = lookup_commit(parent.hash);
+               new_parent = lookup_commit(&parent);
                if (new_parent)
                        pptr = &commit_list_insert(new_parent, pptr)->next;
        }
                int i;
                struct commit *new_parent;
                for (i = 0; i < graft->nr_parent; i++) {
-                       new_parent = lookup_commit(graft->parent[i].hash);
+                       new_parent = lookup_commit(&graft->parent[i]);
                        if (!new_parent)
                                continue;
                        pptr = &commit_list_insert(new_parent, pptr)->next;
 
 static int commit_list_compare_by_date(const void *a, const void *b)
 {
-       unsigned long a_date = ((const struct commit_list *)a)->item->date;
-       unsigned long b_date = ((const struct commit_list *)b)->item->date;
+       timestamp_t a_date = ((const struct commit_list *)a)->item->date;
+       timestamp_t b_date = ((const struct commit_list *)b)->item->date;
        if (a_date < b_date)
                return 1;
        if (a_date > b_date)
 
        for (i = 0; i < a->nr; i++) {
                object = a->objects[i].item;
-               commit = lookup_commit_reference_gently(object->oid.hash, 1);
+               commit = lookup_commit_reference_gently(&object->oid, 1);
                if (commit)
                        clear_commit_marks(commit, mark);
        }
        const char *ident_line;
        size_t ident_len;
        char *date_end;
-       unsigned long date;
+       timestamp_t date;
 
        ident_line = find_commit_header(buffer, "author", &ident_len);
        if (!ident_line)
            !ident.date_begin || !ident.date_end)
                goto fail_exit; /* malformed "author" line */
 
-       date = strtoul(ident.date_begin, &date_end, 10);
+       date = parse_timestamp(ident.date_begin, &date_end, 10);
        if (date_end != ident.date_end)
                goto fail_exit; /* malformed date */
        *(author_date_slab_at(author_date, commit)) = date;
 {
        const struct commit *a = a_, *b = b_;
        struct author_date_slab *author_date = cb_data;
-       unsigned long a_date = *(author_date_slab_at(author_date, a));
-       unsigned long b_date = *(author_date_slab_at(author_date, b));
+       timestamp_t a_date = *(author_date_slab_at(author_date, a));
+       timestamp_t b_date = *(author_date_slab_at(author_date, b));
 
        /* newer commits with larger date first */
        if (a_date < b_date)
        struct object_id oid;
        if (get_sha1(name, oid.hash))
                return NULL;
-       obj = parse_object(oid.hash);
+       obj = parse_object(&oid);
        commit = (struct commit *)peel_to_type(name, 0, obj, OBJ_COMMIT);
        if (commit && !commit->util)
                set_merge_remote_desc(commit, name, obj);