static int try_delta(struct unpacked *trg, struct unpacked *src,
		     unsigned max_depth, unsigned long *mem_usage)
{
	struct object_entry *trg_entry = trg->entry;
	struct object_entry *src_entry = src->entry;
	unsigned long trg_size, src_size, delta_size, sizediff, max_size, sz;
	unsigned ref_depth;
	enum object_type type;
	void *delta_buf;

	/* Don't bother doing diffs between different types */
	if (trg_entry->type != src_entry->type)
		return -1;

	/*
	 * We do not bother to try a delta that we discarded on an
	 * earlier try, but only when reusing delta data.  Note that
	 * src_entry that is marked as the preferred_base should always
	 * be considered, as even if we produce a suboptimal delta against
	 * it, we will still save the transfer cost, as we already know
	 * the other side has it and we won't send src_entry at all.
	 */
	if (reuse_delta && trg_entry->in_pack &&
	    trg_entry->in_pack == src_entry->in_pack &&
	    !src_entry->preferred_base &&
	    trg_entry->in_pack_type != OBJ_REF_DELTA &&
	    trg_entry->in_pack_type != OBJ_OFS_DELTA)
		return 0;

	/* Let's not bust the allowed depth. */
	if (src->depth >= max_depth)
		return 0;

	/* Now some size filtering heuristics. */
	trg_size = trg_entry->size;
	if (!trg_entry->delta) {
		max_size = trg_size/2 - 20;
		ref_depth = 1;
	} else {
		max_size = trg_entry->delta_size;
		ref_depth = trg->depth;
	}
	max_size = (uint64_t)max_size * (max_depth - src->depth) /
						(max_depth - ref_depth + 1);
	if (max_size == 0)
		return 0;
	src_size = src_entry->size;
	sizediff = src_size < trg_size ? trg_size - src_size : 0;
	if (sizediff >= max_size)
		return 0;
	if (trg_size < src_size / 32)
		return 0;

	/* Load data if not already done */
	if (!trg->data) {
		read_lock();
		trg->data = read_sha1_file(trg_entry->idx.sha1, &type, &sz);
		read_unlock();
		if (!trg->data)
			die("object %s cannot be read",
			    sha1_to_hex(trg_entry->idx.sha1));
		if (sz != trg_size)
			die("object %s inconsistent object length (%lu vs %lu)",
			    sha1_to_hex(trg_entry->idx.sha1), sz, trg_size);
		*mem_usage += sz;
	}
	if (!src->data) {
		read_lock();
		src->data = read_sha1_file(src_entry->idx.sha1, &type, &sz);
		read_unlock();
		if (!src->data) {
			if (src_entry->preferred_base) {
				static int warned = 0;
				if (!warned++)
					warning("object %s cannot be read",
						sha1_to_hex(src_entry->idx.sha1));
				/*
				 * Those objects are not included in the
				 * resulting pack.  Be resilient and ignore
				 * them if they can't be read, in case the
				 * pack could be created nevertheless.
				 */
				return 0;
			}
			die("object %s cannot be read",
			    sha1_to_hex(src_entry->idx.sha1));
		}
		if (sz != src_size)
			die("object %s inconsistent object length (%lu vs %lu)",
			    sha1_to_hex(src_entry->idx.sha1), sz, src_size);
		*mem_usage += sz;
	}
	if (!src->index) {
		src->index = create_delta_index(src->data, src_size);
		if (!src->index) {
			static int warned = 0;
			if (!warned++)
				warning("suboptimal pack - out of memory");
			return 0;
		}
		*mem_usage += sizeof_delta_index(src->index);
	}

	delta_buf = create_delta(src->index, trg->data, trg_size, &delta_size, max_size);
	if (!delta_buf)
		return 0;

	if (trg_entry->delta) {
		/* Prefer only shallower same-sized deltas. */
		if (delta_size == trg_entry->delta_size &&
		    src->depth + 1 >= trg->depth) {
			free(delta_buf);
			return 0;
		}
	}

	/*
	 * Handle memory allocation outside of the cache
	 * accounting lock.  Compiler will optimize the strangeness
	 * away when NO_PTHREADS is defined.
	 */
	free(trg_entry->delta_data);
	cache_lock();
	if (trg_entry->delta_data) {
		delta_cache_size -= trg_entry->delta_size;
		trg_entry->delta_data = NULL;
	}
	if (delta_cacheable(src_size, trg_size, delta_size)) {
		delta_cache_size += delta_size;
		cache_unlock();
		trg_entry->delta_data = xrealloc(delta_buf, delta_size);
	} else {
		cache_unlock();
		free(delta_buf);
	}

	trg_entry->delta = src_entry;
	trg_entry->delta_size = delta_size;
	trg->depth = src->depth + 1;

	return 1;
}