improve depth heuristic for maximum delta size

This provides a linear decrement on the penalty related to delta depth
instead of being an 1/x function.  With this another 5% reduction is
observed on packs for both the GIT repo and the Linux kernel repo, as
well as fixing a pack size regression in another sample repo I have.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/pack-objects.c b/pack-objects.c
index b430b02..3375179 100644 (file)
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -1037,9 +1037,12 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
        if (src_entry->depth >= max_depth)
                return 0;
 
-       /* Now some size filtering euristics. */
+       /* Now some size filtering heuristics. */
        size = trg_entry->size;
-       max_size = (size/2 - 20) / (src_entry->depth + 1);
+       max_size = size/2 - 20;
+       max_size = max_size * (max_depth - src_entry->depth) / max_depth;
+       if (max_size == 0)
+               return 0;
        if (trg_entry->delta && trg_entry->delta_size <= max_size)
                max_size = trg_entry->delta_size-1;
        src_size = src_entry->size;