[PATCH 6/6] pack-objects: reuse deltas for thin "have" objects

[Jeff King <peff@peff.net>]

When we calculate the "wants" and "haves" for a pack, we
only add the objects in the boundary commits as preferred
bases. However, we know that every object reachable from the
"haves" could be a preferred base.

We probably don't want to add these to our preferred base
list, because they would clog the delta-search window.
However, there is no reason we cannot reuse an on-disk delta
against such a deep "have" base, avoiding the delta search
for that object altogether.

Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/pack-objects.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 7950c43..92bd682 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -53,6 +53,7 @@ static unsigned long pack_size_limit;
 static int depth = 50;
 static int delta_search_threads;
 static int pack_to_stdout;
+static int thin = 0;
 static int num_preferred_base;
 static struct progress *progress_state;
 static int pack_compression_level = Z_DEFAULT_COMPRESSION;
@@ -1419,6 +1420,20 @@ static void check_object(struct object_entry *entry)
 			base_entry->delta_child = entry;
 			unuse_pack(&w_curs);
 			return;
+		} else if(thin && base_ref && bitmap_have(base_ref)) {
+			entry->type = entry->in_pack_type;
+			entry->delta_size = entry->size;
+			/*
+			 * XXX we'll leak this, but it's probably OK
+			 * since we'll exit immediately after the packing
+			 * is done
+			 */
+			entry->delta = xcalloc(1, sizeof(*entry->delta));
+			hashcpy(entry->delta->idx.sha1, base_ref);
+			entry->delta->preferred_base = 1;
+			entry->delta->filled = 1;
+			unuse_pack(&w_curs);
+			return;
 		}
 
 		if (entry->type) {
@@ -2559,7 +2574,6 @@ static int option_parse_ulong(const struct option *opt,
 int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 {
 	int use_internal_rev_list = 0;
-	int thin = 0;
 	int all_progress_implied = 0;
 	const char *rp_av[6];
 	int rp_ac = 0;
-- 
1.9.1.601.g7ec968e