pack-objects: simplify "thin" pack.

There was a misguided logic to overly prefer using objects that
we are not going to pack as the base object.  This was
unnecessary.  It does not matter to the unpacking side where the
base object is -- it matters more to make the resulting delta
smaller.

Signed-off-by: Junio C Hamano <junkio@cox.net>

pack-objects.c

@@ -32,9 +32,6 @@ struct object_entry {
 				 * be used as the base objectto delta huge
 				 * objects against.
 				 */
-	int based_on_preferred;	/* current delta candidate is a preferred
-				 * one, or delta against a preferred one.
-				 */
 };
 
 /*
@@ -824,8 +821,6 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de
 {
 	struct object_entry *cur_entry = cur->entry;
 	struct object_entry *old_entry = old->entry;
-	int old_preferred = (old_entry->preferred_base ||
-			     old_entry->based_on_preferred);
 	unsigned long size, oldsize, delta_size, sizediff;
 	long max_size;
 	void *delta_buf;
@@ -867,27 +862,8 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de
 	 * delete).
 	 */
 	max_size = size / 2 - 20;
-	if (cur_entry->delta) {
-		if (cur_entry->based_on_preferred) {
-			if (old_preferred)
-				max_size = cur_entry->delta_size-1;
-			else
-				/* trying with non-preferred one when we
-				 * already have a delta based on preferred
-				 * one is pointless.
-				 */
-				return -1;
-		}
-		else if (!old_preferred)
-			max_size = cur_entry->delta_size-1;
-		else
-			/* otherwise...  even if delta with a
-			 * preferred one produces a bigger result than
-			 * what we currently have, which is based on a
-			 * non-preferred one, it is OK.
-			 */
-			;
-	}
+	if (cur_entry->delta)
+		max_size = cur_entry->delta_size-1;
 	if (sizediff >= max_size)
 		return -1;
 	delta_buf = diff_delta(old->data, oldsize,
@@ -897,7 +873,6 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de
 	cur_entry->delta = old_entry;
 	cur_entry->delta_size = delta_size;
 	cur_entry->depth = old_entry->depth + 1;
-	cur_entry->based_on_preferred = old_preferred;
 	free(delta_buf);
 	return 0;
 }
@@ -966,6 +941,15 @@ static void find_deltas(struct object_entry **list, int window, int depth)
 			if (try_delta(n, m, depth) < 0)
 				break;
 		}
+#if 0
+		/* if we made n a delta, and if n is already at max
+		 * depth, leaving it in the window is pointless.  we
+		 * should evict it first.
+		 * ... in theory only; somehow this makes things worse.
+		 */
+		if (entry->delta && depth <= entry->depth)
+			continue;
+#endif
 		idx++;
 		if (idx >= window)
 			idx = 0;