git-pack-objects: do the delta search in reverse size order

Starting from big objects and going backwards means that we end up
picking a delta that goes from a bigger object to a smaller one.  That's
advantageous for two reasons: the bigger object is likely the newer one
(since things tend to grow, rather than shrink), and doing a delete
tends to be smaller than doing an add.

So the deltas don't tend to be top-of-tree, and the packed end result is
just slightly smaller.

diff --git a/pack-objects.c b/pack-objects.c
index aa6cb6d35e6b6935c4d2e2138558384f9d82eca6..286e4501c62941b0979f24223d857c88c1c923ee 100644 (file)
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -280,18 +280,18 @@ struct unpacked {
 };
 
 /*
- * We search for deltas in a list sorted by type and by size, and
- * walk the "old" chain backwards in the list, so if the type
- * has changed or the size difference is too big, there's no point
- * in even continuing the walk, since the other old objects are
- * going to be even smaller or of a different type. So return -1
- * once we determine that there's no point even trying.
+ * We search for deltas _backwards_ in a list sorted by type and
+ * by size, so that we see progressively smaller and smaller files.
+ * That's because we prefer deltas to be from the bigger file
+ * to the smaller - deletes are potentially cheaper, but perhaps
+ * more importantly, the bigger file is likely the more recent
+ * one.
  */
 static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_depth)
 {
        struct object_entry *cur_entry = cur->entry;
        struct object_entry *old_entry = old->entry;
-       unsigned long size, oldsize, delta_size;
+       unsigned long size, oldsize, delta_size, sizediff;
        long max_size;
        void *delta_buf;
 
@@ -299,12 +299,12 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de
        if (cur_entry->type != old_entry->type)
                return -1;
 
-       /* Size is guaranteed to be larger than or equal to oldsize */
        size = cur_entry->size;
        if (size < 50)
                return -1;
        oldsize = old_entry->size;
-       if (size - oldsize > oldsize / 4)
+       sizediff = oldsize > size ? oldsize - size : size - oldsize;
+       if (sizediff > size / 8)
                return -1;
        if (old_entry->depth >= max_depth)
                return 0;
@@ -332,13 +332,14 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de
 
 static void find_deltas(struct object_entry **list, int window, int depth)
 {
-       unsigned int i;
+       int i, idx;
        unsigned int array_size = window * sizeof(struct unpacked);
        struct unpacked *array = xmalloc(array_size);
 
        memset(array, 0, array_size);
-       for (i = 0; i < nr_objects; i++) {
-               unsigned int idx = i % window;
+       i = nr_objects;
+       idx = 0;
+       while (--i >= 0) {
                struct object_entry *entry = list[i];
                struct unpacked *n = array + idx;
                unsigned long size;
@@ -362,6 +363,9 @@ static void find_deltas(struct object_entry **list, int window, int depth)
                        if (try_delta(n, m, depth) < 0)
                                break;
                }
+               idx++;
+               if (idx >= window)
+                       idx = 0;
        }
 }