diff-delta.c: Rationalize culling of hash buckets

The previous hash bucket culling resulted in a somewhat unpredictable
number of hash bucket entries in the order of magnitude of HASH_LIMIT.

Replace this with a Bresenham-like algorithm leaving us with exactly
HASH_LIMIT entries by uniform culling.

Signed-off-by: David Kastrup <dak@gnu.org>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/diff-delta.c b/diff-delta.c
index 7f8a60de18584fa18dba71a454a6f2300674f47f..9e440a9299b902bc96749ad2c86c08df6492eb1c 100644 (file)
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -207,19 +207,40 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
         * the reference buffer.
         */
        for (i = 0; i < hsize; i++) {
-               if (hash_count[i] < HASH_LIMIT)
+               int acc;
+
+               if (hash_count[i] <= HASH_LIMIT)
                        continue;
+
+               entries -= hash_count[i] - HASH_LIMIT;
+               /* We leave exactly HASH_LIMIT entries in the bucket */
+
                entry = hash[i];
+               acc = 0;
                do {
-                       struct unpacked_index_entry *keep = entry;
-                       int skip = hash_count[i] / HASH_LIMIT;
-                       do {
-                               --entries;
-                               entry = entry->next;
-                       } while(--skip && entry);
-                       ++entries;
-                       keep->next = entry;
-               } while(entry);
+                       acc += hash_count[i] - HASH_LIMIT;
+                       if (acc > 0) {
+                               struct unpacked_index_entry *keep = entry;
+                               do {
+                                       entry = entry->next;
+                                       acc -= HASH_LIMIT;
+                               } while (acc > 0);
+                               keep->next = entry->next;
+                       }
+                       entry = entry->next;
+               } while (entry);
+
+               /* Assume that this loop is gone through exactly
+                * HASH_LIMIT times and is entered and left with
+                * acc==0.  So the first statement in the loop
+                * contributes (hash_count[i]-HASH_LIMIT)*HASH_LIMIT
+                * to the accumulator, and the inner loop consequently
+                * is run (hash_count[i]-HASH_LIMIT) times, removing
+                * one element from the list each time.  Since acc
+                * balances out to 0 at the final run, the inner loop
+                * body can't be left with entry==NULL.  So we indeed
+                * encounter entry==NULL in the outer loop only.
+                */
        }
        free(hash_count);