Revert "diff-delta: produce optimal pack data"

This reverts 6b7d25d97bdb8a26719f90d17ff5c9720be68762 commit.

It turns out that the new algorithm has a really bad corner
case, that literally spends minutes for inputs that takes less
than a quater seconds to delta with the old algorithm.  The
resulting delta is 50% smaller which is admirable, but the
performance degradation is simply unacceptable for unconditional
use.

Some example cases are these blobs in Linux 2.6 repository:

    4917ec509720a42846d513addc11cbd25e0e3c4f
    9af06ba723df75fed49f7ccae5b6c9c34bc5115f
    dfc9cd58dc065d17030d875d3fea6e7862ede143

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

diff --git a/diff-delta.c b/diff-delta.c
index 27f83a0..2ed5984 100644 (file)
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -20,11 +20,21 @@
 
 #include <stdlib.h>
 #include <string.h>
+#include <zlib.h>
 #include "delta.h"
 
 
+/* block size: min = 16, max = 64k, power of 2 */
+#define BLK_SIZE 16
+
+#define MIN(a, b) ((a) < (b) ? (a) : (b))
+
+#define GR_PRIME 0x9e370001
+#define HASH(v, shift) (((unsigned int)(v) * GR_PRIME) >> (shift))
+
 struct index {
        const unsigned char *ptr;
+       unsigned int val;
        struct index *next;
 };
 
@@ -32,21 +42,21 @@ static struct index ** delta_index(const unsigned char *buf,
                                   unsigned long bufsize,
                                   unsigned int *hash_shift)
 {
-       unsigned long hsize;
-       unsigned int hshift, i;
+       unsigned int hsize, hshift, entries, blksize, i;
        const unsigned char *data;
        struct index *entry, **hash;
        void *mem;
 
        /* determine index hash size */
-       hsize = bufsize / 4;
-       for (i = 8; (1 << i) < hsize && i < 16; i++);
+       entries = (bufsize + BLK_SIZE - 1) / BLK_SIZE;
+       hsize = entries / 4;
+       for (i = 4; (1 << i) < hsize && i < 16; i++);
        hsize = 1 << i;
-       hshift = i - 8;
+       hshift = 32 - i;
        *hash_shift = hshift;
 
        /* allocate lookup index */
-       mem = malloc(hsize * sizeof(*hash) + bufsize * sizeof(*entry));
+       mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
        if (!mem)
                return NULL;
        hash = mem;
@@ -54,12 +64,17 @@ static struct index ** delta_index(const unsigned char *buf,
        memset(hash, 0, hsize * sizeof(*hash));
 
        /* then populate it */
-       data = buf + bufsize - 2;
-       while (data > buf) {
-               entry->ptr = --data;
-               i = data[0] ^ data[1] ^ (data[2] << hshift);
+       data = buf + entries * BLK_SIZE - BLK_SIZE;
+       blksize = bufsize - (data - buf);
+       while (data >= buf) {
+               unsigned int val = adler32(0, data, blksize);
+               i = HASH(val, hshift);
+               entry->ptr = data;
+               entry->val = val;
                entry->next = hash[i];
                hash[i] = entry++;
+               blksize = BLK_SIZE;
+               data -= BLK_SIZE;
        }
 
        return hash;
@@ -126,27 +141,29 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 
        while (data < top) {
                unsigned int moff = 0, msize = 0;
-               if (data + 2 < top) {
-                       i = data[0] ^ data[1] ^ (data[2] << hash_shift);
-                       for (entry = hash[i]; entry; entry = entry->next) {
-                               const unsigned char *ref = entry->ptr;
-                               const unsigned char *src = data;
-                               unsigned int ref_size = ref_top - ref;
-                               if (ref_size > top - src)
-                                       ref_size = top - src;
-                               if (ref_size > 0x10000)
-                                       ref_size = 0x10000;
-                               if (ref_size <= msize)
+               unsigned int blksize = MIN(top - data, BLK_SIZE);
+               unsigned int val = adler32(0, data, blksize);
+               i = HASH(val, hash_shift);
+               for (entry = hash[i]; entry; entry = entry->next) {
+                       const unsigned char *ref = entry->ptr;
+                       const unsigned char *src = data;
+                       unsigned int ref_size = ref_top - ref;
+                       if (entry->val != val)
+                               continue;
+                       if (ref_size > top - src)
+                               ref_size = top - src;
+                       while (ref_size && *src++ == *ref) {
+                               ref++;
+                               ref_size--;
+                       }
+                       ref_size = ref - entry->ptr;
+                       if (ref_size > msize) {
+                               /* this is our best match so far */
+                               moff = entry->ptr - ref_data;
+                               msize = ref_size;
+                               if (msize >= 0x10000) {
+                                       msize = 0x10000;
                                        break;
-                               while (ref_size && *src++ == *ref) {
-                                       ref++;
-                                       ref_size--;
-                               }
-                               ref_size = ref - entry->ptr;
-                               if (msize < ref - entry->ptr) {
-                                       /* this is our best match so far */
-                                       msize = ref - entry->ptr;
-                                       moff = entry->ptr - ref_data;
                                }
                        }
                }