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Revert "diff-delta: produce optimal pack data"

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This reverts 6b7d25d97b 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>
This commit is contained in:
Junio C Hamano 2006-02-24 00:54:59 -08:00
parent 581845f0b8
commit eae3fe5e50
1 changed files with 47 additions and 30 deletions
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53
diff-delta.c
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@ -20,11 +20,21 @@
#include <stdlib.h> #include <stdlib.h>
#include <string.h> #include <string.h>
#include <zlib.h>
#include "delta.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 { struct index {
const unsigned char *ptr; const unsigned char *ptr;
unsigned int val;
struct index *next; struct index *next;
}; };
@ -32,21 +42,21 @@ static struct index ** delta_index(const unsigned char *buf,
unsigned long bufsize, unsigned long bufsize,
unsigned int *hash_shift) unsigned int *hash_shift)
{ {
unsigned long hsize; unsigned int hsize, hshift, entries, blksize, i;
unsigned int hshift, i;
const unsigned char *data; const unsigned char *data;
struct index *entry, **hash; struct index *entry, **hash;
void *mem; void *mem;
/* determine index hash size */ /* determine index hash size */
hsize = bufsize / 4; entries = (bufsize + BLK_SIZE - 1) / BLK_SIZE;
for (i = 8; (1 << i) < hsize && i < 16; i++); hsize = entries / 4;
for (i = 4; (1 << i) < hsize && i < 16; i++);
hsize = 1 << i; hsize = 1 << i;
hshift = i - 8; hshift = 32 - i;
*hash_shift = hshift; *hash_shift = hshift;
/* allocate lookup index */ /* allocate lookup index */
mem = malloc(hsize * sizeof(*hash) + bufsize * sizeof(*entry)); mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
if (!mem) if (!mem)
return NULL; return NULL;
hash = mem; hash = mem;
@ -54,12 +64,17 @@ static struct index ** delta_index(const unsigned char *buf,
memset(hash, 0, hsize * sizeof(*hash)); memset(hash, 0, hsize * sizeof(*hash));
/* then populate it */ /* then populate it */
data = buf + bufsize - 2; data = buf + entries * BLK_SIZE - BLK_SIZE;
while (data > buf) { blksize = bufsize - (data - buf);
entry->ptr = --data; while (data >= buf) {
i = data[0] ^ data[1] ^ (data[2] << hshift); unsigned int val = adler32(0, data, blksize);
i = HASH(val, hshift);
entry->ptr = data;
entry->val = val;
entry->next = hash[i]; entry->next = hash[i];
hash[i] = entry++; hash[i] = entry++;
blksize = BLK_SIZE;
data -= BLK_SIZE;
} }
return hash; return hash;
@ -126,27 +141,29 @@ void *diff_delta(void *from_buf, unsigned long from_size,
while (data < top) { while (data < top) {
unsigned int moff = 0, msize = 0; unsigned int moff = 0, msize = 0;
if (data + 2 < top) { unsigned int blksize = MIN(top - data, BLK_SIZE);
i = data[0] ^ data[1] ^ (data[2] << hash_shift); unsigned int val = adler32(0, data, blksize);
i = HASH(val, hash_shift);
for (entry = hash[i]; entry; entry = entry->next) { for (entry = hash[i]; entry; entry = entry->next) {
const unsigned char *ref = entry->ptr; const unsigned char *ref = entry->ptr;
const unsigned char *src = data; const unsigned char *src = data;
unsigned int ref_size = ref_top - ref; unsigned int ref_size = ref_top - ref;
if (entry->val != val)
continue;
if (ref_size > top - src) if (ref_size > top - src)
ref_size = top - src; ref_size = top - src;
if (ref_size > 0x10000)
ref_size = 0x10000;
if (ref_size <= msize)
break;
while (ref_size && *src++ == *ref) { while (ref_size && *src++ == *ref) {
ref++; ref++;
ref_size--; ref_size--;
} }
ref_size = ref - entry->ptr; ref_size = ref - entry->ptr;
if (msize < ref - entry->ptr) { if (ref_size > msize) {
/* this is our best match so far */ /* this is our best match so far */
msize = ref - entry->ptr;
moff = entry->ptr - ref_data; moff = entry->ptr - ref_data;
msize = ref_size;
if (msize >= 0x10000) {
msize = 0x10000;
break;
} }
} }
} }