From eae3fe5e509f3d3890bc99015cb02f9b67aa501c Mon Sep 17 00:00:00 2001 From: Junio C Hamano Date: Fri, 24 Feb 2006 00:54:59 -0800 Subject: [PATCH] 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 --- diff-delta.c | 77 ++++++++++++++++++++++++++++++++-------------------- 1 file changed, 47 insertions(+), 30 deletions(-) diff --git a/diff-delta.c b/diff-delta.c index 27f83a085..2ed5984b1 100644 --- a/diff-delta.c +++ b/diff-delta.c @@ -20,11 +20,21 @@ #include #include +#include #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; } } } -- 2.26.2