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/*
* diff-delta.c: generate a delta between two buffers
*
* Many parts of this file have been lifted from LibXDiff version 0.10.
* http://www.xmailserver.org/xdiff-lib.html
*
* LibXDiff was written by Davide Libenzi <davidel@xmailserver.org>
* Copyright (C) 2003 Davide Libenzi
*
* Many mods for GIT usage by Nicolas Pitre <nico@cam.org>, (C) 2005.
*
* This file is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* Use of this within git automatically means that the LGPL
* licensing gets turned into GPLv2 within this project.
*/
#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;
};
static struct index ** delta_index(const unsigned char *buf,
unsigned long bufsize,
unsigned long trg_bufsize,
unsigned int *hash_shift)
{
unsigned int i, hsize, hshift, hlimit, entries, *hash_count;
const unsigned char *data;
struct index *entry, **hash;
void *mem;
/* determine index hash size */
entries = bufsize / BLK_SIZE;
hsize = entries / 4;
for (i = 4; (1 << i) < hsize && i < 31; i++);
hsize = 1 << i;
hshift = 32 - i;
*hash_shift = hshift;
/* allocate lookup index */
mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
if (!mem)
return NULL;
hash = mem;
entry = mem + hsize * sizeof(*hash);
memset(hash, 0, hsize * sizeof(*hash));
/* allocate an array to count hash entries */
hash_count = calloc(hsize, sizeof(*hash_count));
if (!hash_count) {
free(hash);
return NULL;
}
/* then populate the index */
data = buf + entries * BLK_SIZE - BLK_SIZE;
while (data >= buf) {
unsigned int val = adler32(0, data, BLK_SIZE);
i = HASH(val, hshift);
entry->ptr = data;
entry->val = val;
entry->next = hash[i];
hash[i] = entry++;
hash_count[i]++;
data -= BLK_SIZE;
}
/*
* Determine a limit on the number of entries in the same hash
* bucket. This guard us against patological data sets causing
* really bad hash distribution with most entries in the same hash
* bucket that would bring us to O(m*n) computing costs (m and n
* corresponding to reference and target buffer sizes).
*
* The more the target buffer is large, the more it is important to
* have small entry lists for each hash buckets. With such a limit
* the cost is bounded to something more like O(m+n).
*/
hlimit = (1 << 26) / trg_bufsize;
if (hlimit < 4*BLK_SIZE)
hlimit = 4*BLK_SIZE;
/*
* Now make sure none of the hash buckets has more entries than
* we're willing to test. Otherwise we cull the entry list
* uniformly to still preserve a good repartition across
* the reference buffer.
*/
for (i = 0; i < hsize; i++) {
if (hash_count[i] < hlimit)
continue;
entry = hash[i];
do {
struct index *keep = entry;
int skip = hash_count[i] / hlimit / 2;
do {
entry = entry->next;
} while(--skip && entry);
keep->next = entry;
} while(entry);
}
free(hash_count);
return hash;
}
/* provide the size of the copy opcode given the block offset and size */
#define COPYOP_SIZE(o, s) \
(!!(o & 0xff) + !!(o & 0xff00) + !!(o & 0xff0000) + !!(o & 0xff000000) + \
!!(s & 0xff) + !!(s & 0xff00) + 1)
/* the maximum size for any opcode */
#define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)
void *diff_delta(void *from_buf, unsigned long from_size,
void *to_buf, unsigned long to_size,
unsigned long *delta_size,
unsigned long max_size)
{
unsigned int i, outpos, outsize, hash_shift;
int inscnt;
const unsigned char *ref_data, *ref_top, *data, *top;
unsigned char *out;
struct index *entry, **hash;
if (!from_size || !to_size)
return NULL;
hash = delta_index(from_buf, from_size, to_size, &hash_shift);
if (!hash)
return NULL;
outpos = 0;
outsize = 8192;
if (max_size && outsize >= max_size)
outsize = max_size + MAX_OP_SIZE + 1;
out = malloc(outsize);
if (!out) {
free(hash);
return NULL;
}
ref_data = from_buf;
ref_top = from_buf + from_size;
data = to_buf;
top = to_buf + to_size;
/* store reference buffer size */
out[outpos++] = from_size;
from_size >>= 7;
while (from_size) {
out[outpos - 1] |= 0x80;
out[outpos++] = from_size;
from_size >>= 7;
}
/* store target buffer size */
out[outpos++] = to_size;
to_size >>= 7;
while (to_size) {
out[outpos - 1] |= 0x80;
out[outpos++] = to_size;
to_size >>= 7;
}
inscnt = 0;
while (data < top) {
unsigned int moff = 0, msize = 0;
if (data + BLK_SIZE <= top) {
unsigned int val = adler32(0, data, BLK_SIZE);
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;
if (ref_size > 0x10000)
ref_size = 0x10000;
if (ref_size <= msize)
break;
while (ref_size-- && *src++ == *ref)
ref++;
if (msize < ref - entry->ptr) {
/* this is our best match so far */
msize = ref - entry->ptr;
moff = entry->ptr - ref_data;
}
}
}
if (!msize || msize < COPYOP_SIZE(moff, msize)) {
if (!inscnt)
outpos++;
out[outpos++] = *data++;
inscnt++;
if (inscnt == 0x7f) {
out[outpos - inscnt - 1] = inscnt;
inscnt = 0;
}
} else {
unsigned char *op;
if (inscnt) {
while (moff && ref_data[moff-1] == data[-1]) {
if (msize == 0x10000)
break;
/* we can match one byte back */
msize++;
moff--;
data--;
outpos--;
if (--inscnt)
continue;
outpos--; /* remove count slot */
inscnt--; /* make it -1 */
break;
}
out[outpos - inscnt - 1] = inscnt;
inscnt = 0;
}
data += msize;
op = out + outpos++;
i = 0x80;
if (moff & 0xff) { out[outpos++] = moff; i |= 0x01; }
moff >>= 8;
if (moff & 0xff) { out[outpos++] = moff; i |= 0x02; }
moff >>= 8;
if (moff & 0xff) { out[outpos++] = moff; i |= 0x04; }
moff >>= 8;
if (moff & 0xff) { out[outpos++] = moff; i |= 0x08; }
if (msize & 0xff) { out[outpos++] = msize; i |= 0x10; }
msize >>= 8;
if (msize & 0xff) { out[outpos++] = msize; i |= 0x20; }
*op = i;
}
if (outpos >= outsize - MAX_OP_SIZE) {
void *tmp = out;
outsize = outsize * 3 / 2;
if (max_size && outsize >= max_size)
outsize = max_size + MAX_OP_SIZE + 1;
if (max_size && outpos > max_size)
out = NULL;
else
out = realloc(out, outsize);
if (!out) {
free(tmp);
free(hash);
return NULL;
}
}
}
if (inscnt)
out[outpos - inscnt - 1] = inscnt;
free(hash);
*delta_size = outpos;
return out;
}
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