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/*
* Copyright 2011 Avery Pennarun. All rights reserved.
*
* (This license applies to bupsplit.c and bupsplit.h only.)
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
*
* THIS SOFTWARE IS PROVIDED BY AVERY PENNARUN ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include "bupsplit.h"
#include <stdint.h>
#include <memory.h>
#include <stdlib.h>
#include <stdio.h>
// According to librsync/rollsum.h:
// "We should make this something other than zero to improve the
// checksum algorithm: tridge suggests a prime number."
// apenwarr: I unscientifically tried 0 and 7919, and they both ended up
// slightly worse than the librsync value of 31 for my arbitrary test data.
#define ROLLSUM_CHAR_OFFSET 31
typedef struct {
unsigned s1, s2;
uint8_t window[BUP_WINDOWSIZE];
int wofs;
} Rollsum;
// These formulas are based on rollsum.h in the librsync project.
static void rollsum_add(Rollsum *r, uint8_t drop, uint8_t add)
{
r->s1 += add - drop;
r->s2 += r->s1 - (BUP_WINDOWSIZE * (drop + ROLLSUM_CHAR_OFFSET));
}
static void rollsum_init(Rollsum *r)
{
r->s1 = BUP_WINDOWSIZE * ROLLSUM_CHAR_OFFSET;
r->s2 = BUP_WINDOWSIZE * (BUP_WINDOWSIZE-1) * ROLLSUM_CHAR_OFFSET;
r->wofs = 0;
memset(r->window, 0, BUP_WINDOWSIZE);
}
// For some reason, gcc 4.3 (at least) optimizes badly if find_ofs()
// is static and rollsum_roll is an inline function. Let's use a macro
// here instead to help out the optimizer.
#define rollsum_roll(r, ch) do { \
rollsum_add((r), (r)->window[(r)->wofs], (ch)); \
(r)->window[(r)->wofs] = (ch); \
(r)->wofs = ((r)->wofs + 1) % BUP_WINDOWSIZE; \
} while (0)
static uint32_t rollsum_digest(Rollsum *r)
{
return (r->s1 << 16) | (r->s2 & 0xffff);
}
uint32_t
bupsplit_sum(uint8_t *buf, size_t ofs, size_t len)
{
size_t count;
Rollsum r;
rollsum_init(&r);
for (count = ofs; count < len; count++)
rollsum_roll(&r, buf[count]);
return rollsum_digest(&r);
}
int bupsplit_find_ofs(const unsigned char *buf, int len, int *bits)
{
Rollsum r;
int count;
rollsum_init(&r);
for (count = 0; count < len; count++)
{
rollsum_roll(&r, buf[count]);
if ((r.s2 & (BUP_BLOBSIZE-1)) == ((~0) & (BUP_BLOBSIZE-1)))
{
if (bits)
{
unsigned rsum = rollsum_digest(&r);
*bits = BUP_BLOBBITS;
rsum >>= BUP_BLOBBITS;
for (*bits = BUP_BLOBBITS; (rsum >>= 1) & 1; (*bits)++)
;
}
return count+1;
}
}
return 0;
}
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