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