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/* Copyright abandoned; this code is in the public domain. */
/* Provided to GNUnet by peter@horizon.com */
/**
* @file util/crc32.c
* @brief implementation of CRC32
**/
#include <unistd.h>
#include "common.h"
#define POLYNOMIAL (unsigned long)0xedb88320
static unsigned long crc_table[256];
/*
* This routine writes each crc_table entry exactly once,
* with the correct final value. Thus, it is safe to call
* even on a table that someone else is using concurrently.
*/
static void make_crc_table(void) {
unsigned long i, j, h = 1;
crc_table[0] = 0;
for (i = 128; i; i >>= 1) {
h = (h >> 1) ^ ((h & 1) ? POLYNOMIAL : 0);
/* h is now crc_table[i] */
for (j = 0; j < 256; j += 2*i)
crc_table[i+j] = crc_table[j] ^ h;
}
}
/*
* This computes the standard preset and inverted CRC, as used
* by most networking standards. Start by passing in an initial
* chaining value of 0, and then pass in the return value from the
* previous crc32() call. The final return value is the CRC.
* Note that this is a little-endian CRC, which is best used with
* data transmitted lsbit-first, and it should, itself, be appended
* to data in little-endian byte and bit order to preserve the
* property of detecting all burst errors of length 32 bits or less.
*/
unsigned long gbs_crc32(unsigned long crc, const char *buf, size_t len) {
if (crc_table[255] == 0)
make_crc_table();
crc ^= 0xffffffff;
while (len--)
crc = (crc >> 8) ^ crc_table[(crc ^ (unsigned char)*buf++) & 0xff];
return crc ^ 0xffffffff;
}
/* end of crc32.c */
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