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#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
uint8_t crc_table[256];
uint8_t flip_byte (uint8_t value)
{
value = ((value >> 1) & 0x55) | ((value & 0x55) << 1);
value = ((value >> 2) & 0x33) | ((value & 0x33) << 2);
value = ((value >> 4) & 0x0F) | ((value & 0x0F) << 4);
return value;
}
uint8_t
crc8 (uint8_t * data, size_t len)
{
unsigned i, j;
uint8_t crc = 0, b;
for (i = 0; i < len; ++i, ++data)
{
b = *data;
for (j = 0; j < 8; j++)
{
if ((crc >> 7) ^ (b & 1))
{
crc = (crc << 1) ^ 7;
}
else
{
crc = crc << 1;
}
b >>= 1;
}
}
return crc;
}
void
make_table (void)
{
unsigned i;
unsigned j;
uint8_t b;
for (i = 0; i < 256; ++i)
{
b = i;
for (j = 0; j < 8; j++)
{
if (b & 1)
{
b = (b >> 1 ) ^ 0xe0;
}
else
{
b = b >> 1;
}
}
crc_table[i] = b;
}
}
uint8_t
crc8_better (uint8_t * data, size_t len)
{
uint8_t crc = 0;
unsigned i;
for (i = 0; i < len; i++, data++)
{
crc = crc_table[crc ^ *data];
}
return flip_byte(crc);
}
int
main (int argc, char *argv[])
{
uint8_t fish[4] = "\5\0\1\301";
size_t l;
make_table ();
for (l=0;l<256;++l) {
printf("0x%02x, ",crc_table[l]);
}
printf("\n");
for (l=0;l<4;++l ) {
printf ("%u %02x %02x %02x\n",(unsigned)l, crc8 (fish, l), crc8_better (fish, l), fish[l]);
}
return 0;
}
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