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/*
** crc.c - calculate CRC
**
** See Copyright Notice in mruby.h
*/
#include <limits.h>
#include <stdint.h>
#include <stddef.h>
/* Calculate CRC (CRC-16-CCITT)
**
** 0000_0000_0000_0000_0000_0000_0000_0000
** ^|------- CRC -------|- work --|
** carry
*/
#define CRC_16_CCITT 0x11021ul /* x^16+x^12+x^5+1 */
#define CRC_XOR_PATTERN (CRC_16_CCITT << 8)
#define CRC_CARRY_BIT (0x01000000)
uint16_t
calc_crc_16_ccitt(const uint8_t *src, size_t nbytes, uint16_t crc)
{
size_t ibyte;
uint32_t ibit;
uint32_t crcwk = crc << 8;
for (ibyte = 0; ibyte < nbytes; ibyte++) {
crcwk |= *src++;
for (ibit = 0; ibit < CHAR_BIT; ibit++) {
crcwk <<= 1;
if (crcwk & CRC_CARRY_BIT) {
crcwk ^= CRC_XOR_PATTERN;
}
}
}
return (uint16_t)(crcwk >> 8);
}
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