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#include <stdint.h>
#include "crc16.h"
/*
* 16 12 5
* this is the CCITT CRC 16 polynomial X + X + X + 1.
* This works out to be 0x1021, but the way the algorithm works
* lets us use 0x8408 (the reverse of the bit pattern). The high
* bit is always assumed to be set, thus we only use 16 bits to
* represent the 17 bit value.
* The low bit contains the first bit of the data on the line.
* The low byte(bit) contains the first bit of the CRC on the line.
*/
#define POLY 0x8408
uint16_t calc_crc16(uint8_t *data_p, int length)
{
int i;
uint16_t data;
uint16_t crc = 0xffff;
while (length--) {
data = *data_p++;
for (i = 0; i < 8; i++) {
if ((crc & 1) ^ (data & 1))
crc = (crc >> 1) ^ POLY;
else
crc >>= 1;
data >>= 1;
}
}
crc = ~crc;
return (crc);
}
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