PoC code comparing bitwise and bytewise NXP CRC implementations

nxp_crc.c

#include <stdint.h>
#include <stdio.h>

#define CRC_PRESET 0xc7

void nxp_crc(uint8_t *crc, const uint8_t value) {
  /* x^8 + x^4 + x^3 + x^2 + 1 => 0x11d */
  const uint8_t poly = 0x1d;

  *crc ^= value;
  for (int current_bit = 7; current_bit >= 0; current_bit--) {
    int bit_out = (*crc) & 0x80;
    *crc <<= 1;
    if (bit_out)
      *crc ^= poly;
  }
}

void nxp_crc2(uint8_t *crc, const uint8_t value) {
  static const uint8_t nxp_crc_lut[] = {
      0x00, 0x1D, 0x3A, 0x27, 0x74, 0x69, 0x4E, 0x53, 0xE8, 0xF5, 0xD2, 0xCF,
      0x9C, 0x81, 0xA6, 0xBB, 0xCD, 0xD0, 0xF7, 0xEA, 0xB9, 0xA4, 0x83, 0x9E,
      0x25, 0x38, 0x1F, 0x02, 0x51, 0x4C, 0x6B, 0x76, 0x87, 0x9A, 0xBD, 0xA0,
      0xF3, 0xEE, 0xC9, 0xD4, 0x6F, 0x72, 0x55, 0x48, 0x1B, 0x06, 0x21, 0x3C,
      0x4A, 0x57, 0x70, 0x6D, 0x3E, 0x23, 0x04, 0x19, 0xA2, 0xBF, 0x98, 0x85,
      0xD6, 0xCB, 0xEC, 0xF1, 0x13, 0x0E, 0x29, 0x34, 0x67, 0x7A, 0x5D, 0x40,
      0xFB, 0xE6, 0xC1, 0xDC, 0x8F, 0x92, 0xB5, 0xA8, 0xDE, 0xC3, 0xE4, 0xF9,
      0xAA, 0xB7, 0x90, 0x8D, 0x36, 0x2B, 0x0C, 0x11, 0x42, 0x5F, 0x78, 0x65,
      0x94, 0x89, 0xAE, 0xB3, 0xE0, 0xFD, 0xDA, 0xC7, 0x7C, 0x61, 0x46, 0x5B,
      0x08, 0x15, 0x32, 0x2F, 0x59, 0x44, 0x63, 0x7E, 0x2D, 0x30, 0x17, 0x0A,
      0xB1, 0xAC, 0x8B, 0x96, 0xC5, 0xD8, 0xFF, 0xE2, 0x26, 0x3B, 0x1C, 0x01,
      0x52, 0x4F, 0x68, 0x75, 0xCE, 0xD3, 0xF4, 0xE9, 0xBA, 0xA7, 0x80, 0x9D,
      0xEB, 0xF6, 0xD1, 0xCC, 0x9F, 0x82, 0xA5, 0xB8, 0x03, 0x1E, 0x39, 0x24,
      0x77, 0x6A, 0x4D, 0x50, 0xA1, 0xBC, 0x9B, 0x86, 0xD5, 0xC8, 0xEF, 0xF2,
      0x49, 0x54, 0x73, 0x6E, 0x3D, 0x20, 0x07, 0x1A, 0x6C, 0x71, 0x56, 0x4B,
      0x18, 0x05, 0x22, 0x3F, 0x84, 0x99, 0xBE, 0xA3, 0xF0, 0xED, 0xCA, 0xD7,
      0x35, 0x28, 0x0F, 0x12, 0x41, 0x5C, 0x7B, 0x66, 0xDD, 0xC0, 0xE7, 0xFA,
      0xA9, 0xB4, 0x93, 0x8E, 0xF8, 0xE5, 0xC2, 0xDF, 0x8C, 0x91, 0xB6, 0xAB,
      0x10, 0x0D, 0x2A, 0x37, 0x64, 0x79, 0x5E, 0x43, 0xB2, 0xAF, 0x88, 0x95,
      0xC6, 0xDB, 0xFC, 0xE1, 0x5A, 0x47, 0x60, 0x7D, 0x2E, 0x33, 0x14, 0x09,
      0x7F, 0x62, 0x45, 0x58, 0x0B, 0x16, 0x31, 0x2C, 0x97, 0x8A, 0xAD, 0xB0,
      0xE3, 0xFE, 0xD9, 0xC4};
  uint8_t tmp = *crc;
  *crc = nxp_crc_lut[tmp ^ value];
}

#define NUM_ELEMENTS(x) (sizeof(x) / sizeof(*x))

int main(void) {
  uint8_t crc = CRC_PRESET;
  const char test[] = "123456789";
  for (unsigned i = 0; i < NUM_ELEMENTS(test) - 1; ++i) {
    nxp_crc(&crc, test[i]);
  }
  printf("nxp_crc = %u\n", crc);

  crc = CRC_PRESET;
  for (unsigned i = 0; i < NUM_ELEMENTS(test) - 1; ++i) {
    nxp_crc2(&crc, test[i]);
  }
  printf("nxp_crc2 = %u\n", crc);
  return 0;
}