crc16 branchless

crc16_branchless.c

#include <stdint.h>
// SEE https://godbolt.org/z/KroE8d
#ifndef TABLE
  const uint16_t poloynom1 = 0x1021;

  uint16_t crc_update(uint16_t signature, uint8_t inputByte) {
      signature ^= inputByte << 8; // 16 Bit CRC
      uint8_t i = 8;
      while (i-- > 0) { // optimiezed for branchless. function like: p = (signature & 0x8000) ? poloynom1 : 0;
          uint16_t p = poloynom1 & (0xffff * ((signature & 0x8000) != 0));
          signature = (signature << 1) ^ p;
      }
      return signature;
  }
/*
 I started  from this, with -O3 some compiler makes it branchless  shorter.
 uint16_t crc_update_crc1(uint16_t signature, uint8_t inputByte) {
    signature ^= inputByte << 8; // 16 Bit CRC
    for (uint8_t i = 0; i < 8; i++) {
        if ((signature & 0x8000) != 0) { // 16 Bit CRC
            signature = (signature << 1) ^ poloynom1;
        } else {
            signature <<= 1;
        }
    }
    return signature;
 }
 */

#else
  uint16_t crc16_table[256] = {  // table version is really fast.
      0x0000U, 0x1021U, 0x2042U, 0x3063U, 0x4084U, 0x50A5U, 0x60C6U, 0x70E7U, 
      0x8108U, 0x9129U, 0xA14AU, 0xB16BU, 0xC18CU, 0xD1ADU, 0xE1CEU, 0xF1EFU, 
      0x1231U, 0x0210U, 0x3273U, 0x2252U, 0x52B5U, 0x4294U, 0x72F7U, 0x62D6U, 
      0x9339U, 0x8318U, 0xB37BU, 0xA35AU, 0xD3BDU, 0xC39CU, 0xF3FFU, 0xE3DEU, 
      0x2462U, 0x3443U, 0x0420U, 0x1401U, 0x64E6U, 0x74C7U, 0x44A4U, 0x5485U, 
      0xA56AU, 0xB54BU, 0x8528U, 0x9509U, 0xE5EEU, 0xF5CFU, 0xC5ACU, 0xD58DU, 
      0x3653U, 0x2672U, 0x1611U, 0x0630U, 0x76D7U, 0x66F6U, 0x5695U, 0x46B4U, 
      0xB75BU, 0xA77AU, 0x9719U, 0x8738U, 0xF7DFU, 0xE7FEU, 0xD79DU, 0xC7BCU, 
      0x48C4U, 0x58E5U, 0x6886U, 0x78A7U, 0x0840U, 0x1861U, 0x2802U, 0x3823U, 
      0xC9CCU, 0xD9EDU, 0xE98EU, 0xF9AFU, 0x8948U, 0x9969U, 0xA90AU, 0xB92BU, 
      0x5AF5U, 0x4AD4U, 0x7AB7U, 0x6A96U, 0x1A71U, 0x0A50U, 0x3A33U, 0x2A12U, 
      0xDBFDU, 0xCBDCU, 0xFBBFU, 0xEB9EU, 0x9B79U, 0x8B58U, 0xBB3BU, 0xAB1AU, 
      0x6CA6U, 0x7C87U, 0x4CE4U, 0x5CC5U, 0x2C22U, 0x3C03U, 0x0C60U, 0x1C41U, 
      0xEDAEU, 0xFD8FU, 0xCDECU, 0xDDCDU, 0xAD2AU, 0xBD0BU, 0x8D68U, 0x9D49U, 
      0x7E97U, 0x6EB6U, 0x5ED5U, 0x4EF4U, 0x3E13U, 0x2E32U, 0x1E51U, 0x0E70U, 
      0xFF9FU, 0xEFBEU, 0xDFDDU, 0xCFFCU, 0xBF1BU, 0xAF3AU, 0x9F59U, 0x8F78U, 
      0x9188U, 0x81A9U, 0xB1CAU, 0xA1EBU, 0xD10CU, 0xC12DU, 0xF14EU, 0xE16FU, 
      0x1080U, 0x00A1U, 0x30C2U, 0x20E3U, 0x5004U, 0x4025U, 0x7046U, 0x6067U, 
      0x83B9U, 0x9398U, 0xA3FBU, 0xB3DAU, 0xC33DU, 0xD31CU, 0xE37FU, 0xF35EU, 
      0x02B1U, 0x1290U, 0x22F3U, 0x32D2U, 0x4235U, 0x5214U, 0x6277U, 0x7256U, 
      0xB5EAU, 0xA5CBU, 0x95A8U, 0x8589U, 0xF56EU, 0xE54FU, 0xD52CU, 0xC50DU, 
      0x34E2U, 0x24C3U, 0x14A0U, 0x0481U, 0x7466U, 0x6447U, 0x5424U, 0x4405U, 
      0xA7DBU, 0xB7FAU, 0x8799U, 0x97B8U, 0xE75FU, 0xF77EU, 0xC71DU, 0xD73CU, 
      0x26D3U, 0x36F2U, 0x0691U, 0x16B0U, 0x6657U, 0x7676U, 0x4615U, 0x5634U, 
      0xD94CU, 0xC96DU, 0xF90EU, 0xE92FU, 0x99C8U, 0x89E9U, 0xB98AU, 0xA9ABU, 
      0x5844U, 0x4865U, 0x7806U, 0x6827U, 0x18C0U, 0x08E1U, 0x3882U, 0x28A3U, 
      0xCB7DU, 0xDB5CU, 0xEB3FU, 0xFB1EU, 0x8BF9U, 0x9BD8U, 0xABBBU, 0xBB9AU, 
      0x4A75U, 0x5A54U, 0x6A37U, 0x7A16U, 0x0AF1U, 0x1AD0U, 0x2AB3U, 0x3A92U, 
      0xFD2EU, 0xED0FU, 0xDD6CU, 0xCD4DU, 0xBDAAU, 0xAD8BU, 0x9DE8U, 0x8DC9U, 
      0x7C26U, 0x6C07U, 0x5C64U, 0x4C45U, 0x3CA2U, 0x2C83U, 0x1CE0U, 0x0CC1U, 
      0xEF1FU, 0xFF3EU, 0xCF5DU, 0xDF7CU, 0xAF9BU, 0xBFBAU, 0x8FD9U, 0x9FF8U, 
      0x6E17U, 0x7E36U, 0x4E55U, 0x5E74U, 0x2E93U, 0x3EB2U, 0x0ED1U, 0x1EF0U
  };
  uint16_t crc_update(uint16_t signature, uint8_t inputByte) {
      return (crc16_table[(signature >> 8U) ^ inputByte] ^ (signature << 8U)) & 0xffffU;
  }
#endif