Crc.h

#pragma once

#include <stdint.h>

#ifdef __AVR__
// AVR has optimized assembly versions of some crc functions
#include <util/crc16.h>
#else
  // These are the equivalent C implementation as documented in the AVR
  // util/crc16.h header.

  inline uint8_t _crc8_ccitt_update (uint8_t inCrc, uint8_t inData)
  {
    uint8_t   i;
    uint8_t   data;

    data = inCrc ^ inData;

    for ( i = 0; i < 8; i++ )
    {
      if (( data & 0x80 ) != 0 )
      {
        data <<= 1;
        data ^= 0x07;
      }
      else
      {
        data <<= 1;
      }
    }
    return data;
  }

  inline uint16_t _crc_ccitt_update (uint16_t crc, uint8_t data)
  {
    data ^= (crc & 0xff);
    data ^= data << 4;

    return ((((uint16_t)data << 8) | (crc >> 8)) ^ (uint8_t)(data >> 4)
            ^ ((uint16_t)data << 3));
  }

  inline uint16_t _crc16_update(uint16_t crc, uint8_t a) {
    int i;

    crc ^= a;
    for (i = 0; i < 8; ++i) {
      if (crc & 1)
        crc = (crc >> 1) ^ 0xA001;
      else
        crc = (crc >> 1);
    }

    return crc;
  }
#endif

/**
 * Helper class to calculate crcs for transfers. To use it, create an
 * instance, call update() for each byte and/or buffer of bytes to
 * calculate the CRC over, and then call get() to get the result.
 * update() is chainable, so you can do e.g.:
 *
 *   uint8_t crc = Crc<...>().update(first_byte).update(rest_of_bytes, len).get();
 */
template <typename T, T Update(T, uint8_t), T Initial>
class Crc {
  public:
    Crc& update(uint8_t b) {
      this->crc = Update(this->crc, b);
      return *this;
    }

    Crc& update(uint8_t *buf, uint8_t len) {
      for (uint8_t i = 0; i < len; ++i)
        this->update(buf[i]);
      return *this;
    }
    T get() {
      return this->crc;
    }

    Crc& reset() {
      this->crc = Initial;
      return *this;
    }

  private:
    T crc = Initial;
};

using Crc8Ccitt = Crc<uint8_t, _crc8_ccitt_update, 0xff>;
using Crc16Ccitt = Crc<uint16_t, _crc_ccitt_update, 0xffff>;
// Called CRC16-IBM (or CRC16-ANSI or just CRC16) by wikipedia, used by ModBus
using Crc16Ibm = Crc<uint16_t, _crc16_update, 0xffff>;