crc16.cpp

/*
 * Library Name: CRC16
 *
 * Filename: CRC16.cpp
 * Description: library CRC16 implementation
 *
 * generate a ccitt 16 bit cyclic redundancy check (crc)
 * The code in this module generates the crc for a block of data.
 *
 * Version: 1.0.2
 * Author: Joao Alves <jpralves@gmail.com>
 * Required files: crc16.cpp, crc16.h
 *
 * History:
 * 1.0.2 - 2017-03-14 - Initial Version
 *
 * This library is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.

 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.

 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */


//                                 16  12  5
// The CCITT CRC 16 polynomial is X + X + X + 1.
// In binary, this is the bit pattern 1 0001 0000 0010 0001, and in hex it
//  is 0x11021.
// A 17 bit register is simulated by testing the MSB before shifting
//  the data, which affords us the luxury of specifiy the polynomial as a
//  16 bit value, 0x1021.

#include "crc16.h"

#define POLY 0x1021

CRC16::CRC16() {
	crc = 0;
}

uint16_t CRC16::processByte(uint8_t data) {
  uint8_t i;

  crc = crc ^ ((uint16_t)data << 8);
  for (i = 0; i < 8; i++) {
    if (crc & 0x8000)
      crc = (crc << 1) ^ POLY;
    else
      crc <<= 1;
  }
  return crc;
}

uint16_t CRC16::processBuffer(const char *data_p, uint16_t length) {
/*  uint8_t i;
  uint16_t data;*/

  while(length--) {
    processByte(*data_p++);
/*    data = *data_p++;
    crc = crc ^ ((uint16_t)data << 8);
    for (i = 0; i < 8; i++) {
      if (crc & 0x8000)
        crc = (crc << 1) ^ POLY;
      else
        crc <<= 1;
    }*/
  }
  return crc;
}