nes-proj/core/lib/crc16.c

/*
 * Copyright (c) 2005, Swedish Institute of Computer Science
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the Institute nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * This file is part of the Contiki operating system.
 *
 */

/** \addtogroup crc16
 * @{ */

/**
 * \file
 *         Implementation of the CRC16 calculcation
 * \author
 *         Adam Dunkels <adam@sics.se>
 *
 */

/* CITT CRC16 polynomial ^16 + ^12 + ^5 + 1 */
/*---------------------------------------------------------------------------*/
unsigned short
crc16_add(unsigned char b, unsigned short acc)
{
  /*
    acc  = (unsigned char)(acc >> 8) | (acc << 8);
    acc ^= b;
    acc ^= (unsigned char)(acc & 0xff) >> 4;
    acc ^= (acc << 8) << 4;
    acc ^= ((acc & 0xff) << 4) << 1;
  */

  acc ^= b;
  acc  = (acc >> 8) | (acc << 8);
  acc ^= (acc & 0xff00) << 4;
  acc ^= (acc >> 8) >> 4;
  acc ^= (acc & 0xff00) >> 5;
  return acc;
}
/*---------------------------------------------------------------------------*/
unsigned short
crc16_data(const unsigned char *data, int len, unsigned short acc)
{
  int i;
  
  for(i = 0; i < len; ++i) {
    acc = crc16_add(*data, acc);
    ++data;
  }
  return acc;
}
/*---------------------------------------------------------------------------*/

/** @} */