crc16.c

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/*
 * Copyright (c) 2005, Swedish Institute of Computer Science
 * All rights reserved.
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 * 2. Redistributions in binary form must reproduce the above copyright
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 *    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
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 *
 * This file is part of the Contiki operating system.
 *
 * @(#)$Id: crc16.c,v 1.4 2009/05/14 12:05:04 nvt-se Exp $
 */

/** \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;
}
/*---------------------------------------------------------------------------*/

/** @} */