16-Bit CRC checksum : CRC « Security « Java

1. Java
2. Security
3. CRC

16-Bit CRC checksum

     
/*
 * 11/19/04 : 1.0 moved to LGPL.
 * 
 * 02/12/99 : Java Conversion by E.B , javalayer@javazoom.net
 *
 *  @(#) crc.h 1.5, last edit: 6/15/94 16:55:32
 *  @(#) Copyright (C) 1993, 1994 Tobias Bading (bading@cs.tu-berlin.de)
 *  @(#) Berlin University of Technology
 *-----------------------------------------------------------------------
 *   This program is free software; you can redistribute it and/or modify
 *   it under the terms of the GNU Library General Public License as published
 *   by the Free Software Foundation; either version 2 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 Library General Public License for more details.
 *
 *   You should have received a copy of the GNU Library General Public
 *   License along with this program; if not, write to the Free Software
 *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 *----------------------------------------------------------------------
 */
//package javazoom.jl.decoder;

/**
 * 16-Bit CRC checksum
 */
public final class Crc16
{
  private static  short polynomial=(short)0x8005;
  private      short crc;

  /**
   * Dummy Constructor
   */
  public Crc16()
  { 
    crc = (short) 0xFFFF;
  }

  /**
   * Feed a bitstring to the crc calculation (0 < length <= 32).
   */
  public void add_bits (int bitstring, int length)
  {
    int bitmask = 1 << (length - 1);
    do
   if (((crc & 0x8000) == 0) ^ ((bitstring & bitmask) == 0 ))
   {
    crc <<= 1;
    crc ^= polynomial;
   }
   else
    crc <<= 1;
    while ((bitmask >>>= 1) != 0);
  }

  /**
   * Return the calculated checksum.
   * Erase it for next calls to add_bits().
   */
  public short  checksum()
  {
    short sum = crc;
    crc = (short) 0xFFFF;
    return sum;
  }
}

   
    
    
    
    
  

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