crc16.java

一个详细的javaLayer的源码

/*
 * 02/12/99 : Java Conversion by E.B , [e-mail], JavaLayer
 *
 *-----------------------------------------------------------------------
 *  @(#) crc.h 1.5, last edit: 6/15/94 16:55:32
 *  @(#) Copyright (C) 1993, 1994 Tobias Bading ([e-mail])
 *  @(#) Berlin University of Technology
 *
 *  This program 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 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 General Public License for more details.
 *
 *  You should have received a copy of the GNU 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;
  }
}