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/*
* Copyright (C) 2010 Preston Lacey http://javaflacencoder.sourceforge.net/
* All Rights Reserved.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library 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
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
package javaFlacEncoder;
/**
* Class to calculate a CRC16 checksum.
* @author Preston Lacey
*/
public class CRC16 {
/** For Debugging: Higher level equals more debug statements */
public static int DEBUG_LEV = 0;
/** CRC Divisor: 0x8005(implicit 1 at MSB for 0x18005) */
static final int divisorCRC16 = 0x8005;
/** working checksum stored between calls to update(..) */
protected int workingCRC;
private static final short xorTable[] = generateTable();
/**
* Constructor. Creates a CRC16 object that is ready to be used. Next step
* would be to call update(...) with appropriate data.
*/
public CRC16() {
reset();
}
/**
* Resets stored data, preparing object for a new checksum.
*/
public void reset() {
workingCRC = 0;
}
public short checksum() {
return (short)(workingCRC & 0xFFFF);
}
public int update(byte input) {
workingCRC = (workingCRC << 8)^xorTable[((workingCRC >>> 8)^input) & 0xFF];
return workingCRC;
}
public int update(byte[] input, int start, int stop) {
for(int i = start; i < stop; i++) {
byte b = input[i];
workingCRC = (workingCRC << 8) ^ xorTable[((workingCRC >>> 8)^b) & 0xFF];
}
return workingCRC;
}
private static short[] generateTable() {
short[] table = new short[256];
for(int i = 0; i < table.length; i++) {
int polynomial = divisorCRC16;
int xorVal = i << 8;
int topmask = 1 << 16;
for(int x = 0; x < 8; x++ ) {
xorVal = xorVal << 1;
if( (xorVal & topmask) > 0) xorVal = (xorVal) ^ polynomial;
}
table[i] = (short)(xorVal & 0xFFFF);
}
return table;
}
}
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