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1695 bivey 1 
package com.erggroup.mass.ant.tasks.util;
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import java.util.zip.*;
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/**
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 * Checksum calculator for a 16-bit CCITT CRC.
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 *
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 * @author bivey
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 */
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public class CCITT implements Checksum
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{
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	protected long checksum;
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	protected long length;
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	private static final long poly = 0x1021;	// CCITT Generator polynomial (x16 + x12 +x5 +1)
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	private static long[] table = new long[256]; // Lookup table for fast calculation
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	static
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	{
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		// Initialise the lookup table
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		long remainder;
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		for( int dividend = 0; dividend < 256; dividend++ )
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		{
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			remainder = (long)dividend;
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			remainder = reflect(remainder,8);
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			remainder <<= 8;
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			for( int bit = 0; bit < 8; bit++ )
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			{
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				if ( (remainder & 0x8000) != 0 )
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				{
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					remainder = ( remainder << 1 ) ^ poly;
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				}
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				else
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				{
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					remainder = ( remainder << 1 );
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				}
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			}
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			remainder = reflect(remainder, 16);
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			table[dividend] = (remainder & 0xFFFF);
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		}
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	}
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	public CCITT()
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	{
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		reset();
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	}
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	/* Returns the current checksum value.
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	 *
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	 * @see java.util.zip.Checksum#getValue()
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	 */
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	public long getValue()
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	{
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		return checksum & 0xFFFF;
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	}
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	/**
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	 * Returns the length of data used in calculating the checksum.
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	 *
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	 * @return data length
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	 */
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	public long getLength()
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	{
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		return length;
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	}
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	/* Resets the checksum to its initial value.
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	 *
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	 * @see java.util.zip.Checksum#reset()
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	 */
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	public void reset()
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	{
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		checksum = reflect(0, 16);
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		length = 0;
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	}
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	/* Updates the current checksum with the specified array of bytes.
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	 *
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	 * @see java.util.zip.Checksum#update(byte[], int, int)
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	 */
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	public void update(byte[] b, int off, int len)
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	{
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		int count = off + len;
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		for( int i = off; i < count; i++ )
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		{
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			update(b[i]);
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		}
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	}
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	/* Updates the current checksum with the specified byte.
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	 *
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	 * @see java.util.zip.Checksum#update(int)
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	 */
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	public void update(int b)
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	{
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		update((byte)(b & 0xFF));
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	}
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	/**
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	 * Updates the checksum with the specified byte.
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	 *
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	 * @param b The byte
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	 */
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	public void update(byte b)
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	{
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		int index = ((int)(checksum ^ b) & 0xff);
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		checksum >>>= 8;
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		checksum &= 0xFF;
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		checksum ^= (table[index]);
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        length++;
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	}
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    /**
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     * Reflects a value by taking the least significant bits into account
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     *
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     * @param value The value which should be reflected
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     * @param bits The number of bits to be reflected
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     * @return The value with the bottom bits reflected.
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     */
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    private static long reflect(long value, int bits) {
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        long temp = 0L;
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        for( int i = 0; i < bits; i++ )
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        {
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            temp <<= 1L;
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            temp |= (value & 1L);
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            value >>>= 1L;
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        }
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        return (value << bits) | temp;
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    }
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}