/*

 * Copyright (c) 1996, 2005, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code 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

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package java.util.zip;

/**

 * A class that can be used to compute the CRC-32 of a data stream.

 *

 * @see         Checksum

 * @author      David Connelly

 */

public

class CRC32 implements Checksum {

    private int crc = 0xFFFFFFFF;

    /**

     * Creates a new CRC32 object.

     */

    public CRC32() {

    }

    /**

     * Updates the CRC-32 checksum with the specified byte (the low

     * eight bits of the argument b).

     *

     * @param b the byte to update the checksum with

     */

    public void update(int b) {

        byte[] arr = { (byte)b };

        update(arr);

    }

    /**

     * Updates the CRC-32 checksum with the specified array of bytes.

     */

    public void update(byte[] b, int off, int len) {

        if (b == null) {

            throw new NullPointerException();

        }

        if (off < 0 || len < 0 || off > b.length - len) {

            throw new ArrayIndexOutOfBoundsException();

        }

        crc = updateBytes(crc, b, off, len);

    }

    /**

     * Updates the CRC-32 checksum with the specified array of bytes.

     *

     * @param b the array of bytes to update the checksum with

     */

    public void update(byte[] b) {

        crc = updateBytes(crc, b, 0, b.length);

    }

    /**

     * Resets CRC-32 to initial value.

     */

    public void reset() {

        crc = 0;

    }

    /**

     * Returns CRC-32 value.

     */

    public long getValue() {

        return (long)crc & 0xffffffffL;

    }

    // XXX: taken from 

    // http://introcs.cs.princeton.edu/java/51data/CRC32.java.html

    private static int updateBytes(int crc, byte[] arr, int off, int len) {

        int poly = 0xEDB88320;   // reverse polynomial

        while (len-- > 0) {

            byte b = arr[off++];

            int temp = (crc ^ b) & 0xff;

            // read 8 bits one at a time

            for (int i = 0; i < 8; i++) {

                if ((temp & 1) == 1) {

                    temp = (temp >>> 1) ^ poly;

                } else {

                    temp = (temp >>> 1);

                }

            }

            crc = (crc >>> 8) ^ temp;

        }

        return crc ^ 0xffffffff;

    }

}