/*

  * 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

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  */

 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;

     }

 }