/* Adler32.java - Computes Adler32 data checksum of a data stream
Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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Linking this library statically or dynamically with other modules is
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permission to link this library with independent modules to produce an
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exception statement from your version. */
//package java.util.zip;
package com.classpath.zip;

/*
 * Written using on-line Java Platform 1.2 API Specification, as well
 * as "The Java Class Libraries", 2nd edition (Addison-Wesley, 1998).
 * The actual Adler32 algorithm is taken from RFC 1950.
 * Status:  Believed complete and correct.
 */
/**
 * Computes Adler32 checksum for a stream of data. An Adler32
 * checksum is not as reliable as a CRC32 checksum, but a lot faster to
 * compute.
 *<p>
 * The specification for Adler32 may be found in RFC 1950.
 * (ZLIB Compressed Data Format Specification version 3.3)
 *<p>
 *<p>
 * From that document:
 *<p>
 *      "ADLER32 (Adler-32 checksum)
 *       This contains a checksum value of the uncompressed data
 *       (excluding any dictionary data) computed according to Adler-32
 *       algorithm. This algorithm is a 32-bit extension and improvement
 *       of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
 *       standard.
 *<p>
 *       Adler-32 is composed of two sums accumulated per byte: s1 is
 *       the sum of all bytes, s2 is the sum of all s1 values. Both sums
 *       are done modulo 65521. s1 is initialized to 1, s2 to zero.  The
 *       Adler-32 checksum is stored as s2*65536 + s1 in most-
 *       significant-byte first (network) order."
 *<p>
 * "8.2. The Adler-32 algorithm
 *<p>
 *    The Adler-32 algorithm is much faster than the CRC32 algorithm yet
 *    still provides an extremely low probability of undetected errors.
 *<p>
 *    The modulo on unsigned long accumulators can be delayed for 5552
 *    bytes, so the modulo operation time is negligible.  If the bytes
 *    are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
 *    and order sensitive, unlike the first sum, which is just a
 *    checksum.  That 65521 is prime is important to avoid a possible
 *    large class of two-byte errors that leave the check unchanged.
 *    (The Fletcher checksum uses 255, which is not prime and which also
 *    makes the Fletcher check insensitive to single byte changes 0 <->
 *    255.)
 *<p>
 *    The sum s1 is initialized to 1 instead of zero to make the length
 *    of the sequence part of s2, so that the length does not have to be
 *   checked separately. (Any sequence of zeroes has a Fletcher
 *    checksum of zero.)"
 *
 * @author John Leuner, Per Bothner
 * @since JDK 1.1
 *
 * @see InflaterInputStream
 * @see DeflaterOutputStream
 */
public class Adler32 implements Checksum
{
	/** largest prime smaller than 65536 */
	private static final int BASE = 65521;

	private int checksum; //we do all in int.

	//Note that java doesn't have unsigned integers,
	//so we have to be careful with what arithmetic
	//we do. We return the checksum as a long to
	//avoid sign confusion.

	/**
	 * Creates a new instance of the <code>Adler32</code> class.
	 * The checksum starts off with a value of 1.
	 */
	public Adler32()
	{
		reset();
	}

	/**
	 * Resets the Adler32 checksum to the initial value.
	 */
	public void reset()
	{
		checksum = 1; //Initialize to 1
	}

	/**
	 * Updates the checksum with the byte b.
	 *
	 * @param bval the data value to add. The high byte of the int is ignored.
	 */
	public void update(int bval)
	{
		//We could make a length 1 byte array and call update again, but I
		//would rather not have that overhead
		int s1 = checksum & 0xffff;
		int s2 = checksum >>> 16;

		s1 = (s1 + (bval & 0xFF)) % BASE;
		s2 = (s1 + s2) % BASE;

		checksum = (s2 << 16) + s1;
	}

	/**
	 * Updates the checksum with the bytes taken from the array.
	 *
	 * @param buffer an array of bytes
	 */
	public void update(byte[] buffer)
	{
		update(buffer, 0, buffer.length);
	}

	/**
	 * Updates the checksum with the bytes taken from the array.
	 *
	 * @param buf an array of bytes
	 * @param off the start of the data used for this update
	 * @param len the number of bytes to use for this update
	 */
	public void update(byte[] buf, int off, int len)
	{
		//(By Per Bothner)
		int s1 = checksum & 0xffff;
		int s2 = checksum >>> 16;

		while(len > 0)
		{
			// We can defer the modulo operation:
			// s1 maximally grows from 65521 to 65521 + 255 * 3800
			// s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
			int n = 3800;
			if(n > len)
			{
				n = len;
			}
			len -= n;
			while(--n >= 0)
			{
				s1 = s1 + (buf[off++] & 0xFF);
				s2 = s2 + s1;
			}
			s1 %= BASE;
			s2 %= BASE;
		}

		/*Old implementation, borrowed from somewhere:
		int n;

		while (len-- > 0) {

		s1 = (s1 + (bs[offset++] & 0xff)) % BASE;
		s2 = (s2 + s1) % BASE;
		}*/

		checksum = (s2 << 16) | s1;
	}

	/**
	 * Returns the Adler32 data checksum computed so far.
	 */
	public long getValue()
	{
		return (long) checksum & 0xffffffffL;
	}
}
