source: trunk/gcc/libjava/java/util/zip/Adler32.java

/* 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.

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package java.util.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;
  }
}