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+/* 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
+the Free Software Foundation; either version 2, or (at your option)
+any later version.
+
+GNU Classpath 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 for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Classpath; see the file COPYING.  If not, write to the
+Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301 USA.
+
+Linking this library statically or dynamically with other modules is
+making a combined work based on this library.  Thus, the terms and
+conditions of the GNU General Public License cover the whole
+combination.
+
+As a special exception, the copyright holders of this library give you
+permission to link this library with independent modules to produce an
+executable, regardless of the license terms of these independent
+modules, and to copy and distribute the resulting executable under
+terms of your choice, provided that you also meet, for each linked
+independent module, the terms and conditions of the license of that
+module.  An independent module is a module which is not derived from
+or based on this library.  If you modify this library, you may extend
+this exception to your version of the library, but you are not
+obligated to do so.  If you do not wish to do so, delete this
+exception statement from your version. */
+
+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;
+  }
+}