Adler32.java

/* -*-mode:java; c-basic-offset:2; -*- */
/*
Copyright (c) 2000-2011 ymnk, JCraft,Inc. All rights reserved.

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     the documentation and/or other materials provided with the distribution.

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     derived from this software without specific prior written permission.

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/*
 * This program is based on zlib-1.1.3, so all credit should go authors
 * Jean-loup Gailly([email protected]) and Mark Adler([email protected])
 * and contributors of zlib.
 */

package com.jcraft.jzlib;

final public class Adler32 implements Checksum {

  // largest prime smaller than 65536
  static final private int BASE=65521; 
  // NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1
  static final private int NMAX=5552;

  private long s1=1L;
  private long s2=0L;

  public void reset(long init){
    s1=init&0xffff;
    s2=(init>>16)&0xffff;
  }

  public void reset(){
    s1=1L;
    s2=0L;
  }

  public long getValue(){
    return ((s2<<16)|s1);
  }

  public void update(byte[] buf, int index, int len){

    if(len==1){
      s1+=buf[index++]&0xff; s2+=s1;
      s1%=BASE;
      s2%=BASE;
      return;
    }

    int len1 = len/NMAX;
    int len2 = len%NMAX;
    while(len1-->0) {
      int k=NMAX;
      len-=k;
      while(k-->0){
	s1+=buf[index++]&0xff; s2+=s1;
      }
      s1%=BASE;
      s2%=BASE;
    }

    int k=len2;
    len-=k;
    while(k-->0){
      s1+=buf[index++]&0xff; s2+=s1;
    }
    s1%=BASE;
    s2%=BASE;
  }

  public Adler32 copy(){
    Adler32 foo = new Adler32();
    foo.s1 = this.s1;
    foo.s2 = this.s2;
    return foo;
  }

  // The following logic has come from zlib.1.2.
  static long combine(long adler1, long adler2, long len2){
    long BASEL = (long)BASE;
    long sum1;
    long sum2;
    long rem;  // unsigned int

    rem = len2 % BASEL;
    sum1 = adler1 & 0xffffL;
    sum2 = rem * sum1;
    sum2 %= BASEL; // MOD(sum2);
    sum1 += (adler2 & 0xffffL) + BASEL - 1;
    sum2 += ((adler1 >> 16) & 0xffffL) + ((adler2 >> 16) & 0xffffL) + BASEL - rem;
    if (sum1 >= BASEL) sum1 -= BASEL;
    if (sum1 >= BASEL) sum1 -= BASEL;
    if (sum2 >= (BASEL << 1)) sum2 -= (BASEL << 1);
    if (sum2 >= BASEL) sum2 -= BASEL;
    return sum1 | (sum2 << 16);
  }
  
}