com.crashnote.core.util.ChksumUtil Maven / Gradle / Ivy

Go to download
Show more of this group Show more artifacts with this name
Show all versions of crashnote-servlet Show documentation
Reports exceptions from Java servlet apps to crashnote.com
There is a newer version: 0.6.0
/**
 * Copyright
 *
 * We compute the checksum using Broder s implementation of
 * Rabin s fingerprinting algorithm. Fingerprints offer
 * provably strong probabilistic guarantees that two
 * different strings will not have the same fingerprint.
 * Other checksum algorithms, such as MD5 and SHA, do not
 * offer such provable guarantees, and are also more
 * expensive to compute than Rabin fingerprint.
 * 
 * A disadvantage is that these faster functions are
 * efficiently invertible (that is, one can easily build an
 * URL that hashes to a particular location), a fact that
 * might be used by malicious users to nefarious purposes.
 * 

 * Using the Rabin's fingerprinting function, the probability of
 * collision of two strings s1 and s2 can be bounded (in a adversarial
 * model for s1 and s2) by max(|s1|,|s2|)/2**(l-1), where |s1| is the
 * length of the string s1 in bits.
 * 

 * The advantage of choosing Rabin fingerprints (which are based on random
 * irreducible polynomials) rather than some arbitrary hash function is that
 * their probability of collision os well understood. Furthermore Rabin
 * fingerprints can be computed very efficiently in software and we can
 * take advantage of their algebraic properties when we compute the
 * fingerprints of "sliding windows".
 * 

 * M. O. Rabin
 * Fingerprinting by random polynomials.
 * Center for Research in Computing Technology
 * Harvard University Report TR-15-81
 * 1981
 * 
 * A. Z. Broder
 * Some applications of Rabin's fingerprinting method
 * In R.Capicelli, A. De Santis and U. Vaccaro editors
 * Sequences II:Methods in Communications, Security, and Computer Science
 * pages 143-152
 * Springer-Verlag
 * 1993
 */
package com.crashnote.core.util;

import java.io.Serializable;

public final class ChksumUtil implements Serializable {

    // INTERFACE ==================================================================================

    /**
     * Computes the Rabin hash value of a String.
     *
     * @param s the string to be hashed
     * @return the hash value
     */
    public static long hash(final String s) {
        return hash(s.toCharArray());
    }


    // INTERNALS ==================================================================================

    private final static int P_DEGREE = 64;
    private final static int X_P_DEGREE = 1 << (P_DEGREE - 1);
    private static final long POLY = Long.decode("0x004AE1202C306041") | 1 << 63;

    private static final long[] table32, table40, table48, table54;
    private static final long[] table62, table70, table78, table84;

    static {
        table32 = new long[256];
        table40 = new long[256];
        table48 = new long[256];
        table54 = new long[256];
        table62 = new long[256];
        table70 = new long[256];
        table78 = new long[256];
        table84 = new long[256];
        final long[] mods = new long[P_DEGREE];
        mods[0] = POLY;
        for (int i = 0; i < 256; i++) {
            table32[i] = 0;
            table40[i] = 0;
            table48[i] = 0;
            table54[i] = 0;
            table62[i] = 0;
            table70[i] = 0;
            table78[i] = 0;
            table84[i] = 0;
        }
        for (int i = 1; i < P_DEGREE; i++) {
            mods[i] = mods[i - 1] << 1;
            if ((mods[i - 1] & X_P_DEGREE) != 0) {
                mods[i] = mods[i] ^ POLY;
            }
        }
        for (int i = 0; i < 256; i++) {
            long c = i;
            for (int j = 0; j < 8 && c != 0; j++) {
                if ((c & 1) != 0) {
                    table32[i] = table32[i] ^ mods[j];
                    table40[i] = table40[i] ^ mods[j + 8];
                    table48[i] = table48[i] ^ mods[j + 16];
                    table54[i] = table54[i] ^ mods[j + 24];
                    table62[i] = table62[i] ^ mods[j + 32];
                    table70[i] = table70[i] ^ mods[j + 40];
                    table78[i] = table78[i] ^ mods[j + 48];
                    table84[i] = table84[i] ^ mods[j + 56];
                }
                c >>>= 1;
            }
        }
    }

    /**
     * Return the Rabin hash value of an array of chars.
     *
     * @param A the array of chars
     * @return the hash value
     */
    private static long hash(final char[] A) {
        long w = 0;
        final int start = A.length % 4;
        for (int s = 0; s < start; s++) {
            w = (w << 16) ^ (A[s] & 0xFFFF);
        }
        for (int s = start; s < A.length; s += 4) {
            w =
                table32[(int) (w & 0xFF)]
                    ^ table40[(int) ((w >>> 8) & 0xFF)]
                    ^ table48[(int) ((w >>> 16) & 0xFF)]
                    ^ table54[(int) ((w >>> 24) & 0xFF)]
                    ^ table62[(int) ((w >>> 32) & 0xFF)]
                    ^ table70[(int) ((w >>> 40) & 0xFF)]
                    ^ table78[(int) ((w >>> 48) & 0xFF)]
                    ^ table84[(int) ((w >>> 56) & 0xFF)]
                    ^ ((long) (A[s] & 0xFFFF) << 48)
                    ^ ((long) (A[s + 1] & 0xFFFF) << 32)
                    ^ ((long) (A[s + 2] & 0xFFFF) << 16)
                    ^ ((long) (A[s + 3] & 0xFFFF));
        }
        return w;
    }
}