com.carrotsearch.hppcrt.hash.PhiMix Maven / Gradle / Ivy

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package com.carrotsearch.hppcrt.hash;

/**
 * Quickly mixes the bits of integers.
 * 
 * Those methods mixes the bits of the argument by multiplying by the golden
 * ratio and xorshifting the result.
 * 

 * It is borrowed from Koloboke, and it has slightly
 * worse behaviour than {@link MurmurHash3} (in open-addressing hash tables the
 * average number of probes is slightly larger), but it's much faster.
 * 
 * Reciprocal mixing functions are borrowed from fastutil.
 */
public final class PhiMix
{
    /**
     * 2³² · φ, φ = (√5 − 1)/2. (package
     * visibility)
     */
    static final int INT_PHI = 0x9E3779B9;

    /**
     * 2⁶⁴ · φ, φ = (√5 − 1)/2. (package
     * visibility)
     */
    static final long LONG_PHI = 0x9E3779B97F4A7C15L;

    /**
     * The reciprocal of {@link #INT_PHI} modulo 2³².
     */
    private static final int INV_INT_PHI = 0x144cbc89;

    /**
     * The reciprocal of {@link #LONG_PHI} modulo 2⁶⁴.
     */
    private static final long INV_LONG_PHI = 0xf1de83e19937733dL;

    /**
     * = hash((int)0)
     */
    public static final int HASH_0 = 0;

    /**
     * = hash((int)1)
     */
    public static final int HASH_1 = 1640556430;

    private PhiMix() {
        // no instances.
    }

    /**
     * Hashes a 4-byte sequence (Java int).
     * 
     * @param x
     *          an integer.
     * @return a hash value obtained by mixing the bits of {@code x}.
     */
    public static int mix32(final int x) {
        final int h = x * PhiMix.INT_PHI;
        return h ^ (h >> 16);
    }

    /**
     * The inverse of {@link #mix32(int)}. This method is mainly useful to create
     * unit tests.
     * 
     * @param x
     *          an integer.
     * @return a value that passed through {@link #mix32(int)} would give {@code x}
     *         .
     */
    public static int invMix32(final int x) {
        return (x ^ x >>> 16) * PhiMix.INV_INT_PHI;
    }

    /**
     * Hashes an 8-byte sequence (Java long).
     * 
     * @param x
     *          a long integer.
     * @return a hash value obtained by mixing the bits of {@code x}.
     */
    public static long mix64(final long x) {
        long h = x * PhiMix.LONG_PHI;
        h ^= h >> 32;
        return h ^ (h >> 16);
    }

    /**
     * The inverse of {@link #mix64(long)}. This method is mainly useful to create
     * unit tests.
     * 
     * @param x
     *          a long integer.
     * @return a value that passed through {@link #mix64(long)} would give
     *         {@code x}.
     */
    public static long invMix64(long x) {
        x ^= x >>> 32;
        x ^= x >>> 16;
        return (x ^ x >>> 32) * PhiMix.INV_LONG_PHI;
    }
}