/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.jackrabbit.oak.commons.collections;

import java.nio.charset.StandardCharsets;
import org.apache.commons.codec.digest.MurmurHash3;

/**
 * A hash function utility class.
 */
public class HashUtils {

    private HashUtils() {
        // utility class
    }

    /**
     * Calculate a 64-bit hash value from a string.
     *
     * @param s the string
     * @return the hash value
     */
    public static long hash64(String s) {
        return MurmurHash3.hash128(s.getBytes(StandardCharsets.UTF_8))[0];
    }

    /**
     * Calculate a 64-bit hash value from a value, using a seed.
     *
     * The current algorithm used the finalizer of the MurmurHash3 hash function,
     * but callers shouldn't rely on that.
     *
     * @param x    the value
     * @param seed the seed
     * @return the hash value
     */
    public static long hash64(long x, long seed) {
        x += seed;
        x = (x ^ (x >>> 33)) * 0xff51afd7ed558ccdL;
        x = (x ^ (x >>> 33)) * 0xc4ceb9fe1a85ec53L;
        x = x ^ (x >>> 33);
        return x;
    }

    /**
     * Calculate a 64-bit hash value from a value. The input is a 64-bit value and
     * the output is a 64-bit values. Two different inputs are never mapped to the
     * same output. The operation is reversible.
     *
     * @param x the value
     * @return the hash value
     */
    public static long hash64(long x) {
        return hash64(x, 100);
    }

    /**
     * Shrink the hash to a value 0..n. Kind of like modulo, but using
     * multiplication and shift, which are faster to compute.
     *
     * @param hash the hash
     * @param n the maximum of the result
     * @return the reduced value
     */
    public static int reduce(int hash, int n) {
        // http://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
        return (int) (((hash & 0xffffffffL) * (n & 0xffffffffL)) >>> 32);
    }

} 