// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License.

package com.azure.cosmos.implementation.routing;

/*
 * The MurmurHash3 algorithm was created by Austin Appleby and placed in the public domain.
 * This java port was authored by Yonik Seeley and also placed into the public domain.
 * The author hereby disclaims copyright to this source code.
 * <p>
 * This produces exactly the same hash values as the final C++
 * version of MurmurHash3 and is thus suitable for producing the same hash values across
 * platforms.
 * <p>
 * The 32 bit x86 version of this hash should be the fastest variant for relatively short keys like ids.
 * See http://github.com/yonik/java_util for future updates to this file.
 */
public class MurmurHash3_32  {
    /**
     * Returns the MurmurHash3_x86_32 hash.
     *
     * @param data a byte array containing the data to be hashed
     * @param len  an integer indicating the length of data
     * @param seed an integer to be used as hash seed
     * @return     the hash value
     */
    @SuppressWarnings("fallthrough") // Fall through is expected
    public static int hash(byte[] data, int len, int seed) {
        final int c1 = 0xcc9e2d51;
        final int c2 = 0x1b873593;

        int h1 = seed;
        int roundedEnd = (len & 0xfffffffc);  // round down to 4 byte block

        for (int i = 0; i < roundedEnd; i += 4) {
            // little endian load order
            int k1 = (data[i] & 0xff) | ((data[i + 1] & 0xff) << 8) | ((data[i + 2] & 0xff) << 16) | (data[i + 3] << 24);
            k1 *= c1;
            k1 = (k1 << 15) | (k1 >>> 17);  // ROTL32(k1,15);
            k1 *= c2;

            h1 ^= k1;
            h1 = (h1 << 13) | (h1 >>> 19);  // ROTL32(h1,13);
            h1 = h1 * 5 + 0xe6546b64;
        }

        // tail
        int k1 = 0;

        switch (len & 0x03) {
            case 3:
                k1 = (data[roundedEnd + 2] & 0xff) << 16;
                // fallthrough
            case 2:
                k1 |= (data[roundedEnd + 1] & 0xff) << 8;
                // fallthrough
            case 1:
                k1 |= (data[roundedEnd] & 0xff);
                k1 *= c1;
                k1 = (k1 << 15) | (k1 >>> 17);  // ROTL32(k1,15);
                k1 *= c2;
                h1 ^= k1;
        }

        // finalization
        h1 ^= len;

        // fmix(h1);
        h1 ^= h1 >>> 16;
        h1 *= 0x85ebca6b;
        h1 ^= h1 >>> 13;
        h1 *= 0xc2b2ae35;
        h1 ^= h1 >>> 16;

        return h1;
    }
}
