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/*
 * Copyright © 2017 Cask Data, Inc.
 *
 * Licensed 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 co.cask.wrangler.sampling;

import co.cask.wrangler.api.annotations.Public;

import java.util.Random;

/**
 * Implement a random number generator based on the XORShift algorithm discovered by George Marsaglia.
 * This RNG is observed 4.5 times faster than {@link java.util.Random} in benchmark, with the cost
 * that abandon thread-safety. So it's recommended to create a new {@link XORShiftRNG} for each
 * thread.
 *
 * @see XORShift Algorithm Paper
 */
@Public
public class XORShiftRNG extends Random {

  private static final long serialVersionUID = -825722456120842841L;
  private long seed;

  public XORShiftRNG() {
    this(System.nanoTime());
  }

  public XORShiftRNG(long input) {
    super(input);
    this.seed = MathUtils.murmurHash((int) input) ^ MathUtils.murmurHash((int) (input >>> 32));
  }

  /**
   * All other methods like nextInt()/nextDouble()... depends on this, so we just need to overwrite
   * this.
   *
   * @param bits Random bits
   * @return The next pseudorandom value from this random number
   * generator's sequence
   */
  @Override
  public int next(int bits) {
    long nextSeed = seed ^ (seed << 21);
    nextSeed ^= (nextSeed >>> 35);
    nextSeed ^= (nextSeed << 4);
    seed = nextSeed;
    return (int) (nextSeed & ((1L << bits) - 1));
  }
}