/*
    * 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.commons.rng.core.source64;
  
  import java.util.stream.Stream;
  import org.apache.commons.rng.JumpableUniformRandomProvider;
  import org.apache.commons.rng.SplittableUniformRandomProvider;
  import org.apache.commons.rng.UniformRandomProvider;
  import org.apache.commons.rng.core.util.NumberFactory;
  import org.apache.commons.rng.core.util.RandomStreams;
  
  /**
   * A 64-bit all purpose generator.
   *
   * <p>This is a member of the LXM family of generators: L=Linear congruential generator;
   * X=Xor based generator; and M=Mix. This member uses a 64-bit LCG and 1024-bit Xor-based
   * generator. It is named as {@code "L64X1024MixRandom"} in the {@code java.util.random}
   * package introduced in JDK 17; the LXM family is described in further detail in:
   *
   * <blockquote>Steele and Vigna (2021) LXM: better splittable pseudorandom number generators
   * (and almost as fast). Proceedings of the ACM on Programming Languages, Volume 5,
   * Article 148, pp 1–31.</blockquote>
   *
   * <p>Memory footprint is 1184 bits and the period is 2<sup>64</sup> (2<sup>1024</sup> - 1).
   *
   * <p>This generator implements
   * {@link org.apache.commons.rng.LongJumpableUniformRandomProvider LongJumpableUniformRandomProvider}.
   * In addition instances created with a different additive parameter for the LCG are robust
   * against accidental correlation in a multi-threaded setting. The additive parameters must be
   * different in the most significant 63-bits.
   *
   * <p>This generator implements
   * {@link org.apache.commons.rng.SplittableUniformRandomProvider SplittableUniformRandomProvider}.
   * The stream of generators created using the {@code splits} methods support parallelisation
   * and are robust against accidental correlation by using unique values for the additive parameter
   * for each instance in the same stream. The primitive streaming methods support parallelisation
   * but with no assurances of accidental correlation; each thread uses a new instance with a
   * randomly initialised state.
   *
   * @see <a href="https://doi.org/10.1145/3485525">Steele &amp; Vigna (2021) Proc. ACM Programming
   *      Languages 5, 1-31</a>
   * @see <a href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/random/package-summary.html">
   *      JDK 17 java.util.random javadoc</a>
   * @since 1.5
   */
  public class L64X1024Mix extends AbstractL64 implements SplittableUniformRandomProvider {
      /** Size of the seed vector. */
      private static final int SEED_SIZE = 18;
      /** Size of the XBG state vector. */
      private static final int XBG_STATE_SIZE = 16;
      /** Size of the LCG state vector. */
      private static final int LCG_STATE_SIZE = SEED_SIZE - XBG_STATE_SIZE;
      /** LCG multiplier. */
      private static final long M = LXMSupport.M64;
  
      /** State of the XBG. */
      private final long[] x = new long[XBG_STATE_SIZE];
      /** Index in "state" array. */
      private int index;
  
      /**
       * Creates a new instance.
       *
       * @param seed Initial seed.
       * If the length is larger than 18, only the first 18 elements will
       * be used; if smaller, the remaining elements will be automatically
       * set. A seed containing all zeros in the last 16 elements
       * will create a non-functional XBG sub-generator and a low
       * quality output with a period of 2<sup>64</sup>.
       *
       * <p>The 1st element is used to set the LCG increment; the least significant bit
       * is set to odd to ensure a full period LCG. The 2nd element is used
       * to set the LCG state.</p>
       */
      public L64X1024Mix(long[] seed) {
          super(seed = extendSeed(seed, SEED_SIZE));
          System.arraycopy(seed, SEED_SIZE - XBG_STATE_SIZE, x, 0, XBG_STATE_SIZE);
          // Initialising to 15 ensures that (index + 1) % 16 == 0 and the
          // first state picked from the XBG generator is state[0].
          index = XBG_STATE_SIZE - 1;
      }
  
      /**
       * Creates a copy instance.
      *
      * @param source Source to copy.
      */
     protected L64X1024Mix(L64X1024Mix source) {
         super(source);
         System.arraycopy(source.x, 0, x, 0, XBG_STATE_SIZE);
         index = source.index;
     }
 
     /** {@inheritDoc} */
     @Override
     protected byte[] getStateInternal() {
         final long[] s = new long[XBG_STATE_SIZE + 1];
         System.arraycopy(x, 0, s, 0, XBG_STATE_SIZE);
         s[XBG_STATE_SIZE] = index;
         return composeStateInternal(NumberFactory.makeByteArray(s),
                                     super.getStateInternal());
     }
 
     /** {@inheritDoc} */
     @Override
     protected void setStateInternal(byte[] s) {
         final byte[][] c = splitStateInternal(s, (XBG_STATE_SIZE + 1) * Long.BYTES);
 
         final long[] tmp = NumberFactory.makeLongArray(c[0]);
         System.arraycopy(tmp, 0, x, 0, XBG_STATE_SIZE);
         index = (int) tmp[XBG_STATE_SIZE];
 
         super.setStateInternal(c[1]);
     }
 
     /** {@inheritDoc} */
     @Override
     public long next() {
         // LXM generate.
         // Old state is used for the output allowing parallel pipelining
         // on processors that support multiple concurrent instructions.
 
         final int q = index;
         index = (q + 1) & 15;
         final long s0 = x[index];
         long s15 = x[q];
         final long s = ls;
 
         // Mix
         final long z = LXMSupport.lea64(s + s0);
 
         // LCG update
         ls = M * s + la;
 
         // XBG update
         s15 ^= s0;
         x[q] = Long.rotateLeft(s0, 25) ^ s15 ^ (s15 << 27);
         x[index] = Long.rotateLeft(s15, 36);
 
         return z;
     }
 
     /**
      * {@inheritDoc}
      *
      * <p>The jump size is the equivalent of moving the state <em>backwards</em> by
      * (2<sup>1024</sup> - 1) positions. It can provide up to 2<sup>64</sup>
      * non-overlapping subsequences.
      */
     @Override
     public UniformRandomProvider jump() {
         return super.jump();
     }
 
     /**
      * {@inheritDoc}
      *
      * <p>The jump size is the equivalent of moving the state <em>backwards</em> by
      * 2<sup>32</sup> (2<sup>1024</sup> - 1) positions. It can provide up to
      * 2<sup>32</sup> non-overlapping subsequences of length 2<sup>32</sup>
      * (2<sup>1024</sup> - 1); each subsequence can provide up to 2<sup>32</sup>
      * non-overlapping subsequences of length (2<sup>1024</sup> - 1) using the
      * {@link #jump()} method.
      */
     @Override
     public JumpableUniformRandomProvider longJump() {
         return super.longJump();
     }
 
     /** {@inheritDoc} */
     @Override
     AbstractL64 copy() {
         // This exists to ensure the jump function performed in the super class returns
         // the correct class type. It should not be public.
         return new L64X1024Mix(this);
     }
 
     /** {@inheritDoc} */
     @Override
     public SplittableUniformRandomProvider split(UniformRandomProvider source) {
         return create(source.nextLong(), source);
     }
 
     /** {@inheritDoc} */
     @Override
     public Stream<SplittableUniformRandomProvider> splits(long streamSize, SplittableUniformRandomProvider source) {
         return RandomStreams.generateWithSeed(streamSize, source, L64X1024Mix::create);
     }
 
     /**
      * Create a new instance using the given {@code seed} and {@code source} of randomness
      * to initialise the instance.
      *
      * @param seed Seed used to initialise the instance.
      * @param source Source of randomness used to initialise the instance.
      * @return A new instance.
      */
     private static SplittableUniformRandomProvider create(long seed, UniformRandomProvider source) {
         final long[] s = new long[SEED_SIZE];
         // LCG state. The addition uses the input seed.
         // The LCG addition parameter is set to odd so left-shift the seed.
         s[0] = seed << 1;
         s[1] = source.nextLong();
         // XBG state must not be all zero
         long x = 0;
         for (int i = LCG_STATE_SIZE; i < s.length; i++) {
             s[i] = source.nextLong();
             x |= s[i];
         }
         if (x == 0) {
             // SplitMix style seed ensures at least one non-zero value
             x = s[LCG_STATE_SIZE - 1];
             for (int i = LCG_STATE_SIZE; i < s.length; i++) {
                 s[i] = LXMSupport.lea64(x);
                 x += LXMSupport.GOLDEN_RATIO_64;
             }
         }
         return new L64X1024Mix(s);
     }
 }