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package java.util;

import java.math.BigInteger;

import java.util.concurrent.atomic.AtomicLong;

/**

 * A generator of uniform pseudorandom values applicable for use in

 * (among other contexts) isolated parallel computations that may

 * generate subtasks.  Class {@code Xoshiro256StarStar} implements

 * interfaces {@link java.util.Rng} and {@link java.util.LeapableRng},

 * and therefore supports methods for producing pseudorandomly chosen

 * numbers of type {@code int}, {@code long}, {@code float}, and {@code double}

 * as well as creating new {@code Xoshiro256StarStar} objects

 * by "jumping" or "leaping".

 *

 * <p>Series of generated values pass the TestU01 BigCrush and PractRand test suites

 * that measure independence and uniformity properties of random number generators.

 * (Most recently validated with

 * <a href="http://simul.iro.umontreal.ca/testu01/tu01.html">version 1.2.3 of TestU01</a>

 * and <a href="http://pracrand.sourceforge.net">version 0.90 of PractRand</a>.

 * Note that TestU01 BigCrush was used to test not only values produced by the {@code nextLong()}

 * method but also the result of bit-reversing each value produced by {@code nextLong()}.)

 * These tests validate only the methods for certain

 * types and ranges, but similar properties are expected to hold, at

 * least approximately, for others as well.

 *

 * <p>The class {@code Xoshiro256StarStar} uses the {@code xoshiro256} algorithm,

 * version 1.0 (parameters 17, 45), with the "**" scrambler (a mixing function).

 * Its state consists of four {@code long} fields {@code x0}, {@code x1}, {@code x2},

 * and {@code x3}, which can take on any values provided that they are not all zero.

 * The period of this generator is 2<sup>256</sup>-1.

 *

 * <p>The 64-bit values produced by the {@code nextLong()} method are equidistributed.

 * To be precise, over the course of the cycle of length 2<sup>256</sup>-1,

 * each nonzero {@code long} value is generated 2<sup>192</sup> times,

 * but the value 0 is generated only 2<sup>192</sup>-1 times.

 * The values produced by the {@code nextInt()}, {@code nextFloat()}, and {@code nextDouble()}

 * methods are likewise equidistributed.

 *

 * <p>In fact, the 64-bit values produced by the {@code nextLong()} method are 4-equidistributed.

 * To be precise: consider the (overlapping) length-4 subsequences of the cycle of 64-bit

 * values produced by {@code nextLong()} (assuming no other methods are called that would

 * affect the state).  There are 2<sup>256</sup>-1 such subsequences, and each subsequence,

 * which consists of 4 64-bit values, can have one of 2<sup>256</sup> values.  Of those

 * 2<sup>256</sup> subsequence values, each one is generated exactly once over the course

 * of the entire cycle, except that the subsequence (0, 0, 0, 0) never appears.

 * The values produced by the {@code nextInt()}, {@code nextFloat()}, and {@code nextDouble()}

 * methods are likewise 4-equidistributed, but note that that the subsequence (0, 0, 0, 0)

 * can also appear (but occurring somewhat less frequently than all other subsequences),

 * because the values produced by those methods have fewer than 64 randomly chosen bits.

 *

 * <p>Instances {@code Xoshiro256StarStar} are <em>not</em> thread-safe.

 * They are designed to be used so that each thread as its own instance.

 * The methods {@link #jump} and {@link #leap} and {@link #jumps} and {@link #leaps}

 * can be used to construct new instances of {@code Xoshiro256StarStar} that traverse

 * other parts of the state cycle.

 *

 * <p>Instances of {@code Xoshiro256StarStar} are not cryptographically

 * secure.  Consider instead using {@link java.security.SecureRandom}

 * in security-sensitive applications. Additionally,

 * default-constructed instances do not use a cryptographically random

 * seed unless the {@linkplain System#getProperty system property}

 * {@code java.util.secureRandomSeed} is set to {@code true}.

 *

 * @author  Guy Steele

 * @since   1.9

 */

public final class Xoshiro256StarStar implements LeapableRng {

    /*

     * Implementation Overview.

     *

     * This is an implementation of the xoroshiro128** algorithm written

     * in 2018 by David Blackman and Sebastiano Vigna (vigna@acm.org).

     * See http://xoshiro.di.unimi.it and these two papers:

     *

     *    Sebastiano Vigna. 2016. An Experimental Exploration of Marsaglia's

     *    xorshift Generators, Scrambled. ACM Transactions on Mathematical

     *    Software 42, 4, Article 30 (June 2016), 23 pages.

     *    https://doi.org/10.1145/2845077

     *

     *    David Blackman and Sebastiano Vigna.  2018.  Scrambled Linear

     *    Pseudorandom Number Generators.  Computing Research Repository (CoRR).

     *    http://arxiv.org/abs/1805.01407

     *

     * The jump operation moves the current generator forward by 2*128

     * steps; this has the same effect as calling nextLong() 2**128

     * times, but is much faster.  Similarly, the leap operation moves

     * the current generator forward by 2*192 steps; this has the same

     * effect as calling nextLong() 2**192 times, but is much faster.

     * The copy method may be used to make a copy of the current

     * generator.  Thus one may repeatedly and cumulatively copy and

     * jump to produce a sequence of generators whose states are well

     * spaced apart along the overall state cycle (indeed, the jumps()

     * and leaps() methods each produce a stream of such generators).

     * The generators can then be parceled out to other threads.

     *

     * File organization: First static fields, then instance

     * fields, then constructors, then instance methods.

     */

    /* ---------------- static fields ---------------- */

    /**

     * The seed generator for default constructors.

     */

    private static final AtomicLong defaultGen = new AtomicLong(RngSupport.initialSeed());

    /*

     * The period of this generator, which is 2**256 - 1.

     */

    private static final BigInteger thePeriod =

	BigInteger.ONE.shiftLeft(256).subtract(BigInteger.ONE);

    /* ---------------- instance fields ---------------- */

    /**

     * The per-instance state.

     * At least one of the four fields x0, x1, x2, and x3 must be nonzero.

     */

    private long x0, x1, x2, x3;

    /* ---------------- constructors ---------------- */

    /**

     * Basic constructor that initializes all fields from parameters.

     * It then adjusts the field values if necessary to ensure that

     * all constraints on the values of fields are met.

     *

     * @param x0 first word of the initial state

     * @param x1 second word of the initial state

     * @param x2 third word of the initial state

     * @param x3 fourth word of the initial state

     */

    public Xoshiro256StarStar(long x0, long x1, long x2, long x3) {

	this.x0 = x0;

        this.x1 = x1;

        this.x2 = x2;

        this.x3 = x3;

	// If x0, x1, x2, and x3 are all zero, we must choose nonzero values.

        if ((x0 | x1 | x2 | x3) == 0) {

	    // At least three of the four values generated here will be nonzero.

	    this.x0 = RngSupport.mixStafford13(x0 += RngSupport.GOLDEN_RATIO_64);

	    this.x1 = (x0 += RngSupport.GOLDEN_RATIO_64);

	    this.x2 = (x0 += RngSupport.GOLDEN_RATIO_64);

	    this.x3 = (x0 += RngSupport.GOLDEN_RATIO_64);

	}

    }

    /**

     * Creates a new instance of {@code Xoshiro256StarStar} using the

     * specified {@code long} value as the initial seed. Instances of

     * {@code Xoshiro256StarStar} created with the same seed in the same

     * program generate identical sequences of values.

     *

     * @param seed the initial seed

     */

    public Xoshiro256StarStar(long seed) {

	// Using a value with irregularly spaced 1-bits to xor the seed

	// argument tends to improve "pedestrian" seeds such as 0 or

	// other small integers.  We may as well use SILVER_RATIO_64.

	//

	// The x values are then filled in as if by a SplitMix PRNG with

	// GOLDEN_RATIO_64 as the gamma value and Stafford13 as the mixer.

        this(RngSupport.mixStafford13(seed ^= RngSupport.SILVER_RATIO_64),

	     RngSupport.mixStafford13(seed += RngSupport.GOLDEN_RATIO_64),

	     RngSupport.mixStafford13(seed += RngSupport.GOLDEN_RATIO_64),

	     RngSupport.mixStafford13(seed + RngSupport.GOLDEN_RATIO_64));

    }

    /**

     * Creates a new instance of {@code Xoshiro256StarStar} that is likely to

     * generate sequences of values that are statistically independent

     * of those of any other instances in the current program execution,

     * but may, and typically does, vary across program invocations.

     */

    public Xoshiro256StarStar() {

	// Using GOLDEN_RATIO_64 here gives us a good Weyl sequence of values.

        this(defaultGen.getAndAdd(RngSupport.GOLDEN_RATIO_64));

    }

    /**

     * Creates a new instance of {@code Xoshiro256StarStar} using the specified array of

     * initial seed bytes. Instances of {@code Xoshiro256StarStar} created with the same

     * seed array in the same program execution generate identical sequences of values.

     *

     * @param seed the initial seed

     */

    public Xoshiro256StarStar(byte[] seed) {

	// Convert the seed to 4 long values, which are not all zero.

	long[] data = RngSupport.convertSeedBytesToLongs(seed, 4, 4);

	long x0 = data[0], x1 = data[1], x2 = data[2], x3 = data[3];

        this.x0 = x0;

        this.x1 = x1;

        this.x2 = x2;

        this.x3 = x3;

    }

    /* ---------------- public methods ---------------- */

    public Xoshiro256StarStar copy() { return new Xoshiro256StarStar(x0, x1, x2, x3); }

    /**

     * Returns a pseudorandom {@code long} value.

     *

     * @return a pseudorandom {@code long} value

     */

    public long nextLong() {

	final long z = x0;

	long q0 = x0, q1 = x1, q2 = x2, q3 = x3;	

	{ long t = q1 << 17; q2 ^= q0; q3 ^= q1; q1 ^= q2; q0 ^= q3; q2 ^= t; q3 = Long.rotateLeft(q3, 45); }  // xoshiro256 1.0

	x0 = q0; x1 = q1; x2 = q2; x3 = q3;

	return Long.rotateLeft(z * 5, 7) * 9;  // "starstar" mixing function

    }

    public BigInteger period() { return thePeriod; }

    public double defaultJumpDistance() { return 0x1.0p64; }

    public double defaultLeapDistance() { return 0x1.0p96; }

    private static final long[] JUMP_TABLE = {

	0x180ec6d33cfd0abaL, 0xd5a61266f0c9392cL, 0xa9582618e03fc9aaL, 0x39abdc4529b1661cL };

    private static final long[] LEAP_TABLE = {

	0x76e15d3efefdcbbfL, 0xc5004e441c522fb3L, 0x77710069854ee241L, 0x39109bb02acbe635L };

/* This is the jump function for the generator. It is equivalent

   to 2**128 calls to next(); it can be used to generate 2**128

   non-overlapping subsequences for parallel computations. */

    public void jump() { jumpAlgorithm(JUMP_TABLE); }

/* This is the long-jump function for the generator. It is equivalent to

   2**192 calls to next(); it can be used to generate 2**64 starting points,

   from each of which jump() will generate 2**64 non-overlapping

   subsequences for parallel distributed computations. */

    public void leap() { jumpAlgorithm(LEAP_TABLE); }

    private void jumpAlgorithm(long[] table) {

	long s0 = 0, s1 = 0, s2 = 0, s3 = 0;

	for (int i = 0; i < table.length; i++) {

	    for (int b = 0; b < 64; b++) {

		if ((table[i] & (1L << b)) != 0) {

		    s0 ^= x0;

		    s1 ^= x1;

		    s2 ^= x2;

		    s3 ^= x3;

		}

		nextLong();

	    }

	    x0 = s0;

	    x1 = s1;

	    x2 = s2;

	    x3 = s3;

	}

    }

}