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The DSI utilities are a mish mash of classes accumulated during the last ten years in projects developed at the DSI (Dipartimento di Scienze dell'Informazione, i.e., Information Sciences Department), now DI (Dipartimento di Informatica, i.e., Informatics Department), of the Universita` degli Studi di Milano.
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package it.unimi.dsi.util;

/*		 
 * DSI utilities
 *
 * Copyright (C) 2013-2016 Sebastiano Vigna 
 *
 *  This library is free software; you can redistribute it and/or modify it
 *  under the terms of the GNU Lesser General Public License as published by the Free
 *  Software Foundation; either version 3 of the License, or (at your option)
 *  any later version.
 *
 *  This library is distributed in the hope that it will be useful, but
 *  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 *  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
 *  for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public License
 *  along with this program; if not, see .
 *
 */


import it.unimi.dsi.Util;
import it.unimi.dsi.fastutil.HashCommon;

import java.util.Random;

/** A fast, top-quality {@linkplain Random pseudorandom number generator} that
 * combines a long-period instance of George Marsaglia's Xorshift generators (described in “Xorshift RNGs”, Journal of
 * Statistical Software, 8:1−6, 2003) with a multiplication.
 * 
 * More details can be found in my paper “An experimental exploration of 
 * Marsaglia's xorshift generators, scrambled”, ACM Trans. Math. Softw., 2016, and  
 * on the xorshift*/xorshift+ 
 * generators and the PRNG shootout page.
 *
 * 
Note that this is not a cryptographic-strength pseudorandom number generator. Its period is
 * 2¹⁰²⁴ − 1, which is more than enough for any massive parallel application (it is actually
 * possible to define analogously a generator with period 2⁴⁰⁹⁶ − 1,
 * but its interest is eminently academic). 
 * 
 * 
By using the supplied {@link #jump()} method it is possible to generate non-overlapping long sequences
 * for parallel computations. This class provides also a {@link #split()} method to support recursive parallel computations, in the spirit of 
 * Java 8's SplittableRandom.
 * 
 * @see it.unimi.dsi.util
 * @see Random
 * @see XorShift1024StarRandomGenerator
 */
public class XorShift1024StarRandom extends Random {
	private static final long serialVersionUID = 1L;

	/** The internal state of the algorithm. */
	private long[] s;
	private int p;
	
	/** Creates a new generator seeded using {@link Util#randomSeed()}. */
	public XorShift1024StarRandom() {
		this( Util.randomSeed() );
	}

	/** Creates a new generator using a given seed.
	 * 
	 * @param seed a nonzero seed for the generator (if zero, the generator will be seeded with -1).
	 */
	public XorShift1024StarRandom( final long seed ) {
		super( seed );
	}

	@Override
	protected int next( int bits ) {
		return (int)( nextLong() >>> 64 - bits );
	}
	
	@Override
	public long nextLong() {
		final long s0 = s[ p ];
		long s1 = s[ p = ( p + 1 ) & 15 ];
		s1 ^= s1 << 31;
		return ( s[ p ] = s1 ^ s0 ^ ( s1 >>> 11 ) ^ ( s0 >>> 30 ) ) * 1181783497276652981L;
	}

	@Override
	public int nextInt() {
		return (int)nextLong();
	}
	
	@Override
	public int nextInt( final int n ) {
		return (int)nextLong( n );
	}
	
	/** Returns a pseudorandom uniformly distributed {@code long} value
     * between 0 (inclusive) and the specified value (exclusive), drawn from
     * this random number generator's sequence. The algorithm used to generate
     * the value guarantees that the result is uniform, provided that the
     * sequence of 64-bit values produced by this generator is. 
     * 
     * @param n the positive bound on the random number to be returned.
     * @return the next pseudorandom {@code long} value between {@code 0} (inclusive) and {@code n} (exclusive).
     */
	public long nextLong( final long n ) {
        if ( n <= 0 ) throw new IllegalArgumentException();
		// No special provision for n power of two: all our bits are good.
		for(;;) {
			final long bits = nextLong() >>> 1;
			final long value = bits % n;
			if ( bits - value + ( n - 1 ) >= 0 ) return value;
		}
	}
	
	@Override
	public double nextDouble() {
		return Double.longBitsToDouble( nextLong() >>> 12 | 0x3FFL << 52 ) - 1.0;
	}

	@Override
	public float nextFloat() {
		return Float.intBitsToFloat( (int)( nextLong() >>> 41 ) | 0x3F8 << 20 ) - 1.0f;
	}

	@Override
	public boolean nextBoolean() {
		return nextLong() < 0;
	}
	
	@Override
	public void nextBytes( final byte[] bytes ) {
		int i = bytes.length, n = 0;
		while( i != 0 ) {
			n = Math.min( i, 8 );
			for ( long bits = nextLong(); n-- != 0; bits >>= 8 ) bytes[ --i ] = (byte)bits;
		}
	}

	private static final long JUMP[] = { 0x84242f96eca9c41dL,
			0xa3c65b8776f96855L, 0x5b34a39f070b5837L, 0x4489affce4f31a1eL,
			0x2ffeeb0a48316f40L, 0xdc2d9891fe68c022L, 0x3659132bb12fea70L,
			0xaac17d8efa43cab8L, 0xc4cb815590989b13L, 0x5ee975283d71c93bL,
			0x691548c86c1bd540L, 0x7910c41d10a1e6a5L, 0x0b5fc64563b3e2a8L,
			0x047f7684e9fc949dL, 0xb99181f2d8f685caL, 0x284600e3f30e38c3L
	};

	/** The the jump function for this generator. It is equivalent to 2⁵¹² 
	 * calls to {@link #nextLong()}; it can be used to generate 2⁵¹² 
	 * non-overlapping subsequences for parallel computations. */

	public void jump() {
		final long[] t = new long[ 16 ];
		for ( int i = 0; i < JUMP.length; i++ )
			for ( int b = 0; b < 64; b++ ) {
				if ( ( JUMP[ i ] & 1L << b ) != 0 ) for ( int j = 0; j < 16; j++ )
					t[ j ] ^= s[ ( j + p ) & 15 ];
				nextLong();
			}

		for ( int j = 0; j < 16; j++ )
			s[ ( j + p ) & 15 ] = t[ j ];
	}

    /**
     * Returns a new instance that shares no mutable state 
     * with this instance. The sequence generated by the new instance
     * depends deterministically from the state of this instance,
     * but the possibility that the sequence generated by this
     * instance and by the new instance overlap is negligible.
     *
     * @return the new instance.
     */
	public XorShift1024StarRandom split() {
		final XorShift1024StarRandom split = new XorShift1024StarRandom();
		for( int i = s.length; i-- != 0; ) split.s[ i ] = HashCommon.murmurHash3( s[ i ] );
		split.p = p;
		return split;
	}
	
	/** Sets the seed of this generator.
	 * 
	 * 
The argument will be used to seed a {@link SplitMix64RandomGenerator}, whose output
	 * will in turn be used to seed this generator. This approach makes “warmup” unnecessary,
	 * and makes the possibility of starting from a state 
	 * with a large fraction of bits set to zero astronomically small.
	 * 
	 * @param seed a nonzero seed for the generator (if zero, the generator will be seeded with -1).
	 */
	@Override
	public void setSeed( final long seed ) {
		if ( s == null ) s = new long[ 16 ];
		p = 0;
		final SplitMix64RandomGenerator r = new SplitMix64RandomGenerator( seed );
		for( int i = s.length; i-- != 0; ) s[ i ] = r.nextLong();
	}

	/** Sets the state of this generator.
	 * 
	 * The internal state of the generator will be reset, and the state array filled with the provided array.
	 * 
	 * @param state an array of 16 longs; at least one must be nonzero.
	 * @param p the internal index. 
	 */
	public void setState( final long[] state, final int p ) {
		if ( state.length != s.length ) throw new IllegalArgumentException( "The argument array contains " + state.length + " longs instead of " + s.length );
		System.arraycopy( state, 0, s, 0, s.length );
		this.p = p;
	}
}