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The DSI utilities are a mishmash of classes accumulated during the last twenty 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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/*
 * DSI utilities
 *
 * Copyright (C) 2012-2020 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 .
 *
 */

package it.unimi.dsi.util;

import java.io.Serializable;

import org.apache.commons.math3.random.AbstractRandomGenerator;
import org.apache.commons.math3.random.RandomGenerator;

import it.unimi.dsi.Util;

/** A fast, high-quality 64-bit {@linkplain RandomGenerator pseudorandom number generator} described in “Some long-period random number generators using shift
 * and xors”, ANZIAM Journal 48, C188−C202, 2007. */

public class XorGensRandomGenerator extends AbstractRandomGenerator implements Serializable {
	private static final long serialVersionUID = 0L;
	private static final int WLEN = 64;
	private static final int R = 64;
	private static final int S = 53;
	private static final int A = 33;
	private static final int B = 26;
	private static final int C = 27;
	private static final int D = 29;
	private static final long WEYL = 0x61c8864680b583ebL;
	/** State of the Xorshift generator. */
	private final long[] x = new long[R];
	private long weyl;
	private int i;

	/** Creates a new generator, initializing its seed with {@link Util#randomSeed()}. */
	public XorGensRandomGenerator() {
		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 XorGensRandomGenerator(final long seed) {
		setSeed(seed);
	}

	@Override
	public void setSeed(final long seed) {
		long v = seed == 0 ? -1 : seed; /* v must be nonzero */
		for (int k = WLEN; k > 0; k--) { /* Avoid correlations for close seeds */
			v ^= v << 10;
			v ^= v >>> 15; /* Recurrence has period 2**wlen-1 */
			v ^= v << 4;
			v ^= v >>> 13; /* for wlen = 32 or 64 */
		}

		for (int k = 0; k < R; k++) { /* Initialise circular array */
			v ^= v << 10;
			v ^= v >>> 15;
			v ^= v << 4;
			v ^= v >>> 13;
			x[k] = v;
		}

		i = R - 1;

		long t;
		for (int k = 4 * R; k > 0; k--) { /* Discard first 4*r results */
			t = x[i = (i + 1) & (R - 1)];
			t ^= t << A;
			t ^= t >>> B;
			v = x[(i + (R - S)) & (R - 1)];
			v ^= v << C;
			v ^= v >>> D;
			x[i] = t ^ v;
		}
	}

	@Override
	public long nextLong() {
		long t, v;
		t = x[i = (i + 1) & (R - 1)]; /* Assumes that r is a power of two */
		v = x[(i + (R - S)) & (R - 1)]; /* Index is (i-s) mod r */
		t ^= t << A;
		v ^= v << C;
		t ^= t >>> B; /* (I + L^a)(I + R^b) */
		v ^= v >>> D; /* (I + L^c)(I + R^d) */
		x[i] = (v ^= t); /* Update circular array */
		weyl += WEYL;
		return (v + (weyl ^ (weyl >>> 27)));
	}

	@Override
	public int nextInt() {
		return (int)nextLong();
	}

	@Override
	public int nextInt(final int n) {
		return (int)nextLong(n);
	}

	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 (nextLong() >>> 11) * 0x1.0p-53;
	}

	@Override
	public float nextFloat() {
		return (nextLong() >>> 40) * 0x1.0p-24f;
	}

	@Override
	public boolean nextBoolean() {
		return nextLong() < 0;
	}

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