path: root/src/main/java/gregtech/api/objects/XSTR.java

package gregtech.api.objects;

import java.util.Random;
import java.util.concurrent.atomic.AtomicLong;

/*
 * TODO: Check the validity of the algorithm.
 * There is a claim that this particular implementation is not faithful to the articles it links, skewing the
 * distribution.
 */
/**
 * XSTR - Xorshift ThermiteRandom Modified by Bogdan-G 03.06.2016 version 0.0.4
 * <p>
 * A subclass of java.util.random that implements the Xorshift random number generator
 * <p>
 * - it is 30% faster than the generator from Java's library - it produces random sequences of higher quality than
 * java.util.Random - this class also provides a clone() function
 * <p>
 * Usage: XSRandom rand = new XSRandom(); //Instantiation x = rand.nextInt(); //pull a random number
 * <p>
 * To use the class in legacy code, you may also instantiate an XSRandom object and assign it to a java.util.Random
 * object: java.util.Random rand = new XSRandom();
 * <p>
 * for an explanation of the algorithm, see http://demesos.blogspot.com/2011/09/pseudo-random-number-generators.html
 *
 * @author Wilfried Elmenreich University of Klagenfurt/Lakeside Labs http://www.elmenreich.tk
 *         <p>
 *         This code is released under the GNU Lesser General Public License Version 3
 *         http://www.gnu.org/licenses/lgpl-3.0.txt
 */
public class XSTR extends Random {

    private static final long serialVersionUID = 6208727693524452904L;
    private long seed;
    private long last;
    private static final long GAMMA = 0x9e3779b97f4a7c15L;
    private static final int PROBE_INCREMENT = 0x9e3779b9;
    private static final long SEEDER_INCREMENT = 0xbb67ae8584caa73bL;
    private static final double DOUBLE_UNIT = 0x1.0p-53; // 1.0 / (1L << 53)
    private static final float FLOAT_UNIT = 0x1.0p-24f; // 1.0f / (1 << 24)
    private static final AtomicLong seedUniquifier = new AtomicLong(8682522807148012L);
    public static final XSTR XSTR_INSTANCE = new XSTR() {

        @Override
        public synchronized void setSeed(long seed) {
            if (!Thread.currentThread()
                .getStackTrace()[2].getClassName()
                    .equals(Random.class.getName()))
                throw new NoSuchMethodError("This is meant to be shared!, leave seed state alone!");
        }
    };

    /*
     * MODIFIED BY: Robotia Modification: Implemented Random class seed generator
     */
    /**
     * Creates a new pseudo random number generator. The seed is initialized to the current time, as if by
     * <code>setSeed(System.currentTimeMillis());</code>.
     */
    public XSTR() {
        this(seedUniquifier() ^ System.nanoTime());
    }

    private static long seedUniquifier() {
        // L'Ecuyer, "Tables of Linear Congruential Generators of
        // Different Sizes and Good Lattice Structure", 1999
        for (;;) {
            long current = seedUniquifier.get();
            long next = current * 181783497276652981L;
            if (seedUniquifier.compareAndSet(current, next)) {
                return next;
            }
        }
    }

    /**
     * Creates a new pseudo random number generator, starting with the specified seed, using
     * <code>setSeed(seed);</code>.
     *
     * @param seed the initial seed
     */
    public XSTR(long seed) {
        this.seed = seed;
    }

    @Override
    public boolean nextBoolean() {
        return next(1) != 0;
    }

    @Override
    public double nextDouble() {
        return (((long) (next(26)) << 27) + next(27)) * DOUBLE_UNIT;
    }

    /**
     * Returns the current state of the seed, can be used to clone the object
     *
     * @return the current seed
     */
    public synchronized long getSeed() {
        return seed;
    }

    /**
     * Sets the seed for this pseudo random number generator. As described above, two instances of the same random
     * class, starting with the same seed, produce the same results, if the same methods are called.
     *
     * @param seed the new seed
     */
    @Override
    public synchronized void setSeed(long seed) {
        this.seed = seed;
    }

    /**
     * @return Returns an XSRandom object with the same state as the original
     */
    @Override
    public XSTR clone() {
        return new XSTR(getSeed());
    }

    /**
     * Implementation of George Marsaglia's Xorshift random generator that is 30% faster and better quality than the
     * built-in java.util.random.
     *
     * @param nbits number of bits to shift the result for
     * @return a random integer
     * @see <a href="https://www.javamex.com/tutorials/random_numbers/xorshift.shtml">the Xorshift article</a>
     */
    @Override
    public int next(int nbits) {
        long x = seed;
        x ^= (x << 21);
        x ^= (x >>> 35);
        x ^= (x << 4);
        seed = x;
        x &= ((1L << nbits) - 1);
        return (int) x;
    }

    boolean haveNextNextGaussian = false;
    double nextNextGaussian = 0;

    @Override
    public synchronized double nextGaussian() {
        // See Knuth, ACP, Section 3.4.1 Algorithm C.
        if (haveNextNextGaussian) {
            haveNextNextGaussian = false;
            return nextNextGaussian;
        } else {
            double v1, v2, s;
            do {
                v1 = 2 * nextDouble() - 1; // between -1 and 1
                v2 = 2 * nextDouble() - 1; // between -1 and 1
                s = v1 * v1 + v2 * v2;
            } while (s >= 1 || s == 0);
            double multiplier = StrictMath.sqrt(-2 * StrictMath.log(s) / s);
            nextNextGaussian = v2 * multiplier;
            haveNextNextGaussian = true;
            return v1 * multiplier;
        }
    }

    /**
     * Returns a pseudorandom, uniformly distributed {@code int} value between 0 (inclusive) and the specified value
     * (exclusive), drawn from this random number generator's sequence. The general contract of {@code nextInt} is that
     * one {@code int} value in the specified range is pseudorandomly generated and returned. All {@code bound} possible
     * {@code int} values are produced with (approximately) equal probability. The method {@code nextInt(int bound)} is
     * implemented by class {@code Random} as if by:
     *
     * <pre>
     *  {@code
     * public int nextInt(int bound) {
     *   if (bound <= 0)
     *     throw new IllegalArgumentException("bound must be positive");
     *
     *   if ((bound & -bound) == bound)  // i.e., bound is a power of 2
     *     return (int)((bound * (long)next(31)) >> 31);
     *
     *   int bits, val;
     *   do {
     *       bits = next(31);
     *       val = bits % bound;
     *   } while (bits - val + (bound-1) < 0);
     *   return val;
     * }}
     * </pre>
     *
     * <p>
     * The next method is only approximately an unbiased source of independently chosen bits. If it were a perfect
     * source of randomly chosen bits, then the algorithm shown would choose {@code int} values from the stated range
     * with perfect uniformity.
     * <p>
     * The algorithm is slightly tricky. It rejects values that would result in an uneven distribution (due to the fact
     * that 2^31 is not divisible by n). The probability of a value being rejected depends on n. The worst case is
     * n=2^30+1, for which the probability of a reject is 1/2, and the expected number of iterations before the loop
     * terminates is 2.
     * <p>
     * The algorithm treats the case where n is a power of two specially: it returns the correct number of high-order
     * bits from the underlying pseudo-random number generator. In the absence of special treatment, the correct number
     * of <i>low-order</i> bits would be returned. Linear congruential pseudo-random number generators such as the one
     * implemented by this class are known to have short periods in the sequence of values of their low-order bits.
     * Thus, this special case greatly increases the length of the sequence of values returned by successive calls to
     * this method if n is a small power of two.
     *
     * @param bound the upper bound (exclusive). Must be positive.
     * @return the next pseudorandom, uniformly distributed {@code int} value between zero (inclusive) and {@code bound}
     *         (exclusive) from this random number generator's sequence
     * @throws IllegalArgumentException if bound is not positive
     * @since 1.2
     */
    @Override
    public int nextInt(int bound) {
        last = seed ^ (seed << 21);
        last ^= (last >>> 35);
        last ^= (last << 4);
        seed = last;
        int out = (int) last % bound;
        return (out < 0) ? -out : out;
    }

    @Override
    public int nextInt() {
        return next(32);
    }

    @Override
    public float nextFloat() {
        return next(24) * FLOAT_UNIT;
    }

    @Override
    public long nextLong() {
        // it's okay that the bottom word remains signed.
        return ((long) (next(32)) << 32) + next(32);
    }

    @Override
    public void nextBytes(byte[] bytes_arr) {
        for (int iba = 0, lenba = bytes_arr.length; iba < lenba;)
            for (int rndba = nextInt(), nba = Math.min(lenba - iba, Integer.SIZE / Byte.SIZE); nba--
                > 0; rndba >>= Byte.SIZE) bytes_arr[iba++] = (byte) rndba;
    }
}