gistfile1.java

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package org.jruby.util;

import java.math.BigInteger;
import java.util.Arrays;
import java.util.concurrent.atomic.AtomicStampedReference;

public class Random extends AtomicStampedReference<int[]> {
    public static int N = 624;
    private static int M = 397;
    private static int MATRIX_A = 0x9908b0df; /* constant vector a */
    private static int UMASK = 0x80000000; /* most significant w-r bits */
    private static int LMASK = 0x7fffffff; /* least significant r bits */

    private static int MIXBITS(int u, int v) {
        return (u & UMASK) | (v & LMASK);
    }

    private static int TWIST(int u, int v) {
        return (MIXBITS(u, v) >>> 1) ^ (((v & 1) != 0) ? MATRIX_A : 0);
    }

    public Random(int s) {
        super(initGenrand(s), 1);
    }

    public Random(int[] initKey) {
        super(initByArray(initKey), 1);
    }

    public Random(Random orig) {
        super(null, 0);
        int[] rhsStamp = new int[1], rhsState = orig.get(rhsStamp);
        set(Arrays.copyOf(rhsState, N), rhsStamp[0]);
    }

    public Random(int[] state, int left) {
        super(null, 0);
        if (state.length != getReference().length) {
            throw new IllegalStateException("wrong state length: " + state.length);
        }
        // use existing initial state[]
        System.arraycopy(state, 0, getReference(), 0, N);
        set(getReference(), left);
    }

    private static int[][] getAllFrom(Random orig) {
        int[] rhsStamp = new int[1], rhsState = orig.get(rhsStamp);
        return new int[][] {rhsState, rhsStamp};
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        } else if (!(obj instanceof Random)) {
            return false;
        }
        Random rhs = (Random) obj;
        int[] rhsStamp = new int[1], rhsState = rhs.get(rhsStamp);
        int[] lhsStamp = new int[1], lhsState = get(lhsStamp);
        return (lhsStamp[0] == rhsStamp[0]) && Arrays.equals(lhsState, rhsState);
    }

    @Override
    public int hashCode() {
        // Using 17 as the initializer, 37 as the multiplier.
        int[] stamp = new int[1], state = get(stamp);
        return (629 + stamp[0]) * 37 + Arrays.hashCode(state);
    }
    
    private static int[] initGenrand(int s) {
        int[] state = new int[N];
        state[0] = s;
        for (int j = 1; j < N; j++) {
            state[j] = (1812433253 * (state[j - 1] ^ (state[j - 1] >>> 30)) + j);
        }
        return state;
    }

    private static int[] initByArray(int[] initKey) {
        int[] newState = initGenrand(19650218);
        int len = initKey.length;
        int i = 1;
        int j = 0;
        int k = N > len ? N : len;
        int[] state = Arrays.copyOf(newState, N);
        for (; k > 0; k--) {
            state[i] = (state[i] ^ ((state[i - 1] ^ (state[i - 1] >>> 30)) * 1664525)) + initKey[j]
                    + j;
            i++;
            j++;
            if (i >= N) {
                state[0] = state[N - 1];
                i = 1;
            }
            if (j >= len) {
                j = 0;
            }
        }
        for (k = N - 1; k > 0; k--) {
            state[i] = (state[i] ^ ((state[i - 1] ^ (state[i - 1] >>> 30)) * 1566083941)) - i;
            i++;
            if (i >= N) {
                state[0] = state[N - 1];
                i = 1;
            }
        }
        state[0] = 0x80000000;

        return state;
    }

    private static void nextState(int[] state) {
        int p = 0;

        for (int j = N - M + 1; --j > 0; p++) {
            state[p] = state[p + M] ^ TWIST(state[p + 0], state[p + 1]);
        }

        for (int j = M; --j > 0; p++) {
            state[p] = state[p + M - N] ^ TWIST(state[p + 0], state[p + 1]);
        }

        state[p] = state[p + M - N] ^ TWIST(state[p + 0], state[0]);
    }

    public int genrandInt32() {
        int y;
        int[] state;
        int[] stateCopy = null;
        int left;

        while (true) {
            state = getReference();
            left = getStamp();
            if (left > 1) {
                if (compareAndSet(state, state, left, left - 1)) {
                    // success! update left to the actual value, and break out
                    left--;
                    break;
                } else {
                    // someone beaten us to it, respin
                }
            } else {
                // try to update
                // $left should be exactly one at this point
                assert left == 1;

                // nextState() should be idempotent
                if (stateCopy == null) stateCopy = Arrays.copyOf(state, N); // only create new array once
                else System.arraycopy(state, 0, stateCopy, 0, N);
                nextState(stateCopy);

                // allow only one CAS, don't care about the result, respin
                compareAndSet(state, stateCopy, 1, N);
            }
        }

        y = state[N - left];

        /* Tempering */
        y ^= (y >>> 11);
        y ^= (y << 7) & 0x9d2c5680L;
        y ^= (y << 15) & 0xefc60000L;
        y ^= (y >>> 18);

        return y;
    }

    public double genrandReal() {
        int a = genrandInt32() >>> 5;
        int b = genrandInt32() >>> 6;
        return (a * 67108864.0 + b) * (1.0 / 9007199254740992.0);
    }

    public double genrandReal2() {
        int a = genrandInt32();
        int b = genrandInt32();
        return intPairToRealInclusive(a, b);
    }

    private static final BigInteger INTPAIR_CONST = BigInteger.valueOf((1L << 53) + 1);
    private static final double LDEXP_CONST = Math.pow(2.0, -53);

    // c: ldexp((a<< 32)|b) * ((1<<53)+1) >> 64, -53)
    // TODO: not enough prec...
    private double intPairToRealInclusive(int a, int b) {
        BigInteger c = BigInteger.valueOf(a & 0xffffffffL);
        BigInteger d = BigInteger.valueOf(b & 0xffffffffL);
        return c.shiftLeft(32).or(d).multiply(INTPAIR_CONST).shiftRight(64).doubleValue()
                * LDEXP_CONST;
    }

    public int[] getState() {
        return getReference();
    }

    public int getLeft() {
        return getStamp();
    }
}

Why not extend the ASR and get rid of the indirection?

Interesting idea! I don't imagine it would change perf characteristics a great deal, but it might be worth trying.

I made the modification you suggested, and it does actually help performance! I think I also found a bug in the original.

Excellent, the prefetch helps a bit :-)