commons-rng-core/src/main/java/org/apache/commons/rng/core/source64/DotyHumphreySmallFastCounting64.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.rng.core.source64;

import org.apache.commons.rng.core.util.NumberFactory;

/**
 * Implement the Small, Fast, Counting (SFC) 64-bit generator of Chris Doty-Humphrey.
 * The original source is the PractRand test suite by the same author.
 *
 * <p>The state size is 256-bits; the period is a minimum of 2<sup>64</sup> and an
 * average of approximately 2<sup>255</sup>.</p>
 *
 * @see <a href="http://pracrand.sourceforge.net/">PractRand</a>
 * @since 1.3
 */
public class DotyHumphreySmallFastCounting64 extends LongProvider {
    /** Size of the seed. */
    private static final int SEED_SIZE = 3;

    /** State a. */
    private long a;
    /** State b. */
    private long b;
    /** State c. */
    private long c;
    /** Counter. */
    private long counter;

    /**
     * Creates an instance with the given seed.
     *
     * @param seed Initial seed.
     * If the length is larger than 3, only the first 3 elements will
     * be used; if smaller, the remaining elements will be automatically set.
     */
    public DotyHumphreySmallFastCounting64(long[] seed) {
        if (seed.length < SEED_SIZE) {
            final long[] state = new long[SEED_SIZE];
            fillState(state, seed);
            setSeedInternal(state);
        } else {
            setSeedInternal(seed);
        }
    }

    /**
     * Seeds the RNG.
     *
     * @param seed Seed.
     */
    private void setSeedInternal(long[] seed) {
        a = seed[0];
        b = seed[1];
        c = seed[2];
        counter = 1L;
        for (int i = 0; i < 18; i++) {
            next();
        }
    }

    /** {@inheritDoc} */
    @Override
    public final long next() {
        final long tmp = a + b + counter++;
        a = b ^ (b >>> 11);
        b = c + (c << 3);
        c = Long.rotateLeft(c, 24) + tmp;
        return tmp;
    }

    /** {@inheritDoc} */
    @Override
    protected byte[] getStateInternal() {
        return composeStateInternal(NumberFactory.makeByteArray(new long[] {a, b, c, counter}),
                                    super.getStateInternal());
    }

    /** {@inheritDoc} */
    @Override
    protected void setStateInternal(byte[] s) {
        final byte[][] parts = splitStateInternal(s, 4 * 8);

        final long[] tmp = NumberFactory.makeLongArray(parts[0]);
        a = tmp[0];
        b = tmp[1];
        c = tmp[2];
        counter = tmp[3];

        super.setStateInternal(parts[1]);
    }
}