org.apache.mahout.common.RandomUtils.java Source code

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Here is the source code for org.apache.mahout.common.RandomUtils.java

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/**
 * 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.mahout.common;

import java.util.Collections;
import java.util.Map;
import java.util.Random;
import java.util.WeakHashMap;

import com.google.common.primitives.Longs;
import org.apache.commons.math3.primes.Primes;

/**
 * <p>
 * The source of random stuff for the whole project. This lets us make all randomness in the project
 * predictable, if desired, for when we run unit tests, which should be repeatable.
 * </p>
 */
public final class RandomUtils {

    /** The largest prime less than 2<sup>31</sup>-1 that is the smaller of a twin prime pair. */
    public static final int MAX_INT_SMALLER_TWIN_PRIME = 2147482949;

    private static final Map<RandomWrapper, Boolean> INSTANCES = Collections
            .synchronizedMap(new WeakHashMap<RandomWrapper, Boolean>());

    private static boolean testSeed = false;

    private RandomUtils() {
    }

    public static void useTestSeed() {
        testSeed = true;
        synchronized (INSTANCES) {
            for (RandomWrapper rng : INSTANCES.keySet()) {
                rng.resetToTestSeed();
            }
        }
    }

    public static RandomWrapper getRandom() {
        RandomWrapper random = new RandomWrapper();
        if (testSeed) {
            random.resetToTestSeed();
        }
        INSTANCES.put(random, Boolean.TRUE);
        return random;
    }

    public static Random getRandom(long seed) {
        RandomWrapper random = new RandomWrapper(seed);
        INSTANCES.put(random, Boolean.TRUE);
        return random;
    }

    /** @return what {@link Double#hashCode()} would return for the same value */
    public static int hashDouble(double value) {
        return Longs.hashCode(Double.doubleToLongBits(value));
    }

    /** @return what {@link Float#hashCode()} would return for the same value */
    public static int hashFloat(float value) {
        return Float.floatToIntBits(value);
    }

    /**
     * <p>
     * Finds next-largest "twin primes": numbers p and p+2 such that both are prime. Finds the smallest such p
     * such that the smaller twin, p, is greater than or equal to n. Returns p+2, the larger of the two twins.
     * </p>
     */
    public static int nextTwinPrime(int n) {
        if (n > MAX_INT_SMALLER_TWIN_PRIME) {
            throw new IllegalArgumentException();
        }
        if (n <= 3) {
            return 5;
        }
        int next = Primes.nextPrime(n);
        while (!Primes.isPrime(next + 2)) {
            next = Primes.nextPrime(next + 4);
        }
        return next + 2;
    }

}