commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/stat/descriptive/rank/RandomPivotingStrategy.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.math4.legacy.stat.descriptive.rank;

import org.apache.commons.math4.legacy.core.MathArrays;
import org.apache.commons.rng.RestorableUniformRandomProvider;
import org.apache.commons.rng.simple.RandomSource;

/**
 * A strategy of selecting random index between begin and end indices.
 *
 * @since 3.4
 */
public class RandomPivotingStrategy implements PivotingStrategy {
    /** Source of randomness. */
    private final RandomSource randomSource;
    /** Random generator to use for selecting pivot. */
    private transient RestorableUniformRandomProvider random;

    /**
     * Simple constructor.
     *
     * @param randomSource RNG to use for selecting pivot.
     * @param seed Seed for initializing the RNG.
     *
     * @since 4.0
     */
    public RandomPivotingStrategy(RandomSource randomSource,
                                  long seed) {
        this.randomSource = randomSource;
        random = randomSource.create(seed);
    }

    /**
     * {@inheritDoc}
     *
     * A uniform random pivot selection between begin and end indices.
     *
     * @return The index corresponding to a random uniformly selected
     * value between first and the last indices of the array slice
     * @throws org.apache.commons.math4.legacy.exception.MathIllegalArgumentException MathIllegalArgumentException when indices exceeds range
     */
    @Override
    public int pivotIndex(final double[] work, final int begin, final int end) {
        MathArrays.verifyValues(work, begin, end - begin);
        return begin + random.nextInt(end - begin - 1);
    }
}