001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017
018package org.apache.commons.rng.sampling;
019
020import org.apache.commons.rng.UniformRandomProvider;
021
022/**
023 * Class for representing <a href="https://en.wikipedia.org/wiki/Combination">combinations</a>
024 * of a sequence of integers.
025 *
026 * <p>A combination is a selection of items from a collection, such that (unlike
027 * permutations) the order of selection <strong>does not matter</strong>. This
028 * sampler can be used to generate a combination in an unspecified order and is
029 * faster than the corresponding {@link PermutationSampler}.
030 *
031 * <p>Note that the sample order is unspecified. For example a sample
032 * combination of 2 from 4 may return {@code [0,1]} or {@code [1,0]} as the two are
033 * equivalent, and the order of a given combination may change in subsequent samples.
034 *
035 * <p>The sampler can be used to generate indices to select subsets where the
036 * order of the subset is not important.
037 *
038 * @see PermutationSampler
039 */
040public class CombinationSampler {
041    /** Domain of the combination. */
042    private final int[] domain;
043    /** The number of steps of a full shuffle to perform. */
044    private final int steps;
045    /**
046     * The section to copy the domain from after a partial shuffle.
047     */
048    private final boolean upper;
049    /** RNG. */
050    private final UniformRandomProvider rng;
051
052    /**
053     * Creates a generator of combinations.
054     *
055     * <p>The {@link #sample()} method will generate an integer array of
056     * length {@code k} whose entries are selected randomly, without
057     * repetition, from the integers 0, 1, ..., {@code n}-1 (inclusive).
058     * The returned array represents a combination of {@code n} taken
059     * {@code k}.
060     *
061     * <p>In contrast to a permutation, the returned array is <strong>not
062     * guaranteed</strong> to be in a random order. The {@link #sample()}
063     * method returns the array in an unspecified order.
064     *
065     * <p>If {@code n <= 0} or {@code k <= 0} or {@code k > n} then no combination
066     * is required and an exception is raised.
067     *
068     * @param rng Generator of uniformly distributed random numbers.
069     * @param n   Domain of the combination.
070     * @param k   Size of the combination.
071     * @throws IllegalArgumentException if {@code n <= 0} or {@code k <= 0} or
072     *                                  {@code k > n}.
073     */
074    public CombinationSampler(UniformRandomProvider rng,
075                              int n,
076                              int k) {
077        SubsetSamplerUtils.checkSubset(n, k);
078        domain = PermutationSampler.natural(n);
079        // The sample can be optimised by only performing the first k or (n - k) steps
080        // from a full Fisher-Yates shuffle from the end of the domain to the start.
081        // The upper positions will then contain a random sample from the domain. The
082        // lower half is then by definition also a random sample (just not in a random order).
083        // The sample is then picked using the upper or lower half depending which
084        // makes the number of steps smaller.
085        upper = k <= n / 2;
086        steps = upper ? k : n - k;
087        this.rng = rng;
088    }
089
090    /**
091     * Return a combination of {@code k} whose entries are selected randomly,
092     * without repetition, from the integers 0, 1, ..., {@code n}-1 (inclusive).
093     *
094     * <p>The order of the returned array is not guaranteed to be in a random order
095     * as the order of a combination <strong>does not matter</strong>.
096     *
097     * @return a random combination.
098     */
099    public int[] sample() {
100        return SubsetSamplerUtils.partialSample(domain, steps, rng, upper);
101    }
102}