| /* |
| * 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.sampling; |
| |
| import org.apache.commons.rng.UniformRandomProvider; |
| |
| /** |
| * Class for representing <a href="https://en.wikipedia.org/wiki/Combination">combinations</a> |
| * of a sequence of integers. |
| * |
| * <p>A combination is a selection of items from a collection, such that (unlike |
| * permutations) the order of selection <strong>does not matter</strong>. This |
| * sampler can be used to generate a combination in an unspecified order and is |
| * faster than the corresponding {@link PermutationSampler}. |
| * |
| * <p>Note that the sample order is unspecified. For example a sample |
| * combination of 2 from 4 may return {@code [0,1]} or {@code [1,0]} as the two are |
| * equivalent, and the order of a given combination may change in subsequent samples. |
| * |
| * <p>The sampler can be used to generate indices to select subsets where the |
| * order of the subset is not important. |
| * |
| * @see PermutationSampler |
| */ |
| public class CombinationSampler { |
| /** Domain of the combination. */ |
| private final int[] domain; |
| /** The number of steps of a full shuffle to perform. */ |
| private final int steps; |
| /** |
| * The section to copy the domain from after a partial shuffle. |
| */ |
| private final boolean upper; |
| /** RNG. */ |
| private final UniformRandomProvider rng; |
| |
| /** |
| * Creates a generator of combinations. |
| * |
| * <p>The {@link #sample()} method will generate an integer array of |
| * length {@code k} whose entries are selected randomly, without |
| * repetition, from the integers 0, 1, ..., {@code n}-1 (inclusive). |
| * The returned array represents a combination of {@code n} taken |
| * {@code k}. |
| * |
| * <p>In contrast to a permutation, the returned array is <strong>not |
| * guaranteed</strong> to be in a random order. The {@link #sample()} |
| * method returns the array in an unspecified order. |
| * |
| * <p>If {@code n <= 0} or {@code k <= 0} or {@code k > n} then no combination |
| * is required and an exception is raised. |
| * |
| * @param rng Generator of uniformly distributed random numbers. |
| * @param n Domain of the combination. |
| * @param k Size of the combination. |
| * @throws IllegalArgumentException if {@code n <= 0} or {@code k <= 0} or |
| * {@code k > n}. |
| */ |
| public CombinationSampler(UniformRandomProvider rng, |
| int n, |
| int k) { |
| SubsetSamplerUtils.checkSubset(n, k); |
| domain = PermutationSampler.natural(n); |
| // The sample can be optimised by only performing the first k or (n - k) steps |
| // from a full Fisher-Yates shuffle from the end of the domain to the start. |
| // The upper positions will then contain a random sample from the domain. The |
| // lower half is then by definition also a random sample (just not in a random order). |
| // The sample is then picked using the upper or lower half depending which |
| // makes the number of steps smaller. |
| upper = k <= n / 2; |
| steps = upper ? k : n - k; |
| this.rng = rng; |
| } |
| |
| /** |
| * Return a combination of {@code k} whose entries are selected randomly, |
| * without repetition, from the integers 0, 1, ..., {@code n}-1 (inclusive). |
| * |
| * <p>The order of the returned array is not guaranteed to be in a random order |
| * as the order of a combination <strong>does not matter</strong>. |
| * |
| * @return a random combination. |
| */ |
| public int[] sample() { |
| return SubsetSamplerUtils.partialSample(domain, steps, rng, upper); |
| } |
| } |