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 java.util.Collection;
021import java.util.List;
022import java.util.ArrayList;
023
024import org.apache.commons.rng.UniformRandomProvider;
025
026/**
027 * Sampling from a {@link Collection}.
028 *
029 * @param <T> Type of items in the collection.
030 *
031 * @since 1.0
032 */
033public class CollectionSampler<T> {
034    /** Collection to be sampled from. */
035    private final List<T> items;
036    /** RNG. */
037    private final UniformRandomProvider rng;
038
039    /**
040     * Creates a sampler.
041     *
042     * @param rng Generator of uniformly distributed random numbers.
043     * @param collection Collection to be sampled.
044     * A (shallow) copy will be stored in the created instance.
045     * @throws IllegalArgumentException if {@code collection.size() <= 0}.
046     */
047    public CollectionSampler(UniformRandomProvider rng,
048                             Collection<T> collection) {
049        if (collection.size() <= 0) {
050            throw new IllegalArgumentException("Empty collection");
051        }
052
053        this.rng = rng;
054        items = new ArrayList<T>(collection);
055    }
056
057    /**
058     * Picks one of the items in the given {@code collection}.
059     *
060     * <p>
061     * Sampling is without replacement; but if the source collection
062     * contains identical objects, the sample may include repeats.
063     * </p>
064     * <p>
065     * There is no guarantee that the concrete type of the returned
066     * collection is the same as the source collection.
067     * </p>
068     *
069     * @return a random sample.
070     */
071    public T sample() {
072        return items.get(rng.nextInt(items.size()));
073    }
074}