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 * <p>Sampling uses {@link UniformRandomProvider#nextInt(int)}.</p>
030 *
031 * @param <T> Type of items in the collection.
032 *
033 * @since 1.0
034 */
035public class CollectionSampler<T> implements SharedStateObjectSampler<T> {
036    /** Collection to be sampled from. */
037    private final List<T> items;
038    /** RNG. */
039    private final UniformRandomProvider rng;
040
041    /**
042     * Creates a sampler.
043     *
044     * @param rng Generator of uniformly distributed random numbers.
045     * @param collection Collection to be sampled.
046     * A (shallow) copy will be stored in the created instance.
047     * @throws IllegalArgumentException if {@code collection} is empty.
048     */
049    public CollectionSampler(UniformRandomProvider rng,
050                             Collection<T> collection) {
051        this(rng, toList(collection));
052    }
053
054    /**
055     * @param rng Generator of uniformly distributed random numbers.
056     * @param collection Collection to be sampled.
057     */
058    private CollectionSampler(UniformRandomProvider rng,
059                              List<T> collection) {
060        this.rng = rng;
061        items = collection;
062    }
063
064    /**
065     * Picks one of the items from the
066     * {@link #CollectionSampler(UniformRandomProvider,Collection)
067     * collection passed to the constructor}.
068     *
069     * @return a random sample.
070     */
071    @Override
072    public T sample() {
073        return items.get(rng.nextInt(items.size()));
074    }
075
076    /**
077     * {@inheritDoc}
078     *
079     * @since 1.3
080     */
081    @Override
082    public CollectionSampler<T> withUniformRandomProvider(UniformRandomProvider rng) {
083        return new CollectionSampler<>(rng, this.items);
084    }
085
086    /**
087     * Convert the collection to a list (shallow) copy.
088     *
089     * <p>This method exists to raise an exception before invocation of the
090     * private constructor; this mitigates Finalizer attacks
091     * (see SpotBugs CT_CONSTRUCTOR_THROW).
092     *
093     * @param <T> Type of items in the collection.
094     * @param collection Collection.
095     * @return the list copy
096     * @throws IllegalArgumentException if {@code collection} is empty.
097     */
098    private static <T> List<T> toList(Collection<T> collection) {
099        if (collection.isEmpty()) {
100            throw new IllegalArgumentException("Empty collection");
101        }
102        return new ArrayList<>(collection);
103    }
104}