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 */
017package org.apache.commons.collections4.iterators;
018
019import java.util.ArrayList;
020import java.util.Arrays;
021import java.util.Collection;
022import java.util.HashMap;
023import java.util.Iterator;
024import java.util.List;
025import java.util.Map;
026import java.util.NoSuchElementException;
027import java.util.Objects;
028
029/**
030 * This iterator creates permutations of an input collection, using the
031 * Steinhaus-Johnson-Trotter algorithm (also called plain changes).
032 * <p>
033 * The iterator will return exactly n! permutations of the input collection.
034 * The {@code remove()} operation is not supported, and will throw an
035 * {@code UnsupportedOperationException}.
036 * <p>
037 * NOTE: in case an empty collection is provided, the iterator will
038 * return exactly one empty list as result, as 0! = 1.
039 *
040 * @param <E>  the type of the objects being permuted
041 *
042 * @since 4.0
043 */
044public class PermutationIterator<E> implements Iterator<List<E>> {
045
046    /**
047     * Permutation is done on these keys to handle equal objects.
048     */
049    private final int[] keys;
050
051    /**
052     * Mapping between keys and objects.
053     */
054    private final Map<Integer, E> objectMap;
055
056    /**
057     * Direction table used in the algorithm:
058     * <ul>
059     *   <li>false is left</li>
060     *   <li>true is right</li>
061     * </ul>
062     */
063    private final boolean[] direction;
064
065    /**
066     * Next permutation to return. When a permutation is requested
067     * this instance is provided and the next one is computed.
068     */
069    private List<E> nextPermutation;
070
071    /**
072     * Standard constructor for this class.
073     * @param collection  the collection to generate permutations for
074     * @throws NullPointerException if coll is null
075     */
076    public PermutationIterator(final Collection<? extends E> collection) {
077        Objects.requireNonNull(collection, "collection");
078        keys = new int[collection.size()];
079        direction = new boolean[collection.size()];
080        Arrays.fill(direction, false);
081        int value = 1;
082        objectMap = new HashMap<>();
083        for (final E e : collection) {
084            objectMap.put(Integer.valueOf(value), e);
085            keys[value - 1] = value;
086            value++;
087        }
088        nextPermutation = new ArrayList<>(collection);
089    }
090
091    /**
092     * Indicates if there are more permutation available.
093     * @return true if there are more permutations, otherwise false
094     */
095    @Override
096    public boolean hasNext() {
097        return nextPermutation != null;
098    }
099
100    /**
101     * Returns the next permutation of the input collection.
102     * @return a list of the permutator's elements representing a permutation
103     * @throws NoSuchElementException if there are no more permutations
104     */
105    @Override
106    public List<E> next() {
107        if (!hasNext()) {
108            throw new NoSuchElementException();
109        }
110
111        // find the largest mobile integer k
112        int indexOfLargestMobileInteger = -1;
113        int largestKey = -1;
114        for (int i = 0; i < keys.length; i++) {
115            if (direction[i] && i < keys.length - 1 && keys[i] > keys[i + 1] ||
116                !direction[i] && i > 0 && keys[i] > keys[i - 1]) {
117                if (keys[i] > largestKey) { // NOPMD
118                    largestKey = keys[i];
119                    indexOfLargestMobileInteger = i;
120                }
121            }
122        }
123        if (largestKey == -1) {
124            final List<E> toReturn = nextPermutation;
125            nextPermutation = null;
126            return toReturn;
127        }
128
129        // swap k and the adjacent integer it is looking at
130        final int offset = direction[indexOfLargestMobileInteger] ? 1 : -1;
131        final int tmpKey = keys[indexOfLargestMobileInteger];
132        keys[indexOfLargestMobileInteger] = keys[indexOfLargestMobileInteger + offset];
133        keys[indexOfLargestMobileInteger + offset] = tmpKey;
134        final boolean tmpDirection = direction[indexOfLargestMobileInteger];
135        direction[indexOfLargestMobileInteger] = direction[indexOfLargestMobileInteger + offset];
136        direction[indexOfLargestMobileInteger + offset] = tmpDirection;
137
138        // reverse the direction of all integers larger than k and build the result
139        final List<E> nextP = new ArrayList<>();
140        for (int i = 0; i < keys.length; i++) {
141            if (keys[i] > largestKey) {
142                direction[i] = !direction[i];
143            }
144            nextP.add(objectMap.get(Integer.valueOf(keys[i])));
145        }
146        final List<E> result = nextPermutation;
147        nextPermutation = nextP;
148        return result;
149    }
150
151    @Override
152    public void remove() {
153        throw new UnsupportedOperationException("remove() is not supported");
154    }
155
156}