/*
* Copyright (C) 2008 The Guava Authors
*
* Licensed 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 com.google.common.collect;
import static com.google.common.base.Preconditions.*;
import static com.google.common.math.LongMath.binomial;
import com.google.common.annotations.*;
import com.google.common.base.*;
import com.google.common.math.IntMath;
import com.google.common.primitives.Ints;
import java.util.*;
import javax.annotation.Nullable;
/**
* Provides static methods for working with {@code Collection} instances.
*
* @author Chris Povirk
* @author Mike Bostock
* @author Jared Levy
* @since 2.0 (imported from Google Collections Library)
*/
@GwtCompatible
public final class Collections2 {
private Collections2() {}
/**
* Returns the elements of {@code unfiltered} that satisfy a predicate. The
* returned collection is a live view of {@code unfiltered}; changes to one
* affect the other.
*
* <p>The resulting collection's iterator does not support {@code remove()},
* but all other collection methods are supported. When given an element that
* doesn't satisfy the predicate, the collection's {@code add()} and {@code
* addAll()} methods throw an {@link IllegalArgumentException}. When methods
* such as {@code removeAll()} and {@code clear()} are called on the filtered
* collection, only elements that satisfy the filter will be removed from the
* underlying collection.
*
* <p>The returned collection isn't threadsafe or serializable, even if
* {@code unfiltered} is.
*
* <p>Many of the filtered collection's methods, such as {@code size()},
* iterate across every element in the underlying collection and determine
* which elements satisfy the filter. When a live view is <i>not</i> needed,
* it may be faster to copy {@code Iterables.filter(unfiltered, predicate)}
* and use the copy.
*
* <p><b>Warning:</b> {@code predicate} must be <i>consistent with equals</i>,
* as documented at {@link Predicate#apply}. Do not provide a predicate such
* as {@code Predicates.instanceOf(ArrayList.class)}, which is inconsistent
* with equals. (See {@link Iterables#filter(Iterable, Class)} for related
* functionality.)
*/
// TODO(kevinb): how can we omit that Iterables link when building gwt
// javadoc?
public static <E> Collection<E> filter(
Collection<E> unfiltered, Predicate<? super E> predicate) {
if (unfiltered instanceof FilteredCollection) {
// Support clear(), removeAll(), and retainAll() when filtering a filtered
// collection.
return ((FilteredCollection<E>) unfiltered).createCombined(predicate);
}
return new FilteredCollection<E>(
checkNotNull(unfiltered), checkNotNull(predicate));
}
/**
* Delegates to {@link Collection#contains}. Returns {@code false} if the
* {@code contains} method throws a {@code ClassCastException}.
*/
static boolean safeContains(Collection<?> collection, Object object) {
try {
return collection.contains(object);
} catch (ClassCastException e) {
return false;
}
}
static class FilteredCollection<E> implements Collection<E> {
final Collection<E> unfiltered;
final Predicate<? super E> predicate;
FilteredCollection(Collection<E> unfiltered,
Predicate<? super E> predicate) {
this.unfiltered = unfiltered;
this.predicate = predicate;
}
FilteredCollection<E> createCombined(Predicate<? super E> newPredicate) {
return new FilteredCollection<E>(unfiltered,
Predicates.<E>and(predicate, newPredicate));
// .<E> above needed to compile in JDK 5
}
@Override
public boolean add(E element) {
checkArgument(predicate.apply(element));
return unfiltered.add(element);
}
@Override
public boolean addAll(Collection<? extends E> collection) {
for (E element : collection) {
checkArgument(predicate.apply(element));
}
return unfiltered.addAll(collection);
}
@Override
public void clear() {
Iterables.removeIf(unfiltered, predicate);
}
@Override
public boolean contains(Object element) {
try {
// unsafe cast can result in a CCE from predicate.apply(), which we
// will catch
@SuppressWarnings("unchecked")
E e = (E) element;
/*
* We check whether e satisfies the predicate, when we really mean to
* check whether the element contained in the set does. This is ok as
* long as the predicate is consistent with equals, as required.
*/
return predicate.apply(e) && unfiltered.contains(element);
} catch (NullPointerException e) {
return false;
} catch (ClassCastException e) {
return false;
}
}
@Override
public boolean containsAll(Collection<?> collection) {
for (Object element : collection) {
if (!contains(element)) {
return false;
}
}
return true;
}
@Override
public boolean isEmpty() {
return !Iterators.any(unfiltered.iterator(), predicate);
}
@Override
public Iterator<E> iterator() {
return Iterators.filter(unfiltered.iterator(), predicate);
}
@Override
public boolean remove(Object element) {
try {
// unsafe cast can result in a CCE from predicate.apply(), which we
// will catch
@SuppressWarnings("unchecked")
E e = (E) element;
// See comment in contains() concerning predicate.apply(e)
return predicate.apply(e) && unfiltered.remove(element);
} catch (NullPointerException e) {
return false;
} catch (ClassCastException e) {
return false;
}
}
@Override
public boolean removeAll(final Collection<?> collection) {
checkNotNull(collection);
Predicate<E> combinedPredicate = new Predicate<E>() {
@Override
public boolean apply(E input) {
return predicate.apply(input) && collection.contains(input);
}
};
return Iterables.removeIf(unfiltered, combinedPredicate);
}
@Override
public boolean retainAll(final Collection<?> collection) {
checkNotNull(collection);
Predicate<E> combinedPredicate = new Predicate<E>() {
@Override
public boolean apply(E input) {
// See comment in contains() concerning predicate.apply(e)
return predicate.apply(input) && !collection.contains(input);
}
};
return Iterables.removeIf(unfiltered, combinedPredicate);
}
@Override
public int size() {
return Iterators.size(iterator());
}
@Override
public Object[] toArray() {
// creating an ArrayList so filtering happens once
return Lists.newArrayList(iterator()).toArray();
}
@Override
public <T> T[] toArray(T[] array) {
return Lists.newArrayList(iterator()).toArray(array);
}
@Override public String toString() {
return Iterators.toString(iterator());
}
}
/**
* Returns a collection that applies {@code function} to each element of
* {@code fromCollection}. The returned collection is a live view of {@code
* fromCollection}; changes to one affect the other.
*
* <p>The returned collection's {@code add()} and {@code addAll()} methods
* throw an {@link UnsupportedOperationException}. All other collection
* methods are supported, as long as {@code fromCollection} supports them.
*
* <p>The returned collection isn't threadsafe or serializable, even if
* {@code fromCollection} is.
*
* <p>When a live view is <i>not</i> needed, it may be faster to copy the
* transformed collection and use the copy.
*
* <p>If the input {@code Collection} is known to be a {@code List}, consider
* {@link Lists#transform}. If only an {@code Iterable} is available, use
* {@link Iterables#transform}.
*/
public static <F, T> Collection<T> transform(Collection<F> fromCollection,
Function<? super F, T> function) {
return new TransformedCollection<F, T>(fromCollection, function);
}
static class TransformedCollection<F, T> extends AbstractCollection<T> {
final Collection<F> fromCollection;
final Function<? super F, ? extends T> function;
TransformedCollection(Collection<F> fromCollection,
Function<? super F, ? extends T> function) {
this.fromCollection = checkNotNull(fromCollection);
this.function = checkNotNull(function);
}
@Override public void clear() {
fromCollection.clear();
}
@Override public boolean isEmpty() {
return fromCollection.isEmpty();
}
@Override public Iterator<T> iterator() {
return Iterators.transform(fromCollection.iterator(), function);
}
@Override public int size() {
return fromCollection.size();
}
}
/**
* Returns {@code true} if the collection {@code self} contains all of the
* elements in the collection {@code c}.
*
* <p>This method iterates over the specified collection {@code c}, checking
* each element returned by the iterator in turn to see if it is contained in
* the specified collection {@code self}. If all elements are so contained,
* {@code true} is returned, otherwise {@code false}.
*
* @param self a collection which might contain all elements in {@code c}
* @param c a collection whose elements might be contained by {@code self}
*/
static boolean containsAllImpl(Collection<?> self, Collection<?> c) {
checkNotNull(self);
for (Object o : c) {
if (!self.contains(o)) {
return false;
}
}
return true;
}
/**
* An implementation of {@link Collection#toString()}.
*/
static String toStringImpl(final Collection<?> collection) {
StringBuilder sb
= newStringBuilderForCollection(collection.size()).append('[');
STANDARD_JOINER.appendTo(
sb, Iterables.transform(collection, new Function<Object, Object>() {
@Override public Object apply(Object input) {
return input == collection ? "(this Collection)" : input;
}
}));
return sb.append(']').toString();
}
/**
* Returns best-effort-sized StringBuilder based on the given collection size.
*/
static StringBuilder newStringBuilderForCollection(int size) {
checkArgument(size >= 0, "size must be non-negative");
return new StringBuilder((int) Math.min(size * 8L, Ints.MAX_POWER_OF_TWO));
}
/**
* Used to avoid http://bugs.sun.com/view_bug.do?bug_id=6558557
*/
static <T> Collection<T> cast(Iterable<T> iterable) {
return (Collection<T>) iterable;
}
static final Joiner STANDARD_JOINER = Joiner.on(", ").useForNull("null");
/**
* Returns a {@link Collection} of all the permutations of the specified
* {@link Iterable}.
*
* <p><i>Notes:</i> This is an implementation of the algorithm for
* Lexicographical Permutations Generation, described in Knuth's "The Art of
* Computer Programming", Volume 4, Chapter 7, Section 7.2.1.2. The
* iteration order follows the lexicographical order. This means that
* the first permutation will be in ascending order, and the last will be in
* descending order.
*
* <p>Duplicate elements are considered equal. For example, the list [1, 1]
* will have only one permutation, instead of two. This is why the elements
* have to implement {@link Comparable}.
*
* <p>An empty iterable has only one permutation, which is an empty list.
*
* <p>This method is equivalent to
* {@code Collections2.orderedPermutations(list, Ordering.natural())}.
*
* @param elements the original iterable whose elements have to be permuted.
* @return an immutable {@link Collection} containing all the different
* permutations of the original iterable.
* @throws NullPointerException if the specified iterable is null or has any
* null elements.
* @since 12.0
*/
@Beta public static <E extends Comparable<? super E>>
Collection<List<E>> orderedPermutations(Iterable<E> elements) {
return orderedPermutations(elements, Ordering.natural());
}
/**
* Returns a {@link Collection} of all the permutations of the specified
* {@link Iterable} using the specified {@link Comparator} for establishing
* the lexicographical ordering.
*
* <p>Examples: <pre> {@code
*
* for (List<String> perm : orderedPermutations(asList("b", "c", "a"))) {
* println(perm);
* }
* // -> ["a", "b", "c"]
* // -> ["a", "c", "b"]
* // -> ["b", "a", "c"]
* // -> ["b", "c", "a"]
* // -> ["c", "a", "b"]
* // -> ["c", "b", "a"]
*
* for (List<Integer> perm : orderedPermutations(asList(1, 2, 2, 1))) {
* println(perm);
* }
* // -> [1, 1, 2, 2]
* // -> [1, 2, 1, 2]
* // -> [1, 2, 2, 1]
* // -> [2, 1, 1, 2]
* // -> [2, 1, 2, 1]
* // -> [2, 2, 1, 1]}</pre>
*
* <p><i>Notes:</i> This is an implementation of the algorithm for
* Lexicographical Permutations Generation, described in Knuth's "The Art of
* Computer Programming", Volume 4, Chapter 7, Section 7.2.1.2. The
* iteration order follows the lexicographical order. This means that
* the first permutation will be in ascending order, and the last will be in
* descending order.
*
* <p>Elements that compare equal are considered equal and no new permutations
* are created by swapping them.
*
* <p>An empty iterable has only one permutation, which is an empty list.
*
* @param elements the original iterable whose elements have to be permuted.
* @param comparator a comparator for the iterable's elements.
* @return an immutable {@link Collection} containing all the different
* permutations of the original iterable.
* @throws NullPointerException If the specified iterable is null, has any
* null elements, or if the specified comparator is null.
* @since 12.0
*/
@Beta public static <E> Collection<List<E>> orderedPermutations(
Iterable<E> elements, Comparator<? super E> comparator) {
return new OrderedPermutationCollection<E>(elements, comparator);
}
private static final class OrderedPermutationCollection<E>
extends AbstractCollection<List<E>> {
final ImmutableList<E> inputList;
final Comparator<? super E> comparator;
final int size;
OrderedPermutationCollection(Iterable<E> input,
Comparator<? super E> comparator) {
this.inputList = Ordering.from(comparator).immutableSortedCopy(input);
this.comparator = comparator;
this.size = calculateSize(inputList, comparator);
}
/**
* The number of permutations with repeated elements is calculated as
* follows:
* <ul>
* <li>For an empty list, it is 1 (base case).</li>
* <li>When r numbers are added to a list of n-r elements, the number of
* permutations is increased by a factor of (n choose r).</li>
* </ul>
*/
private static <E> int calculateSize(
List<E> sortedInputList, Comparator<? super E> comparator) {
long permutations = 1;
int n = 1;
int r = 1;
while (n < sortedInputList.size()) {
int comparison = comparator.compare(
sortedInputList.get(n - 1), sortedInputList.get(n));
if (comparison < 0) {
// We move to the next non-repeated element.
permutations *= binomial(n, r);
r = 0;
if (!isPositiveInt(permutations)) {
return Integer.MAX_VALUE;
}
}
n++;
r++;
}
permutations *= binomial(n, r);
if (!isPositiveInt(permutations)) {
return Integer.MAX_VALUE;
}
return (int) permutations;
}
@Override public int size() {
return size;
}
@Override public boolean isEmpty() {
return false;
}
@Override public Iterator<List<E>> iterator() {
return new OrderedPermutationIterator<E>(inputList, comparator);
}
@Override public boolean contains(@Nullable Object obj) {
if (obj instanceof List) {
List<?> list = (List<?>) obj;
return isPermutation(inputList, list);
}
return false;
}
@Override public String toString() {
return "orderedPermutationCollection(" + inputList + ")";
}
}
private static final class OrderedPermutationIterator<E>
extends AbstractIterator<List<E>> {
List<E> nextPermutation;
final Comparator<? super E> comparator;
OrderedPermutationIterator(List<E> list,
Comparator<? super E> comparator) {
this.nextPermutation = Lists.newArrayList(list);
this.comparator = comparator;
}
@Override protected List<E> computeNext() {
if (nextPermutation == null) {
return endOfData();
}
ImmutableList<E> next = ImmutableList.copyOf(nextPermutation);
calculateNextPermutation();
return next;
}
void calculateNextPermutation() {
int j = findNextJ();
if (j == -1) {
nextPermutation = null;
return;
}
int l = findNextL(j);
Collections.swap(nextPermutation, j, l);
int n = nextPermutation.size();
Collections.reverse(nextPermutation.subList(j + 1, n));
}
int findNextJ() {
for (int k = nextPermutation.size() - 2; k >= 0; k--) {
if (comparator.compare(nextPermutation.get(k),
nextPermutation.get(k + 1)) < 0) {
return k;
}
}
return -1;
}
int findNextL(int j) {
E ak = nextPermutation.get(j);
for (int l = nextPermutation.size() - 1; l > j; l--) {
if (comparator.compare(ak, nextPermutation.get(l)) < 0) {
return l;
}
}
throw new AssertionError("this statement should be unreachable");
}
}
/**
* Returns a {@link Collection} of all the permutations of the specified
* {@link Collection}.
*
* <p><i>Notes:</i> This is an implementation of the Plain Changes algorithm
* for permutations generation, described in Knuth's "The Art of Computer
* Programming", Volume 4, Chapter 7, Section 7.2.1.2.
*
* <p>If the input list contains equal elements, some of the generated
* permutations will be equal.
*
* <p>An empty collection has only one permutation, which is an empty list.
*
* @param elements the original collection whose elements have to be permuted.
* @return an immutable {@link Collection} containing all the different
* permutations of the original collection.
* @throws NullPointerException if the specified collection is null or has any
* null elements.
* @since 12.0
*/
@Beta public static <E> Collection<List<E>> permutations(
Collection<E> elements) {
return new PermutationCollection<E>(ImmutableList.copyOf(elements));
}
private static final class PermutationCollection<E>
extends AbstractCollection<List<E>> {
final ImmutableList<E> inputList;
PermutationCollection(ImmutableList<E> input) {
this.inputList = input;
}
@Override public int size() {
return IntMath.factorial(inputList.size());
}
@Override public boolean isEmpty() {
return false;
}
@Override public Iterator<List<E>> iterator() {
return new PermutationIterator<E>(inputList);
}
@Override public boolean contains(@Nullable Object obj) {
if (obj instanceof List) {
List<?> list = (List<?>) obj;
return isPermutation(inputList, list);
}
return false;
}
@Override public String toString() {
return "permutations(" + inputList + ")";
}
}
private static class PermutationIterator<E>
extends AbstractIterator<List<E>> {
final List<E> list;
final int[] c;
final int[] o;
int j;
PermutationIterator(List<E> list) {
this.list = new ArrayList<E>(list);
int n = list.size();
c = new int[n];
o = new int[n];
for (int i = 0; i < n; i++) {
c[i] = 0;
o[i] = 1;
}
j = Integer.MAX_VALUE;
}
@Override protected List<E> computeNext() {
if (j <= 0) {
return endOfData();
}
ImmutableList<E> next = ImmutableList.copyOf(list);
calculateNextPermutation();
return next;
}
void calculateNextPermutation() {
j = list.size() - 1;
int s = 0;
// Handle the special case of an empty list. Skip the calculation of the
// next permutation.
if (j == -1) {
return;
}
while (true) {
int q = c[j] + o[j];
if (q < 0) {
switchDirection();
continue;
}
if (q == j + 1) {
if (j == 0) {
break;
}
s++;
switchDirection();
continue;
}
Collections.swap(list, j - c[j] + s, j - q + s);
c[j] = q;
break;
}
}
void switchDirection() {
o[j] = -o[j];
j--;
}
}
/**
* Returns {@code true} if the second list is a permutation of the first.
*/
private static boolean isPermutation(List<?> first,
List<?> second) {
if (first.size() != second.size()) {
return false;
}
Multiset<?> firstSet = HashMultiset.create(first);
Multiset<?> secondSet = HashMultiset.create(second);
return firstSet.equals(secondSet);
}
private static boolean isPositiveInt(long n) {
return n >= 0 && n <= Integer.MAX_VALUE;
}
}