com.google.common.collect.Collections2 Maven / Gradle / Ivy
/*
* 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Predicates.and;
import static com.google.common.base.Predicates.in;
import static com.google.common.base.Predicates.not;
import static com.google.common.collect.CollectPreconditions.checkNonnegative;
import static com.google.common.math.LongMath.binomial;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import com.google.common.base.Joiner;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.math.IntMath;
import com.google.common.primitives.Ints;
import java.util.AbstractCollection;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
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.
*
* 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.
*
*
The returned collection isn't threadsafe or serializable, even if
* {@code unfiltered} is.
*
*
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 not needed,
* it may be faster to copy {@code Iterables.filter(unfiltered, predicate)}
* and use the copy.
*
*
Warning: {@code predicate} must be consistent with equals,
* 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 Collection filter(
Collection unfiltered, Predicate predicate) {
if (unfiltered instanceof FilteredCollection) {
// Support clear(), removeAll(), and retainAll() when filtering a filtered
// collection.
return ((FilteredCollection) unfiltered).createCombined(predicate);
}
return new FilteredCollection(
checkNotNull(unfiltered), checkNotNull(predicate));
}
/**
* Delegates to {@link Collection#contains}. Returns {@code false} if the
* {@code contains} method throws a {@code ClassCastException} or
* {@code NullPointerException}.
*/
static boolean safeContains(
Collection collection, @Nullable Object object) {
checkNotNull(collection);
try {
return collection.contains(object);
} catch (ClassCastException e) {
return false;
} catch (NullPointerException e) {
return false;
}
}
/**
* Delegates to {@link Collection#remove}. Returns {@code false} if the
* {@code remove} method throws a {@code ClassCastException} or
* {@code NullPointerException}.
*/
static boolean safeRemove(Collection collection, @Nullable Object object) {
checkNotNull(collection);
try {
return collection.remove(object);
} catch (ClassCastException e) {
return false;
} catch (NullPointerException e) {
return false;
}
}
static class FilteredCollection extends AbstractCollection {
final Collection unfiltered;
final Predicate predicate;
FilteredCollection(Collection unfiltered,
Predicate predicate) {
this.unfiltered = unfiltered;
this.predicate = predicate;
}
FilteredCollection createCombined(Predicate newPredicate) {
return new FilteredCollection(unfiltered,
Predicates.and(predicate, newPredicate));
// . 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 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(@Nullable Object element) {
if (safeContains(unfiltered, element)) {
@SuppressWarnings("unchecked") // element is in unfiltered, so it must be an E
E e = (E) element;
return predicate.apply(e);
}
return false;
}
@Override
public boolean containsAll(Collection collection) {
return containsAllImpl(this, collection);
}
@Override
public boolean isEmpty() {
return !Iterables.any(unfiltered, predicate);
}
@Override
public Iterator iterator() {
return Iterators.filter(unfiltered.iterator(), predicate);
}
@Override
public boolean remove(Object element) {
return contains(element) && unfiltered.remove(element);
}
@Override
public boolean removeAll(final Collection collection) {
return Iterables.removeIf(unfiltered, and(predicate, in(collection)));
}
@Override
public boolean retainAll(final Collection collection) {
return Iterables.removeIf(unfiltered, and(predicate, not(in(collection))));
}
@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[] toArray(T[] array) {
return Lists.newArrayList(iterator()).toArray(array);
}
}
/**
* 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.
*
* 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.
*
*
The returned collection isn't threadsafe or serializable, even if
* {@code fromCollection} is.
*
*
When a live view is not needed, it may be faster to copy the
* transformed collection and use the copy.
*
*
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 Collection transform(Collection fromCollection,
Function function) {
return new TransformedCollection(fromCollection, function);
}
static class TransformedCollection extends AbstractCollection {
final Collection fromCollection;
final Function function;
TransformedCollection(Collection fromCollection,
Function 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 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}.
*
* 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) {
return Iterables.all(c, Predicates.in(self));
}
/**
* 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() {
@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) {
checkNonnegative(size, "size");
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 Collection cast(Iterable iterable) {
return (Collection) iterable;
}
static final Joiner STANDARD_JOINER = Joiner.on(", ").useForNull("null");
/**
* Returns a {@link Collection} of all the permutations of the specified
* {@link Iterable}.
*
* Notes: 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.
*
*
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}.
*
*
An empty iterable has only one permutation, which is an empty list.
*
*
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 >
Collection> orderedPermutations(Iterable 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.
*
* Examples:
{@code
*
* for (List 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 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]}
*
* Notes: 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.
*
*
Elements that compare equal are considered equal and no new permutations
* are created by swapping them.
*
*
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 Collection> orderedPermutations(
Iterable elements, Comparator comparator) {
return new OrderedPermutationCollection(elements, comparator);
}
private static final class OrderedPermutationCollection
extends AbstractCollection> {
final ImmutableList inputList;
final Comparator comparator;
final int size;
OrderedPermutationCollection(Iterable input,
Comparator 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:
*
* - For an empty list, it is 1 (base case).
* - 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).
*
*/
private static int calculateSize(
List sortedInputList, Comparator 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> iterator() {
return new OrderedPermutationIterator(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
extends AbstractIterator> {
List nextPermutation;
final Comparator comparator;
OrderedPermutationIterator(List list,
Comparator comparator) {
this.nextPermutation = Lists.newArrayList(list);
this.comparator = comparator;
}
@Override protected List computeNext() {
if (nextPermutation == null) {
return endOfData();
}
ImmutableList 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}.
*
* Notes: 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.
*
*
If the input list contains equal elements, some of the generated
* permutations will be equal.
*
*
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 Collection> permutations(
Collection elements) {
return new PermutationCollection(ImmutableList.copyOf(elements));
}
private static final class PermutationCollection
extends AbstractCollection> {
final ImmutableList inputList;
PermutationCollection(ImmutableList input) {
this.inputList = input;
}
@Override public int size() {
return IntMath.factorial(inputList.size());
}
@Override public boolean isEmpty() {
return false;
}
@Override public Iterator> iterator() {
return new PermutationIterator(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
extends AbstractIterator> {
final List list;
final int[] c;
final int[] o;
int j;
PermutationIterator(List list) {
this.list = new ArrayList(list);
int n = list.size();
c = new int[n];
o = new int[n];
Arrays.fill(c, 0);
Arrays.fill(o, 1);
j = Integer.MAX_VALUE;
}
@Override protected List computeNext() {
if (j <= 0) {
return endOfData();
}
ImmutableList 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 firstMultiset = HashMultiset.create(first);
Multiset secondMultiset = HashMultiset.create(second);
return firstMultiset.equals(secondMultiset);
}
private static boolean isPositiveInt(long n) {
return n >= 0 && n <= Integer.MAX_VALUE;
}
}