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/*
* Copyright (C) 2007 Google Inc.
*
* 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 com.google.common.annotations.VisibleForTesting;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.collect.Multisets.checkNonnegative;
import com.google.common.collect.Serialization.FieldSetter;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.AbstractSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import javax.annotation.Nullable;
/**
* A multiset that supports concurrent modifications and that provides atomic
* versions of most {@code Multiset} operations (exceptions where noted). Null
* elements are not supported.
*
* @author Cliff L. Biffle
*/
public final class ConcurrentHashMultiset extends AbstractMultiset
implements Serializable {
/*
* The ConcurrentHashMultiset's atomic operations are implemented in terms of
* ConcurrentMap's atomic operations. Many of them, such as add(E, int), are
* read-modify-write sequences, and so are implemented as loops that wrap
* ConcurrentMap's compare-and-set operations (like putIfAbsent).
*/
/** The number of occurrences of each element. */
private final transient ConcurrentMap countMap;
// This constant allows the deserialization code to set a final field. This
// holder class makes sure it is not initialized unless an instance is
// deserialized.
private static class FieldSettersHolder {
@SuppressWarnings("unchecked")
// eclipse doesn't like the raw type here, but it's harmless
static final FieldSetter COUNT_MAP_FIELD_SETTER
= Serialization.getFieldSetter(
ConcurrentHashMultiset.class, "countMap");
}
/**
* Creates a new, empty {@code ConcurrentHashMultiset} using the default
* initial capacity, load factor, and concurrency settings.
*/
public static ConcurrentHashMultiset create() {
return new ConcurrentHashMultiset(new ConcurrentHashMap());
}
/**
* Creates a new {@code ConcurrentHashMultiset} containing the specified
* elements, using the default initial capacity, load factor, and concurrency
* settings.
*
* @param elements the elements that the multiset should contain
*/
public static ConcurrentHashMultiset create(
Iterable elements) {
ConcurrentHashMultiset multiset = ConcurrentHashMultiset.create();
Iterables.addAll(multiset, elements);
return multiset;
}
/**
* Creates an instance using {@code countMap} to store elements and their
* counts.
*
* This instance will assume ownership of {@code countMap}, and other code
* should not maintain references to the map or modify it in any way.
*
* @param countMap backing map for storing the elements in the multiset and
* their counts. It must be empty.
* @throws IllegalArgumentException if {@code countMap} is not empty
*/
@VisibleForTesting ConcurrentHashMultiset(
ConcurrentMap countMap) {
checkArgument(countMap.isEmpty());
this.countMap = countMap;
}
// Query Operations
/**
* Returns the number of occurrences of {@code element} in this multiset.
*
* @param element the element to look for
* @return the nonnegative number of occurrences of the element
*/
@Override public int count(@Nullable Object element) {
try {
return unbox(countMap.get(element));
} catch (NullPointerException e) {
return 0;
} catch (ClassCastException e) {
return 0;
}
}
/**
* {@inheritDoc}
*
* If the data in the multiset is modified by any other threads during this
* method, it is undefined which (if any) of these modifications will be
* reflected in the result.
*/
@Override public int size() {
long sum = 0L;
for (Integer value : countMap.values()) {
sum += value;
}
return (int) Math.min(sum, Integer.MAX_VALUE);
}
/*
* Note: the superclass toArray() methods assume that size() gives a correct
* answer, which ours does not.
*/
@Override public Object[] toArray() {
return snapshot().toArray();
}
@Override public T[] toArray(T[] array) {
return snapshot().toArray(array);
}
/*
* We'd love to use 'new ArrayList(this)' or 'list.addAll(this)', but
* either of these would recurse back to us again!
*/
private List snapshot() {
List list = Lists.newArrayListWithExpectedSize(size());
for (Multiset.Entry entry : entrySet()) {
E element = entry.getElement();
for (int i = entry.getCount(); i > 0; i--) {
list.add(element);
}
}
return list;
}
// Modification Operations
/**
* Adds a number of occurrences of the specified element to this multiset.
*
* @param element the element to add
* @param occurrences the number of occurrences to add
* @return the previous count of the element before the operation; possibly
* zero
* @throws IllegalArgumentException if {@code occurrences} is negative, or if
* the resulting amount would exceed {@link Integer#MAX_VALUE}
*/
@Override public int add(E element, int occurrences) {
if (occurrences == 0) {
return count(element);
}
checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
while (true) {
int current = count(element);
if (current == 0) {
if (countMap.putIfAbsent(element, occurrences) == null) {
return 0;
}
} else {
checkArgument(occurrences <= Integer.MAX_VALUE - current,
"Overflow adding %s occurrences to a count of %s",
occurrences, current);
int next = current + occurrences;
if (countMap.replace(element, current, next)) {
return current;
}
}
// If we're still here, there was a race, so just try again.
}
}
/**
* Removes a number of occurrences of the specified element from this
* multiset. If the multiset contains fewer than this number of occurrences to
* begin with, all occurrences will be removed.
*
* @param element the element whose occurrences should be removed
* @param occurrences the number of occurrences of the element to remove
* @return the count of the element before the operation; possibly zero
* @throws IllegalArgumentException if {@code occurrences} is negative
*/
@Override public int remove(@Nullable Object element, int occurrences) {
if (occurrences == 0) {
return count(element);
}
checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
while (true) {
int current = count(element);
if (current == 0) {
return 0;
}
if (occurrences >= current) {
if (countMap.remove(element, current)) {
return current;
}
} else {
// We know it's an "E" because it already exists in the map.
@SuppressWarnings("unchecked")
E casted = (E) element;
if (countMap.replace(casted, current, current - occurrences)) {
return current;
}
}
// If we're still here, there was a race, so just try again.
}
}
/**
* Removes all occurrences of the specified element from this multiset.
* This method complements {@link Multiset#remove(Object)}, which removes only
* one occurrence at a time.
*
* @param element the element whose occurrences should all be removed
* @return the number of occurrences successfully removed, possibly zero
*/
private int removeAllOccurrences(@Nullable Object element) {
try {
return unbox(countMap.remove(element));
} catch (NullPointerException e) {
return 0;
} catch (ClassCastException e) {
return 0;
}
}
/**
* Removes exactly the specified number of occurrences of {@code element}, or
* makes no change if this is not possible.
*
* This method, in contrast to {@link #remove(Object, int)}, has no effect
* when the element count is smaller than {@code occurrences}.
*
* @param element the element to remove
* @param occurrences the number of occurrences of {@code element} to remove
* @return {@code true} if the removal was possible (including if {@code
* occurrences} is zero)
*/
public boolean removeExactly(@Nullable Object element, int occurrences) {
if (occurrences == 0) {
return true;
}
checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
while (true) {
int current = count(element);
if (occurrences > current) {
return false;
}
if (occurrences == current) {
if (countMap.remove(element, occurrences)) {
return true;
}
} else {
@SuppressWarnings("unchecked") // it's in the map, must be an "E"
E casted = (E) element;
if (countMap.replace(casted, current, current - occurrences)) {
return true;
}
}
// If we're still here, there was a race, so just try again.
}
}
/**
* Adds or removes occurrences of {@code element} such that the {@link #count}
* of the element becomes {@code count}.
*
* @return the count of {@code element} in the multiset before this call
* @throws IllegalArgumentException if {@code count} is negative
*/
@Override public int setCount(E element, int count) {
checkNonnegative(count, "count");
return (count == 0)
? removeAllOccurrences(element)
: unbox(countMap.put(element, count));
}
/**
* Sets the number of occurrences of {@code element} to {@code newCount}, but
* only if the count is currently {@code oldCount}. If {@code element} does
* not appear in the multiset exactly {@code oldCount} times, no changes will
* be made.
*
* @return {@code true} if the change was successful. This usually indicates
* that the multiset has been modified, but not always: in the case that
* {@code oldCount == newCount}, the method will return {@code true} if
* the condition was met.
* @throws IllegalArgumentException if {@code oldCount} or {@code newCount} is
* negative
*/
@Override public boolean setCount(E element, int oldCount, int newCount) {
checkNonnegative(oldCount, "oldCount");
checkNonnegative(newCount, "newCount");
if (newCount == 0) {
if (oldCount == 0) {
// No change to make, but must return true if the element is not present
return !countMap.containsKey(element);
} else {
return countMap.remove(element, oldCount);
}
}
if (oldCount == 0) {
return countMap.putIfAbsent(element, newCount) == null;
}
return countMap.replace(element, oldCount, newCount);
}
// Views
@Override Set createElementSet() {
final Set delegate = countMap.keySet();
return new ForwardingSet() {
@Override protected Set delegate() {
return delegate;
}
@Override public boolean remove(Object object) {
try {
return delegate.remove(object);
} catch (NullPointerException e) {
return false;
} catch (ClassCastException e) {
return false;
}
}
};
}
private volatile transient EntrySet entrySet;
@Override public Set> entrySet() {
EntrySet result = entrySet;
if (result == null) {
entrySet = result = new EntrySet();
}
return result;
}
private class EntrySet extends AbstractSet> {
@Override public int size() {
return countMap.size();
}
@Override public boolean isEmpty() {
return countMap.isEmpty();
}
@Override public boolean contains(Object object) {
if (object instanceof Multiset.Entry) {
Multiset.Entry entry = (Multiset.Entry) object;
Object element = entry.getElement();
int entryCount = entry.getCount();
return entryCount > 0 && count(element) == entryCount;
}
return false;
}
@Override public Iterator> iterator() {
final Iterator> backingIterator
= countMap.entrySet().iterator();
return new Iterator>() {
public boolean hasNext() {
return backingIterator.hasNext();
}
public Multiset.Entry next() {
Map.Entry backingEntry = backingIterator.next();
return Multisets.immutableEntry(
backingEntry.getKey(), backingEntry.getValue());
}
public void remove() {
backingIterator.remove();
}
};
}
/*
* Note: the superclass toArray() methods assume that size() gives a correct
* answer, which ours does not.
*/
@Override public Object[] toArray() {
return snapshot().toArray();
}
@Override public T[] toArray(T[] array) {
return snapshot().toArray(array);
}
/*
* We'd love to use 'new ArrayList(this)' or 'list.addAll(this)', but
* either of these would recurse back to us again!
*/
private List> snapshot() {
List> list = Lists.newArrayListWithExpectedSize(size());
for (Multiset.Entry entry : this) {
list.add(entry);
}
return list;
}
@Override public boolean remove(Object object) {
if (object instanceof Multiset.Entry) {
Multiset.Entry entry = (Multiset.Entry) object;
Object element = entry.getElement();
int entryCount = entry.getCount();
return countMap.remove(element, entryCount);
}
return false;
}
@Override public void clear() {
countMap.clear();
}
/**
* The hash code is the same as countMap's, though the objects aren't equal.
*/
@Override public int hashCode() {
return countMap.hashCode();
}
}
/**
* We use a special form of unboxing that treats null as zero.
*/
private static int unbox(Integer i) {
return (i == null) ? 0 : i;
}
/**
* @serialData the number of distinct elements, the first element, its count,
* the second element, its count, and so on
*/
private void writeObject(ObjectOutputStream stream) throws IOException {
stream.defaultWriteObject();
// creating HashMultiset to handle concurrent changes
Serialization.writeMultiset(HashMultiset.create(this), stream);
}
private void readObject(ObjectInputStream stream)
throws IOException, ClassNotFoundException {
stream.defaultReadObject();
FieldSettersHolder.COUNT_MAP_FIELD_SETTER.set(
this, new ConcurrentHashMap());
Serialization.populateMultiset(this, stream);
}
private static final long serialVersionUID = 0L;
}