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	/*
	* Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/
	package java.lang;

	import java.lang.ref.Reference;
	import java.lang.ref.ReferenceQueue;
	import java.lang.ref.WeakReference;
	import java.util.Collection;
	import java.util.Objects;
	import java.util.concurrent.ConcurrentHashMap;
	import java.util.function.BiFunction;

	/**
	* A WeakHashMap-like data structure that uses a pair of weakly-referenced keys
	* with identity equality semantics to associate a strongly-referenced value.
	* Unlike WeakHashMap, this data structure is thread-safe.
	*
	* @param <K1> the type of 1st key in key pair
	* @param <K2> the type of 2nd key in key pair
	* @param <V> the type of value
	* @author Peter Levart
	*/
	final class WeakPairMap<K1, K2, V> {

	private final ConcurrentHashMap<Pair<K1, K2>, V> map = new ConcurrentHashMap<>();
	private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

	/**
	* Tests if the specified pair of keys are associated with a value
	* in the WeakPairMap.
	*
	* @param k1 the 1st of the pair of keys
	* @param k2 the 2nd of the pair of keys
	* @return true if and only if the specified key pair is in this WeakPairMap,
	* as determined by the identity comparison; false otherwise
	* @throws NullPointerException if any of the specified keys is null
	*/
	public boolean containsKeyPair(K1 k1, K2 k2) {
	expungeStaleAssociations();
	return map.containsKey(Pair.lookup(k1, k2));
	}

	/**
	* Returns the value to which the specified pair of keys is mapped, or null
	* if this WeakPairMap contains no mapping for the key pair.
	* <p>More formally, if this WeakPairMap contains a mapping from a key pair
	* {@code (_k1, _k2)} to a value {@code v} such that
	* {@code k1 == _k1 && k2 == _k2}, then this method returns {@code v};
	* otherwise it returns {@code null}.
	* (There can be at most one such mapping.)
	*
	* @param k1 the 1st of the pair of keys for which the mapped value is to
	* be returned
	* @param k2 the 2nd of the pair of keys for which the mapped value is to
	* be returned
	* @return the value to which the specified key pair is mapped, or null if
	* this map contains no mapping for the key pair
	* @throws NullPointerException if any of the specified keys is null
	*/
	public V get(K1 k1, K2 k2) {
	expungeStaleAssociations();
	return map.get(Pair.lookup(k1, k2));
	}

	/**
	* Maps the specified key pair to the specified value in this WeakPairMap.
	* Neither the keys nor the value can be null.
	* <p>The value can be retrieved by calling the {@link #get} method
	* with the same keys (compared by identity).
	*
	* @param k1 the 1st of the pair of keys with which the specified value is to
	* be associated
	* @param k2 the 2nd of the pair of keys with which the specified value is to
	* be associated
	* @param v value to be associated with the specified key pair
	* @return the previous value associated with key pair, or {@code null} if
	* there was no mapping for key pair
	* @throws NullPointerException if any of the specified keys or value is null
	*/
	public V put(K1 k1, K2 k2, V v) {
	expungeStaleAssociations();
	return map.put(Pair.weak(k1, k2, queue), v);
	}

	/**
	* If the specified key pair is not already associated with a value,
	* associates it with the given value and returns {@code null}, else does
	* nothing and returns the currently associated value.
	*
	* @param k1 the 1st of the pair of keys with which the specified value is to
	* be associated
	* @param k2 the 2nd of the pair of keys with which the specified value is to
	* be associated
	* @param v value to be associated with the specified key pair
	* @return the previous value associated with key pair, or {@code null} if
	* there was no mapping for key pair
	* @throws NullPointerException if any of the specified keys or value is null
	*/
	public V putIfAbsent(K1 k1, K2 k2, V v) {
	expungeStaleAssociations();
	return map.putIfAbsent(Pair.weak(k1, k2, queue), v);
	}

	/**
	* If the specified key pair is not already associated with a value,
	* attempts to compute its value using the given mapping function
	* and enters it into this WeakPairMap unless {@code null}. The entire
	* method invocation is performed atomically, so the function is
	* applied at most once per key pair. Some attempted update operations
	* on this WeakPairMap by other threads may be blocked while computation
	* is in progress, so the computation should be short and simple,
	* and must not attempt to update any other mappings of this WeakPairMap.
	*
	* @param k1 the 1st of the pair of keys with which the
	* computed value is to be associated
	* @param k2 the 2nd of the pair of keys with which the
	* computed value is to be associated
	* @param mappingFunction the function to compute a value
	* @return the current (existing or computed) value associated with
	* the specified key pair, or null if the computed value is null
	* @throws NullPointerException if any of the specified keys or
	* mappingFunction is null
	* @throws IllegalStateException if the computation detectably
	* attempts a recursive update to this map
	* that would otherwise never complete
	* @throws RuntimeException or Error if the mappingFunction does so, in
	* which case the mapping is left unestablished
	*/
	public V computeIfAbsent(K1 k1, K2 k2,
	BiFunction<? super K1, ? super K2, ? extends V>
	mappingFunction) {
	expungeStaleAssociations();
	try {
	return map.computeIfAbsent(
	Pair.weak(k1, k2, queue),
	pair -> mappingFunction.apply(pair.first(), pair.second()));
	} finally {
	Reference.reachabilityFence(k1);
	Reference.reachabilityFence(k2);
	}
	}

	/**
	* Returns a {@link Collection} view of the values contained in this
	* WeakPairMap. The collection is backed by the WeakPairMap, so changes to
	* the map are reflected in the collection, and vice-versa. The collection
	* supports element removal, which removes the corresponding
	* mapping from this map, via the {@code Iterator.remove},
	* {@code Collection.remove}, {@code removeAll},
	* {@code retainAll}, and {@code clear} operations. It does not
	* support the {@code add} or {@code addAll} operations.
	*
	* @return the collection view
	*/
	public Collection<V> values() {
	expungeStaleAssociations();
	return map.values();
	}

	/**
	* Removes associations from this WeakPairMap for which at least one of the
	* keys in key pair has been found weakly-reachable and corresponding
	* WeakRefPeer(s) enqueued. Called as part of each public operation.
	*/
	private void expungeStaleAssociations() {
	WeakRefPeer<?> peer;
	while ((peer = (WeakRefPeer<?>) queue.poll()) != null) {
	map.remove(peer.weakPair());
	}
	}

	/**
	* Common interface of both {@link Weak} and {@link Lookup} key pairs.
	*/
	private interface Pair<K1, K2> {

	static <K1, K2> Pair<K1, K2> weak(K1 k1, K2 k2,
	ReferenceQueue<Object> queue) {
	return new Weak<>(k1, k2, queue);
	}

	static <K1, K2> Pair<K1, K2> lookup(K1 k1, K2 k2) {
	return new Lookup<>(k1, k2);
	}

	/**
	* @return The 1st of the pair of keys (may be null for {@link Weak}
	* when it gets cleared)
	*/
	K1 first();

	/**
	* @return The 2nd of the pair of keys (may be null for {@link Weak}
	* when it gets cleared)
	*/
	K2 second();

	static int hashCode(Object first, Object second) {
	// assert first != null && second != null;
	return System.identityHashCode(first) ^
	System.identityHashCode(second);
	}

	static boolean equals(Object first, Object second, Pair<?, ?> p) {
	return first != null && second != null &&
	first == p.first() && second == p.second();
	}

	/**
	* A Pair where both keys are weakly-referenced.
	* It is composed of two instances of {@link WeakRefPeer}s:
	* <pre>{@code
	*
	* +-referent-> [K1] +-referent-> [K2]
	* \| \|
	* +----------------+ +----------------+
	* \| Pair.Weak <: \|-----peer----->\| (anonymous) <: \|
	* \| WeakRefPeer, \| \| WeakRefPeer \|
	* \| Pair \|<--weakPair()--\| \|
	* +----------------+ +----------------+
	* \| ^
	* \| \|
	* +-weakPair()-+
	*
	* }</pre>
	* <p>
	* Pair.Weak is used for CHM keys. Both peers are associated with the
	* same {@link ReferenceQueue} so when either of their referents
	* becomes weakly-reachable, the corresponding entries can be
	* {@link #expungeStaleAssociations() expunged} from the map.
	*/
	final class Weak<K1, K2> extends WeakRefPeer<K1> implements Pair<K1, K2> {

	// saved hash so it can be retrieved after the reference is cleared
	private final int hash;
	// link to <K2> peer
	private final WeakRefPeer<K2> peer;

	Weak(K1 k1, K2 k2, ReferenceQueue<Object> queue) {
	super(k1, queue);
	hash = Pair.hashCode(k1, k2);
	peer = new WeakRefPeer<>(k2, queue) {
	// link back to <K1> peer
	@Override
	Weak<?, ?> weakPair() { return Weak.this; }
	};
	}

	@Override
	Weak<?, ?> weakPair() {
	return this;
	}

	@Override
	public K1 first() {
	return get();
	}

	@Override
	public K2 second() {
	return peer.get();
	}

	@Override
	public int hashCode() {
	return hash;
	}

	@Override
	public boolean equals(Object obj) {
	return this == obj \|\|
	(obj instanceof Pair &&
	Pair.equals(first(), second(), (Pair<?, ?>) obj));
	}
	}

	/**
	* Optimized lookup Pair, used as lookup key in methods like
	* {@link java.util.Map#get(Object)} or
	* {@link java.util.Map#containsKey(Object)}) where
	* there is a great chance its allocation is eliminated
	* by escape analysis when such lookups are inlined by JIT.
	* All its methods are purposely designed so that 'this' is never
	* passed to any other method or used as identity.
	*/
	final class Lookup<K1, K2> implements Pair<K1, K2> {
	private final K1 k1;
	private final K2 k2;

	Lookup(K1 k1, K2 k2) {
	this.k1 = Objects.requireNonNull(k1);
	this.k2 = Objects.requireNonNull(k2);
	}

	@Override
	public K1 first() {
	return k1;
	}

	@Override
	public K2 second() {
	return k2;
	}

	@Override
	public int hashCode() {
	return Pair.hashCode(k1, k2);
	}

	@Override
	public boolean equals(Object obj) {
	return obj instanceof Pair &&
	Pair.equals(k1, k2, (Pair<?, ?>) obj);
	}
	}
	}

	/**
	* Common abstract supertype of a pair of WeakReference peers.
	*/
	private static abstract class WeakRefPeer<K> extends WeakReference<K> {

	WeakRefPeer(K k, ReferenceQueue<Object> queue) {
	super(Objects.requireNonNull(k), queue);
	}

	/**
	* @return the {@link Pair.Weak} side of the pair of peers.
	*/
	abstract Pair.Weak<?, ?> weakPair();
	}
	}

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