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/*
* Copyright 2018 dorkbox, llc
*
* 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 dorkbox.util.collections;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
/**
* A bimap (or "bidirectional map") is a map that preserves the uniqueness of its values as well as that of its keys. This constraint
* enables bimaps to support an "inverse view", which is another bimap containing the same entries as this bimap but with reversed keys and values.
*
* This class uses the "single-writer-principle" for lock-free publication.
*
* Since there are only 2 methods to guarantee that modifications can only be called one-at-a-time (either it is only called by
* one thread, or only one thread can access it at a time) -- we chose the 2nd option -- and use 'synchronized' to make sure that only
* one thread can access this modification methods at a time. Getting or checking the presence of values can then happen in a lock-free
* manner.
*
* According to my benchmarks, this is approximately 25% faster than ConcurrentHashMap for (all types of) reads, and a lot slower for
* contended writes.
*
* This data structure is for many-read/few-write scenarios
*/
@SuppressWarnings("WeakerAccess")
public final
class LockFreeBiMap {
// Recommended for best performance while adhering to the "single writer principle". Must be static-final
private static final AtomicReferenceFieldUpdater forwardREF =
AtomicReferenceFieldUpdater.newUpdater(LockFreeBiMap.class,
HashMap.class,
"forwardHashMap");
private static final AtomicReferenceFieldUpdater reverseREF =
AtomicReferenceFieldUpdater.newUpdater(LockFreeBiMap.class,
HashMap.class,
"reverseHashMap");
private volatile HashMap forwardHashMap;
private volatile HashMap reverseHashMap;
private final LockFreeBiMap inverse;
// synchronized is used here to ensure the "single writer principle", and make sure that ONLY one thread at a time can enter this
// section. Because of this, we can have unlimited reader threads all going at the same time, without contention (which is our
// use-case 99% of the time)
public
LockFreeBiMap() {
forwardHashMap = new HashMap();
reverseHashMap = new HashMap();
this.inverse = new LockFreeBiMap(reverseHashMap, forwardHashMap, this);
}
private
LockFreeBiMap(final HashMap forwardHashMap, final HashMap reverseHashMap, final LockFreeBiMap inverse) {
this.forwardHashMap = forwardHashMap;
this.reverseHashMap = reverseHashMap;
this.inverse = inverse;
}
/**
* Removes all of the mappings from this bimap.
* The bimap will be empty after this call returns.
*/
public synchronized
void clear() {
forwardHashMap.clear();
reverseHashMap.clear();
}
/**
* @return the inverse view of this bimap, which maps each of this bimap's values to its associated key.
*/
public
LockFreeBiMap inverse() {
return inverse;
}
/**
* Replaces all of the mappings from the specified map to this bimap.
* These mappings will replace any mappings that this map had for
* any of the keys currently in the specified map.
*
* @param hashMap mappings to be stored in this map
*
* @throws NullPointerException if the specified map is null
*
* @throws IllegalArgumentException if a given value in the map is already bound to a different key in this bimap. The bimap will remain
* unmodified in this event. To avoid this exception, call {@link #replaceAllForce(Map)} replaceAllForce(map) instead.
*/
public synchronized
void replaceAll(final Map hashMap) throws IllegalArgumentException {
if (hashMap == null) {
throw new NullPointerException("hashMap");
}
LockFreeBiMap biMap = new LockFreeBiMap();
try {
biMap.putAll(hashMap);
} catch (IllegalArgumentException e) {
// do nothing if there is an exception
throw e;
}
// only if there are no problems with the creation of the new bimap.
this.forwardHashMap.clear();
this.reverseHashMap.clear();
this.forwardHashMap.putAll(biMap.forwardHashMap);
this.reverseHashMap.putAll(biMap.reverseHashMap);
}
/**
* Replaces all of the mappings from the specified map to this bimap.
* These mappings will replace any mappings that this map had for
* any of the keys currently in the specified map. This is an alternate
* form of {@link #replaceAll(Map)} replaceAll(K, V) that will silently
* ignore duplicates
*
* @param hashMap mappings to be stored in this map
*
* @throws NullPointerException if the specified map is null
*/
public synchronized
void replaceAllForce(final Map hashMap) {
if (hashMap == null) {
throw new NullPointerException("hashMap");
}
// only if there are no problems with the creation of the new bimap.
this.forwardHashMap.clear();
this.reverseHashMap.clear();
putAllForce(hashMap);
}
/**
* Associates the specified value with the specified key in this bimap.
* If the bimap previously contained a mapping for the key, the old
* value is replaced. If the given value is already bound to a different
* key in this bimap, the bimap will remain unmodified. To avoid throwing
* an exception, call {@link #putForce(Object, Object)} putForce(K, V) instead.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
*
* @return the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
*
* @throws IllegalArgumentException if the given value is already bound to a different key in this bimap. The bimap will remain
* unmodified in this event. To avoid this exception, call {@link #putForce(Object, Object)} putForce(K, V) instead.
*/
public synchronized
V put(final K key, final V value) throws IllegalArgumentException {
V prevForwardValue = this.forwardHashMap.put(key, value);
if (prevForwardValue != null) {
reverseHashMap.remove(prevForwardValue);
}
K prevReverseValue = this.reverseHashMap.put(value, key);
if (prevReverseValue != null) {
// put the old value back
if (prevForwardValue != null) {
this.forwardHashMap.put(key, prevForwardValue);
}
else {
this.forwardHashMap.remove(key);
}
this.reverseHashMap.put(value, prevReverseValue);
throw new IllegalArgumentException("Value already exists. Keys and values must both be unique!");
}
return prevForwardValue;
}
/**
* Associates the specified value with the specified key in this bimap.
* If the bimap previously contained a mapping for the key, the old
* value is replaced. This is an alternate form of {@link #put(Object, Object)} put(K, V)
* that will silently ignore duplicates
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
*
* @return the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
*/
public synchronized
V putForce(final K key, final V value) {
V prevForwardValue = this.forwardHashMap.put(key, value);
if (prevForwardValue != null) {
reverseHashMap.remove(prevForwardValue);
}
K prevReverseValue = this.reverseHashMap.put(value, key);
if (prevReverseValue != null) {
forwardHashMap.remove(prevReverseValue);
}
return prevForwardValue;
}
/**
* Copies all of the mappings from the specified map to this map.
* These mappings will replace any mappings that this map had for
* any of the keys currently in the specified map.
*
* @param hashMap mappings to be stored in this map
*
* @throws NullPointerException if the specified map is null
*
* @throws IllegalArgumentException if the given value is already bound to a different key in this bimap. The bimap will remain
* unmodified in this event. To avoid this exception, call {@link #putAllForce(Map)} putAllForce(K, V) instead.
*/
public synchronized
void putAll(final Map hashMap) throws IllegalArgumentException {
LockFreeBiMap biMap = new LockFreeBiMap();
try {
for (Map.Entry entry : hashMap.entrySet()) {
K key = entry.getKey();
V value = entry.getValue();
biMap.put(key, value);
// we have to verify that the keys/values between the bimaps are unique
if (this.forwardHashMap.containsKey(key)) {
throw new IllegalArgumentException("Key already exists. Keys and values must both be unique!");
}
if (this.reverseHashMap.containsKey(value)) {
throw new IllegalArgumentException("Value already exists. Keys and values must both be unique!");
}
}
} catch (IllegalArgumentException e) {
// do nothing if there is an exception
throw e;
}
// only if there are no problems with the creation of the new bimap AND the uniqueness constrain is guaranteed
this.forwardHashMap.putAll(biMap.forwardHashMap);
this.reverseHashMap.putAll(biMap.reverseHashMap);
}
/**
* Copies all of the mappings from the specified map to this map.
* These mappings will replace any mappings that this map had for
* any of the keys currently in the specified map. This is an alternate
* form of {@link #putAll(Map)} putAll(K, V) that will silently
* ignore duplicates
*
* @param hashMap mappings to be stored in this map
*
* @throws NullPointerException if the specified map is null
*/
public synchronized
void putAllForce(final Map hashMap) {
for (Map.Entry entry : hashMap.entrySet()) {
K key = entry.getKey();
V value = entry.getValue();
putForce(key, value);
}
}
/**
* Removes the mapping for the specified key from this map if present.
*
* @param key key whose mapping is to be removed from the map
*
* @return the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
*/
public synchronized
V remove(final K key) {
V value = forwardHashMap.remove(key);
reverseHashMap.remove(value);
return value;
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
*
More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
*
A return value of {@code null} does not necessarily
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link HashMap#containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/
@SuppressWarnings("unchecked")
public
V get(final K key) {
// use the SWP to get a lock-free get of the value
return (V) forwardREF.get(this).get(key);
}
/**
* Returns the reverse key to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
*
More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
*
A return value of {@code null} does not necessarily
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link HashMap#containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/
@SuppressWarnings("unchecked")
public
K getReverse(final V key) {
// use the SWP to get a lock-free get of the value
return (K) reverseREF.get(this).get(key);
}
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own remove operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the Iterator.remove,
* Collection.remove, removeAll,
* retainAll and clear operations. It does not
* support the add or addAll operations.
*
* @return a view of the values contained in this map
*/
@SuppressWarnings("unchecked")
public
Collection values() {
// use the SWP to get a lock-free get of the value
return forwardREF.get(this).values();
}
/**
* Returns true if this bimap contains no key-value mappings.
*
* @return true if this bimap contains no key-value mappings
*/
public
boolean isEmpty() {
// use the SWP to get a lock-free get of the value
return forwardREF.get(this)
.isEmpty();
}
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own remove operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the Iterator.remove,
* Collection.remove, removeAll,
* retainAll and clear operations. It does not
* support the add or addAll operations.
*
* @return a view of the values contained in this map
*/
@SuppressWarnings("unchecked")
public
Collection reverseValues() {
// use the SWP to get a lock-free get of the value
return reverseREF.get(this).values();
}
}