/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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 org.apache.hadoop.util;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import com.google.common.base.Preconditions;
/**
* The IdentityHashStore stores (key, value) mappings in an array.
* It is similar to java.util.HashTable, but much more lightweight.
* Neither inserting nor removing an element ever leads to any garbage
* getting created (assuming the array doesn't need to be enlarged).
*
* Unlike HashTable, it compares keys using
* {@link System#identityHashCode(Object)} and the identity operator.
* This is useful for types like ByteBuffer which have expensive hashCode
* and equals operators.
*
* We use linear probing to resolve collisions. This avoids the need for
* the overhead of linked list data structures. It also means that it is
* expensive to attempt to remove an element that isn't there, since we
* have to look at the entire array to be sure that it doesn't exist.
*
* @param <K> The key type to use.
* @param <V> THe value type to use.
*/
@InterfaceAudience.Private
@InterfaceStability.Evolving
@SuppressWarnings("unchecked")
public final class IdentityHashStore<K, V> {
/**
* Even elements are keys; odd elements are values.
* The array has size 1 + Math.pow(2, capacity).
*/
private Object buffer[];
private int numInserted = 0;
private int capacity;
/**
* The default maxCapacity value to use.
*/
private static final int DEFAULT_MAX_CAPACITY = 2;
public IdentityHashStore(int capacity) {
Preconditions.checkArgument(capacity >= 0);
if (capacity == 0) {
this.capacity = 0;
this.buffer = null;
} else {
// Round the capacity we need up to a power of 2.
realloc((int)Math.pow(2,
Math.ceil(Math.log(capacity) / Math.log(2))));
}
}
private void realloc(int newCapacity) {
Preconditions.checkArgument(newCapacity > 0);
Object prevBuffer[] = buffer;
this.capacity = newCapacity;
// Each element takes two array slots -- one for the key,
// and another for the value. We also want a load factor
// of 0.50. Combine those together and you get 4 * newCapacity.
this.buffer = new Object[4 * newCapacity];
this.numInserted = 0;
if (prevBuffer != null) {
for (int i = 0; i < prevBuffer.length; i += 2) {
if (prevBuffer[i] != null) {
putInternal(prevBuffer[i], prevBuffer[i + 1]);
}
}
}
}
private void putInternal(Object k, Object v) {
final int hash = System.identityHashCode(k);
final int numEntries = buffer.length >> 1;
//computing modulo with the assumption buffer.length is power of 2
int index = hash & (numEntries-1);
while (true) {
if (buffer[2 * index] == null) {
buffer[2 * index] = k;
buffer[1 + (2 * index)] = v;
numInserted++;
return;
}
index = (index + 1) % numEntries;
}
}
/**
* Add a new (key, value) mapping.
*
* Inserting a new (key, value) never overwrites a previous one.
* In other words, you can insert the same key multiple times and it will
* lead to multiple entries.
*/
public void put(K k, V v) {
Preconditions.checkNotNull(k);
if (buffer == null) {
realloc(DEFAULT_MAX_CAPACITY);
} else if (numInserted + 1 > capacity) {
realloc(capacity * 2);
}
putInternal(k, v);
}
private int getElementIndex(K k) {
if (buffer == null) {
return -1;
}
final int numEntries = buffer.length >> 1;
final int hash = System.identityHashCode(k);
//computing modulo with the assumption buffer.length is power of 2
int index = hash & (numEntries -1);
int firstIndex = index;
do {
if (buffer[2 * index] == k) {
return index;
}
index = (index + 1) % numEntries;
} while (index != firstIndex);
return -1;
}
/**
* Retrieve a value associated with a given key.
*/
public V get(K k) {
int index = getElementIndex(k);
if (index < 0) {
return null;
}
return (V)buffer[1 + (2 * index)];
}
/**
* Retrieve a value associated with a given key, and delete the
* relevant entry.
*/
public V remove(K k) {
int index = getElementIndex(k);
if (index < 0) {
return null;
}
V val = (V)buffer[1 + (2 * index)];
buffer[2 * index] = null;
buffer[1 + (2 * index)] = null;
numInserted--;
return val;
}
public boolean isEmpty() {
return numInserted == 0;
}
public int numElements() {
return numInserted;
}
public int capacity() {
return capacity;
}
public interface Visitor<K, V> {
void accept(K k, V v);
}
/**
* Visit all key, value pairs in the IdentityHashStore.
*/
public void visitAll(Visitor<K, V> visitor) {
int length = buffer == null ? 0 : buffer.length;
for (int i = 0; i < length; i += 2) {
if (buffer[i] != null) {
visitor.accept((K)buffer[i], (V)buffer[i + 1]);
}
}
}
}