opennlp.model.IndexHashTable Maven / Gradle / Ivy
/*
* 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 opennlp.model;
/**
* The {@link IndexHashTable} is a hash table which maps entries
* of an array to their index in the array. All entries in the array must
* be unique otherwise a well-defined mapping is not possible.
*
* The entry objects must implement {@link Object#equals(Object)} and
* {@link Object#hashCode()} otherwise the behavior of this class is
* undefined.
*
* The implementation uses a hash table with open addressing and linear probing.
*
* The table is thread safe and can concurrently accessed by multiple threads,
* thread safety is achieved through immutability. Though its not strictly immutable
* which means, that the table must still be safely published to other threads.
*/
public class IndexHashTable {
private final Object keys[];
private final int values[];
private final int size;
/**
* Initializes the current instance. The specified array is copied into the
* table and later changes to the array do not affect this table in any way.
*
* @param mapping
* the values to be indexed, all values must be unique otherwise a
* well-defined mapping of an entry to an index is not possible
* @param loadfactor
* the load factor, usually 0.7
*
* @throws IllegalArgumentException
* if the entries are not unique
*/
public IndexHashTable(T mapping[], double loadfactor) {
if (loadfactor <= 0 || loadfactor > 1) {
throw new IllegalArgumentException("loadfactor must be larger than 0 "
+ "and equal to or smaller than 1 but is " + loadfactor + "!");
}
int arraySize = (int) (mapping.length / loadfactor) + 1;
keys = new Object[arraySize];
values = new int[arraySize];
size = mapping.length;
for (int i = 0; i < mapping.length; i++) {
int startIndex = indexForHash(mapping[i].hashCode(), keys.length);
int index = searchKey(startIndex, null, true);
if (index == -1)
throw new IllegalArgumentException(
"Array must contain only unique keys!");
keys[index] = mapping[i];
values[index] = i;
}
}
private static int indexForHash(int h, int length) {
return (h & 0x7fffffff) % length;
}
private int searchKey(int startIndex, Object key, boolean insert) {
for (int index = startIndex; true; index = (index + 1) % keys.length) {
// The keys array contains at least one null element, which guarantees
// termination of the loop
if (keys[index] == null) {
if (insert)
return index;
else
return -1;
}
if (keys[index].equals(key)) {
if (!insert)
return index;
else
return -1;
}
}
}
/**
* Retrieves the index for the specified key.
*
* @param key
* @return the index or -1 if there is no entry to the keys
*/
public int get(T key) {
int startIndex = indexForHash(key.hashCode(), keys.length);
int index = searchKey(startIndex, key, false);
if (index != -1) {
return values[index];
} else {
return -1;
}
}
/**
* Retrieves the size.
*
* @return the number of elements in this map.
*/
public int size() {
return size;
}
@SuppressWarnings("unchecked")
public T[] toArray(T array[]) {
for (int i = 0; i < keys.length; i++) {
if (keys[i] != null)
array[values[i]] = (T) keys[i];
}
return array;
}
}