package org.apache.lucene.util;
/**
* 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.
*/
/**
* Simple lockless and memory barrier free String intern cache that is guaranteed
* to return the same String instance as String.intern()
* does.
*
* @lucene.internal
*/
public class SimpleStringInterner extends StringInterner {
private static class Entry {
final private String str;
final private int hash;
private Entry next;
private Entry(String str, int hash, Entry next) {
this.str = str;
this.hash = hash;
this.next = next;
}
}
private final Entry[] cache;
private final int maxChainLength;
/**
* @param tableSize Size of the hash table, should be a power of two.
* @param maxChainLength Maximum length of each bucket, after which the oldest item inserted is dropped.
*/
public SimpleStringInterner(int tableSize, int maxChainLength) {
cache = new Entry[Math.max(1,BitUtil.nextHighestPowerOfTwo(tableSize))];
this.maxChainLength = Math.max(2,maxChainLength);
}
@Override
public String intern(String s) {
int h = s.hashCode();
// In the future, it may be worth augmenting the string hash
// if the lower bits need better distribution.
int slot = h & (cache.length-1);
Entry first = this.cache[slot];
Entry nextToLast = null;
int chainLength = 0;
for(Entry e=first; e!=null; e=e.next) {
if (e.hash == h && (e.str == s || e.str.compareTo(s)==0)) {
// if (e.str == s || (e.hash == h && e.str.compareTo(s)==0)) {
return e.str;
}
chainLength++;
if (e.next != null) {
nextToLast = e;
}
}
// insertion-order cache: add new entry at head
s = s.intern();
this.cache[slot] = new Entry(s, h, first);
if (chainLength >= maxChainLength) {
// prune last entry
nextToLast.next = null;
}
return s;
}
}