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////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Copyright (c) 2018-2022 Saxonica Limited
// This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
// If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
// This Source Code Form is "Incompatible With Secondary Licenses", as defined by the Mozilla Public License, v. 2.0.
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
package net.sf.saxon.expr.sort;
import net.sf.saxon.transpile.CSharpTypeBounds;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
/**
* A "Least Frequently Used" cache. When the cache grows too large, we discard
* entries whose reference counters are below some threshold. This is cheaper
* than an LRU (least recently used) algorithm, because access to the map
* doesn't require any locking against concurrent activity.
*
* Unlike classic LFU algorithms where the cache size has a fixed upper bound,
* we allow it to grow beyond the target size and then do a bulk deletion of
* infrequently used entries. We delete all entries whose usage counter is below
* some threshold, learning the best threshold to apply from experience of
* previous reorganisations.
*
* There is no synchronization. The underlying map itself is thread-safe, but we
* make no attempt to prevent two concurrent threads garbage collecting at the same
* time: if this happens, no harm is caused, because it's only a cache and lost
* entries will simply be recomputed. Equally, the incrementing of counters isn't
* synchronized because they don't need to be exact.
*/
@CSharpTypeBounds(bounds={"V:class"})
public class LFUCache {
private final boolean concurrent;
private int targetSize;
private int retentionThreshold = 1;
private Map> map;
/**
* Creates a new LCU cache, suitable for use only within a single thread.
*
* @param cacheSize the target number of entries to be kept in this cache.
*/
public LFUCache(final int cacheSize) {
this(cacheSize, false);
targetSize = cacheSize;
}
/**
* Creates a new LCU cache, with the option of making it thread-safe
*
* @param cacheSize the maximum number of entries that will be kept in this cache.
* @param concurrent set to true if concurrent access is required, so that the
* underlying map will be thread-safe
*/
public LFUCache(final int cacheSize, boolean concurrent) {
this.concurrent = concurrent;
targetSize = cacheSize;
map = makeMap(cacheSize);
}
private Map> makeMap(int cacheSize) {
if (concurrent) {
return new ConcurrentHashMap>(cacheSize);
} else {
return new HashMap>(cacheSize);
}
}
/**
* Retrieves an entry from the cache.
* The usage count of the entry is incremented.
*
* @param key the key whose associated value is to be returned.
* @return the value associated to this key, or null if no value with this key exists in the cache.
*/
public V get(K key) {
LFUCacheEntryWithCounter entry = map.get(key);
if (entry == null) {
return null;
} else {
entry.counter++;
return entry.value;
}
}
/**
* Ask whether a given key is present in the cache.
* The usage count of the entry is incremented if the entry exists.
* @param key the key to be tested
* @return true if the key is present
*/
public boolean containsKey(K key) {
LFUCacheEntryWithCounter entry = map.get(key);
if (entry == null) {
return false;
} else {
entry.counter++;
return true;
}
}
/**
* Adds an entry to this cache. It is assumed that the caller has
* already checked that the cache doesn't already contain a suitable
* entry, so we treat each entry as a new one.
* If the cache is exceeds a threshold size of three times the target
* size, it is rebuilt with infrequently used entries purged.
*
* @param key the key with which the specified value is to be associated.
* @param value a value to be associated with the specified key.
*/
public void put(K key, V value) {
map.put(key, new LFUCacheEntryWithCounter(value));
// Consider purging rarely-used entries
if (map.size() > 3*targetSize) {
rebuild();
}
}
private void rebuild() {
Map> m2 = makeMap(targetSize);
int retained = 0;
for (Map.Entry> entry : map.entrySet()) {
if (entry.getValue().counter > retentionThreshold) {
m2.put(entry.getKey(), new LFUCacheEntryWithCounter(entry.getValue().value));
retained++;
}
}
// System.err.println("Rebuilt LCUCache; retained size = " + retained + " retention threshold = " + retentionThreshold);
// Consider adjusting the threshold for next time
if (retained > 1.5*targetSize) {
// We retained too many entries, try to do better next time
retentionThreshold++;
} else if (retentionThreshold > 0 && retained < targetSize) {
// We discarded too many entries, try to do better next time
retentionThreshold--;
}
// Replace the map. Note this update isn't thread-safe; it doesn't matter if we lose it, or if some
// other thread is doing the same thing concurrently.
map = m2;
}
/**
* Clear the cache
*/
public void clear() {
map.clear();
}
/**
* Get the number of entries in the cache
*
* @return the number of entries
*/
public int size() {
return map.size();
}
}