/** * SimpleARC * a Simple Adaptive Replacement Cache * Copyright 2009 by Michael Peter Christen, mc@yacy.net, Frankfurt a. M., Germany * First released 17.04.2009 at http://yacy.net * * $LastChangedDate$ * $LastChangedRevision$ * $LastChangedBy$ * * This library is free software; you can redistribute it and/or * modify it under the terms of the GNU Lesser General Public * License as published by the Free Software Foundation; either * version 2.1 of the License, or (at your option) any later version. * * This library is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * Lesser General Public License for more details. * * You should have received a copy of the GNU Lesser General Public License * along with this program in the file lgpl21.txt * If not, see <http://www.gnu.org/licenses/>. */ package net.yacy.cora.storage; import java.util.AbstractMap; import java.util.ArrayList; import java.util.Collection; import java.util.HashSet; import java.util.Iterator; import java.util.Map; import java.util.Set; /** * This is a simple cache using two generations of hashtables to store the content with a LFU strategy. * The Algorithm is described in a slightly more complex version as Adaptive Replacement Cache, "ARC". * For details see http://www.almaden.ibm.com/cs/people/dmodha/ARC.pdf * or http://en.wikipedia.org/wiki/Adaptive_Replacement_Cache * This version omits the ghost entry handling which is described in ARC, and keeps both cache levels * at the same size. * * This class is defined abstract because it shall be used with either the HashARC or the ComparableARC classes */ abstract class SimpleARC<K, V> extends AbstractMap<K, V> implements Map<K, V>, Iterable<Map.Entry<K, V>>, ARC<K, V> { protected int cacheSize; protected Map<K, V> levelA, levelB; // we can assume that these maps are synchronized /** * put a value to the cache. * @param s * @param v */ @Override public final synchronized void insert(final K s, final V v) { if (this.levelB.containsKey(s)) { this.levelB.put(s, v); assert (this.levelB.size() <= this.cacheSize); // the cache should shrink automatically } else { this.levelA.put(s, v); assert (this.levelA.size() <= this.cacheSize); // the cache should shrink automatically } } /** * put a value to the cache if there was not an entry before * do not return a previous content value * @param s * @param v */ @Override public void insertIfAbsent(final K s, final V v) { if (this.levelB.containsKey(s)) { return; } else if (this.levelA.containsKey(s)) { return; } else { synchronized (this) { // we must repeat the tests again because we did this in a not synchronized environment if (this.levelB.containsKey(s)) { return; } else if (this.levelA.containsKey(s)) { return; } else { this.levelA.put(s, v); assert (this.levelA.size() <= this.cacheSize); // the cache should shrink automatically } } } } /** * put a value to the cache if there was not an entry before * return a previous content value * @param s * @param v * @return the value before inserting the new value */ @Override public V putIfAbsent(final K s, final V v) { synchronized (this) { V o = this.levelB.get(s); if (o != null) return o; o = this.levelA.get(s); if (o != null) return o; this.levelA.put(s, v); assert (this.levelA.size() <= this.cacheSize); // the cache should shrink automatically return null; } } /** * put a value to the cache. * @param s * @param v */ @Override public final synchronized V put(final K s, final V v) { if (this.levelB.containsKey(s)) { final V r = this.levelB.put(s, v); assert (this.levelB.size() <= this.cacheSize); // the cache should shrink automatically return r; } final V r = this.levelA.put(s, v); assert (this.levelA.size() <= this.cacheSize); // the cache should shrink automatically return r; } /** * get a value from the cache. * @param s * @return the value */ @SuppressWarnings("unchecked") @Override public final V get(final Object s) { V v = this.levelB.get(s); if (v != null) return v; synchronized (this) { // we must repeat the get here because another thread may have moved the // entry from A to B meanwhile v = this.levelB.get(s); if (v != null) return v; // now get and move the entry to B v = this.levelA.remove(s); if (v == null) return null; // move value from A to B; since it was already removed from A, just put it to B //System.out.println("ARC: moving A->B, size(A) = " + this.levelA.size() + ", size(B) = " + this.levelB.size()); this.levelB.put((K) s, v); assert (this.levelB.size() <= this.cacheSize); // the cache should shrink automatically } return v; } /** * check if the map contains the value * @param value * @return the keys that have the given value */ @Override public Collection<K> getKeys(final V value) { final ArrayList<K> keys = new ArrayList<K>(); synchronized (this.levelB) { for (final Map.Entry<K, V> entry: this.levelB.entrySet()) { if (value.equals(entry.getValue())) keys.add(entry.getKey()); } } synchronized (this) { for (final Map.Entry<K, V> entry: this.levelA.entrySet()) { if (value.equals(entry.getValue())) keys.add(entry.getKey()); } } return keys; } /** * check if the map contains the key * @param s * @return */ @Override public final boolean containsKey(final Object s) { if (this.levelB.containsKey(s)) return true; return this.levelA.containsKey(s); } /** * remove an entry from the cache * @param s * @return the old value */ @Override public final synchronized V remove(final Object s) { final V r = this.levelB.remove(s); if (r != null) return r; return this.levelA.remove(s); } /** * clear the cache */ @Override public final synchronized void clear() { this.levelA.clear(); this.levelB.clear(); } /** * get the size of the ARC. this returns the sum of main and ghost cache * @return the complete number of entries in the ARC cache */ @Override public final synchronized int size() { return this.levelA.size() + this.levelB.size(); } /** * iterator implements the Iterable interface */ @Override public final Iterator<Map.Entry<K, V>> iterator() { return entrySet().iterator(); } /** * Return a Set view of the mappings contained in this map. * This method is the basis for all methods that are implemented * by a AbstractMap implementation * * @return a set view of the mappings contained in this map */ @Override public final synchronized Set<Map.Entry<K, V>> entrySet() { final Set<Map.Entry<K, V>> m = new HashSet<Map.Entry<K, V>>(); for (final Map.Entry<K, V> entry: this.levelA.entrySet()) m.add(entry); for (final Map.Entry<K, V> entry: this.levelB.entrySet()) m.add(entry); return m; } /** * a hash code for this ARC * @return the hash code of one of the ARC partial hash tables */ @Override public final int hashCode() { return this.levelA.hashCode(); } }