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132 lines
4.5 KiB
132 lines
4.5 KiB
// SimpleARC.java
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// a Simple Adaptive Replacement Cache
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// (C) 2009 by Michael Peter Christen; mc@yacy.net, Frankfurt a. M., Germany
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// first published 17.04.2009 on http://yacy.net
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//
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// $LastChangedDate: 2006-04-02 22:40:07 +0200 (So, 02 Apr 2006) $
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// $LastChangedRevision: 1986 $
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// $LastChangedBy: orbiter $
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//
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// LICENSE
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//
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// This program is free software; you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation; either version 2 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program; if not, write to the Free Software
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// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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package net.yacy.kelondro.index;
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import java.util.LinkedHashMap;
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import java.util.Map;
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/**
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* This is a simple cache using two generations of hashtables to store the content with a LFU strategy.
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* The Algorithm is described in a slightly more complex version as Adaptive Replacement Cache, "ARC".
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* For details see http://www.almaden.ibm.com/cs/people/dmodha/ARC.pdf
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* or http://en.wikipedia.org/wiki/Adaptive_Replacement_Cache
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* This version omits the ghost entry handling which is described in ARC, and keeps both cache levels
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* at the same size.
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*/
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public final class SimpleARC<K, V> implements ARC<K, V> {
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public final static boolean accessOrder = false; // if false, then a insertion-order is used
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protected final int cacheSize;
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private final Map<K, V> levelA, levelB;
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public SimpleARC(final int cacheSize) {
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this.cacheSize = cacheSize / 2;
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this.levelA = new LinkedHashMap<K, V>(cacheSize, 0.1f, accessOrder) {
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private static final long serialVersionUID = 1L;
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@Override protected boolean removeEldestEntry(final Map.Entry<K, V> eldest) {
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return size() > SimpleARC.this.cacheSize;
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}
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};
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this.levelB = new LinkedHashMap<K, V>(cacheSize, 0.1f, accessOrder) {
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private static final long serialVersionUID = 1L;
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@Override protected boolean removeEldestEntry(final Map.Entry<K, V> eldest) {
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return size() > SimpleARC.this.cacheSize;
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}
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};
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}
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/**
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* put a value to the cache.
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* @param s
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* @param v
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*/
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public final synchronized void put(final K s, final V v) {
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if (this.levelB.containsKey(s)) {
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this.levelB.put(s, v);
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assert (this.levelB.size() <= cacheSize); // the cache should shrink automatically
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} else {
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this.levelA.put(s, v);
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assert (this.levelA.size() <= cacheSize); // the cache should shrink automatically
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}
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}
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/**
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* get a value from the cache.
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* @param s
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* @return the value
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*/
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public final synchronized V get(final K s) {
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V v = this.levelB.get(s);
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if (v != null) return v;
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v = this.levelA.remove(s);
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if (v == null) return null;
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// move value from A to B; since it was already removed from A, just put it to B
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//System.out.println("ARC: moving A->B, size(A) = " + this.levelA.size() + ", size(B) = " + this.levelB.size());
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this.levelB.put(s, v);
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assert (this.levelB.size() <= cacheSize); // the cache should shrink automatically
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return v;
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}
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/**
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* check if the map contains the key
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* @param s
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* @return
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*/
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public final synchronized boolean containsKey(final K s) {
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if (this.levelB.containsKey(s)) return true;
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return this.levelA.containsKey(s);
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}
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/**
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* remove an entry from the cache
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* @param s
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* @return the old value
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*/
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public final synchronized V remove(final K s) {
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final V r = this.levelB.remove(s);
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if (r != null) return r;
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return this.levelA.remove(s);
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}
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/**
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* clear the cache
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*/
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public final synchronized void clear() {
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this.levelA.clear();
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this.levelB.clear();
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}
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/**
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* get the size of the ARC. this returns the sum of main and ghost cache
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* @return the complete number of entries in the ARC cache
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*/
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public final synchronized int size() {
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return this.levelA.size() + this.levelB.size();
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}
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}
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