LRU hashmap with rudimentary performance tracking code.

git-svn-id: https://samskivert.googlecode.com/svn/trunk@873 6335cc39-0255-0410-8fd6-9bcaacd3b74c
2002-10-18 01:43:46 +00:00
parent 72a05fa745
commit b483031902
1 changed files with 99 additions and 0 deletions
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+//
+// $Id: LRUHashMap.java,v 1.1 2002/10/18 01:43:46 ray Exp $
+
+package com.samskivert.util;
+
+import java.util.HashSet;
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+/**
+ * A HashMap with LRU functionality and rudimentary performance tracking
+ * facilities.
+ */
+public class LRUHashMap extends LinkedHashMap
+{
+    /**
+     * Construct a LRUHashMap with the specified maximum size.
+     */
+    public LRUHashMap (int maxSize)
+    {
+        super(Math.min(1024, Math.max(16, maxSize)), .75f, true);
+        _maxSize = maxSize;
+    }
+
+    /**
+     * Turn performance tracking on/off.
+     */
+    public void setTracking (boolean track)
+    {
+        if (track != _tracking) {
+            _tracking = track;
+            if (track) {
+                _seenKeys = new HashSet();
+                _misses = _hits = 0;
+
+                // oh boy, but to properly track we need to clear the hash
+                clear();
+            } else {
+                _seenKeys = null;
+            }
+        }
+    }
+
+    /**
+     * Return a measure of the effectiveness of this cache, the ratio of
+     * hits to misses.
+     *
+     * @return an array containing {hits, misses}
+     */
+    public int[] getTrackedEffectiveness ()
+    {
+        return new int[] {_hits, _misses};
+    }
+
+    // documentation inherited
+    public Object get (Object key)
+    {
+        Object result = super.get(key);
+
+        if (_tracking) {
+            if (result == null) {
+                if (_seenKeys.contains(key)) {
+                    // only count a miss if we've seen the key before
+                    _misses++;
+                }
+            } else {
+                _hits++;
+            }
+        }
+
+        return result;
+    }
+
+    // documentation inherited
+    public Object put (Object key, Object value)
+    {
+        Object result = super.put(key, value);
+
+        if (_tracking) {
+            _seenKeys.add(key);
+        }
+
+        return result;
+    }
+
+    // documentation inherited
+    protected boolean removeEldestEntry (Map.Entry eldest)
+    {
+        return size() > _maxSize;
+    }
+
+    /** The maximum size of this cache. */
+    protected int _maxSize;
+
+    /** Tracking info. */
+    protected boolean _tracking;
+    protected HashSet _seenKeys;
+    protected int _hits, _misses;
+}