Reduce cache density

Author: ngallagher

snap-common/src/main/java/org/snapscript/common/CopyOnWriteCache.java

@@ -10,8 +10,16 @@ public class CopyOnWriteCache<K, V> implements Cache<K, V> {
    private volatile Map<K, V> cache;
 
    public CopyOnWriteCache() {
+      this(16);
+   }
+   
+   public CopyOnWriteCache(int size) {
+      this(16, 0.5f);
+   }
+   
+   public CopyOnWriteCache(int size, float density) {
+      this.updater = new MapUpdater(size, density);
       this.cache = new HashMap<K, V>();
-      this.updater = new MapUpdater();
    }
 
    @Override
@@ -57,17 +65,22 @@ public int size() {
    private class MapUpdater {
 
       private final Map<K, V> empty;
+      private final float density;
+      private final int size;
 
-      public MapUpdater() {
+      public MapUpdater(int size, float density) {
          this.empty = new HashMap<K, V>();
+         this.density = density;
+         this.size = size;
       }
 
       public synchronized void cache(K key, V value) {
          V existing = cache.get(key);
 
          if(existing != value) { // reduce churn
-            Map<K, V> copy = new HashMap<K, V>(cache);
+            Map<K, V> copy = new HashMap<K, V>(size, density);
 
+            copy.putAll(cache);
             copy.put(key, value);
             cache = copy;
          }
@@ -77,8 +90,9 @@ public synchronized V take(K key) {
          V existing = cache.get(key);
 
          if(existing != null) {
-            Map<K, V> copy = new HashMap<K, V>(cache);
-           
+            Map<K, V> copy = new HashMap<K, V>(size, density);
+            
+            copy.putAll(cache);
             copy.remove(key);
             cache = copy;
          }

snap-core/src/test/java/org/snapscript/core/HashMapLoadFactorTest.java

@@ -0,0 +1,42 @@
+package org.snapscript.core;
+
+import java.security.SecureRandom;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Random;
+
+import junit.framework.TestCase;
+
+public class HashMapLoadFactorTest extends TestCase {
+   
+   private static final int ITERATIONS = 10;
+   private static final int CAPACITY = 1000000;
+   
+   public void testLoadFactor() throws Exception{
+      Random random = new SecureRandom();
+      String[] keys = new String[CAPACITY];
+      
+      for(int i = 0; i< keys.length; i++){
+         keys[i] = "key-" + random.nextInt(CAPACITY);
+      }
+      for(int i = 0; i < ITERATIONS; i++) {
+         measurePerformance(keys, 0.75f);    
+         measurePerformance(keys, 0.50f);  
+         measurePerformance(keys, 0.35f); 
+      }
+   }
+   
+   private void measurePerformance(String[] keys, float loadFactor) {
+      Map<String, String> map = new HashMap<String, String>(keys.length, loadFactor);
+      for(int i = 0; i< keys.length; i++){
+         map.put(keys[i], keys[i]);
+      }
+      long start = System.currentTimeMillis();
+      for(int i = 0; i< keys.length; i++){
+         map.get(i);
+      }
+      long finish = System.currentTimeMillis();
+      System.err.println("loadFactor="+loadFactor+" time="+(finish-start));
+   }
+
+}