--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/gov/nasa/jpf/util/HashPool.java	Fri Jan 23 10:14:01 2015 -0800
@@ -0,0 +1,90 @@
+/*
+ * Copyright (C) 2014, United States Government, as represented by the
+ * Administrator of the National Aeronautics and Space Administration.
+ * All rights reserved.
+ *
+ * The Java Pathfinder core (jpf-core) platform is licensed under the
+ * Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ * 
+ *        http://www.apache.org/licenses/LICENSE-2.0. 
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and 
+ * limitations under the License.
+ */
+package gov.nasa.jpf.util;
+
+import java.util.ArrayList;
+
+
+/**
+ * data structure used to do hash collapsing. All the major state components
+ * (fields, Monitors, StackFrames, uThreadData) are stored in pools to
+ * determine if they are new. Only the pool index values are used to
+ * compute hash values.
+ * <p>
+ * 2006-06-14 - major rewrite by pcd
+ */
+public final class HashPool<V> {
+  private IntTable<V> pool;
+  private ArrayList<V> vect;
+  
+  public HashPool() {
+    this(8); // default to 256 slots
+  }
+  
+  public HashPool(int pow) {
+    pool = new IntTable<V>(pow);
+    vect = new ArrayList<V>(1 << pow);
+  }
+
+  /** optionally called only once after creation to link null to 0. */ 
+  public HashPool<V> addNull() {
+    if (size() == 0) {
+      pool.add(null, 0);
+      vect.add(null);
+      return this;
+    } else {
+      throw new IllegalStateException();
+    }
+  }
+  
+  public IntTable.Entry<V> getEntry (V o) {
+    int sz = pool.size(); // == vect.size();
+    
+    IntTable.Entry<V> e = pool.pool(o);
+    if (e.val == sz) {
+      vect.add(o);
+    }
+    return e;
+  }
+
+  public int getIndex (V o) {
+    return getEntry(o).val;
+  }
+
+  public V get (V o) {
+    return getEntry(o).key;
+  }
+
+  public V getObject (int idx) {
+    return vect.get(idx);
+  }
+
+  public void print () {
+    System.out.println("{");
+
+    for (IntTable.Entry<V> entry : pool) {
+      System.out.println("\t" + entry);
+    }
+
+    System.out.println("}");
+  }
+
+  public int size () {
+    return pool.size();
+  }
+}