--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/graal/com.oracle.max.base/src/com/sun/max/util/ArrayValueHistory.java	Sat Dec 17 19:59:18 2011 +0100
@@ -0,0 +1,157 @@
+/*
+ * Copyright (c) 2009, 2011, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code 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 General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package com.sun.max.util;
+
+import java.util.*;
+
+import com.sun.max.program.*;
+
+/**
+ * An array-based recording of the history of a value, with
+ * time expressed as the number of generations back from the current generation (0).
+ */
+public final class ArrayValueHistory<E> {
+
+    private final ArrayDeque<E> generations;
+    private final int limit;
+    private int age = -1;
+
+    public ArrayValueHistory(int limit) {
+        this.generations = new ArrayDeque<E>();
+        this.limit = limit;
+    }
+
+    public ArrayValueHistory() {
+        this (Integer.MAX_VALUE);
+    }
+
+    /**
+     * Adds a new value, which becomes the current generation.
+     * The generation of all previously recorded values increases by 1.
+     */
+    public void addNew(E newValue) {
+//        if (generations.size() > 0) {
+//            if (newValue.equals(generations.getFirst())) {
+//                if (age >= 0) {
+//                    age++;
+//                }
+//            } else {
+//                age = 0;
+//            }
+//        }
+        generations.addFirst(newValue);
+        if (generations.size() > limit) {
+            generations.removeLast();
+        }
+        this.age = currentAge();
+    }
+
+    /**
+     * Replaces the current value in the history.
+     *
+     * @param newValue value which becomes current
+     * @throws ProgramError if no values have been recorded.
+     */
+    public void updateCurrent(E newValue) {
+        if (generations.size() > 0) {
+            if (!newValue.equals(generations.getFirst())) {
+                generations.pop();
+                addNew(newValue);
+            }
+        } else {
+            throw ProgramError.unexpected("attempt to update empty history");
+        }
+    }
+
+    /**
+     * Gets the historical value at some generation, 0 is current.
+     *
+     * @return The value at a specified generation.
+     * @throws ProgramError if the index is out of range of the current history
+     */
+    public E value(int generation) {
+        if (generation == 0 && generations.size() > 0) {
+            return generations.getFirst();
+        }
+        final Iterator<E> iterator = generations.iterator();
+        int index = 0;
+        while (iterator.hasNext()) {
+            if (index == generation) {
+                return iterator.next();
+            }
+            index++;
+        }
+        throw ProgramError.unexpected("exceeded history");
+    }
+
+    /**
+     * @return the age, in generations, of the current value, since recording began.
+     * 0 if different from immediate predecessor; -1 if no different value ever recorded
+     * Comparison uses {@linkplain Object#equals(Object) equals}.
+     */
+    public int currentValueAge() {
+        return age;
+    }
+
+    /**
+     * @return the maximum number of generations that can be recorded.
+     */
+    public int getLimit() {
+        return limit;
+    }
+
+    /**
+     * @return the number of generations recorded; initially 0.
+     */
+    public int size() {
+        return generations.size();
+    }
+
+    /**
+     * @return iteration of the values recorded in the history, starting with the current
+     * generation and proceeding backward in time.
+     */
+    public Iterator<E> generations() {
+        return generations.iterator();
+    }
+
+    /**
+     * Computes the age of the current generation, defined to be the number of
+     * preceding values that are equal, or -1 if no different value exists.
+     */
+    private int currentAge() {
+        assert generations.size() > 0;
+        final Iterator<E> iterator = generations.iterator();
+        E currentValue = iterator.next();
+        int duplicates = 0;
+        while (iterator.hasNext()) {
+            if (iterator.next().equals(currentValue)) {
+                duplicates++;
+            } else {
+                return duplicates;
+            }
+        }
+        return -1;
+    }
+
+}