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+/*
+ * Copyright (C) 2015-2016 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
+ */
+
+#pragma once
+
+#include <wtf/Vector.h>
+
+namespace WTF {
+
+// This is a map for keys that have an index(). It's super efficient for BasicBlocks. It's only
+// efficient for Values if you don't create too many of these maps, since Values can have very
+// sparse indices and there are a lot of Values.
+
+template<typename Key, typename Value>
+class IndexMap {
+public:
+    explicit IndexMap(size_t size = 0)
+    {
+        m_vector.fill(Value(), size);
+    }
+
+    void resize(size_t size)
+    {
+        m_vector.fill(Value(), size);
+    }
+
+    void clear()
+    {
+        m_vector.fill(Value(), m_vector.size());
+    }
+
+    size_t size() const { return m_vector.size(); }
+
+    Value& operator[](size_t index)
+    {
+        return m_vector[index];
+    }
+
+    const Value& operator[](size_t index) const
+    {
+        return m_vector[index];
+    }
+    
+    Value& operator[](Key* key)
+    {
+        return m_vector[key->index()];
+    }
+    
+    const Value& operator[](Key* key) const
+    {
+        return m_vector[key->index()];
+    }
+
+private:
+    Vector<Value> m_vector;
+};
+
+} // namespace WTF
+
+using WTF::IndexMap;