RingBuffer<T, N> utility with unit test; use for GC history.

runtime/vm/ring_buffer.h

Index: runtime/vm/ring_buffer.h
===================================================================
--- runtime/vm/ring_buffer.h	(revision 0)
+++ runtime/vm/ring_buffer.h	(revision 0)
@@ -0,0 +1,41 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#ifndef VM_RING_BUFFER_H_
+#define VM_RING_BUFFER_H_
+
+#include "platform/assert.h"
+#include "platform/utils.h"
+
+namespace dart {
+
+// Fixed-capacity ring buffer.
+template<typename T, int N>
+class RingBuffer {
+ public:
+  RingBuffer() : count_(0) { }
+
+  void Add(const T& t) {
+    data_[count_++ & kMask] = t;
+  }
+
+  // Returns the i'th most recently added element. Requires 0 <= i < Size().
+  const T& Get(int i) const {
+    ASSERT(0 <= i && i < Size());
+    return data_[(count_ - i - 1) & kMask];
+  }
+
+  // Returns the number of elements currently stored in this buffer (at most N).
+  int Size() const { return Utils::Minimum(count_, static_cast<int64_t>(N)); }
+
+ private:
+  static const int kMask = N - 1;
+  COMPILE_ASSERT((N & kMask) == 0, N_must_be_power_of_two);
+  T data_[N];
+  int64_t count_;
+};
+
+}  // namespace dart
+
+#endif  // VM_RING_BUFFER_H_