Bug 1147766 - Part 1: Add a mozilla::RangedArray class, for fixed length arrays with a non-zero base index. r=waldo

mfbt/RangedArray.h

@@ -0,0 +1,44 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * A compile-time constant-length array, with bounds-checking assertions -- but
+ * unlike mozilla::Array, with indexes biased by a constant.
+ *
+ * Thus where mozilla::Array<int, 3> is a three-element array indexed by [0, 3),
+ * mozilla::RangedArray<int, 8, 3> is a three-element array indexed by [8, 11).
+ */
+
+#ifndef mozilla_RangedArray_h
+#define mozilla_RangedArray_h
+
+#include "mozilla/Array.h"
+
+namespace mozilla {
+
+template<typename T, size_t MinIndex, size_t Length>
+class RangedArray
+{
+public:
+  T& operator[](size_t aIndex)
+  {
+    MOZ_ASSERT(aIndex >= MinIndex);
+    return mArr[aIndex - MinIndex];
+  }
+
+  const T& operator[](size_t aIndex) const
+  {
+    MOZ_ASSERT(aIndex >= MinIndex);
+    return mArr[aIndex - MinIndex];
+  }
+
+private:
+  Array<T, Length> mArr;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_RangedArray_h

mfbt/moz.build

@@ -59,6 +59,7 @@ EXPORTS.mozilla = [
     'PodOperations.h',
     'Poison.h',
     'Range.h',
+    'RangedArray.h',
     'RangedPtr.h',
     'RefCountType.h',
     'ReentrancyGuard.h',