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diff --git a/src/compile_assert.h b/src/compile_assert.h
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+// Copyright 2008 Google Inc.
+// Authors: Zhanyong Wan, Lincoln Smith
+//
+// 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.
+
+#ifndef OPEN_VCDIFF_COMPILE_ASSERT_H_
+#define OPEN_VCDIFF_COMPILE_ASSERT_H_
+
+#include <config.h>
+
+namespace open_vcdiff {
+
+// The VCD_COMPILE_ASSERT macro can be used to verify that a compile-time
+// expression is true. For example, you could use it to verify the
+// size of a static array:
+//
+//   VCD_COMPILE_ASSERT(ARRAYSIZE(content_type_names) == CONTENT_NUM_TYPES,
+//                      content_type_names_incorrect_size);
+//
+// or to make sure a struct is smaller than a certain size:
+//
+//   VCD_COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large);
+//
+// For the second argument to VCD_COMPILE_ASSERT, the programmer should supply
+// a variable name that meets C++ naming rules, but that provides
+// a description of the compile-time rule that has been violated.
+// (In the example above, the name used is "foo_too_large".)
+// If the expression is false, most compilers will issue a warning/error
+// containing the name of the variable.
+// This refinement (adding a descriptive variable name argument)
+// is what differentiates VCD_COMPILE_ASSERT from Boost static asserts.
+
+template <bool>
+struct CompileAssert {
+};
+
+}  // namespace open_vcdiff
+
+#define VCD_COMPILE_ASSERT(expr, msg) \
+  typedef open_vcdiff::CompileAssert<static_cast<bool>(expr)> \
+      msg[static_cast<bool>(expr) ? 1 : -1]
+
+// Implementation details of VCD_COMPILE_ASSERT:
+//
+// - VCD_COMPILE_ASSERT works by defining an array type that has -1
+//   elements (and thus is invalid) when the expression is false.
+//
+// - The simpler definition
+//
+//     #define VCD_COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1]
+//
+//   does not work, as gcc supports variable-length arrays whose sizes
+//   are determined at run-time (this is gcc's extension and not part
+//   of the C++ standard).  As a result, gcc fails to reject the
+//   following code with the simple definition:
+//
+//     int foo;
+//     VCD_COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is
+//                               // not a compile-time constant.
+//
+// - By using the type CompileAssert<(static_cast<bool>(expr))>, we ensure that
+//   expr is a compile-time constant.  (Template arguments must be
+//   determined at compile-time.)
+//
+// - The array size is (static_cast<bool>(expr) ? 1 : -1), instead of simply
+//
+//     ((expr) ? 1 : -1).
+//
+//   This is to avoid running into a bug in MS VC 7.1, which
+//   causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1.
+
+#endif  // OPEN_VCDIFF_COMPILE_ASSERT_H_