Add a basic utility for checked addition of unsigned integers

Summary: A very basic function that if done incorrectly will get optimized away as undefined behavior. For now keep things simple and only support addition of unsigned values.

Reviewed By: yfeldblum

Differential Revision: D15609542

fbshipit-source-id: 70d5fde784f57d3d52e6122352a885d7a835f104

folly/lang/CheckedMath.h

+/*
+ * Copyright 2011-present Facebook, Inc.
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include <cassert>
+#include <limits>
+#include <type_traits>
+
+#include <folly/Likely.h>
+
+namespace folly {
+template <typename T, typename = std::enable_if_t<std::is_unsigned<T>::value>>
+bool checked_add(T* result, T a, T b) {
+  assert(result != nullptr);
+  if (FOLLY_LIKELY(a < std::numeric_limits<T>::max() - b)) {
+    *result = a + b;
+    return true;
+  } else {
+    *result = {};
+    return false;
+  }
+}
+} // namespace folly

folly/lang/test/CheckedMathTest.cpp

+/*
+ * Copyright 2011-present Facebook, Inc.
+ *
+ * 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.
+ */
+
+#include <folly/lang/CheckedMath.h>
+
+#include <limits>
+
+#include <folly/portability/GTest.h>
+
+TEST(CheckedMath, checked_add_no_overflow) {
+  unsigned int a;
+
+  EXPECT_TRUE(folly::checked_add(&a, 5u, 4u));
+  EXPECT_EQ(a, 9);
+}
+
+TEST(CheckedMath, checked_add_overflow) {
+  unsigned int a;
+
+  EXPECT_FALSE(
+      folly::checked_add(&a, std::numeric_limits<unsigned int>::max(), 4u));
+  EXPECT_EQ(a, {});
+}
+
+TEST(CheckedMath, checked_add_size_t_overflow) {
+  size_t a;
+
+  EXPECT_FALSE(folly::checked_add<size_t>(
+      &a, std::numeric_limits<size_t>::max() - 7, 9));
+  EXPECT_EQ(a, {});
+}