Add SafeMin() and SafeMax(), which accept args of different types

webrtc/base/safe_minmax.h

+/*
+ *  Copyright 2017 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+// Minimum and maximum
+// ===================
+//
+//   rtc::SafeMin(x, y)
+//   rtc::SafeMax(x, y)
+//
+// Accept two arguments of either any two integral or any two floating-point
+// types, and return the smaller and larger value, respectively, with no
+// truncation or wrap-around. If only one of the input types is statically
+// guaranteed to be able to represent the result, the return type is that type;
+// if either one would do, the result type is the smaller type. (One of these
+// two cases always applies.)
+//
+// (The case with one floating-point and one integral type is not allowed,
+// because the floating-point type will have greater range, but may not have
+// sufficient precision to represent the integer value exactly.)
+//
+// Requesting a specific return type
+// =================================
+//
+// Both functions allow callers to explicitly specify the return type as a
+// template parameter, overriding the default return type. E.g.
+//
+//   rtc::SafeMin<int>(x, y)  // returns an int
+//
+// If the requested type is statically guaranteed to be able to represent the
+// result, then everything's fine, and the return type is as requested. But if
+// the requested type is too small, a static_assert is triggered.
+
+#ifndef WEBRTC_BASE_SAFE_MINMAX_H_
+#define WEBRTC_BASE_SAFE_MINMAX_H_
+
+#include <limits>
+#include <type_traits>
+
+#include "webrtc/base/checks.h"
+#include "webrtc/base/safe_compare.h"
+#include "webrtc/base/type_traits.h"
+
+namespace rtc {
+
+namespace safe_minmax_impl {
+
+// Make the range of a type available via something other than a constexpr
+// function, to work around MSVC limitations. See
+// https://blogs.msdn.microsoft.com/vcblog/2015/12/02/partial-support-for-expression-sfinae-in-vs-2015-update-1/
+template <typename T>
+struct Limits {
+  static constexpr T lowest = std::numeric_limits<T>::lowest();
+  static constexpr T max = std::numeric_limits<T>::max();
+};
+
+template <typename T, bool is_enum = std::is_enum<T>::value>
+struct UnderlyingType;
+
+template <typename T>
+struct UnderlyingType<T, false> {
+  using type = T;
+};
+
+template <typename T>
+struct UnderlyingType<T, true> {
+  using type = typename std::underlying_type<T>::type;
+};
+
+// Given two types T1 and T2, find types that can hold the smallest (in
+// ::min_t) and the largest (in ::max_t) of the two values.
+template <typename T1,
+          typename T2,
+          bool int1 = IsIntlike<T1>::value,
+          bool int2 = IsIntlike<T2>::value>
+struct MType {
+  static_assert(int1 == int2,
+                "You may not mix integral and floating-point arguments");
+};
+
+// Specialization for when neither type is integral (and therefore presumably
+// floating-point).
+template <typename T1, typename T2>
+struct MType<T1, T2, false, false> {
+  using min_t = typename std::common_type<T1, T2>::type;
+  static_assert(std::is_same<min_t, T1>::value ||
+                    std::is_same<min_t, T2>::value,
+                "");
+
+  using max_t = typename std::common_type<T1, T2>::type;
+  static_assert(std::is_same<max_t, T1>::value ||
+                    std::is_same<max_t, T2>::value,
+                "");
+};
+
+// Specialization for when both types are integral.
+template <typename T1, typename T2>
+struct MType<T1, T2, true, true> {
+  // The type with the lowest minimum value. In case of a tie, the type with
+  // the lowest maximum value. In case that too is a tie, the types have the
+  // same range, and we arbitrarily pick T1.
+  using min_t = typename std::conditional<
+      safe_cmp::Lt(Limits<T1>::lowest, Limits<T2>::lowest),
+      T1,
+      typename std::conditional<
+          safe_cmp::Gt(Limits<T1>::lowest, Limits<T2>::lowest),
+          T2,
+          typename std::conditional<safe_cmp::Le(Limits<T1>::max,
+                                                 Limits<T2>::max),
+                                    T1,
+                                    T2>::type>::type>::type;
+  static_assert(std::is_same<min_t, T1>::value ||
+                    std::is_same<min_t, T2>::value,
+                "");
+
+  // The type with the highest maximum value. In case of a tie, the types have
+  // the same range (because in C++, integer types with the same maximum also
+  // have the same minimum).
+  static_assert(safe_cmp::Ne(Limits<T1>::max, Limits<T2>::max) ||
+                    safe_cmp::Eq(Limits<T1>::lowest, Limits<T2>::lowest),
+                "integer types with the same max should have the same min");
+  using max_t = typename std::
+      conditional<safe_cmp::Ge(Limits<T1>::max, Limits<T2>::max), T1, T2>::type;
+  static_assert(std::is_same<max_t, T1>::value ||
+                    std::is_same<max_t, T2>::value,
+                "");
+};
+
+// A dummy type that we pass around at compile time but never actually use.
+// Declared but not defined.
+struct DefaultType;
+
+// ::type is A, except we fall back to B if A is DefaultType. We static_assert
+// that the chosen type can hold all values that B can hold.
+template <typename A, typename B>
+struct TypeOr {
+  using type = typename std::
+      conditional<std::is_same<A, DefaultType>::value, B, A>::type;
+  static_assert(safe_cmp::Le(Limits<type>::lowest, Limits<B>::lowest) &&
+                    safe_cmp::Ge(Limits<type>::max, Limits<B>::max),
+                "The specified type isn't large enough");
+  static_assert(IsIntlike<type>::value == IsIntlike<B>::value &&
+                    std::is_floating_point<type>::value ==
+                        std::is_floating_point<type>::value,
+                "float<->int conversions not allowed");
+};
+
+}  // namespace safe_minmax_impl
+
+template <typename R = safe_minmax_impl::DefaultType,
+          typename T1 = safe_minmax_impl::DefaultType,
+          typename T2 = safe_minmax_impl::DefaultType,
+          typename R2 = typename safe_minmax_impl::TypeOr<
+              R,
+              typename safe_minmax_impl::UnderlyingType<
+                  typename safe_minmax_impl::MType<T1, T2>::min_t>::type>::type>
+constexpr R2 SafeMin(T1 a, T2 b) {
+  static_assert(IsIntlike<T1>::value || std::is_floating_point<T1>::value,
+                "The first argument must be integral or floating-point");
+  static_assert(IsIntlike<T2>::value || std::is_floating_point<T2>::value,
+                "The second argument must be integral or floating-point");
+  return safe_cmp::Lt(a, b) ? static_cast<R2>(a) : static_cast<R2>(b);
+}
+
+template <typename R = safe_minmax_impl::DefaultType,
+          typename T1 = safe_minmax_impl::DefaultType,
+          typename T2 = safe_minmax_impl::DefaultType,
+          typename R2 = typename safe_minmax_impl::TypeOr<
+              R,
+              typename safe_minmax_impl::UnderlyingType<
+                  typename safe_minmax_impl::MType<T1, T2>::max_t>::type>::type>
+constexpr R2 SafeMax(T1 a, T2 b) {
+  static_assert(IsIntlike<T1>::value || std::is_floating_point<T1>::value,
+                "The first argument must be integral or floating-point");
+  static_assert(IsIntlike<T2>::value || std::is_floating_point<T2>::value,
+                "The second argument must be integral or floating-point");
+  return safe_cmp::Gt(a, b) ? static_cast<R2>(a) : static_cast<R2>(b);
+}
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_SAFE_MINMAX_H_