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 all_int = IsIntlike<T1>::value&& IsIntlike<T2>::value>
+struct MType;
+
+// Specialization for when at least one of the types is floating-point.
+template <typename T1, typename T2>
+struct MType<T1, T2, 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> {
+  // 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<

[kwiberg-webrtc, 2017/04/13 00:56:05]

I also changed it so that the return type is the underlying type of an enum
type, instead of the enum type itself, since the returned value could easily be
one that's not listed in the enum.

+                  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");
+  static_assert(IsIntlike<T1>::value == IsIntlike<T2>::value,
+                "You may not mix integral and floating-point arguments");
+  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");
+  static_assert(IsIntlike<T1>::value == IsIntlike<T2>::value,
+                "You may not mix integral and floating-point arguments");
+  return safe_cmp::Gt(a, b) ? static_cast<R2>(a) : static_cast<R2>(b);
+}
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_SAFE_MINMAX_H_