chromium/third_party/webrtc/rtc_base/numerics/safe_compare.h

/*
 *  Copyright 2016 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.
 */

// This file defines six constexpr functions:
//
//   rtc::SafeEq  // ==
//   rtc::SafeNe  // !=
//   rtc::SafeLt  // <
//   rtc::SafeLe  // <=
//   rtc::SafeGt  // >
//   rtc::SafeGe  // >=
//
// They each accept two arguments of arbitrary types, and in almost all cases,
// they simply call the appropriate comparison operator. However, if both
// arguments are integers, they don't compare them using C++'s quirky rules,
// but instead adhere to the true mathematical definitions. It is as if the
// arguments were first converted to infinite-range signed integers, and then
// compared, although of course nothing expensive like that actually takes
// place. In practice, for signed/signed and unsigned/unsigned comparisons and
// some mixed-signed comparisons with a compile-time constant, the overhead is
// zero; in the remaining cases, it is just a few machine instructions (no
// branches).

#ifndef RTC_BASE_NUMERICS_SAFE_COMPARE_H_
#define RTC_BASE_NUMERICS_SAFE_COMPARE_H_

#include <stddef.h>
#include <stdint.h>

#include <type_traits>

#include "rtc_base/type_traits.h"

namespace rtc {

namespace safe_cmp_impl {

template <size_t N>
struct LargerIntImpl : std::false_type { … };
template <>
struct LargerIntImpl<sizeof(int8_t)> : std::true_type { … };
template <>
struct LargerIntImpl<sizeof(int16_t)> : std::true_type { … };
template <>
struct LargerIntImpl<sizeof(int32_t)> : std::true_type { … };

// LargerInt<T1, T2>::value is true iff there's a signed type that's larger
// than T1 (and no larger than the larger of T2 and int*, for performance
// reasons); and if there is such a type, LargerInt<T1, T2>::type is an alias
// for it.
template <typename T1, typename T2>
struct LargerInt
    : LargerIntImpl<sizeof(T1) < sizeof(T2) || sizeof(T1) < sizeof(int*)
                        ? sizeof(T1)
                        : 0> { … };

template <typename T>
constexpr typename std::make_unsigned<T>::type MakeUnsigned(T a) { … }

// Overload for when both T1 and T2 have the same signedness.
template <typename Op,
          typename T1,
          typename T2,
          typename std::enable_if<std::is_signed<T1>::value ==
                                  std::is_signed<T2>::value>::type* = nullptr>
constexpr bool Cmp(T1 a, T2 b) { … }

// Overload for signed - unsigned comparison that can be promoted to a bigger
// signed type.
template <typename Op,
          typename T1,
          typename T2,
          typename std::enable_if<std::is_signed<T1>::value &&
                                  std::is_unsigned<T2>::value &&
                                  LargerInt<T2, T1>::value>::type* = nullptr>
constexpr bool Cmp(T1 a, T2 b) { … }

// Overload for unsigned - signed comparison that can be promoted to a bigger
// signed type.
template <typename Op,
          typename T1,
          typename T2,
          typename std::enable_if<std::is_unsigned<T1>::value &&
                                  std::is_signed<T2>::value &&
                                  LargerInt<T1, T2>::value>::type* = nullptr>
constexpr bool Cmp(T1 a, T2 b) { … }

// Overload for signed - unsigned comparison that can't be promoted to a bigger
// signed type.
template <typename Op,
          typename T1,
          typename T2,
          typename std::enable_if<std::is_signed<T1>::value &&
                                  std::is_unsigned<T2>::value &&
                                  !LargerInt<T2, T1>::value>::type* = nullptr>
constexpr bool Cmp(T1 a, T2 b) { … }

// Overload for unsigned - signed comparison that can't be promoted to a bigger
// signed type.
template <typename Op,
          typename T1,
          typename T2,
          typename std::enable_if<std::is_unsigned<T1>::value &&
                                  std::is_signed<T2>::value &&
                                  !LargerInt<T1, T2>::value>::type* = nullptr>
constexpr bool Cmp(T1 a, T2 b) { … }

#define RTC_SAFECMP_MAKE_OP …
RTC_SAFECMP_MAKE_OP(EqOp, ==)
RTC_SAFECMP_MAKE_OP(NeOp, !=)
RTC_SAFECMP_MAKE_OP(LtOp, <)
RTC_SAFECMP_MAKE_OP
RTC_SAFECMP_MAKE_OP
RTC_SAFECMP_MAKE_OP
#undef RTC_SAFECMP_MAKE_OP

}  // namespace safe_cmp_impl

#define RTC_SAFECMP_MAKE_FUN …
RTC_SAFECMP_MAKE_FUN
RTC_SAFECMP_MAKE_FUN
RTC_SAFECMP_MAKE_FUN
RTC_SAFECMP_MAKE_FUN
RTC_SAFECMP_MAKE_FUN
RTC_SAFECMP_MAKE_FUN
#undef RTC_SAFECMP_MAKE_FUN

}  // namespace rtc

#endif  // RTC_BASE_NUMERICS_SAFE_COMPARE_H_