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/*
* 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 <utility>
#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 {
using type = int16_t;
};
template <>
struct LargerIntImpl<sizeof(int16_t)> : std::true_type {
using type = int32_t;
};
template <>
struct LargerIntImpl<sizeof(int32_t)> : std::true_type {
using type = int64_t;
};
// 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) {
return static_cast<typename std::make_unsigned<T>::type>(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) {
return Op::Op(a, 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) {
return Op::Op(a, static_cast<typename LargerInt<T2, T1>::type>(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) {
return Op::Op(static_cast<typename LargerInt<T1, T2>::type>(a), 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) {
return a < 0 ? Op::Op(-1, 0) : Op::Op(safe_cmp_impl::MakeUnsigned(a), 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) {
return b < 0 ? Op::Op(0, -1) : Op::Op(a, safe_cmp_impl::MakeUnsigned(b));
}
#define RTC_SAFECMP_MAKE_OP(name, op) \
struct name { \
template <typename T1, typename T2> \
static constexpr bool Op(T1 a, T2 b) { \
return a op b; \
} \
};
RTC_SAFECMP_MAKE_OP(EqOp, ==)
RTC_SAFECMP_MAKE_OP(NeOp, !=)
RTC_SAFECMP_MAKE_OP(LtOp, <)
RTC_SAFECMP_MAKE_OP(LeOp, <=)
RTC_SAFECMP_MAKE_OP(GtOp, >)
RTC_SAFECMP_MAKE_OP(GeOp, >=)
#undef RTC_SAFECMP_MAKE_OP
} // namespace safe_cmp_impl
#define RTC_SAFECMP_MAKE_FUN(name) \
template <typename T1, typename T2> \
constexpr \
typename std::enable_if<IsIntlike<T1>::value && IsIntlike<T2>::value, \
bool>::type Safe##name(T1 a, T2 b) { \
/* Unary plus here turns enums into real integral types. */ \
return safe_cmp_impl::Cmp<safe_cmp_impl::name##Op>(+a, +b); \
} \
template <typename T1, typename T2> \
constexpr \
typename std::enable_if<!IsIntlike<T1>::value || !IsIntlike<T2>::value, \
bool>::type Safe##name(const T1& a, \
const T2& b) { \
return safe_cmp_impl::name##Op::Op(a, b); \
}
RTC_SAFECMP_MAKE_FUN(Eq)
RTC_SAFECMP_MAKE_FUN(Ne)
RTC_SAFECMP_MAKE_FUN(Lt)
RTC_SAFECMP_MAKE_FUN(Le)
RTC_SAFECMP_MAKE_FUN(Gt)
RTC_SAFECMP_MAKE_FUN(Ge)
#undef RTC_SAFECMP_MAKE_FUN
} // namespace rtc
#endif // RTC_BASE_NUMERICS_SAFE_COMPARE_H_
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