177 lines
6.5 KiB
C++
177 lines
6.5 KiB
C++
/*
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* Copyright 2016 The WebRTC Project Authors. All rights reserved.
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*
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* Use of this source code is governed by a BSD-style license
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* that can be found in the LICENSE file in the root of the source
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* tree. An additional intellectual property rights grant can be found
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* in the file PATENTS. All contributing project authors may
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* be found in the AUTHORS file in the root of the source tree.
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*/
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// This file defines six constexpr functions:
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//
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// rtc::SafeEq // ==
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// rtc::SafeNe // !=
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// rtc::SafeLt // <
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// rtc::SafeLe // <=
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// rtc::SafeGt // >
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// rtc::SafeGe // >=
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//
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// They each accept two arguments of arbitrary types, and in almost all cases,
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// they simply call the appropriate comparison operator. However, if both
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// arguments are integers, they don't compare them using C++'s quirky rules,
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// but instead adhere to the true mathematical definitions. It is as if the
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// arguments were first converted to infinite-range signed integers, and then
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// compared, although of course nothing expensive like that actually takes
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// place. In practice, for signed/signed and unsigned/unsigned comparisons and
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// some mixed-signed comparisons with a compile-time constant, the overhead is
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// zero; in the remaining cases, it is just a few machine instructions (no
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// branches).
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#ifndef RTC_BASE_NUMERICS_SAFE_COMPARE_H_
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#define RTC_BASE_NUMERICS_SAFE_COMPARE_H_
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#include <stddef.h>
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#include <stdint.h>
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#include <type_traits>
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#include <utility>
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#include "rtc_base/type_traits.h"
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namespace rtc {
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namespace safe_cmp_impl {
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template <size_t N>
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struct LargerIntImpl : std::false_type {};
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template <>
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struct LargerIntImpl<sizeof(int8_t)> : std::true_type {
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using type = int16_t;
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};
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template <>
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struct LargerIntImpl<sizeof(int16_t)> : std::true_type {
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using type = int32_t;
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};
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template <>
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struct LargerIntImpl<sizeof(int32_t)> : std::true_type {
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using type = int64_t;
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};
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// LargerInt<T1, T2>::value is true iff there's a signed type that's larger
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// than T1 (and no larger than the larger of T2 and int*, for performance
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// reasons); and if there is such a type, LargerInt<T1, T2>::type is an alias
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// for it.
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template <typename T1, typename T2>
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struct LargerInt
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: LargerIntImpl<sizeof(T1) < sizeof(T2) || sizeof(T1) < sizeof(int*)
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? sizeof(T1)
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: 0> {};
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template <typename T>
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constexpr typename std::make_unsigned<T>::type MakeUnsigned(T a) {
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return static_cast<typename std::make_unsigned<T>::type>(a);
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}
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// Overload for when both T1 and T2 have the same signedness.
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template <typename Op,
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typename T1,
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typename T2,
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typename std::enable_if<std::is_signed<T1>::value ==
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std::is_signed<T2>::value>::type* = nullptr>
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constexpr bool Cmp(T1 a, T2 b) {
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return Op::Op(a, b);
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}
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// Overload for signed - unsigned comparison that can be promoted to a bigger
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// signed type.
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template <typename Op,
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typename T1,
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typename T2,
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typename std::enable_if<std::is_signed<T1>::value &&
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std::is_unsigned<T2>::value &&
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LargerInt<T2, T1>::value>::type* = nullptr>
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constexpr bool Cmp(T1 a, T2 b) {
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return Op::Op(a, static_cast<typename LargerInt<T2, T1>::type>(b));
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}
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// Overload for unsigned - signed comparison that can be promoted to a bigger
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// signed type.
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template <typename Op,
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typename T1,
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typename T2,
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typename std::enable_if<std::is_unsigned<T1>::value &&
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std::is_signed<T2>::value &&
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LargerInt<T1, T2>::value>::type* = nullptr>
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constexpr bool Cmp(T1 a, T2 b) {
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return Op::Op(static_cast<typename LargerInt<T1, T2>::type>(a), b);
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}
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// Overload for signed - unsigned comparison that can't be promoted to a bigger
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// signed type.
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template <typename Op,
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typename T1,
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typename T2,
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typename std::enable_if<std::is_signed<T1>::value &&
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std::is_unsigned<T2>::value &&
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!LargerInt<T2, T1>::value>::type* = nullptr>
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constexpr bool Cmp(T1 a, T2 b) {
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return a < 0 ? Op::Op(-1, 0) : Op::Op(safe_cmp_impl::MakeUnsigned(a), b);
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}
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// Overload for unsigned - signed comparison that can't be promoted to a bigger
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// signed type.
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template <typename Op,
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typename T1,
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typename T2,
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typename std::enable_if<std::is_unsigned<T1>::value &&
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std::is_signed<T2>::value &&
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!LargerInt<T1, T2>::value>::type* = nullptr>
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constexpr bool Cmp(T1 a, T2 b) {
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return b < 0 ? Op::Op(0, -1) : Op::Op(a, safe_cmp_impl::MakeUnsigned(b));
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}
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#define RTC_SAFECMP_MAKE_OP(name, op) \
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struct name { \
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template <typename T1, typename T2> \
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static constexpr bool Op(T1 a, T2 b) { \
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return a op b; \
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} \
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};
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RTC_SAFECMP_MAKE_OP(EqOp, ==)
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RTC_SAFECMP_MAKE_OP(NeOp, !=)
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RTC_SAFECMP_MAKE_OP(LtOp, <)
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RTC_SAFECMP_MAKE_OP(LeOp, <=)
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RTC_SAFECMP_MAKE_OP(GtOp, >)
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RTC_SAFECMP_MAKE_OP(GeOp, >=)
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#undef RTC_SAFECMP_MAKE_OP
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} // namespace safe_cmp_impl
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#define RTC_SAFECMP_MAKE_FUN(name) \
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template <typename T1, typename T2> \
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constexpr \
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typename std::enable_if<IsIntlike<T1>::value && IsIntlike<T2>::value, \
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bool>::type Safe##name(T1 a, T2 b) { \
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/* Unary plus here turns enums into real integral types. */ \
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return safe_cmp_impl::Cmp<safe_cmp_impl::name##Op>(+a, +b); \
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} \
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template <typename T1, typename T2> \
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constexpr \
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typename std::enable_if<!IsIntlike<T1>::value || !IsIntlike<T2>::value, \
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bool>::type Safe##name(const T1& a, \
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const T2& b) { \
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return safe_cmp_impl::name##Op::Op(a, b); \
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}
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RTC_SAFECMP_MAKE_FUN(Eq)
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RTC_SAFECMP_MAKE_FUN(Ne)
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RTC_SAFECMP_MAKE_FUN(Lt)
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RTC_SAFECMP_MAKE_FUN(Le)
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RTC_SAFECMP_MAKE_FUN(Gt)
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RTC_SAFECMP_MAKE_FUN(Ge)
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#undef RTC_SAFECMP_MAKE_FUN
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} // namespace rtc
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#endif // RTC_BASE_NUMERICS_SAFE_COMPARE_H_
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