text: // Copyright 2017 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  #ifndef BASE_NUMERICS_CHECKED_MATH_H_
  #define BASE_NUMERICS_CHECKED_MATH_H_
  
  #include <stddef.h>
  
  #include <limits>
  #include <type_traits>
  
  #include "base/numerics/checked_math_impl.h"
  
  namespace base {
  namespace internal {
  
  template <typename T>
  class CheckedNumeric {
    static_assert(std::is_arithmetic<T>::value,
                  "CheckedNumeric<T>: T must be a numeric type.");
  
   public:
    using type = T;
  
    constexpr CheckedNumeric() {}
  
    // Copy constructor.
    template <typename Src>
    constexpr CheckedNumeric(const CheckedNumeric<Src>& rhs)
        : state_(rhs.state_.value(), rhs.IsValid()) {}
  
    template <typename Src>
    friend class CheckedNumeric;
  
    // This is not an explicit constructor because we implicitly upgrade regular
    // numerics to CheckedNumerics to make them easier to use.
    template <typename Src>
    constexpr CheckedNumeric(Src value)  // NOLINT(runtime/explicit)
        : state_(value) {
      static_assert(std::is_arithmetic<Src>::value, "Argument must be numeric.");
    }
  
    // This is not an explicit constructor because we want a seamless conversion
    // from StrictNumeric types.
    template <typename Src>
    constexpr CheckedNumeric(
        StrictNumeric<Src> value)  // NOLINT(runtime/explicit)
        : state_(static_cast<Src>(value)) {}
  
    // IsValid() - The public API to test if a CheckedNumeric is currently valid.
    // A range checked destination type can be supplied using the Dst template
    // parameter.
    template <typename Dst = T>
    constexpr bool IsValid() const {
      return state_.is_valid() &&
             IsValueInRangeForNumericType<Dst>(state_.value());
    }
  
    // AssignIfValid(Dst) - Assigns the underlying value if it is currently valid
    // and is within the range supported by the destination type. Returns true if
    // successful and false otherwise.
    template <typename Dst>
    constexpr bool AssignIfValid(Dst* result) const {
      return IsValid<Dst>() ? ((*result = static_cast<Dst>(state_.value())), true)
                            : false;
    }
  
    // ValueOrDie() - The primary accessor for the underlying value. If the
    // current state is not valid it will CHECK and crash.
    // A range checked destination type can be supplied using the Dst template
    // parameter, which will trigger a CHECK if the value is not in bounds for
    // the destination.
    // The CHECK behavior can be overridden by supplying a handler as a
    // template parameter, for test code, etc. However, the handler cannot access
    // the underlying value, and it is not available through other means.
    template <typename Dst = T, class CheckHandler = CheckOnFailure>
    constexpr StrictNumeric<Dst> ValueOrDie() const {
      return IsValid<Dst>() ? static_cast<Dst>(state_.value())
                            : CheckHandler::template HandleFailure<Dst>();
    }
  
    // ValueOrDefault(T default_value) - A convenience method that returns the
    // current value if the state is valid, and the supplied default_value for
    // any other state.
    // A range checked destination type can be supplied using the Dst template
    // parameter. WARNING: This function may fail to compile or CHECK at runtime
    // if the supplied default_value is not within range of the destination type.
    template <typename Dst = T, typename Src>
    constexpr StrictNumeric<Dst> ValueOrDefault(const Src default_value) const {
      return IsValid<Dst>() ? static_cast<Dst>(state_.value())
                            : checked_cast<Dst>(default_value);
    }
  
    // Returns a checked numeric of the specified type, cast from the current
    // CheckedNumeric. If the current state is invalid or the destination cannot
    // represent the result then the returned CheckedNumeric will be invalid.
    template <typename Dst>
    constexpr CheckedNumeric<typename UnderlyingType<Dst>::type> Cast() const {
      return *this;
    }
  
    // This friend method is available solely for providing more detailed logging
    // in the the tests. Do not implement it in production code, because the
    // underlying values may change at any time.
    template <typename U>
    friend U GetNumericValueForTest(const CheckedNumeric<U>& src);
  
    // Prototypes for the supported arithmetic operator overloads.
    template <typename Src>
    CheckedNumeric& operator+=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator-=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator*=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator/=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator%=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator<<=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator>>=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator&=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator|=(const Src rhs);
    template <typename Src>
    CheckedNumeric& operator^=(const Src rhs);
  
    constexpr CheckedNumeric operator-() const {
      // The negation of two's complement int min is int min, so we simply
      // check for that in the constexpr case.
      // We use an optimized code path for a known run-time variable.
      return MustTreatAsConstexpr(state_.value()) || !std::is_signed<T>::value ||
                     std::is_floating_point<T>::value
                 ? CheckedNumeric<T>(
                       NegateWrapper(state_.value()),
                       IsValid() && (!std::is_signed<T>::value ||
                                     std::is_floating_point<T>::value ||
                                     NegateWrapper(state_.value()) !=
                                         std::numeric_limits<T>::lowest()))
                 : FastRuntimeNegate();
    }
  
    constexpr CheckedNumeric operator~() const {
      return CheckedNumeric<decltype(InvertWrapper(T()))>(
          InvertWrapper(state_.value()), IsValid());
    }
  
    constexpr CheckedNumeric Abs() const {
      return !IsValueNegative(state_.value()) ? *this : -*this;
    }
  
    template <typename U>
    constexpr CheckedNumeric<typename MathWrapper<CheckedMaxOp, T, U>::type> Max(
        const U rhs) const {
      using R = typename UnderlyingType<U>::type;
      using result_type = typename MathWrapper<CheckedMaxOp, T, U>::type;
      // TODO(jschuh): This can be converted to the MathOp version and remain
      // constexpr once we have C++14 support.
      return CheckedNumeric<result_type>(
          static_cast<result_type>(
              IsGreater<T, R>::Test(state_.value(), Wrapper<U>::value(rhs))
                  ? state_.value()
                  : Wrapper<U>::value(rhs)),
          state_.is_valid() && Wrapper<U>::is_valid(rhs));
    }
  
    template <typename U>
    constexpr CheckedNumeric<typename MathWrapper<CheckedMinOp, T, U>::type> Min(
        const U rhs) const {
      using R = typename UnderlyingType<U>::type;
      using result_type = typename MathWrapper<CheckedMinOp, T, U>::type;
      // TODO(jschuh): This can be converted to the MathOp version and remain
      // constexpr once we have C++14 support.
      return CheckedNumeric<result_type>(
          static_cast<result_type>(
              IsLess<T, R>::Test(state_.value(), Wrapper<U>::value(rhs))
                  ? state_.value()
                  : Wrapper<U>::value(rhs)),
          state_.is_valid() && Wrapper<U>::is_valid(rhs));
    }
  
    // This function is available only for integral types. It returns an unsigned
    // integer of the same width as the source type, containing the absolute value
    // of the source, and properly handling signed min.
    constexpr CheckedNumeric<typename UnsignedOrFloatForSize<T>::type>
    UnsignedAbs() const {
      return CheckedNumeric<typename UnsignedOrFloatForSize<T>::type>(
          SafeUnsignedAbs(state_.value()), state_.is_valid());
    }
  
    CheckedNumeric& operator++() {
      *this += 1;
      return *this;
    }
  
    CheckedNumeric operator++(int) {
      CheckedNumeric value = *this;
      *this += 1;
      return value;
    }
  
    CheckedNumeric& operator--() {
      *this -= 1;
      return *this;
    }
  
    CheckedNumeric operator--(int) {
      CheckedNumeric value = *this;
      *this -= 1;
      return value;
    }
  
    // These perform the actual math operations on the CheckedNumerics.
    // Binary arithmetic operations.
    template <template <typename, typename, typename> class M,
              typename L,
              typename R>
    static CheckedNumeric MathOp(const L lhs, const R rhs) {
      using Math = typename MathWrapper<M, L, R>::math;
      T result = 0;
      bool is_valid =
          Wrapper<L>::is_valid(lhs) && Wrapper<R>::is_valid(rhs) &&
          Math::Do(Wrapper<L>::value(lhs), Wrapper<R>::value(rhs), &result);
      return CheckedNumeric<T>(result, is_valid);
    }
  
    // Assignment arithmetic operations.
    template <template <typename, typename, typename> class M, typename R>
    CheckedNumeric& MathOp(const R rhs) {
      using Math = typename MathWrapper<M, T, R>::math;
      T result = 0;  // Using T as the destination saves a range check.
      bool is_valid = state_.is_valid() && Wrapper<R>::is_valid(rhs) &&
                      Math::Do(state_.value(), Wrapper<R>::value(rhs), &result);
      *this = CheckedNumeric<T>(result, is_valid);
      return *this;
    }
  
   private:
    CheckedNumericState<T> state_;
  
    CheckedNumeric FastRuntimeNegate() const {
      T result;
      bool success = CheckedSubOp<T, T>::Do(T(0), state_.value(), &result);
      return CheckedNumeric<T>(result, IsValid() && success);
    }
  
    template <typename Src>
    constexpr CheckedNumeric(Src value, bool is_valid)
        : state_(value, is_valid) {}
  
    // These wrappers allow us to handle state the same way for both
    // CheckedNumeric and POD arithmetic types.
    template <typename Src>
    struct Wrapper {
      static constexpr bool is_valid(Src) { return true; }
      static constexpr Src value(Src value) { return value; }
    };
  
    template <typename Src>
    struct Wrapper<CheckedNumeric<Src>> {
      static constexpr bool is_valid(const CheckedNumeric<Src> v) {
        return v.IsValid();
      }
      static constexpr Src value(const CheckedNumeric<Src> v) {
        return v.state_.value();
      }
    };
  
    template <typename Src>
    struct Wrapper<StrictNumeric<Src>> {
      static constexpr bool is_valid(const StrictNumeric<Src>) { return true; }
      static constexpr Src value(const StrictNumeric<Src> v) {
        return static_cast<Src>(v);
      }
    };
  };
  
  // Convenience functions to avoid the ugly template disambiguator syntax.
  template <typename Dst, typename Src>
  constexpr bool IsValidForType(const CheckedNumeric<Src> value) {
    return value.template IsValid<Dst>();
  }
  
  template <typename Dst, typename Src>
  constexpr StrictNumeric<Dst> ValueOrDieForType(
      const CheckedNumeric<Src> value) {
    return value.template ValueOrDie<Dst>();
  }
  
  template <typename Dst, typename Src, typename Default>
  constexpr StrictNumeric<Dst> ValueOrDefaultForType(
      const CheckedNumeric<Src> value,
      const Default default_value) {
    return value.template ValueOrDefault<Dst>(default_value);
  }
  
  // Convience wrapper to return a new CheckedNumeric from the provided arithmetic
  // or CheckedNumericType.
  template <typename T>
  constexpr CheckedNumeric<typename UnderlyingType<T>::type> MakeCheckedNum(
      const T value) {
    return value;
  }
  
  // These implement the variadic wrapper for the math operations.
  template <template <typename, typename, typename> class M,
            typename L,
            typename R>
  CheckedNumeric<typename MathWrapper<M, L, R>::type> CheckMathOp(const L lhs,
                                                                  const R rhs) {
    using Math = typename MathWrapper<M, L, R>::math;
    return CheckedNumeric<typename Math::result_type>::template MathOp<M>(lhs,
                                                                          rhs);
  }
  
  // General purpose wrapper template for arithmetic operations.
  template <template <typename, typename, typename> class M,
            typename L,
            typename R,
            typename... Args>
  CheckedNumeric<typename ResultType<M, L, R, Args...>::type>
  CheckMathOp(const L lhs, const R rhs, const Args... args) {
    auto tmp = CheckMathOp<M>(lhs, rhs);
    return tmp.IsValid() ? CheckMathOp<M>(tmp, args...)
                         : decltype(CheckMathOp<M>(tmp, args...))(tmp);
  }
  
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Add, +, +=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Sub, -, -=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Mul, *, *=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Div, /, /=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Mod, %, %=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Lsh, <<, <<=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Rsh, >>, >>=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, And, &, &=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Or, |, |=)
  BASE_NUMERIC_ARITHMETIC_OPERATORS(Checked, Check, Xor, ^, ^=)
  BASE_NUMERIC_ARITHMETIC_VARIADIC(Checked, Check, Max)
  BASE_NUMERIC_ARITHMETIC_VARIADIC(Checked, Check, Min)
  
  // These are some extra StrictNumeric operators to support simple pointer
  // arithmetic with our result types. Since wrapping on a pointer is always
  // bad, we trigger the CHECK condition here.
  template <typename L, typename R>
  L* operator+(L* lhs, const StrictNumeric<R> rhs) {
    uintptr_t result = CheckAdd(reinterpret_cast<uintptr_t>(lhs),
                                CheckMul(sizeof(L), static_cast<R>(rhs)))
                           .template ValueOrDie<uintptr_t>();
    return reinterpret_cast<L*>(result);
  }
  
  template <typename L, typename R>
  L* operator-(L* lhs, const StrictNumeric<R> rhs) {
    uintptr_t result = CheckSub(reinterpret_cast<uintptr_t>(lhs),
                                CheckMul(sizeof(L), static_cast<R>(rhs)))
                           .template ValueOrDie<uintptr_t>();
    return reinterpret_cast<L*>(result);
  }
  
  }  // namespace internal
  
  using internal::CheckedNumeric;
  using internal::IsValidForType;
  using internal::ValueOrDieForType;
  using internal::ValueOrDefaultForType;
  using internal::MakeCheckedNum;
  using internal::CheckMax;
  using internal::CheckMin;
  using internal::CheckAdd;
  using internal::CheckSub;
  using internal::CheckMul;
  using internal::CheckDiv;
  using internal::CheckMod;
  using internal::CheckLsh;
  using internal::CheckRsh;
  using internal::CheckAnd;
  using internal::CheckOr;
  using internal::CheckXor;
  
  }  // namespace base
  
  #endif  // BASE_NUMERICS_CHECKED_MATH_H_
  