text: // Copyright 2014 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 PDFIUM_THIRD_PARTY_BASE_NUMERICS_SAFE_CONVERSIONS_H_
  #define PDFIUM_THIRD_PARTY_BASE_NUMERICS_SAFE_CONVERSIONS_H_
  
  #include <stddef.h>
  
  #include <limits>
  #include <ostream>
  #include <type_traits>
  
  #include "safe_conversions_impl.h"
  
  namespace pdfium {
  namespace base {
  
  // The following are helper constexpr template functions and classes for safely
  // performing a range of conversions, assignments, and tests:
  //
  //  checked_cast<> - Analogous to static_cast<> for numeric types, except
  //      that it CHECKs that the specified numeric conversion will not overflow
  //      or underflow. NaN source will always trigger a CHECK.
  //      The default CHECK triggers a crash, but the handler can be overriden.
  //  saturated_cast<> - Analogous to static_cast<> for numeric types, except
  //      that it returns a saturated result when the specified numeric conversion
  //      would otherwise overflow or underflow. An NaN source returns 0 by
  //      default, but can be overridden to return a different result.
  //  strict_cast<> - Analogous to static_cast<> for numeric types, except that
  //      it will cause a compile failure if the destination type is not large
  //      enough to contain any value in the source type. It performs no runtime
  //      checking and thus introduces no runtime overhead.
  //  IsValueInRangeForNumericType<>() - A convenience function that returns true
  //      if the type supplied to the template parameter can represent the value
  //      passed as an argument to the function.
  //  IsValueNegative<>() - A convenience function that will accept any arithmetic
  //      type as an argument and will return whether the value is less than zero.
  //      Unsigned types always return false.
  //  SafeUnsignedAbs() - Returns the absolute value of the supplied integer
  //      parameter as an unsigned result (thus avoiding an overflow if the value
  //      is the signed, two's complement minimum).
  //  StrictNumeric<> - A wrapper type that performs assignments and copies via
  //      the strict_cast<> template, and can perform valid arithmetic comparisons
  //      across any range of arithmetic types. StrictNumeric is the return type
  //      for values extracted from a CheckedNumeric class instance. The raw
  //      arithmetic value is extracted via static_cast to the underlying type.
  //  MakeStrictNum() - Creates a new StrictNumeric from the underlying type of
  //      the supplied arithmetic or StrictNumeric type.
  
  // Convenience function that returns true if the supplied value is in range
  // for the destination type.
  template <typename Dst, typename Src>
  constexpr bool IsValueInRangeForNumericType(Src value) {
    return internal::DstRangeRelationToSrcRange<Dst>(value).IsValid();
  }
  
  // Forces a crash, like a CHECK(false). Used for numeric boundary errors.
  struct CheckOnFailure {
    template <typename T>
    static T HandleFailure() {
  #if defined(__GNUC__) || defined(__clang__)
      __builtin_trap();
  #else
      ((void)(*(volatile char*)0 = 0));
  #endif
      return T();
    }
  };
  
  // checked_cast<> is analogous to static_cast<> for numeric types,
  // except that it CHECKs that the specified numeric conversion will not
  // overflow or underflow. NaN source will always trigger a CHECK.
  template <typename Dst, class CheckHandler = CheckOnFailure, typename Src>
  constexpr Dst checked_cast(Src value) {
    // This throws a compile-time error on evaluating the constexpr if it can be
    // determined at compile-time as failing, otherwise it will CHECK at runtime.
    using SrcType = typename internal::UnderlyingType<Src>::type;
    return IsValueInRangeForNumericType<Dst, SrcType>(value)
               ? static_cast<Dst>(static_cast<SrcType>(value))
               : CheckHandler::template HandleFailure<Dst>();
  }
  
  // Default boundaries for integral/float: max/infinity, lowest/-infinity, 0/NaN.
  template <typename T>
  struct SaturationDefaultHandler {
    static constexpr T NaN() {
      return std::numeric_limits<T>::has_quiet_NaN
                 ? std::numeric_limits<T>::quiet_NaN()
                 : T();
    }
    static constexpr T max() { return std::numeric_limits<T>::max(); }
    static constexpr T Overflow() {
      return std::numeric_limits<T>::has_infinity
                 ? std::numeric_limits<T>::infinity()
                 : std::numeric_limits<T>::max();
    }
    static constexpr T lowest() { return std::numeric_limits<T>::lowest(); }
    static constexpr T Underflow() {
      return std::numeric_limits<T>::has_infinity
                 ? std::numeric_limits<T>::infinity() * -1
                 : std::numeric_limits<T>::lowest();
    }
  };
  
  namespace internal {
  
  template <typename Dst, template <typename> class S, typename Src>
  constexpr Dst saturated_cast_impl(Src value, RangeCheck constraint) {
    // For some reason clang generates much better code when the branch is
    // structured exactly this way, rather than a sequence of checks.
    return !constraint.IsOverflowFlagSet()
               ? (!constraint.IsUnderflowFlagSet() ? static_cast<Dst>(value)
                                                   : S<Dst>::Underflow())
               // Skip this check for integral Src, which cannot be NaN.
               : (std::is_integral<Src>::value || !constraint.IsUnderflowFlagSet()
                      ? S<Dst>::Overflow()
                      : S<Dst>::NaN());
  }
  
  // saturated_cast<> is analogous to static_cast<> for numeric types, except
  // that the specified numeric conversion will saturate by default rather than
  // overflow or underflow, and NaN assignment to an integral will return 0.
  // All boundary condition behaviors can be overriden with a custom handler.
  template <typename Dst,
            template <typename>
            class SaturationHandler = SaturationDefaultHandler,
            typename Src>
  constexpr Dst saturated_cast(Src value) {
    using SrcType = typename UnderlyingType<Src>::type;
    return saturated_cast_impl<Dst, SaturationHandler, SrcType>(
        value,
        DstRangeRelationToSrcRange<Dst, SaturationHandler, SrcType>(value));
  }
  
  // strict_cast<> is analogous to static_cast<> for numeric types, except that
  // it will cause a compile failure if the destination type is not large enough
  // to contain any value in the source type. It performs no runtime checking.
  template <typename Dst, typename Src>
  constexpr Dst strict_cast(Src value) {
    using SrcType = typename UnderlyingType<Src>::type;
    static_assert(UnderlyingType<Src>::is_numeric, "Argument must be numeric.");
    static_assert(std::is_arithmetic<Dst>::value, "Result must be numeric.");
  
    // If you got here from a compiler error, it's because you tried to assign
    // from a source type to a destination type that has insufficient range.
    // The solution may be to change the destination type you're assigning to,
    // and use one large enough to represent the source.
    // Alternatively, you may be better served with the checked_cast<> or
    // saturated_cast<> template functions for your particular use case.
    static_assert(StaticDstRangeRelationToSrcRange<Dst, SrcType>::value ==
                      NUMERIC_RANGE_CONTAINED,
                  "The source type is out of range for the destination type. "
                  "Please see strict_cast<> comments for more information.");
  
    return static_cast<Dst>(static_cast<SrcType>(value));
  }
  
  // Some wrappers to statically check that a type is in range.
  template <typename Dst, typename Src, class Enable = void>
  struct IsNumericRangeContained {
    static const bool value = false;
  };
  
  template <typename Dst, typename Src>
  struct IsNumericRangeContained<
      Dst,
      Src,
      typename std::enable_if<ArithmeticOrUnderlyingEnum<Dst>::value &&
                              ArithmeticOrUnderlyingEnum<Src>::value>::type> {
    static const bool value = StaticDstRangeRelationToSrcRange<Dst, Src>::value ==
                              NUMERIC_RANGE_CONTAINED;
  };
  
  // StrictNumeric implements compile time range checking between numeric types by
  // wrapping assignment operations in a strict_cast. This class is intended to be
  // used for function arguments and return types, to ensure the destination type
  // can always contain the source type. This is essentially the same as enforcing
  // -Wconversion in gcc and C4302 warnings on MSVC, but it can be applied
  // incrementally at API boundaries, making it easier to convert code so that it
  // compiles cleanly with truncation warnings enabled.
  // This template should introduce no runtime overhead, but it also provides no
  // runtime checking of any of the associated mathematical operations. Use
  // CheckedNumeric for runtime range checks of the actual value being assigned.
  template <typename T>
  class StrictNumeric {
   public:
    using type = T;
  
    constexpr StrictNumeric() : value_(0) {}
  
    // Copy constructor.
    template <typename Src>
    constexpr StrictNumeric(const StrictNumeric<Src>& rhs)
        : value_(strict_cast<T>(rhs.value_)) {}
  
    // This is not an explicit constructor because we implicitly upgrade regular
    // numerics to StrictNumerics to make them easier to use.
    template <typename Src>
    constexpr StrictNumeric(Src value)  // NOLINT(runtime/explicit)
        : value_(strict_cast<T>(value)) {}
  
    // If you got here from a compiler error, it's because you tried to assign
    // from a source type to a destination type that has insufficient range.
    // The solution may be to change the destination type you're assigning to,
    // and use one large enough to represent the source.
    // If you're assigning from a CheckedNumeric<> class, you may be able to use
    // the AssignIfValid() member function, specify a narrower destination type to
    // the member value functions (e.g. val.template ValueOrDie<Dst>()), use one
    // of the value helper functions (e.g. ValueOrDieForType<Dst>(val)).
    // If you've encountered an _ambiguous overload_ you can use a static_cast<>
    // to explicitly cast the result to the destination type.
    // If none of that works, you may be better served with the checked_cast<> or
    // saturated_cast<> template functions for your particular use case.
    template <typename Dst,
              typename std::enable_if<
                  IsNumericRangeContained<Dst, T>::value>::type* = nullptr>
    constexpr operator Dst() const {
      return static_cast<typename ArithmeticOrUnderlyingEnum<Dst>::type>(value_);
    }
  
   private:
    const T value_;
  };
  
  // Convience wrapper returns a StrictNumeric from the provided arithmetic type.
  template <typename T>
  constexpr StrictNumeric<typename UnderlyingType<T>::type> MakeStrictNum(
      const T value) {
    return value;
  }
  
  // Overload the ostream output operator to make logging work nicely.
  template <typename T>
  std::ostream& operator<<(std::ostream& os, const StrictNumeric<T>& value) {
    os << static_cast<T>(value);
    return os;
  }
  
  #define STRICT_COMPARISON_OP(NAME, OP)                               \
    template <typename L, typename R,                                  \
              typename std::enable_if<                                 \
                  internal::IsStrictOp<L, R>::value>::type* = nullptr> \
    constexpr bool operator OP(const L lhs, const R rhs) {             \
      return SafeCompare<NAME, typename UnderlyingType<L>::type,       \
                         typename UnderlyingType<R>::type>(lhs, rhs);  \
    }
  
  STRICT_COMPARISON_OP(IsLess, <);
  STRICT_COMPARISON_OP(IsLessOrEqual, <=);
  STRICT_COMPARISON_OP(IsGreater, >);
  STRICT_COMPARISON_OP(IsGreaterOrEqual, >=);
  STRICT_COMPARISON_OP(IsEqual, ==);
  STRICT_COMPARISON_OP(IsNotEqual, !=);
  
  #undef STRICT_COMPARISON_OP
  };
  
  using internal::strict_cast;
  using internal::saturated_cast;
  using internal::SafeUnsignedAbs;
  using internal::StrictNumeric;
  using internal::MakeStrictNum;
  using internal::IsValueNegative;
  
  // Explicitly make a shorter size_t alias for convenience.
  using SizeT = StrictNumeric<size_t>;
  
  }  // namespace base
  }  // namespace pdfium
  
  #endif  // PDFIUM_THIRD_PARTY_BASE_NUMERICS_SAFE_CONVERSIONS_H_
  