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#pragma once
/**
* @file Checked arithmetic with classes that make it hard to accidentally make something an unchecked operation.
*/
#include <compare>
#include <concepts> // IWYU pragma: keep
#include <exception>
#include <ostream>
#include <limits>
#include <optional>
#include <type_traits>
namespace nix::checked {
class DivideByZero : std::exception
{};
/**
* Numeric value enforcing checked arithmetic. Performing mathematical operations on such values will return a Result type which needs to be checked.
*/
template<std::integral T>
struct Checked
{
using Inner = T;
// TODO: this must be a "trivial default constructor", which means it
// cannot set the value to NOT DO UB on uninit.
T value;
Checked() = default;
explicit Checked(T const value) : value{value} {}
Checked(Checked<T> const & other) = default;
Checked(Checked<T> && other) = default;
Checked<T> & operator=(Checked<T> const & other) = default;
std::strong_ordering operator<=>(Checked<T> const & other) const = default;
std::strong_ordering operator<=>(T const & other) const
{
return value <=> other;
}
explicit operator T() const
{
return value;
}
enum class OverflowKind {
NoOverflow,
Overflow,
DivByZero,
};
class Result
{
T value;
OverflowKind overflowed_;
public:
Result(T value, bool overflowed) : value{value}, overflowed_{overflowed ? OverflowKind::Overflow : OverflowKind::NoOverflow} {}
Result(T value, OverflowKind overflowed) : value{value}, overflowed_{overflowed} {}
bool operator==(Result other) const
{
return value == other.value && overflowed_ == other.overflowed_;
}
std::optional<T> valueChecked() const
{
if (overflowed_ != OverflowKind::NoOverflow) {
return std::nullopt;
} else {
return value;
}
}
/**
* Returns the result as if the arithmetic were performed as wrapping arithmetic.
*
* \throws DivideByZero if the operation was a divide by zero.
*/
T valueWrapping() const
{
if (overflowed_ == OverflowKind::DivByZero) {
throw DivideByZero{};
}
return value;
}
bool overflowed() const
{
return overflowed_ == OverflowKind::Overflow;
}
bool divideByZero() const
{
return overflowed_ == OverflowKind::DivByZero;
}
};
Result operator+(Checked<T> const other) const
{
return (*this) + other.value;
}
Result operator+(T const other) const
{
T result;
bool overflowed = __builtin_add_overflow(value, other, &result);
return Result{result, overflowed};
}
Result operator-(Checked<T> const other) const
{
return (*this) - other.value;
}
Result operator-(T const other) const
{
T result;
bool overflowed = __builtin_sub_overflow(value, other, &result);
return Result{result, overflowed};
}
Result operator*(Checked<T> const other) const
{
return (*this) * other.value;
}
Result operator*(T const other) const
{
T result;
bool overflowed = __builtin_mul_overflow(value, other, &result);
return Result{result, overflowed};
}
Result operator/(Checked<T> const other) const
{
return (*this) / other.value;
}
/**
* Performs a checked division.
*
* If the right hand side is zero, the result is marked as a DivByZero and
* valueWrapping will throw.
*/
Result operator/(T const other) const
{
constexpr T const minV = std::numeric_limits<T>::min();
// It's only possible to overflow with signed division since doing so
// requires crossing the two's complement limits by MIN / -1 (since
// two's complement has one more in range in the negative direction
// than in the positive one).
if (std::is_signed<T>() && (value == minV && other == -1)) {
return Result{minV, true};
} else if (other == 0) {
return Result{0, OverflowKind::DivByZero};
} else {
T result = value / other;
return Result{result, false};
}
}
};
template<std::integral T>
std::ostream & operator<<(std::ostream & ios, Checked<T> v)
{
ios << v.value;
return ios;
}
}
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