compare.hpp

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// Copyright Take Vos 2022.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#pragma once
 
#include "concepts.hpp"
#include "type_traits.hpp"
#include <concepts>
#include <compare>
 
namespace hi { inline namespace v1 {
 
template<typename Lhs, typename Rhs>
struct three_way_comparison;
 
template<std::unsigned_integral Lhs, std::unsigned_integral Rhs>
struct three_way_comparison<Lhs, Rhs> {
    [[nodiscard]] constexpr std::strong_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
    {
        return lhs <=> rhs;
    }
};
 
template<std::signed_integral Lhs, std::signed_integral Rhs>
struct three_way_comparison<Lhs, Rhs> {
    [[nodiscard]] constexpr std::strong_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
    {
        return lhs <=> rhs;
    }
};
 
template<std::floating_point Lhs, std::floating_point Rhs>
struct three_way_comparison<Lhs, Rhs> {
    [[nodiscard]] constexpr std::partial_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
    {
        return lhs <=> rhs;
    }
};
 
template<std::signed_integral Lhs, std::unsigned_integral Rhs>
struct three_way_comparison<Lhs, Rhs> {
    [[nodiscard]] constexpr std::strong_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
        requires(sizeof(Rhs) < sizeof(long long))
    {
        return lhs <=> static_cast<long long>(rhs);
    }
 
    [[nodiscard]] constexpr std::strong_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
        requires(sizeof(Rhs) == sizeof(long long))
    {
        if (lhs < 0) {
            return std::strong_ordering::less;
        } else {
            return static_cast<unsigned long long>(lhs) <=> rhs;
        }
    }
};
 
template<std::unsigned_integral Lhs, std::signed_integral Rhs>
struct three_way_comparison<Lhs, Rhs> {
    [[nodiscard]] constexpr std::strong_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
        requires(sizeof(Lhs) < sizeof(long long))
    {
        return static_cast<long long>(lhs) <=> rhs;
    }
 
    [[nodiscard]] constexpr std::strong_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
        requires(sizeof(Lhs) == sizeof(long long))
    {
        if (rhs < 0) {
            return std::strong_ordering::greater;
        } else {
            return lhs <=> static_cast<unsigned long long>(rhs);
        }
    }
};
 
template<std::floating_point Lhs, std::integral Rhs>
struct three_way_comparison<Lhs, Rhs> {
    [[nodiscard]] constexpr std::partial_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
    {
        if constexpr (sizeof(Rhs) < sizeof(float)) {
            // All 16-bit integers can be exactly represented as float.
            return lhs <=> static_cast<float>(rhs);
        } else if constexpr (sizeof(Rhs) < sizeof(double)) {
            // All 32-bit integers can be exactly represented as double.
            return lhs <=> static_cast<double>(rhs);
        } else {
            return lhs <=> static_cast<long double>(rhs);
        }
    }
};
 
template<std::integral Lhs, std::floating_point Rhs>
struct three_way_comparison<Lhs, Rhs> {
    [[nodiscard]] constexpr std::partial_ordering operator()(Lhs const& lhs, Rhs const& rhs) const noexcept
    {
        if constexpr (sizeof(Lhs) < sizeof(float)) {
            // All 16-bit integers can be exactly represented as float.
            return static_cast<float>(lhs) <=> rhs;
        } else if constexpr (sizeof(Lhs) < sizeof(double)) {
            // All 32-bit integers can be exactly represented as double.
            return static_cast<double>(lhs) <=> rhs;
        } else {
            return static_cast<long double>(lhs) <=> rhs;
        }
    }
};
 
template<arithmetic Lhs, arithmetic Rhs>
[[nodiscard]] constexpr auto three_way_compare(Lhs const& lhs, Rhs const& rhs) noexcept
{
    return three_way_comparison<Lhs, Rhs>{}(lhs, rhs);
}
 
}} // namespace hi::v1