constexpr_math.h

 
#pragma once
 
#include <concepts>
#include <limits>
#include <cmath>
#include <bit>
 
namespace ghassanpl::constexpr_math {}
namespace ghassanpl { namespace cem = ghassanpl::constexpr_math; }
 
namespace ghassanpl::constexpr_math
{
    template <typename T>
    concept arithmetic = std::is_arithmetic_v<T>;
 
#ifdef __cpp_lib_constexpr_cmath
    using std::pow;
    using std::floor;
    using std::sign;
    using std::signbit;
    using std::ceil;
    using std::trunc;
    using std::abs;
    using std::fma;
    using std::fmod;
    using std::isnan;
#else

 
    [[nodiscard]] constexpr bool isnan(std::integral auto f) noexcept { return false; }
    [[nodiscard]] constexpr bool isnan(float f) {
        if (std::is_constant_evaluated())
            return (std::bit_cast<uint32_t>(f) & 0x7FFFFFFFu) > 0x7F800000u;
        else
            return std::isnan(f);
    }
    [[nodiscard]] constexpr bool isnan(double f) {
        if (std::is_constant_evaluated())
            return (std::bit_cast<uint64_t>(f) & 0x7FFFFFFFFFFFFFFFull) > 0x7FF0000000000000ull;
        else
            return std::isnan(f);
    }
    [[nodiscard]] constexpr bool isfinite(std::integral auto f) noexcept { return true; }
    [[nodiscard]] constexpr bool isfinite(float f) {
        if (std::is_constant_evaluated())
            return (std::bit_cast<uint32_t>(f) & 0x7F800000u) != 0x7F800000u;
        else
            return std::isfinite(f);
    }
    [[nodiscard]] constexpr bool isfinite(double f) {
        if (std::is_constant_evaluated())
            return (std::bit_cast<uint64_t>(f) & 0x7FF0000000000000ull) != 0x7FF0000000000000ull;
        else
            return std::isfinite(f);
    }

 
    template <std::floating_point T, typename RESULT = T>
    [[nodiscard]] constexpr RESULT floor(T num) noexcept
    {
        if (std::is_constant_evaluated())
        {
            if (num == -std::numeric_limits<T>::infinity())
                return static_cast<RESULT>(-std::numeric_limits<T>::infinity());
            else if (num == std::numeric_limits<T>::infinity())
                return static_cast<RESULT>(std::numeric_limits<T>::infinity());
            else if (num != num)
                return static_cast<RESULT>(std::numeric_limits<T>::quiet_NaN());
 
            const auto res = static_cast<int64_t>(num);
            return static_cast<RESULT>((res > num) ? res - 1 : res);
        }
        else
            return std::floor(num);
    }
 
    template <typename T>
    requires std::is_signed_v<T>
    [[nodiscard]] constexpr bool signbit(T num)
    {
        if (std::is_constant_evaluated())
        {
            if constexpr (std::floating_point<T> && std::numeric_limits<T>::is_iec559)
            {
                if constexpr (sizeof(T) == sizeof(uint64_t))
                    return (std::bit_cast<uint64_t>(num) & 0x8000000000000000ull) != 0;
                else if constexpr (sizeof(T) == sizeof(uint32_t))
                    return (std::bit_cast<uint32_t>(num) & 0x80000000u) != 0;
                else if constexpr (sizeof(T) == sizeof(uint16_t))
                    return (std::bit_cast<uint16_t>(num) & 0x8000u) != 0;
                else if constexpr (sizeof(T) == sizeof(uint8_t))
                    return (std::bit_cast<uint8_t>(num) & 0x80u) != 0;
                else
                    static_assert(sizeof(T) == sizeof(uint8_t), "unsupported floating point type");
            }
            else
            {
                return num < T{};
            }
        }
        else
            return std::signbit(num);
    }
 
    template <typename T>
    requires (!std::is_signed_v<T>) 
    [[nodiscard]] constexpr bool signbit(T val) noexcept
    {
        return val < T{};
    }
 
    template <typename T>
    [[nodiscard]] constexpr int sign(T val)
    {
        return (T{} < val) - (val < T{});
        /*
        if (::ghassanpl::cem::signbit(val))
            return -1;
        else if (val == T{})
            return 0;
        return 1;
        */
    }
 
    template <std::floating_point T, typename RESULT = T>
    [[nodiscard]] constexpr RESULT ceil(T num) noexcept
    {
        if (std::is_constant_evaluated())
        {
            if (num == -std::numeric_limits<T>::infinity())
                return static_cast<RESULT>(-std::numeric_limits<T>::infinity());
            else if (num == std::numeric_limits<T>::infinity())
                return static_cast<RESULT>(std::numeric_limits<T>::infinity());
            else if (::ghassanpl::cem::isnan(num))
                return static_cast<RESULT>(std::numeric_limits<T>::quiet_NaN());
 
            const auto res = static_cast<int64_t>(num);
            return static_cast<RESULT>((res >= num) ? res : res + 1);
        }
        else
            return std::ceil(num);
    }
 
    template <std::floating_point T, typename RESULT = T>
    [[nodiscard]] constexpr RESULT trunc(T num) noexcept
    {
        if (std::is_constant_evaluated())
            return num < T{} ? -::ghassanpl::cem::floor<T, RESULT>(-num) : ::ghassanpl::cem::floor<T, RESULT>(num);
        else
            return std::trunc(num);
    }
 
    // TODO: round
 
    template <arithmetic T>
    [[nodiscard]] constexpr auto abs(T num)
    {
        if (std::is_constant_evaluated())
            return ::ghassanpl::cem::signbit(num) ? -num : num;
        else if constexpr (std::is_signed_v<T>)
            return std::abs(num);
        else
            return num;
    }
 
    template <arithmetic T, arithmetic U, arithmetic V>
    [[nodiscard]] constexpr auto fma(T a, U b, V c) noexcept
    {
        if (std::is_constant_evaluated())
            return a * b + c;
        else
        {
            using CT = std::conditional_t<
                std::floating_point<T> || std::floating_point<U> || std::floating_point<V>,
                decltype(a* b + c),
                double
            >;
            return std::fma((CT)a, (CT)b, (CT)c);
        }
    }
 
    template <arithmetic T, arithmetic U>
    [[nodiscard]] constexpr auto fmod(T a, U b) noexcept
    {
        using CT = std::conditional_t<
            std::floating_point<T> || std::floating_point<U>,
            decltype(fma(trunc(a / b), -b, a)),
            double
        >;
        if (std::is_constant_evaluated())
        {
            if (b == 0)
                return std::numeric_limits<CT>::quiet_NaN();
            else if (a == 0)
                return CT{};
            else if (a == std::numeric_limits<T>::infinity() || a == -std::numeric_limits<T>::infinity())
                return std::numeric_limits<CT>::quiet_NaN();
            else if (b == std::numeric_limits<U>::infinity() || b == -std::numeric_limits<U>::infinity())
                return CT{};
            else if (::ghassanpl::cem::isnan(a) || ::ghassanpl::cem::isnan(b))
                return std::numeric_limits<CT>::quiet_NaN();
 
            return ::ghassanpl::cem::fma(::ghassanpl::cem::trunc((CT)a / (CT)b), -(CT)b, (CT)a);
        }
        else
            return std::fmod((CT)a, (CT)b);
    }
#endif
 
    #include "constexpr_math.inl"
 
    template <arithmetic T>
    [[nodiscard]] constexpr auto sqrt(T value) noexcept
    {
        using CT = std::conditional_t<std::floating_point<T>, T, double>;
        if (std::is_constant_evaluated())
        {
            static_assert(std::numeric_limits<double>::is_iec559, "sqrt only works for IEEE 754 floating point types");
            return (CT)::ghassanpl::cem::detail::sqrt_impl((double)value);
        }
        else
            return (CT)std::sqrt(value);
    }
 
    template <arithmetic T, arithmetic U>
    [[nodiscard]] constexpr auto pow(T base, U exponent) noexcept
    {
        using CT = decltype(base * exponent);
        if (std::is_constant_evaluated())
        {
            if constexpr (std::integral<T> && std::integral<U>)
            {
                CT result = static_cast<CT>(1);
                while (exponent-- > 0)
                    result = result * base;
                return result;
            }
            else
            {
                static_assert(std::numeric_limits<double>::is_iec559, "pow only works for IEEE 754 floating point types");
                return (CT)::ghassanpl::cem::detail::pow_impl((double)base, (double)exponent);
            }
        }
        else
            return (CT)std::pow(base, exponent);
    }
 
    template <std::integral T>
    [[nodiscard]] constexpr unsigned ilog2(T val) noexcept
    {
        unsigned result = 0;
        if (val >= (1ULL << 32)) { result += 32; val >>= 32ULL; }
        if (val >= (1ULL << 16)) { result += 16; val >>= 16ULL; }
        if (val >= (1ULL << 8)) { result += 8; val >>= 8ULL; }
        if (val >= (1ULL << 4)) { result += 4; val >>= 4ULL; }
        if (val >= (1ULL << 2)) { result += 2; val >>= 2ULL; }
        if (val >= (1ULL << 1)) result += 1;
        return result;
    }
    
    [[nodiscard]] constexpr inline double unlerp(const double a, const double b, const double t) noexcept
    {
        return (t - a) / (b - a);
    }

}