/*

 * Copyright (c) 2022, Jelle Raaijmakers <jelle@gmta.nl>

 *

 * SPDX-License-Identifier: BSD-2-Clause

 */

#pragma once

#include <AK/BitCast.h>

#include <AK/StdLibExtras.h>

#include <AK/Types.h>

namespace AK {

template<typename T>

struct FloatExtractor;

#ifdef AK_HAS_FLOAT_128

template<>

struct FloatExtractor<f128> {

    static constexpr FloatExtractor<f128> from_float(f128 f) { return bit_cast<FloatExtractor<f128>>(f); }

    constexpr f128 to_float() const { return bit_cast<f128>(*this); }

    using ComponentType = unsigned __int128;

    static constexpr int mantissa_bits = 112;

    static constexpr ComponentType mantissa_max = (((ComponentType)1) << 112) - 1;

    static constexpr int exponent_bias = 16383;

    static constexpr int exponent_bits = 15;

    static constexpr unsigned exponent_max = 32767;

    ComponentType mantissa : 112;

    ComponentType exponent : 15;

    ComponentType sign : 1;

};

// Validate that f128 and the FloatExtractor struct are 128 bits.

static_assert(AssertSize<f128, 16>());

static_assert(AssertSize<FloatExtractor<f128>, sizeof(f128)>());

#endif

#ifdef AK_HAS_FLOAT_80

template<>

struct FloatExtractor<f80> {

    static constexpr FloatExtractor<f80> from_float(f80 f) { return bit_cast<FloatExtractor<f80>>(f); }

    constexpr f80 to_float() const { return bit_cast<f80>(*this); }

    using ComponentType = unsigned long long;

    static constexpr int mantissa_bits = 64;

    static constexpr ComponentType mantissa_max = ~0ull;

    static constexpr int exponent_bias = 16383;

    static constexpr int exponent_bits = 15;

    static constexpr unsigned exponent_max = 32767;

    // This is technically wrong: Extended floating point values really only have 63 bits of mantissa

    // and an "integer bit" that behaves in various strange, unintuitive and non-IEEE-754 ways.

    // However, since all bit-fiddling float code assumes IEEE floats, it cannot handle this properly.

    // If we pretend that 80-bit floats are IEEE floats with 64-bit mantissas, almost everything works correctly

    // and we just need a few special cases.

    ComponentType mantissa : 64;

    ComponentType exponent : 15;

    ComponentType sign : 1;

};

static_assert(AssertSize<FloatExtractor<f80>, sizeof(f80)>());

#endif

template<>

struct FloatExtractor<f64> {

    static constexpr FloatExtractor<f64> from_float(f64 f) { return bit_cast<FloatExtractor<f64>>(f); }

    constexpr f64 to_float() const { return bit_cast<f64>(*this); }

    using ComponentType = unsigned long long;

    static constexpr int mantissa_bits = 52;

    static constexpr ComponentType mantissa_max = (1ull << 52) - 1;

    static constexpr int exponent_bias = 1023;

    static constexpr int exponent_bits = 11;

    static constexpr unsigned exponent_max = 2047;

    // FIXME: These types have to all be the same, otherwise this struct

    //        goes from being a bitfield describing the layout of an f64

    //        into being a multibyte mess on windows.

    //        Technically, '-mno-ms-bitfields' is supposed to disable this

    //        very intuitive and portable behaviour on windows, but it doesn't

    //        work with the msvc ABI.

    //        See <https://github.com/llvm/llvm-project/issues/24757>

    ComponentType mantissa : 52;

    ComponentType exponent : 11;

    ComponentType sign : 1;

};

static_assert(AssertSize<FloatExtractor<f64>, sizeof(f64)>());

#ifdef AK_LONG_DOUBLE_IS_DOUBLE

template<>

struct FloatExtractor<long double> : FloatExtractor<double> {

};

static_assert(AssertSize<FloatExtractor<long double>, sizeof(FloatExtractor<double>)>());

#endif

template<>

struct FloatExtractor<f32> {

    static constexpr FloatExtractor<f32> from_float(f32 f) { return bit_cast<FloatExtractor<f32>>(f); }

    constexpr f32 to_float() const { return bit_cast<f32>(*this); }

    using ComponentType = unsigned;

    static constexpr int mantissa_bits = 23;

    static constexpr ComponentType mantissa_max = (1 << 23) - 1;

    static constexpr int exponent_bias = 127;

    static constexpr int exponent_bits = 8;

    static constexpr ComponentType exponent_max = 255;

    ComponentType mantissa : 23;

    ComponentType exponent : 8;

    ComponentType sign : 1;

};

static_assert(AssertSize<FloatExtractor<f32>, sizeof(f32)>());

template<size_t S, size_t E, size_t M>

requires(S <= 1 && E >= 1 && M >= 1 && (S + E + M) <= 64) class FloatingPointBits final {

public:

    static size_t const signbit = S;

    static size_t const exponentbits = E;

    static size_t const mantissabits = M;

    template<typename T>

    requires(IsIntegral<T> && IsUnsigned<T> && sizeof(T) <= 8) constexpr FloatingPointBits(T bits)

        : m_bits(bits)

    {

    }

    constexpr FloatingPointBits(double value)

        : m_bits(bit_cast<u64>(value))

    {

    }

    constexpr FloatingPointBits(float value)

        : m_bits(bit_cast<u32>(value))

    {

    }

    double as_double() const

    requires(S == 1 && E == 11 && M == 52)

    {

        return bit_cast<double>(m_bits);

    }

    float as_float() const

    requires(S == 1 && E == 8 && M == 23)

    {

        return bit_cast<float>(static_cast<u32>(m_bits));

    }

    u64 bits() const { return m_bits; }

private:

    u64 m_bits;

};

typedef FloatingPointBits<1, 8, 23> SingleFloatingPointBits;

typedef FloatingPointBits<1, 11, 52> DoubleFloatingPointBits;

/**

 * Convert between two IEEE 754 floating point types in any arrangement of sign, exponent and mantissa bits.

 */

template<typename To, typename From>

constexpr To float_to_float(From const input)

{

    constexpr u64 from_exponent_nonnumber = (1ull << From::exponentbits) - 1;

    constexpr u64 from_exponent_bias = (1ull << (From::exponentbits - 1)) - 1;

    constexpr u64 to_exponent_nonnumber = (1ull << To::exponentbits) - 1;

    constexpr u64 to_exponent_bias = (1ull << (To::exponentbits - 1)) - 1;

    constexpr u64 to_exponent_max = (1ull << To::exponentbits) - 2;

    // Deconstruct input bits to float components

    u64 from_sign = (input.bits() >> (From::exponentbits + From::mantissabits)) & From::signbit;

    u64 from_exponent = (input.bits() >> From::mantissabits) & ((1ull << From::exponentbits) - 1);

    u64 from_mantissa = input.bits() & ((1ull << From::mantissabits) - 1);

    u64 to_sign = from_sign & To::signbit;

    u64 to_exponent;

    u64 to_mantissa;

    auto target_value = [&to_sign, &to_exponent, &to_mantissa]() {

        return To((to_sign << (To::exponentbits + To::mantissabits)) | (to_exponent << To::mantissabits) | to_mantissa);

    };

    auto shift_mantissa = [](u64 mantissa) -> u64 {

        if constexpr (From::mantissabits < To::mantissabits)

            return mantissa << (To::mantissabits - From::mantissabits);

        else

            return mantissa >> (From::mantissabits - To::mantissabits);

    };

    // If target is unsigned and source is negative, clamp to 0 or keep NaN

    if constexpr (To::signbit == 0) {

        if (from_sign == 1) {

            if (from_exponent == from_exponent_nonnumber && from_mantissa > 0) {

                to_exponent = to_exponent_nonnumber;

                to_mantissa = 1;

            } else {

                to_exponent = 0;

                to_mantissa = 0;

            }

            return target_value();

        }

    }

    // If the source floating point is denormalized;

    if (from_exponent == 0) {

        // If the source mantissa is 0, the value is +/-0

        if (from_mantissa == 0) {

            to_exponent = 0;

            to_mantissa = 0;

            return target_value();

        }

        // If the source has more exponent bits than the target, then the largest possible

        // source mantissa still cannot be represented in the target denormalized value.

        if constexpr (From::exponentbits > To::exponentbits) {

            to_exponent = 0;

            to_mantissa = 0;

            return target_value();

        }

        // If the source and target have the same number of exponent bits, we only need to

        // shift the mantissa.

        if constexpr (From::exponentbits == To::exponentbits) {

            to_exponent = 0;

            to_mantissa = shift_mantissa(from_mantissa);

            return target_value();

        }

        // The target has more exponent bits, so our denormalized value can be represented

        // as a normalized value in the target floating point. Normalized values have an

        // implicit leading 1, so we shift the mantissa left until we find our explicit

        // leading 1 which is then dropped.

        int adjust_exponent = -1;

        to_mantissa = from_mantissa;

        do {

            ++adjust_exponent;

            to_mantissa <<= 1;

        } while ((to_mantissa & (1ull << From::mantissabits)) == 0);

        to_exponent = to_exponent_bias - from_exponent_bias - adjust_exponent;

        // Drop the most significant bit from the mantissa

        to_mantissa &= (1ull << From::mantissabits) - 1;

        to_mantissa = shift_mantissa(to_mantissa);

        return target_value();

    }

    // If the source is NaN or +/-Inf, keep it that way

    if (from_exponent == from_exponent_nonnumber) {

        to_exponent = to_exponent_nonnumber;

        to_mantissa = (from_mantissa == 0) ? 0 : 1;

        return target_value();

    }

    // Determine the target exponent

    to_exponent = to_exponent_bias - from_exponent_bias + from_exponent;

    // If the calculated exponent exceeds the target's capacity, clamp both the exponent and the

    // mantissa to their maximum values.

    if (to_exponent > to_exponent_max) {

        to_exponent = to_exponent_max;

        to_mantissa = (1ull << To::mantissabits) - 1;

        return target_value();

    }

    // If the new exponent is less than 1, we can only represent this value as a denormalized number

    if (to_exponent < 1) {

        to_exponent = 0;

        // Add a leading 1 and shift the mantissa right

        int adjust_exponent = 1 - to_exponent_bias - from_exponent + from_exponent_bias;

        to_mantissa = ((1ull << From::mantissabits) | from_mantissa) >> adjust_exponent;

        to_mantissa = shift_mantissa(to_mantissa);

        return target_value();

    }

    // New exponent fits; shift the mantissa to fit as well

    to_mantissa = shift_mantissa(from_mantissa);

    return target_value();

}

template<typename O>

constexpr O convert_from_native_double(double input) { return float_to_float<O>(DoubleFloatingPointBits(input)); }

template<typename O>

constexpr O convert_from_native_float(float input) { return float_to_float<O>(SingleFloatingPointBits(input)); }

template<typename I>

constexpr double convert_to_native_double(I input) { return float_to_float<DoubleFloatingPointBits>(input).as_double(); }

template<typename I>

constexpr float convert_to_native_float(I input) { return float_to_float<SingleFloatingPointBits>(input).as_float(); }

}

#if USING_AK_GLOBALLY

using AK::DoubleFloatingPointBits;

using AK::FloatExtractor;

using AK::FloatingPointBits;

using AK::SingleFloatingPointBits;

using AK::convert_from_native_double;

using AK::convert_from_native_float;

using AK::convert_to_native_double;

using AK::convert_to_native_float;

using AK::float_to_float;

#endif