/* This file is part of the dynarmic project.
 * Copyright (c) 2018 MerryMage
 * This software may be used and distributed according to the terms of the GNU
 * General Public License version 2 or any later version.
 */

#include <array>
#include <tuple>

#include "common/assert.h"
#include "common/common_types.h"
#include "common/fp/fpcr.h"
#include "common/fp/fpsr.h"
#include "common/fp/info.h"
#include "common/fp/op/FPRecipEstimate.h"
#include "common/fp/process_exception.h"
#include "common/fp/process_nan.h"
#include "common/fp/unpacked.h"
#include "common/safe_ops.h"

namespace Dynarmic::FP {

/// Input is a u0.9 fixed point number. Only values in [0.5, 1.0) are valid.
/// Output is a u0.8 fixed point number, with an implied 1 prefixed.
/// i.e.: The output is a value in [1.0, 2.0).
static u8 RecipEstimate(u64 a) {
    constexpr u64 offset = 256;
    using LUT = std::array<u8, 256>;

    static const LUT lut = [] {
        LUT result{};
        for (u64 i = 0; i < result.size(); i++) {
            u64 a = i + offset;

            a = a * 2 + 1;
            u64 b = (1u << 19) / a;
            result[i] = static_cast<u8>((b + 1) / 2);
        }
        return result;
    }();

    return lut[a - offset];
}

template<typename FPT>
FPT FPRecipEstimate(FPT op, FPCR fpcr, FPSR& fpsr) {
    FPType type;
    bool sign;
    FPUnpacked value;
    std::tie(type, sign, value) = FPUnpack<FPT>(op, fpcr, fpsr);

    if (type == FPType::SNaN || type == FPType::QNaN) {
        return FPProcessNaN(type, op, fpcr, fpsr);
    }

    if (type == FPType::Infinity) {
        return FPInfo<FPT>::Zero(sign);
    }

    if (type == FPType::Zero) {
        FPProcessException(FPExc::DivideByZero, fpcr, fpsr);
        return FPInfo<FPT>::Infinity(sign);
    }

    if (value.exponent < FPInfo<FPT>::exponent_min - 2) {
        const bool overflow_to_inf = [&]{
            switch (fpcr.RMode()) {
            case RoundingMode::ToNearest_TieEven:
                return true;
            case RoundingMode::TowardsPlusInfinity:
                return !sign;
            case RoundingMode::TowardsMinusInfinity:
                return sign;
            case RoundingMode::TowardsZero:
                return false;
            default:
                UNREACHABLE();
            }
            return false;
        }();

        FPProcessException(FPExc::Overflow, fpcr, fpsr);
        FPProcessException(FPExc::Inexact, fpcr, fpsr);
        return overflow_to_inf ? FPInfo<FPT>::Infinity(sign) : FPInfo<FPT>::MaxNormal(sign);
    }

    if ((fpcr.FZ() && !std::is_same_v<FPT, u16>) || (fpcr.FZ16() && std::is_same_v<FPT, u16>)) {
        if (value.exponent >= -FPInfo<FPT>::exponent_min) {
            fpsr.UFC(true);
            return FPInfo<FPT>::Zero(sign);
        }
    }

    const u64 scaled = Safe::LogicalShiftRight(value.mantissa, normalized_point_position - 8);
    u64 estimate = static_cast<u64>(RecipEstimate(scaled)) << (FPInfo<FPT>::explicit_mantissa_width - 8);
    int result_exponent = -value.exponent;
    if (result_exponent < FPInfo<FPT>::exponent_min) {
        switch (result_exponent) {
        case (FPInfo<FPT>::exponent_min - 1):
            estimate |= FPInfo<FPT>::implicit_leading_bit;
            estimate >>= 1;
            break;
        case (FPInfo<FPT>::exponent_min - 2):
            estimate |= FPInfo<FPT>::implicit_leading_bit;
            estimate >>= 2;
            result_exponent = 0;
            break;
        default:
            UNREACHABLE();
        }
    }

    const FPT bits_exponent = static_cast<FPT>(result_exponent + FPInfo<FPT>::exponent_bias);
    const FPT bits_mantissa = static_cast<FPT>(estimate);
    return (bits_exponent << FPInfo<FPT>::explicit_mantissa_width) | (bits_mantissa & FPInfo<FPT>::mantissa_mask);
}

template u32 FPRecipEstimate<u32>(u32 op, FPCR fpcr, FPSR& fpsr);
template u64 FPRecipEstimate<u64>(u64 op, FPCR fpcr, FPSR& fpsr);

} // namespace Dynarmic::FP