bitfield.hh

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/*
 
MIT License
 
Copyright (c) 2025 PCSX-Redux authors
 
Permission is hereby granted, free of charge, to any person obtaining a copy
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SOFTWARE.
 
*/
 
#pragma once
 
#include <stdint.h>
 
#include <concepts>
#include <type_traits>
 
#include "util.h"
 
namespace Utilities {
 
namespace BitFieldInternal {
 
template <typename T>
concept IntegralLike = std::is_integral_v<T> || std::is_enum_v<T>;
 
template <IntegralLike T>
struct DefaultBitSize {
    static constexpr unsigned size = sizeof(T) * 8;
};
 
template <>
struct DefaultBitSize<bool> {
    static constexpr unsigned size = 1;
};
 
template <typename... T>
struct ComputeStorage {
    static constexpr unsigned size() { return (sizeInBits() + 7) / 8; }
 
  private:
    static constexpr unsigned sizeInBits() { return recSize<0, T...>(); }
    template <unsigned index>
    static constexpr unsigned recSize() {
        return 0;
    }
    template <unsigned index, typename One, typename... Rest>
    static constexpr unsigned recSize() {
        return One::Width + recSize<index + 1, Rest...>();
    }
};
 
template <typename Target, typename... T>
struct ComputeOffset {
    static constexpr unsigned offset() { return recOffset<0, T...>(); }
 
  private:
    template <unsigned index>
    static constexpr unsigned recOffset() {
        return 0;
    }
    template <unsigned index, typename One, typename... Rest>
    static constexpr unsigned recOffset() {
        if constexpr (std::is_same_v<Target, One>) {
            return 0;
        } else {
            return recOffset<index + 1, Rest...>() + One::Width;
        }
    }
};
 
template <std::integral T, std::integral U = T>
using SignedType = typename std::conditional_t<std::is_signed_v<T>, std::make_signed_t<U>, std::make_unsigned_t<U>>;
 
template <unsigned span>
using StorageType = typename std::conditional_t<
    span <= 8, uint8_t,
    typename std::conditional_t<span <= 16, uint16_t, typename std::conditional_t<span <= 32, uint32_t, void>>>;
 
template <unsigned span, std::integral T>
using SignedStorageType = SignedType<T, StorageType<span>>;
 
template <unsigned Offset, unsigned Width, unsigned storageSize, std::integral T>
struct BitFieldHelper {
    static constexpr unsigned offset = Offset;
    static constexpr unsigned width = Width;
    static constexpr unsigned firstByteOffset = offset / 8;
    static constexpr unsigned lastByteOffset = (offset + width - 1) / 8;
    static constexpr unsigned bytesCount = lastByteOffset - firstByteOffset + 1;
    static constexpr unsigned shift = offset % 8;
    static constexpr uint32_t mask = (1 << width) - 1;
    static constexpr bool isAlignedAndSafe =
        ((firstByteOffset % sizeof(T)) == 0) && (firstByteOffset + sizeof(T)) <= storageSize;
    static constexpr bool fullBytes = ((width % 8) == 0) && ((offset % 8) == 0);
    BitFieldHelper() {
        static_assert(bytesCount <= 4, "Type too large");
        static_assert(width > 0, "Width must be greater than 0");
        static_assert(width <= 32, "Width must be less than or equal to 32");
        static_assert(offset + width <= storageSize * 8, "Offset + Width must be less than or equal to storage size");
    }
};
 
enum Dummy : int;
 
}  // namespace BitFieldInternal
 
template <BitFieldInternal::IntegralLike T, unsigned width = BitFieldInternal::DefaultBitSize<T>::size>
struct BitSpan {
    static constexpr unsigned Width = width;
    using Type = T;
    using Underlying =
        std::conditional_t<std::is_enum_v<T>,
                           std::underlying_type_t<std::conditional_t<std::is_enum_v<T>, T, BitFieldInternal::Dummy>>,
                           T>;
};
 
template <typename... T>
struct BitField {
    template <typename Field>
    constexpr Field::Type get() const {
        constexpr unsigned offset = BitFieldInternal::ComputeOffset<Field, T...>::offset();
        auto ret = get<offset, Field::Width,
                       BitFieldInternal::SignedStorageType<(offset % 8) + Field::Width, typename Field::Underlying>>();
        return static_cast<Field::Type>(ret);
    }
    template <typename Field>
    constexpr void set(Field::Type v_) {
        constexpr unsigned offset = BitFieldInternal::ComputeOffset<Field, T...>::offset();
        auto v = static_cast<Field::Underlying>(v_);
        set<offset, Field::Width,
            BitFieldInternal::SignedStorageType<(offset % 8) + Field::Width, typename Field::Underlying>>(v);
    }
    void clear() {
        for (unsigned i = 0; i < sizeof(storage); i++) {
            storage[i] = 0;
        }
    }
 
  private:
    template <unsigned offset, unsigned width, std::integral U>
    constexpr U get() const {
        using helper = BitFieldInternal::BitFieldHelper<offset, width, sizeof(storage), U>;
        if constexpr (helper::isAlignedAndSafe) {
            return reinterpret_cast<const U*>(storage)[helper::firstByteOffset / sizeof(U)] >> helper::shift &
                   helper::mask;
        } else {
            return (loadUnaligned<U, helper::bytesCount>(storage + helper::firstByteOffset) >> helper::shift) &
                   helper::mask;
        }
        return 0;
    }
    template <unsigned offset, unsigned width, std::integral U>
    constexpr void set(U v) {
        using helper = BitFieldInternal::BitFieldHelper<offset, width, sizeof(storage), U>;
        if constexpr (helper::fullBytes) {
            if constexpr (helper::bytesCount == 1) {
                storage[helper::firstByteOffset] = static_cast<uint8_t>(v);
            } else if constexpr (helper::bytesCount == 2) {
                if constexpr (helper::isAlignedAndSafe) {
                    *reinterpret_cast<U*>(storage + helper::firstByteOffset) = v;
                } else {
                    storeUnaligned<U>(storage + helper::firstByteOffset, v);
                }
            } else if constexpr (helper::bytesCount == 3) {
                if constexpr ((helper::firstByteOffset % 2) == 0) {
                    *reinterpret_cast<uint16_t*>(storage + helper::firstByteOffset) = static_cast<uint16_t>(v);
                    storage[helper::firstByteOffset + 2] = static_cast<uint8_t>(v >> 16);
                } else {
                    storage[helper::firstByteOffset] = static_cast<uint8_t>(v);
                    *reinterpret_cast<uint16_t*>(storage + helper::firstByteOffset + 1) = static_cast<uint16_t>(v >> 8);
                }
            } else if constexpr (helper::bytesCount == 4) {
                if constexpr (helper::isAlignedAndSafe) {
                    *reinterpret_cast<U*>(storage + helper::firstByteOffset) = v;
                } else {
                    storeUnaligned<U>(storage + helper::firstByteOffset, v);
                }
            }
        } else if constexpr (helper::isAlignedAndSafe) {
            U* ptr = reinterpret_cast<U*>(storage);
            ptr[helper::firstByteOffset / sizeof(U)] &= ~(helper::mask << helper::shift);
            ptr[helper::firstByteOffset / sizeof(U)] |= (v & helper::mask) << helper::shift;
        } else {
            U span = loadUnaligned<U, helper::bytesCount>(storage + helper::firstByteOffset);
            span &= ~(helper::mask << helper::shift);
            span |= (v & helper::mask) << helper::shift;
            storeUnaligned<U, helper::bytesCount>(storage + helper::firstByteOffset, span);
        }
    }
    uint8_t storage[BitFieldInternal::ComputeStorage<T...>::size()];
};
 
}  // namespace Utilities