ladybird/Userland/Libraries/LibGfx/BMPLoader.cpp

/*
 * Copyright (c) 2020, Matthew Olsson <mattco@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Debug.h>
#include <AK/Function.h>
#include <AK/LexicalPath.h>
#include <AK/MappedFile.h>
#include <LibGfx/BMPLoader.h>

namespace Gfx {

const u8 bmp_header_size = 14;
const u32 color_palette_limit = 1024;

// Compression flags
struct Compression {
    enum : u32 {
        RGB = 0,
        RLE8,
        RLE4,
        BITFIELDS,
        RLE24, // doubles as JPEG for V4+, but that is unsupported
        PNG,
        ALPHABITFIELDS,
        CMYK = 11,
        CMYKRLE8,
        CMYKRLE4,
    };
};

struct DIBCore {
    // u16 for BITMAPHEADERCORE, but i32 for everything else. If the dib type is
    // BITMAPHEADERCORE, this is range checked.
    i32 width;
    i32 height;
    u16 bpp;
};

struct DIBInfo {
    u32 compression { Compression::RGB };
    u32 image_size { 0 };
    i32 horizontal_resolution { 0 };
    i32 vertical_resolution { 0 };
    u32 number_of_palette_colors { 0 };
    u32 number_of_important_palette_colors { number_of_palette_colors };

    // Introduced in the BITMAPV2INFOHEADER and would ideally be stored in the
    // DIBV2 struct, however with a compression value of BI_BITFIELDS or
    // BI_ALPHABITFIELDS, these can be specified with the Info header.
    Vector<u32> masks;
    Vector<i8> mask_shifts;
    Vector<u8> mask_sizes;
};

struct DIBOSV2 {
    u16 recording;
    u16 halftoning;
    u16 size1;
    u16 size2;
};

template<typename T>
struct Endpoint {
    T x;
    T y;
    T z;
};

}

namespace AK {

template<typename T>
struct Formatter<Gfx::Endpoint<T>> : Formatter<StringView> {
    void format(FormatBuilder& builder, const Gfx::Endpoint<T>& value)
    {
        Formatter<StringView>::format(builder, String::formatted("({}, {}, {})", value.x, value.y, value.z));
    }
};

}

namespace Gfx {

struct DIBV4 {
    u32 color_space { 0 };
    Endpoint<i32> red_endpoint { 0, 0, 0 };
    Endpoint<i32> green_endpoint { 0, 0, 0 };
    Endpoint<i32> blue_endpoint { 0, 0, 0 };
    Endpoint<u32> gamma_endpoint { 0, 0, 0 };
};

struct DIBV5 {
    u32 intent { 0 };
    u32 profile_data { 0 };
    u32 profile_size { 0 };
};

struct DIB {
    DIBCore core;
    DIBInfo info;
    DIBOSV2 osv2;
    DIBV4 v4;
    DIBV5 v5;
};

enum class DIBType {
    Core = 0,
    OSV2Short,
    OSV2,
    Info,
    V2,
    V3,
    V4,
    V5
};

struct BMPLoadingContext {
    enum class State {
        NotDecoded = 0,
        HeaderDecoded,
        DIBDecoded,
        ColorTableDecoded,
        PixelDataDecoded,
        Error,
    };
    State state { State::NotDecoded };

    const u8* file_bytes { nullptr };
    size_t file_size { 0 };
    u32 data_offset { 0 };

    DIB dib;
    DIBType dib_type;

    Vector<u32> color_table;
    RefPtr<Gfx::Bitmap> bitmap;

    u32 dib_size() const
    {
        switch (dib_type) {
        case DIBType::Core:
            return 12;
        case DIBType::OSV2Short:
            return 16;
        case DIBType::OSV2:
            return 64;
        case DIBType::Info:
            return 40;
        case DIBType::V2:
            return 52;
        case DIBType::V3:
            return 56;
        case DIBType::V4:
            return 108;
        case DIBType::V5:
            return 124;
        }

        VERIFY_NOT_REACHED();
    }
};

static RefPtr<Bitmap> load_bmp_impl(const u8*, size_t);

RefPtr<Gfx::Bitmap> load_bmp(String const& path)
{
    auto file_or_error = MappedFile::map(path);
    if (file_or_error.is_error())
        return nullptr;
    auto bitmap = load_bmp_impl((const u8*)file_or_error.value()->data(), file_or_error.value()->size());
    if (bitmap)
        bitmap->set_mmap_name(String::formatted("Gfx::Bitmap [{}] - Decoded BMP: {}", bitmap->size(), LexicalPath::canonicalized_path(path)));
    return bitmap;
}

RefPtr<Gfx::Bitmap> load_bmp_from_memory(const u8* data, size_t length)
{
    auto bitmap = load_bmp_impl(data, length);
    if (bitmap)
        bitmap->set_mmap_name(String::formatted("Gfx::Bitmap [{}] - Decoded BMP: <memory>", bitmap->size()));
    return bitmap;
}

class InputStreamer {
public:
    InputStreamer(const u8* data, size_t size)
        : m_data_ptr(data)
        , m_size_remaining(size)
    {
    }

    u8 read_u8()
    {
        VERIFY(m_size_remaining >= 1);
        m_size_remaining--;
        return *(m_data_ptr++);
    }

    u16 read_u16()
    {
        return read_u8() | (read_u8() << 8);
    }

    u32 read_u24()
    {
        return read_u8() | (read_u8() << 8) | (read_u8() << 16);
    }

    i32 read_i32()
    {
        return static_cast<i32>(read_u16() | (read_u16() << 16));
    }

    u32 read_u32()
    {
        return read_u16() | (read_u16() << 16);
    }

    void drop_bytes(u8 num_bytes)
    {
        VERIFY(m_size_remaining >= num_bytes);
        m_size_remaining -= num_bytes;
        m_data_ptr += num_bytes;
    }

    bool at_end() const { return !m_size_remaining; }

    bool has_u8() const { return m_size_remaining >= 1; }
    bool has_u16() const { return m_size_remaining >= 2; }
    bool has_u24() const { return m_size_remaining >= 3; }
    bool has_u32() const { return m_size_remaining >= 4; }

    size_t remaining() const { return m_size_remaining; }

private:
    const u8* m_data_ptr { nullptr };
    size_t m_size_remaining { 0 };
};

// Lookup table for distributing all possible 2-bit numbers evenly into 8-bit numbers
static u8 scaling_factors_2bit[4] = {
    0x00,
    0x55,
    0xaa,
    0xff,
};

// Lookup table for distributing all possible 3-bit numbers evenly into 8-bit numbers
static u8 scaling_factors_3bit[8] = {
    0x00,
    0x24,
    0x48,
    0x6d,
    0x91,
    0xb6,
    0xdb,
    0xff,
};

static u8 scale_masked_8bit_number(u8 number, u8 bits_set)
{
    // If there are more than 4 bit set, an easy way to scale the number is to
    // just copy the most significant bits into the least significant bits
    if (bits_set >= 4)
        return number | (number >> bits_set);
    if (!bits_set)
        return 0;
    if (bits_set == 1)
        return number ? 0xff : 0;
    if (bits_set == 2)
        return scaling_factors_2bit[number >> 6];
    return scaling_factors_3bit[number >> 5];
}

static u8 get_scaled_color(u32 data, u8 mask_size, i8 mask_shift)
{
    // A negative mask_shift indicates we actually need to left shift
    // the result in order to get out a valid 8-bit color (for example, the blue
    // value in an RGB555 encoding is XXXBBBBB, which needs to be shifted to the
    // left by 3, hence it would have a "mask_shift" value of -3).
    if (mask_shift < 0)
        return scale_masked_8bit_number(data << -mask_shift, mask_size);
    return scale_masked_8bit_number(data >> mask_shift, mask_size);
}

// Scales an 8-bit number with "mask_size" bits set (and "8 - mask_size" bits
//   ignored). This function scales the number appropriately over the entire
//   256 value color spectrum.
// Note that a much simpler scaling can be done by simple bit shifting. If you
//   just ignore the bottom 8-mask_size bits, then you get *close*. However,
//   consider, as an example, a 5 bit number (so the bottom 3 bits are ignored).
//   The purest white you could get is 0xf8, which is 248 in RGB-land. We need
//   to scale the values in order to reach the proper value of 255.
static u32 int_to_scaled_rgb(BMPLoadingContext& context, u32 data)
{
    dbgln_if(BMP_DEBUG, "DIB info sizes before access: #masks={}, #mask_sizes={}, #mask_shifts={}",
        context.dib.info.masks.size(),
        context.dib.info.mask_sizes.size(),
        context.dib.info.mask_shifts.size());

    u8 r = get_scaled_color(data & context.dib.info.masks[0], context.dib.info.mask_sizes[0], context.dib.info.mask_shifts[0]);
    u8 g = get_scaled_color(data & context.dib.info.masks[1], context.dib.info.mask_sizes[1], context.dib.info.mask_shifts[1]);
    u8 b = get_scaled_color(data & context.dib.info.masks[2], context.dib.info.mask_sizes[2], context.dib.info.mask_shifts[2]);
    u32 color = (r << 16) | (g << 8) | b;

    if (context.dib.info.masks.size() == 4) {
        // The bitmap has an alpha mask
        u8 a = get_scaled_color(data & context.dib.info.masks[3], context.dib.info.mask_sizes[3], context.dib.info.mask_shifts[3]);
        color |= (a << 24);
    } else {
        color |= 0xff000000;
    }

    return color;
}

static void populate_dib_mask_info_if_needed(BMPLoadingContext& context)
{
    if (context.dib.info.masks.is_empty())
        return;

    // Mask shift is the number of right shifts needed to align the MSb of the
    // mask to the MSb of the LSB. Note that this can be a negative number.
    // Mask size is the number of set bits in the mask. This is required for
    // color scaling (for example, ensuring that a 4-bit color value spans the
    // entire 256 value color spectrum.
    auto& masks = context.dib.info.masks;
    auto& mask_shifts = context.dib.info.mask_shifts;
    auto& mask_sizes = context.dib.info.mask_sizes;

    if (!mask_shifts.is_empty() && !mask_sizes.is_empty())
        return;

    VERIFY(mask_shifts.is_empty() && mask_sizes.is_empty());

    mask_shifts.ensure_capacity(masks.size());
    mask_sizes.ensure_capacity(masks.size());

    for (size_t i = 0; i < masks.size(); ++i) {
        u32 mask = masks[i];
        if (!mask) {
            mask_shifts.append(0);
            mask_sizes.append(0);
            continue;
        }
        int trailing_zeros = count_trailing_zeroes_32(mask);
        // If mask is exactly `0xFFFFFFFF`, then we might try to count the trailing zeros of 0x00000000 here, so we need the safe version:
        int size = count_trailing_zeroes_32_safe(~(mask >> trailing_zeros));
        if (size > 8) {
            // Drop lowest bits if mask is longer than 8 bits.
            trailing_zeros += size - 8;
            size = 8;
        }
        mask_shifts.append(size + trailing_zeros - 8);
        mask_sizes.append(size);
    }
}

static bool check_for_invalid_bitmask_combinations(BMPLoadingContext& context)
{
    auto& bpp = context.dib.core.bpp;
    auto& compression = context.dib.info.compression;

    if (compression == Compression::ALPHABITFIELDS && context.dib_type != DIBType::Info)
        return false;

    switch (context.dib_type) {
    case DIBType::Core:
        if (bpp == 2 || bpp == 16 || bpp == 32)
            return false;
        break;
    case DIBType::Info:
        switch (compression) {
        case Compression::BITFIELDS:
        case Compression::ALPHABITFIELDS:
            if (bpp != 16 && bpp != 32)
                return false;
            break;
        case Compression::RGB:
            break;
        case Compression::RLE8:
            if (bpp > 8)
                return false;
            break;
        case Compression::RLE4:
            // TODO: This is a guess
            if (bpp > 4)
                return false;
            break;
        default:
            // Other compressions are not officially supported.
            // Technically, we could even drop ALPHABITFIELDS.
            return false;
        }
        break;
    case DIBType::OSV2Short:
    case DIBType::OSV2:
    case DIBType::V2:
    case DIBType::V3:
    case DIBType::V4:
    case DIBType::V5:
        if (compression == Compression::BITFIELDS && bpp != 16 && bpp != 32)
            return false;
        break;
    }

    return true;
}

static bool set_dib_bitmasks(BMPLoadingContext& context, InputStreamer& streamer)
{
    if (!check_for_invalid_bitmask_combinations(context))
        return false;

    auto& bpp = context.dib.core.bpp;
    if (bpp <= 8 || bpp == 24)
        return true;

    auto& compression = context.dib.info.compression;
    auto& type = context.dib_type;

    if (type > DIBType::OSV2 && bpp == 16 && compression == Compression::RGB) {
        context.dib.info.masks.extend({ 0x7c00, 0x03e0, 0x001f });
        context.dib.info.mask_shifts.extend({ 7, 2, -3 });
        context.dib.info.mask_sizes.extend({ 5, 5, 5 });
    } else if (type == DIBType::Info && (compression == Compression::BITFIELDS || compression == Compression::ALPHABITFIELDS)) {
        // Consume the extra BITFIELDS bytes
        auto number_of_mask_fields = compression == Compression::ALPHABITFIELDS ? 4 : 3;

        for (auto i = 0; i < number_of_mask_fields; i++) {
            if (!streamer.has_u32())
                return false;
            context.dib.info.masks.append(streamer.read_u32());
        }
    }

    populate_dib_mask_info_if_needed(context);
    return true;
}

static bool decode_bmp_header(BMPLoadingContext& context)
{
    if (context.state == BMPLoadingContext::State::Error)
        return false;

    if (context.state >= BMPLoadingContext::State::HeaderDecoded)
        return true;

    if (!context.file_bytes || context.file_size < bmp_header_size) {
        dbgln_if(BMP_DEBUG, "Missing BMP header");
        context.state = BMPLoadingContext::State::Error;
        return false;
    }

    InputStreamer streamer(context.file_bytes, bmp_header_size);

    u16 header = streamer.read_u16();
    if (header != 0x4d42) {
        dbgln_if(BMP_DEBUG, "BMP has invalid magic header number: {:#04x}", header);
        context.state = BMPLoadingContext::State::Error;
        return false;
    }

    // The reported size of the file in the header is actually not important
    // for decoding the file. Some specifications say that this value should
    // be the size of the header instead, so we just rely on the known file
    // size, instead of a possibly-correct-but-also-possibly-incorrect reported
    // value of the file size.
    streamer.drop_bytes(4);

    // Ignore reserved bytes
    streamer.drop_bytes(4);
    context.data_offset = streamer.read_u32();

    if constexpr (BMP_DEBUG) {
        dbgln("BMP file size: {}", context.file_size);
        dbgln("BMP data offset: {}", context.data_offset);
    }

    if (context.data_offset >= context.file_size) {
        dbgln_if(BMP_DEBUG, "BMP data offset is beyond file end?!");
        return false;
    }

    context.state = BMPLoadingContext::State::HeaderDecoded;
    return true;
}

static bool decode_bmp_core_dib(BMPLoadingContext& context, InputStreamer& streamer)
{
    auto& core = context.dib.core;

    // The width and height are u16 fields in the actual BITMAPCOREHEADER format.
    if (context.dib_type == DIBType::Core) {
        core.width = streamer.read_u16();
        core.height = streamer.read_u16();
    } else {
        core.width = streamer.read_i32();
        core.height = streamer.read_i32();
    }

    if (core.width < 0) {
        dbgln("BMP has a negative width: {}", core.width);
        return false;
    }

    if (static_cast<size_t>(core.width) > maximum_width_for_decoded_images || static_cast<size_t>(abs(core.height)) > maximum_height_for_decoded_images) {
        dbgln("This BMP is too large for comfort: {}x{}", core.width, abs(core.height));
        return false;
    }

    auto color_planes = streamer.read_u16();
    if (color_planes != 1) {
        dbgln("BMP has an invalid number of color planes: {}", color_planes);
        return false;
    }

    core.bpp = streamer.read_u16();
    switch (core.bpp) {
    case 1:
    case 2:
    case 4:
    case 8:
    case 16:
    case 24:
    case 32:
        break;
    default:
        dbgln("BMP has an invalid bpp: {}", core.bpp);
        context.state = BMPLoadingContext::State::Error;
        return false;
    }

    if constexpr (BMP_DEBUG) {
        dbgln("BMP width: {}", core.width);
        dbgln("BMP height: {}", core.height);
        dbgln("BMP bits_per_pixel: {}", core.bpp);
    }

    return true;
}

ALWAYS_INLINE static bool is_supported_compression_format(BMPLoadingContext& context, u32 compression)
{
    return compression == Compression::RGB || compression == Compression::BITFIELDS
        || compression == Compression::ALPHABITFIELDS || compression == Compression::RLE8
        || compression == Compression::RLE4 || (compression == Compression::RLE24 && context.dib_type <= DIBType::OSV2);
}

static bool decode_bmp_osv2_dib(BMPLoadingContext& context, InputStreamer& streamer, bool short_variant = false)
{
    auto& core = context.dib.core;

    core.width = streamer.read_u32();
    core.height = streamer.read_u32();

    if (core.width < 0) {
        dbgln("BMP has a negative width: {}", core.width);
        return false;
    }

    auto color_planes = streamer.read_u16();
    if (color_planes != 1) {
        dbgln("BMP has an invalid number of color planes: {}", color_planes);
        return false;
    }

    core.bpp = streamer.read_u16();
    switch (core.bpp) {
    case 1:
    case 2:
    case 4:
    case 8:
    case 24:
        break;
    default:
        // OS/2 didn't expect 16- or 32-bpp to be popular.
        dbgln("BMP has an invalid bpp: {}", core.bpp);
        context.state = BMPLoadingContext::State::Error;
        return false;
    }

    if constexpr (BMP_DEBUG) {
        dbgln("BMP width: {}", core.width);
        dbgln("BMP height: {}", core.height);
        dbgln("BMP bits_per_pixel: {}", core.bpp);
    }

    if (short_variant)
        return true;

    auto& info = context.dib.info;
    auto& osv2 = context.dib.osv2;

    info.compression = streamer.read_u32();
    info.image_size = streamer.read_u32();
    info.horizontal_resolution = streamer.read_u32();
    info.vertical_resolution = streamer.read_u32();
    info.number_of_palette_colors = streamer.read_u32();
    info.number_of_important_palette_colors = streamer.read_u32();

    if (!is_supported_compression_format(context, info.compression)) {
        dbgln("BMP has unsupported compression value: {}", info.compression);
        return false;
    }

    if (info.number_of_palette_colors > color_palette_limit || info.number_of_important_palette_colors > color_palette_limit) {
        dbgln("BMP header indicates too many palette colors: {}", info.number_of_palette_colors);
        return false;
    }

    // Units (2) + reserved (2)
    streamer.drop_bytes(4);

    osv2.recording = streamer.read_u16();
    osv2.halftoning = streamer.read_u16();
    osv2.size1 = streamer.read_u32();
    osv2.size2 = streamer.read_u32();

    // ColorEncoding (4) + Identifier (4)
    streamer.drop_bytes(8);

    if constexpr (BMP_DEBUG) {
        dbgln("BMP compression: {}", info.compression);
        dbgln("BMP image size: {}", info.image_size);
        dbgln("BMP horizontal res: {}", info.horizontal_resolution);
        dbgln("BMP vertical res: {}", info.vertical_resolution);
        dbgln("BMP colors: {}", info.number_of_palette_colors);
        dbgln("BMP important colors: {}", info.number_of_important_palette_colors);
    }

    return true;
}

static bool decode_bmp_info_dib(BMPLoadingContext& context, InputStreamer& streamer)
{
    if (!decode_bmp_core_dib(context, streamer))
        return false;

    auto& info = context.dib.info;

    auto compression = streamer.read_u32();
    info.compression = compression;
    if (!is_supported_compression_format(context, compression)) {
        dbgln("BMP has unsupported compression value: {}", compression);
        return false;
    }

    info.image_size = streamer.read_u32();
    info.horizontal_resolution = streamer.read_i32();
    info.vertical_resolution = streamer.read_i32();
    info.number_of_palette_colors = streamer.read_u32();
    info.number_of_important_palette_colors = streamer.read_u32();

    if (info.number_of_palette_colors > color_palette_limit || info.number_of_important_palette_colors > color_palette_limit) {
        dbgln("BMP header indicates too many palette colors: {}", info.number_of_palette_colors);
        return false;
    }

    if (info.number_of_important_palette_colors == 0)
        info.number_of_important_palette_colors = info.number_of_palette_colors;

    if constexpr (BMP_DEBUG) {
        dbgln("BMP compression: {}", info.compression);
        dbgln("BMP image size: {}", info.image_size);
        dbgln("BMP horizontal res: {}", info.horizontal_resolution);
        dbgln("BMP vertical res: {}", info.vertical_resolution);
        dbgln("BMP colors: {}", info.number_of_palette_colors);
        dbgln("BMP important colors: {}", info.number_of_important_palette_colors);
    }

    return true;
}

static bool decode_bmp_v2_dib(BMPLoadingContext& context, InputStreamer& streamer)
{
    if (!decode_bmp_info_dib(context, streamer))
        return false;

    context.dib.info.masks.append(streamer.read_u32());
    context.dib.info.masks.append(streamer.read_u32());
    context.dib.info.masks.append(streamer.read_u32());

    if constexpr (BMP_DEBUG) {
        dbgln("BMP red mask: {:#08x}", context.dib.info.masks[0]);
        dbgln("BMP green mask: {:#08x}", context.dib.info.masks[1]);
        dbgln("BMP blue mask: {:#08x}", context.dib.info.masks[2]);
    }

    return true;
}

static bool decode_bmp_v3_dib(BMPLoadingContext& context, InputStreamer& streamer)
{
    if (!decode_bmp_v2_dib(context, streamer))
        return false;

    // There is zero documentation about when alpha masks actually get applied.
    // Well, there's some, but it's not even close to comprehensive. So, this is
    // in no way based off of any spec, it's simply based off of the BMP test
    // suite results.
    if (context.dib.info.compression == Compression::ALPHABITFIELDS) {
        context.dib.info.masks.append(streamer.read_u32());
        dbgln_if(BMP_DEBUG, "BMP alpha mask: {:#08x}", context.dib.info.masks[3]);
    } else if (context.dib_size() >= 56 && context.dib.core.bpp >= 16) {
        auto mask = streamer.read_u32();
        if ((context.dib.core.bpp == 32 && mask != 0) || context.dib.core.bpp == 16) {
            context.dib.info.masks.append(mask);
            dbgln_if(BMP_DEBUG, "BMP alpha mask: {:#08x}", mask);
        }
    } else {
        streamer.drop_bytes(4);
    }

    return true;
}

static bool decode_bmp_v4_dib(BMPLoadingContext& context, InputStreamer& streamer)
{
    if (!decode_bmp_v3_dib(context, streamer))
        return false;

    auto& v4 = context.dib.v4;
    v4.color_space = streamer.read_u32();
    v4.red_endpoint = { streamer.read_i32(), streamer.read_i32(), streamer.read_i32() };
    v4.green_endpoint = { streamer.read_i32(), streamer.read_i32(), streamer.read_i32() };
    v4.blue_endpoint = { streamer.read_i32(), streamer.read_i32(), streamer.read_i32() };
    v4.gamma_endpoint = { streamer.read_u32(), streamer.read_u32(), streamer.read_u32() };

    if constexpr (BMP_DEBUG) {
        dbgln("BMP color space: {}", v4.color_space);
        dbgln("BMP red endpoint: {}", v4.red_endpoint);
        dbgln("BMP green endpoint: {}", v4.green_endpoint);
        dbgln("BMP blue endpoint: {}", v4.blue_endpoint);
        dbgln("BMP gamma endpoint: {}", v4.gamma_endpoint);
    }

    return true;
}

static bool decode_bmp_v5_dib(BMPLoadingContext& context, InputStreamer& streamer)
{
    if (!decode_bmp_v4_dib(context, streamer))
        return false;

    auto& v5 = context.dib.v5;
    v5.intent = streamer.read_u32();
    v5.profile_data = streamer.read_u32();
    v5.profile_size = streamer.read_u32();

    if constexpr (BMP_DEBUG) {
        dbgln("BMP intent: {}", v5.intent);
        dbgln("BMP profile data: {}", v5.profile_data);
        dbgln("BMP profile size: {}", v5.profile_size);
    }

    return true;
}

static bool decode_bmp_dib(BMPLoadingContext& context)
{
    if (context.state == BMPLoadingContext::State::Error)
        return false;

    if (context.state >= BMPLoadingContext::State::DIBDecoded)
        return true;

    if (context.state < BMPLoadingContext::State::HeaderDecoded && !decode_bmp_header(context))
        return false;

    if (context.file_size < bmp_header_size + 4)
        return false;

    InputStreamer streamer(context.file_bytes + bmp_header_size, 4);
    u32 dib_size = streamer.read_u32();

    if (context.file_size < bmp_header_size + dib_size)
        return false;
    if (context.data_offset < bmp_header_size + dib_size) {
        dbgln("Shenanigans! BMP pixel data and header usually don't overlap.");
        return false;
    }

    streamer = InputStreamer(context.file_bytes + bmp_header_size + 4, context.data_offset - bmp_header_size - 4);

    dbgln_if(BMP_DEBUG, "BMP dib size: {}", dib_size);

    bool error = false;

    if (dib_size == 12) {
        context.dib_type = DIBType::Core;
        if (!decode_bmp_core_dib(context, streamer))
            error = true;
    } else if (dib_size == 64) {
        context.dib_type = DIBType::OSV2;
        if (!decode_bmp_osv2_dib(context, streamer))
            error = true;
    } else if (dib_size == 16) {
        context.dib_type = DIBType::OSV2Short;
        if (!decode_bmp_osv2_dib(context, streamer, true))
            error = true;
    } else if (dib_size == 40) {
        context.dib_type = DIBType::Info;
        if (!decode_bmp_info_dib(context, streamer))
            error = true;
    } else if (dib_size == 52) {
        context.dib_type = DIBType::V2;
        if (!decode_bmp_v2_dib(context, streamer))
            error = true;
    } else if (dib_size == 56) {
        context.dib_type = DIBType::V3;
        if (!decode_bmp_v3_dib(context, streamer))
            error = true;
    } else if (dib_size == 108) {
        context.dib_type = DIBType::V4;
        if (!decode_bmp_v4_dib(context, streamer))
            error = true;
    } else if (dib_size == 124) {
        context.dib_type = DIBType::V5;
        if (!decode_bmp_v5_dib(context, streamer))
            error = true;
    } else {
        dbgln("Unsupported BMP DIB size: {}", dib_size);
        error = true;
    }

    switch (context.dib.info.compression) {
    case Compression::RGB:
    case Compression::RLE8:
    case Compression::RLE4:
    case Compression::BITFIELDS:
    case Compression::RLE24:
    case Compression::PNG:
    case Compression::ALPHABITFIELDS:
    case Compression::CMYK:
    case Compression::CMYKRLE8:
    case Compression::CMYKRLE4:
        break;
    default:
        error = true;
    }

    if (!error && !set_dib_bitmasks(context, streamer))
        error = true;

    if (error) {
        dbgln("BMP has an invalid DIB");
        context.state = BMPLoadingContext::State::Error;
        return false;
    }

    context.state = BMPLoadingContext::State::DIBDecoded;

    return true;
}

static bool decode_bmp_color_table(BMPLoadingContext& context)
{
    if (context.state == BMPLoadingContext::State::Error)
        return false;

    if (context.state < BMPLoadingContext::State::DIBDecoded && !decode_bmp_dib(context))
        return false;

    if (context.state >= BMPLoadingContext::State::ColorTableDecoded)
        return true;

    if (context.dib.core.bpp > 8) {
        context.state = BMPLoadingContext::State::ColorTableDecoded;
        return true;
    }

    auto bytes_per_color = context.dib_type == DIBType::Core ? 3 : 4;
    u32 max_colors = 1 << context.dib.core.bpp;
    VERIFY(context.data_offset >= bmp_header_size + context.dib_size());
    auto size_of_color_table = context.data_offset - bmp_header_size - context.dib_size();

    if (context.dib_type <= DIBType::OSV2) {
        // Partial color tables are not supported, so the space of the color
        // table must be at least enough for the maximum amount of colors
        if (size_of_color_table < 3 * max_colors) {
            // This is against the spec, but most viewers process it anyways
            dbgln("BMP with CORE header does not have enough colors. Has: {}, expected: {}", size_of_color_table, (3 * max_colors));
        }
    }

    InputStreamer streamer(context.file_bytes + bmp_header_size + context.dib_size(), size_of_color_table);
    for (u32 i = 0; !streamer.at_end() && i < max_colors; ++i) {
        if (bytes_per_color == 4) {
            if (!streamer.has_u32())
                return false;
            context.color_table.append(streamer.read_u32());
        } else {
            if (!streamer.has_u24())
                return false;
            context.color_table.append(streamer.read_u24());
        }
    }

    context.state = BMPLoadingContext::State::ColorTableDecoded;
    return true;
}

struct RLEState {
    enum : u8 {
        PixelCount = 0,
        PixelValue,
        Meta, // Represents just consuming a null byte, which indicates something special
    };
};

static bool uncompress_bmp_rle_data(BMPLoadingContext& context, ByteBuffer& buffer)
{
    // RLE-compressed images cannot be stored top-down
    if (context.dib.core.height < 0) {
        dbgln_if(BMP_DEBUG, "BMP is top-down and RLE compressed");
        context.state = BMPLoadingContext::State::Error;
        return false;
    }

    InputStreamer streamer(context.file_bytes + context.data_offset, context.file_size - context.data_offset);

    auto compression = context.dib.info.compression;

    u32 total_rows = static_cast<u32>(context.dib.core.height);
    u32 total_columns = round_up_to_power_of_two(static_cast<u32>(context.dib.core.width), 4);
    u32 column = 0;
    u32 row = 0;
    auto currently_consuming = RLEState::PixelCount;
    i16 pixel_count = 0;

    // ByteBuffer asserts that allocating the memory never fails.
    // FIXME: ByteBuffer should return either RefPtr<> or Optional<>.
    // Decoding the RLE data on-the-fly might actually be faster, and avoids this topic entirely.
    u32 buffer_size;
    if (compression == Compression::RLE24) {
        buffer_size = total_rows * round_up_to_power_of_two(total_columns, 4) * 4;
    } else {
        buffer_size = total_rows * round_up_to_power_of_two(total_columns, 4);
    }
    if (buffer_size > 300 * MiB) {
        dbgln("Suspiciously large amount of RLE data");
        return false;
    }
    auto buffer_result = ByteBuffer::create_zeroed(buffer_size);
    if (!buffer_result.has_value()) {
        dbgln("Not enough memory for buffer allocation");
        return false;
    }
    buffer = buffer_result.release_value();

    // Avoid as many if statements as possible by pulling out
    // compression-dependent actions into separate lambdas
    Function<u32()> get_buffer_index;
    Function<bool(u32, bool)> set_byte;
    Function<Optional<u32>()> read_byte;

    if (compression == Compression::RLE8) {
        get_buffer_index = [&]() -> u32 { return row * total_columns + column; };
    } else if (compression == Compression::RLE4) {
        get_buffer_index = [&]() -> u32 { return (row * total_columns + column) / 2; };
    } else {
        get_buffer_index = [&]() -> u32 { return (row * total_columns + column) * 3; };
    }

    if (compression == Compression::RLE8) {
        set_byte = [&](u32 color, bool) -> bool {
            if (column >= total_columns) {
                column = 0;
                row++;
            }
            auto index = get_buffer_index();
            if (index >= buffer.size()) {
                dbgln("BMP has badly-formatted RLE data");
                return false;
            }
            buffer[index] = color;
            column++;
            return true;
        };
    } else if (compression == Compression::RLE24) {
        set_byte = [&](u32 color, bool) -> bool {
            if (column >= total_columns) {
                column = 0;
                row++;
            }
            auto index = get_buffer_index();
            if (index + 3 >= buffer.size()) {
                dbgln("BMP has badly-formatted RLE data");
                return false;
            }
            ((u32&)buffer[index]) = color;
            column++;
            return true;
        };
    } else {
        set_byte = [&](u32 byte, bool rle4_set_second_nibble) -> bool {
            if (column >= total_columns) {
                column = 0;
                row++;
            }

            u32 index = get_buffer_index();
            if (index >= buffer.size() || (rle4_set_second_nibble && index + 1 >= buffer.size())) {
                dbgln("BMP has badly-formatted RLE data");
                return false;
            }

            if (column % 2) {
                buffer[index] |= byte >> 4;
                if (rle4_set_second_nibble) {
                    buffer[index + 1] |= byte << 4;
                    column++;
                }
            } else {
                if (rle4_set_second_nibble) {
                    buffer[index] = byte;
                    column++;
                } else {
                    buffer[index] |= byte & 0xf0;
                }
            }

            column++;
            return true;
        };
    }

    if (compression == Compression::RLE24) {
        read_byte = [&]() -> Optional<u32> {
            if (!streamer.has_u24()) {
                dbgln("BMP has badly-formatted RLE data");
                return {};
            }
            return streamer.read_u24();
        };
    } else {
        read_byte = [&]() -> Optional<u32> {
            if (!streamer.has_u8()) {
                dbgln("BMP has badly-formatted RLE data");
                return {};
            }
            return streamer.read_u8();
        };
    }

    while (true) {
        u32 byte;

        switch (currently_consuming) {
        case RLEState::PixelCount:
            if (!streamer.has_u8())
                return false;
            byte = streamer.read_u8();
            if (!byte) {
                currently_consuming = RLEState::Meta;
            } else {
                pixel_count = byte;
                currently_consuming = RLEState::PixelValue;
            }
            break;
        case RLEState::PixelValue: {
            auto result = read_byte();
            if (!result.has_value())
                return false;
            byte = result.value();
            for (u16 i = 0; i < pixel_count; ++i) {
                if (compression != Compression::RLE4) {
                    if (!set_byte(byte, true))
                        return false;
                } else {
                    if (!set_byte(byte, i != pixel_count - 1))
                        return false;
                    i++;
                }
            }

            currently_consuming = RLEState::PixelCount;
            break;
        }
        case RLEState::Meta:
            if (!streamer.has_u8())
                return false;
            byte = streamer.read_u8();
            if (!byte) {
                column = 0;
                row++;
                currently_consuming = RLEState::PixelCount;
                continue;
            }
            if (byte == 1)
                return true;
            if (byte == 2) {
                if (!streamer.has_u8())
                    return false;
                u8 offset_x = streamer.read_u8();
                if (!streamer.has_u8())
                    return false;
                u8 offset_y = streamer.read_u8();
                column += offset_x;
                if (column >= total_columns) {
                    column -= total_columns;
                    row++;
                }
                row += offset_y;
                currently_consuming = RLEState::PixelCount;
                continue;
            }

            // Consume literal bytes
            pixel_count = byte;
            i16 i = byte;

            while (i >= 1) {
                auto result = read_byte();
                if (!result.has_value())
                    return false;
                byte = result.value();
                if (!set_byte(byte, i != 1))
                    return false;
                i--;
                if (compression == Compression::RLE4)
                    i--;
            }

            // Optionally consume a padding byte
            if (compression != Compression::RLE4) {
                if (pixel_count % 2) {
                    if (!streamer.has_u8())
                        return false;
                    byte = streamer.read_u8();
                }
            } else {
                if (((pixel_count + 1) / 2) % 2) {
                    if (!streamer.has_u8())
                        return false;
                    byte = streamer.read_u8();
                }
            }
            currently_consuming = RLEState::PixelCount;
            break;
        }
    }

    VERIFY_NOT_REACHED();
}

static bool decode_bmp_pixel_data(BMPLoadingContext& context)
{
    if (context.state == BMPLoadingContext::State::Error)
        return false;

    if (context.state <= BMPLoadingContext::State::ColorTableDecoded && !decode_bmp_color_table(context))
        return false;

    const u16 bits_per_pixel = context.dib.core.bpp;

    BitmapFormat format = [&]() -> BitmapFormat {
        switch (bits_per_pixel) {
        case 1:
            return BitmapFormat::Indexed1;
        case 2:
            return BitmapFormat::Indexed2;
        case 4:
            return BitmapFormat::Indexed4;
        case 8:
            return BitmapFormat::Indexed8;
        case 16:
            if (context.dib.info.masks.size() == 4)
                return BitmapFormat::BGRA8888;
            return BitmapFormat::BGRx8888;
        case 24:
            return BitmapFormat::BGRx8888;
        case 32:
            return BitmapFormat::BGRA8888;
        default:
            return BitmapFormat::Invalid;
        }
    }();

    if (format == BitmapFormat::Invalid) {
        dbgln("BMP has invalid bpp of {}", bits_per_pixel);
        context.state = BMPLoadingContext::State::Error;
        return false;
    }

    const u32 width = abs(context.dib.core.width);
    const u32 height = abs(context.dib.core.height);
    context.bitmap = Bitmap::try_create(format, { static_cast<int>(width), static_cast<int>(height) });
    if (!context.bitmap) {
        dbgln("BMP appears to have overly large dimensions");
        return false;
    }

    ByteBuffer rle_buffer;
    ReadonlyBytes bytes { context.file_bytes + context.data_offset, context.file_size - context.data_offset };

    if (context.dib.info.compression == Compression::RLE4 || context.dib.info.compression == Compression::RLE8
        || context.dib.info.compression == Compression::RLE24) {
        if (!uncompress_bmp_rle_data(context, rle_buffer))
            return false;
        bytes = rle_buffer.bytes();
    }

    InputStreamer streamer(bytes.data(), bytes.size());

    auto process_row = [&](u32 row) -> bool {
        u32 space_remaining_before_consuming_row = streamer.remaining();

        for (u32 column = 0; column < width;) {
            switch (bits_per_pixel) {
            case 1: {
                if (!streamer.has_u8())
                    return false;
                u8 byte = streamer.read_u8();
                u8 mask = 8;
                while (column < width && mask > 0) {
                    mask -= 1;
                    context.bitmap->scanline_u8(row)[column++] = (byte >> mask) & 0x1;
                }
                break;
            }
            case 2: {
                if (!streamer.has_u8())
                    return false;
                u8 byte = streamer.read_u8();
                u8 mask = 8;
                while (column < width && mask > 0) {
                    mask -= 2;
                    context.bitmap->scanline_u8(row)[column++] = (byte >> mask) & 0x3;
                }
                break;
            }
            case 4: {
                if (!streamer.has_u8())
                    return false;
                u8 byte = streamer.read_u8();
                context.bitmap->scanline_u8(row)[column++] = (byte >> 4) & 0xf;
                if (column < width)
                    context.bitmap->scanline_u8(row)[column++] = byte & 0xf;
                break;
            }
            case 8:
                if (!streamer.has_u8())
                    return false;
                context.bitmap->scanline_u8(row)[column++] = streamer.read_u8();
                break;
            case 16: {
                if (!streamer.has_u16())
                    return false;
                context.bitmap->scanline(row)[column++] = int_to_scaled_rgb(context, streamer.read_u16());
                break;
            }
            case 24: {
                if (!streamer.has_u24())
                    return false;
                context.bitmap->scanline(row)[column++] = streamer.read_u24();
                break;
            }
            case 32:
                if (!streamer.has_u32())
                    return false;
                if (context.dib.info.masks.is_empty()) {
                    context.bitmap->scanline(row)[column++] = streamer.read_u32() | 0xff000000;
                } else {
                    context.bitmap->scanline(row)[column++] = int_to_scaled_rgb(context, streamer.read_u32());
                }
                break;
            }
        }

        auto consumed = space_remaining_before_consuming_row - streamer.remaining();

        // Calculate padding
        u8 bytes_to_drop = [consumed]() -> u8 {
            switch (consumed % 4) {
            case 0:
                return 0;
            case 1:
                return 3;
            case 2:
                return 2;
            case 3:
                return 1;
            }
            VERIFY_NOT_REACHED();
        }();
        if (streamer.remaining() < bytes_to_drop)
            return false;
        streamer.drop_bytes(bytes_to_drop);

        return true;
    };

    if (context.dib.core.height < 0) {
        // BMP is stored top-down
        for (u32 row = 0; row < height; ++row) {
            if (!process_row(row))
                return false;
        }
    } else {
        for (i32 row = height - 1; row >= 0; --row) {
            if (!process_row(row))
                return false;
        }
    }

    for (size_t i = 0; i < context.color_table.size(); ++i)
        context.bitmap->set_palette_color(i, Color::from_rgb(context.color_table[i]));

    context.state = BMPLoadingContext::State::PixelDataDecoded;

    return true;
}

static RefPtr<Bitmap> load_bmp_impl(const u8* data, size_t data_size)
{
    BMPLoadingContext context;
    context.file_bytes = data;
    context.file_size = data_size;

    // Forces a decode of the header, dib, and color table as well
    if (!decode_bmp_pixel_data(context)) {
        context.state = BMPLoadingContext::State::Error;
        return nullptr;
    }

    return context.bitmap;
}

BMPImageDecoderPlugin::BMPImageDecoderPlugin(const u8* data, size_t data_size)
{
    m_context = make<BMPLoadingContext>();
    m_context->file_bytes = data;
    m_context->file_size = data_size;
}

BMPImageDecoderPlugin::~BMPImageDecoderPlugin()
{
}

IntSize BMPImageDecoderPlugin::size()
{
    if (m_context->state == BMPLoadingContext::State::Error)
        return {};

    if (m_context->state < BMPLoadingContext::State::DIBDecoded && !decode_bmp_dib(*m_context))
        return {};

    return { m_context->dib.core.width, abs(m_context->dib.core.height) };
}

RefPtr<Gfx::Bitmap> BMPImageDecoderPlugin::bitmap()
{
    if (m_context->state == BMPLoadingContext::State::Error)
        return nullptr;

    if (m_context->state < BMPLoadingContext::State::PixelDataDecoded && !decode_bmp_pixel_data(*m_context))
        return nullptr;

    VERIFY(m_context->bitmap);
    return m_context->bitmap;
}

void BMPImageDecoderPlugin::set_volatile()
{
    if (m_context->bitmap)
        m_context->bitmap->set_volatile();
}

bool BMPImageDecoderPlugin::set_nonvolatile(bool& was_purged)
{
    if (!m_context->bitmap)
        return false;
    return m_context->bitmap->set_nonvolatile(was_purged);
}

bool BMPImageDecoderPlugin::sniff()
{
    return decode_bmp_header(*m_context);
}

bool BMPImageDecoderPlugin::is_animated()
{
    return false;
}

size_t BMPImageDecoderPlugin::loop_count()
{
    return 0;
}

size_t BMPImageDecoderPlugin::frame_count()
{
    return 1;
}

ImageFrameDescriptor BMPImageDecoderPlugin::frame(size_t i)
{
    if (i > 0)
        return {};
    return { bitmap(), 0 };
}

}