/*
 * Copyright (c) 2018-2025, Andreas Kling <andreas@ladybird.org>
 * Copyright (c) 2021, the SerenityOS developers.
 * Copyright (c) 2021-2025, Sam Atkins <sam@ladybird.org>
 * Copyright (c) 2024, Matthew Olsson <mattco@serenityos.org>
 * Copyright (c) 2025, Callum Law <callumlaw1709@outlook.com>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include "FontComputer.h"
#include <LibGfx/Font/FontDatabase.h>
#include <LibGfx/Font/WOFF/Loader.h>
#include <LibGfx/Font/WOFF2/Loader.h>
#include <LibWeb/CSS/CSSFontFaceRule.h>
#include <LibWeb/CSS/CSSFontFeatureValuesRule.h>
#include <LibWeb/CSS/CSSStyleSheet.h>
#include <LibWeb/CSS/ComputedProperties.h>
#include <LibWeb/CSS/Fetch.h>
#include <LibWeb/CSS/FontFace.h>
#include <LibWeb/CSS/FontFaceSet.h>
#include <LibWeb/CSS/StyleValues/CustomIdentStyleValue.h>
#include <LibWeb/CSS/StyleValues/KeywordStyleValue.h>
#include <LibWeb/CSS/StyleValues/StringStyleValue.h>
#include <LibWeb/CSS/StyleValues/StyleValueList.h>
#include <LibWeb/DOM/Document.h>
#include <LibWeb/DOM/Element.h>
#include <LibWeb/DOM/ShadowRoot.h>
#include <LibWeb/Fetch/Infrastructure/HTTP/MIME.h>
#include <LibWeb/Fetch/Response.h>
#include <LibWeb/MimeSniff/Resource.h>
#include <LibWeb/Platform/FontPlugin.h>

namespace Web::CSS {

GC_DEFINE_ALLOCATOR(FontComputer);
GC_DEFINE_ALLOCATOR(FontLoader);

}

namespace AK {

template<>
struct Traits<Web::CSS::FontFaceKey> : public DefaultTraits<Web::CSS::FontFaceKey> {
    static unsigned hash(Web::CSS::FontFaceKey const& key) { return key.hash(); }
};

template<>
struct Traits<Web::CSS::ComputedFontCacheKey> : public DefaultTraits<Web::CSS::ComputedFontCacheKey> {
    static unsigned hash(Web::CSS::ComputedFontCacheKey const& key)
    {
        unsigned hash = 0;
        for (auto const& family_value : key.font_family->as_value_list().values()) {
            if (family_value->is_keyword())
                hash = pair_int_hash(hash, to_underlying(family_value->as_keyword().keyword()));
            else
                hash = string_from_style_value(family_value).hash();
        }

        hash = pair_int_hash(hash, to_underlying(key.font_optical_sizing));
        hash = pair_int_hash(hash, Traits<Web::CSSPixels>::hash(key.font_size));
        hash = pair_int_hash(hash, key.font_slope);
        hash = pair_int_hash(hash, Traits<double>::hash(key.font_weight));
        hash = pair_int_hash(hash, Traits<double>::hash(key.font_width.value()));
        for (auto const& [variation_name, variation_value] : key.font_variation_settings)
            hash = pair_int_hash(hash, pair_int_hash(variation_name.hash(), Traits<double>::hash(variation_value)));
        hash = pair_int_hash(hash, Traits<Web::CSS::FontFeatureData>::hash(key.font_feature_data));

        return hash;
    }
};

}

namespace Web::CSS {

FontLoader::FontLoader(FontComputer& font_computer, RuleOrDeclaration rule_or_declaration, FlyString family_name, Vector<Gfx::UnicodeRange> unicode_ranges, Vector<URL> urls, GC::Ptr<GC::Function<void(RefPtr<Gfx::Typeface const>)>> on_load)
    : m_font_computer(font_computer)
    , m_rule_or_declaration(rule_or_declaration)
    , m_family_name(move(family_name))
    , m_unicode_ranges(move(unicode_ranges))
    , m_urls(move(urls))
    , m_on_load(move(on_load))
{
}

FontLoader::~FontLoader() = default;

void FontLoader::visit_edges(Visitor& visitor)
{
    Base::visit_edges(visitor);
    visitor.visit(m_font_computer);
    if (auto* rule = m_rule_or_declaration.value.get_pointer<RuleOrDeclaration::Rule>())
        visitor.visit(rule->parent_style_sheet);
    else if (auto* block = m_rule_or_declaration.value.get_pointer<RuleOrDeclaration::StyleDeclaration>())
        visitor.visit(block->parent_rule);
    visitor.visit(m_fetch_controller);
    visitor.visit(m_on_load);
}

bool FontLoader::is_loading() const
{
    return m_fetch_controller && !m_typeface;
}

RefPtr<Gfx::Font const> FontLoader::font_with_point_size(float point_size, Gfx::FontVariationSettings const& variations, Gfx::ShapeFeatures const& shape_features)
{
    if (!m_typeface) {
        if (!m_fetch_controller)
            start_loading_next_url();
        return nullptr;
    }
    return m_typeface->font(point_size, variations, shape_features);
}

void FontLoader::start_loading_next_url()
{
    // FIXME: Load local() fonts somehow.
    if (m_fetch_controller && m_fetch_controller->state() == Fetch::Infrastructure::FetchController::State::Ongoing)
        return;
    if (m_urls.is_empty())
        return;

    // https://drafts.csswg.org/css-fonts-4/#fetch-a-font
    // To fetch a font given a selected <url> url for @font-face rule, fetch url, with ruleOrDeclaration being rule,
    // destination "font", CORS mode "cors", and processResponse being the following steps given response res and null,
    // failure or a byte stream stream:
    m_fetch_controller = fetch_a_style_resource(m_urls.take_first(), m_rule_or_declaration, Fetch::Infrastructure::Request::Destination::Font, CorsMode::Cors,
        [loader = this](auto response, auto stream) {
            // 1. If stream is null, return.
            // 2. Load a font from stream according to its type.

            // NB: We need to fetch the next source if this one fails to fetch OR decode. So, first try to decode it.
            RefPtr<Gfx::Typeface const> typeface;
            if (auto* bytes = stream.template get_pointer<ByteBuffer>()) {
                if (auto maybe_typeface = loader->try_load_font(response, *bytes); !maybe_typeface.is_error())
                    typeface = maybe_typeface.release_value();
            }

            if (!typeface) {
                // NB: If we have other sources available, try the next one.
                if (loader->m_urls.is_empty()) {
                    loader->font_did_load_or_fail(nullptr);
                } else {
                    loader->m_fetch_controller = nullptr;
                    loader->start_loading_next_url();
                }
            } else {
                loader->font_did_load_or_fail(move(typeface));
            }
        });

    if (!m_fetch_controller)
        font_did_load_or_fail(nullptr);
}

void FontLoader::font_did_load_or_fail(RefPtr<Gfx::Typeface const> typeface)
{
    if (typeface) {
        m_typeface = typeface.release_nonnull();
        m_font_computer->clear_computed_font_cache(m_family_name);
        if (m_on_load)
            m_on_load->function()(m_typeface);
    } else {
        if (m_on_load)
            m_on_load->function()(nullptr);
    }
    m_fetch_controller = nullptr;
}

ErrorOr<NonnullRefPtr<Gfx::Typeface const>> FontLoader::try_load_font(Fetch::Infrastructure::Response const& response, ByteBuffer const& bytes)
{
    // FIXME: This could maybe use the format() provided in @font-face as well, since often the mime type is just application/octet-stream and we have to try every format
    auto mime_type = Fetch::Infrastructure::extract_mime_type(response.header_list());
    if (!mime_type.has_value() || !mime_type->is_font()) {
        mime_type = MimeSniff::Resource::sniff(bytes, MimeSniff::SniffingConfiguration { .sniffing_context = MimeSniff::SniffingContext::Font });
    }
    if (mime_type.has_value()) {
        if (mime_type->essence() == "font/ttf"sv || mime_type->essence() == "application/x-font-ttf"sv || mime_type->essence() == "font/otf"sv) {
            if (auto result = Gfx::Typeface::try_load_from_temporary_memory(bytes); !result.is_error()) {
                return result;
            }
        }
        if (mime_type->essence() == "font/woff"sv || mime_type->essence() == "application/font-woff"sv) {
            if (auto result = WOFF::try_load_from_bytes(bytes); !result.is_error()) {
                return result;
            }
        }
        if (mime_type->essence() == "font/woff2"sv || mime_type->essence() == "application/font-woff2"sv) {
            if (auto result = WOFF2::try_load_from_bytes(bytes); !result.is_error()) {
                return result;
            }
        }
    }

    return Error::from_string_literal("Automatic format detection failed");
}

struct FontComputer::MatchingFontCandidate {
    FontFaceKey key;
    Gfx::Typeface const* system_typeface { nullptr };

    [[nodiscard]] RefPtr<Gfx::FontCascadeList const> font_with_point_size(HashMap<FontFaceKey, Vector<GC::Ref<FontFace>>> const& font_faces, float point_size, Gfx::FontVariationSettings const& variations, FontFeatureData const& font_feature_data, HashMap<FontFeatureValueKey, Vector<u32>> const& font_feature_values) const
    {
        auto const& shape_features = font_feature_data.to_shape_features(font_feature_values);

        if (system_typeface) {
            auto font_list = Gfx::FontCascadeList::create();
            font_list->add(system_typeface->font(point_size, variations, shape_features));
            return font_list;
        }

        auto it = font_faces.find(key);
        if (it == font_faces.end())
            return {};

        auto font_list = Gfx::FontCascadeList::create();
        for (auto const& face : it->value) {
            if (auto face_fonts = face->font_with_point_size(point_size, variations, shape_features))
                font_list->extend(*face_fonts);
        }
        if (font_list->is_empty())
            return {};
        return font_list;
    }
};

void FontComputer::visit_edges(Visitor& visitor)
{
    Base::visit_edges(visitor);
    visitor.visit(m_document);
    for (auto& [_, faces] : m_font_faces)
        visitor.visit(faces);
}

RefPtr<Gfx::FontCascadeList const> FontComputer::find_matching_font_weight_ascending(Vector<MatchingFontCandidate> const& candidates, int target_weight, float font_size_in_pt, Gfx::FontVariationSettings const& variations, FontFeatureData const& font_feature_data, HashMap<FontFeatureValueKey, Vector<u32>> const& font_feature_values, bool inclusive) const
{
    using Fn = AK::Function<bool(MatchingFontCandidate const&)>;
    auto pred = inclusive ? Fn([&](auto const& matching_font_candidate) { return matching_font_candidate.key.weight.min >= target_weight; })
                          : Fn([&](auto const& matching_font_candidate) { return matching_font_candidate.key.weight.min > target_weight; });
    auto it = find_if(candidates.begin(), candidates.end(), pred);
    for (; it != candidates.end(); ++it) {
        if (auto found_font = it->font_with_point_size(m_font_faces, font_size_in_pt, variations, font_feature_data, font_feature_values))
            return found_font;
    }
    return {};
}

RefPtr<Gfx::FontCascadeList const> FontComputer::find_matching_font_weight_descending(Vector<MatchingFontCandidate> const& candidates, int target_weight, float font_size_in_pt, Gfx::FontVariationSettings const& variations, FontFeatureData const& font_feature_data, HashMap<FontFeatureValueKey, Vector<u32>> const& font_feature_values, bool inclusive) const
{
    using Fn = AK::Function<bool(MatchingFontCandidate const&)>;
    auto pred = inclusive ? Fn([&](auto const& matching_font_candidate) { return matching_font_candidate.key.weight.max <= target_weight; })
                          : Fn([&](auto const& matching_font_candidate) { return matching_font_candidate.key.weight.max < target_weight; });
    auto it = find_if(candidates.rbegin(), candidates.rend(), pred);
    for (; it != candidates.rend(); ++it) {
        if (auto found_font = it->font_with_point_size(m_font_faces, font_size_in_pt, variations, font_feature_data, font_feature_values))
            return found_font;
    }
    return {};
}

// Partial implementation of the font-matching algorithm: https://www.w3.org/TR/css-fonts-4/#font-matching-algorithm
// FIXME: This should be replaced by the full CSS font selection algorithm.
RefPtr<Gfx::FontCascadeList const> FontComputer::font_matching_algorithm(FlyString const& family_name, int weight, int slope, float font_size_in_pt, Gfx::FontVariationSettings const& variations, FontFeatureData const& font_feature_data, HashMap<FontFeatureValueKey, Vector<u32>> const& font_feature_values) const
{
    // If a font family match occurs, the user agent assembles the set of font faces in that family and then
    // narrows the set to a single face using other font properties in the order given below.
    Vector<MatchingFontCandidate> matching_family_fonts;
    // FIXME: URL-backed faces with no typeface yet should trigger a load on demand, matching other engines.
    for (auto const& [map_key, faces] : m_font_faces) {
        if (map_key.family_name.equals_ignoring_ascii_case(family_name))
            matching_family_fonts.empend(map_key);
    }
    Gfx::FontDatabase::the().for_each_typeface_with_family_name(family_name, [&](Gfx::Typeface const& typeface) {
        matching_family_fonts.append({
            .key = {
                .family_name = typeface.family(),
                // FIXME: Support system fonts that have a range of weights, etc.
                .weight = { static_cast<int>(typeface.weight()), static_cast<int>(typeface.weight()) },
                .slope = typeface.slope(),
            },
            .system_typeface = &typeface,
        });
    });

    if (matching_family_fonts.is_empty())
        return {};

    quick_sort(matching_family_fonts, [](auto const& a, auto const& b) {
        return a.key.weight.min < b.key.weight.min;
    });
    // FIXME: 1. font-width is tried first.
    // FIXME: 2. font-style is tried next.
    // We don't have complete support of italic and oblique fonts, so matching on font-style can be simplified to:
    // If a matching slope is found, all faces which don't have that matching slope are excluded from the matching set.
    auto style_it = find_if(matching_family_fonts.begin(), matching_family_fonts.end(),
        [&](auto const& matching_font_candidate) { return matching_font_candidate.key.slope == slope; });
    if (style_it != matching_family_fonts.end()) {
        matching_family_fonts.remove_all_matching([&](auto const& matching_font_candidate) {
            return matching_font_candidate.key.slope != slope;
        });
    }
    // 3. font-weight is matched next.
    // If a font does not have any concept of varying strengths of weights, its weight is mapped according list in the
    // property definition. If bolder/lighter relative weights are used, the effective weight is calculated based on
    // the inherited weight value, as described in the definition of the font-weight property.
    // FIXME: "varying strengths of weights"
    // If the matching set after performing the steps above includes faces with weight values containing the
    // font-weight desired value, faces with weight values which do not include the desired font-weight value are
    // removed from the matching set.

    // FIXME: This whole function currently just returns the first match instead of progressing further, so we'll do that here too.
    auto matching_weight_it = matching_family_fonts.find_if([weight](auto const& candidate) {
        return candidate.key.weight.contains_inclusive(weight);
    });
    for (; matching_weight_it != matching_family_fonts.end(); ++matching_weight_it) {
        if (auto found_font = matching_weight_it->font_with_point_size(m_font_faces, font_size_in_pt, variations, font_feature_data, font_feature_values))
            return found_font;
    }

    // If there is no face which contains the desired value, a weight value is chosen using the rules below:

    // - If the desired weight is inclusively between 400 and 500, weights greater than or equal to the target weight
    //   are checked in ascending order until 500 is hit and checked, followed by weights less than the target weight
    //   in descending order, followed by weights greater than 500, until a match is found.
    if (weight >= 400 && weight <= 500) {
        auto it = find_if(matching_family_fonts.begin(), matching_family_fonts.end(),
            [&](auto const& matching_font_candidate) { return matching_font_candidate.key.weight.min >= weight; });
        for (; it != matching_family_fonts.end() && it->key.weight.min <= 500; ++it) {
            if (auto found_font = it->font_with_point_size(m_font_faces, font_size_in_pt, variations, font_feature_data, font_feature_values))
                return found_font;
        }
        if (auto found_font = find_matching_font_weight_descending(matching_family_fonts, weight, font_size_in_pt, variations, font_feature_data, font_feature_values, false))
            return found_font;
        for (; it != matching_family_fonts.end(); ++it) {
            if (auto found_font = it->font_with_point_size(m_font_faces, font_size_in_pt, variations, font_feature_data, font_feature_values))
                return found_font;
        }
    }
    // - If the desired weight is less than 400, weights less than or equal to the desired weight are checked in
    //   descending order followed by weights above the desired weight in ascending order until a match is found.
    if (weight < 400) {
        if (auto found_font = find_matching_font_weight_descending(matching_family_fonts, weight, font_size_in_pt, variations, font_feature_data, font_feature_values, true))
            return found_font;
        if (auto found_font = find_matching_font_weight_ascending(matching_family_fonts, weight, font_size_in_pt, variations, font_feature_data, font_feature_values, false))
            return found_font;
    }
    // - If the desired weight is greater than 500, weights greater than or equal to the desired weight are checked in
    //   ascending order followed by weights below the desired weight in descending order until a match is found.
    if (weight > 500) {
        if (auto found_font = find_matching_font_weight_ascending(matching_family_fonts, weight, font_size_in_pt, variations, font_feature_data, font_feature_values, true))
            return found_font;
        if (auto found_font = find_matching_font_weight_descending(matching_family_fonts, weight, font_size_in_pt, variations, font_feature_data, font_feature_values, false))
            return found_font;
    }

    return {};
}

HashMap<FontFeatureValueKey, Vector<u32>> const& FontComputer::font_feature_values_for_family(FlyString const& family_name) const
{
    return m_font_feature_values_cache.ensure(family_name, [&]() {
        HashMap<FontFeatureValueKey, Vector<u32>> font_feature_values;

        // https://drafts.csswg.org/css-fonts/#font-feature-values-syntax
        // For each <family-name> in the @font-feature-values prelude, each font feature value declaration defines a
        // mapping between a (family name, feature block name, declaration name) tuple and the list of one or more
        // integers from the declaration’s value. If the same tuple appears more than once in a document (such as if a
        // single block), the last-defined one is used.

        // FIXME: We only account for Author stylesheets here, we should also account for UserAgent and User
        m_document->style_scope().for_each_active_css_style_sheet([&](CSS::CSSStyleSheet const& sheet) {
            // FIXME: Account for @font-feature-values within grouping (i.e. @media, @supports) rules as well.
            for (auto const& rule : sheet.rules()) {
                if (auto const* font_feature_values_rule = as_if<CSSFontFeatureValuesRule>(*rule)) {
                    if (!font_feature_values_rule->font_families().contains_slow(family_name))
                        continue;

                    font_feature_values.update(font_feature_values_rule->to_hash_map());
                }
            }
        });

        return font_feature_values;
    });

    return m_font_feature_values_cache.get(family_name).value();
}

NonnullRefPtr<Gfx::FontCascadeList const> FontComputer::compute_font_for_style_values(StyleValue const& font_family, CSSPixels const& font_size, int font_slope, double font_weight, Percentage const& font_width, FontOpticalSizing font_optical_sizing, HashMap<FlyString, double> const& font_variation_settings, FontFeatureData const& font_feature_data) const
{
    ComputedFontCacheKey cache_key {
        .font_family = font_family,
        .font_optical_sizing = font_optical_sizing,
        .font_size = font_size,
        .font_slope = font_slope,
        .font_weight = font_weight,
        .font_width = font_width,
        .font_variation_settings = font_variation_settings,
        .font_feature_data = font_feature_data,
    };

    return m_computed_font_cache.ensure(cache_key, [&]() {
        return compute_font_for_style_values_impl(font_family, font_size, font_slope, font_weight, font_width, font_optical_sizing, font_variation_settings, font_feature_data);
    });
}

NonnullRefPtr<Gfx::FontCascadeList const> FontComputer::compute_font_for_style_values_impl(StyleValue const& font_family, CSSPixels const& font_size, int slope, double font_weight, Percentage const& font_width, FontOpticalSizing font_optical_sizing, HashMap<FlyString, double> const& font_variation_settings, FontFeatureData const& font_feature_data) const
{
    // FIXME: We round to int here as that is what is expected by our font infrastructure below
    auto weight = round_to<int>(font_weight);

    // FIXME: We need to respect `font-size-adjust` once that is implemented.
    auto font_size_used_value = font_size.to_float();

    Gfx::FontVariationSettings variation;
    variation.set_weight(font_weight);
    variation.set_width(font_width.value());

    // NB: The spec recommends that we use the 'used value' of font-size for 'opsz' when font-optical-sizing is 'auto'.
    // FIXME: User agents must not select a value for the "opsz" axis which is not supported by the font used for
    //        rendering the text. This can be accomplished by clamping a chosen value to the range supported by the
    //        font. https://drafts.csswg.org/css-fonts/#font-optical-sizing-def
    if (font_optical_sizing == FontOpticalSizing::Auto)
        variation.set_optical_sizing(font_size_used_value);

    for (auto const& [tag_string, value] : font_variation_settings) {
        auto string_view = tag_string.bytes_as_string_view();
        if (string_view.length() != 4)
            continue;

        auto tag = Gfx::FourCC(string_view.characters_without_null_termination());

        variation.axes.set(tag, value);
    }

    // FIXME: Implement the full font-matching algorithm: https://www.w3.org/TR/css-fonts-4/#font-matching-algorithm
    float const font_size_in_pt = font_size_used_value * 0.75f;

    auto find_font = [&](FlyString const& family) -> RefPtr<Gfx::FontCascadeList const> {
        auto const& font_feature_values = font_feature_values_for_family(family);

        // OPTIMIZATION: Look for an exact match in loaded fonts first.
        // FIXME: Respect the other font-* descriptors
        FontFaceKey lookup_key {
            .family_name = family,
            .weight = { weight, weight },
            .slope = slope,
        };
        if (auto it = m_font_faces.find(lookup_key); it != m_font_faces.end()) {
            auto shape_features = font_feature_data.to_shape_features(font_feature_values);
            auto result = Gfx::FontCascadeList::create();
            for (auto const& font_face : it->value) {
                if (auto face_fonts = font_face->font_with_point_size(font_size_in_pt, variation, shape_features))
                    result->extend(*face_fonts);
            }
            if (!result->is_empty())
                return result;
        }

        if (auto found_font = font_matching_algorithm(family, weight, slope, font_size_in_pt, variation, font_feature_data, font_feature_values); found_font && !found_font->is_empty())
            return found_font;

        return {};
    };

    auto find_generic_font = [&](Keyword font_id) -> RefPtr<Gfx::FontCascadeList const> {
        Platform::GenericFont generic_font {};
        switch (font_id) {
        case Keyword::Monospace:
        case Keyword::UiMonospace:
            generic_font = Platform::GenericFont::Monospace;
            break;
        case Keyword::Serif:
            generic_font = Platform::GenericFont::Serif;
            break;
        case Keyword::Fantasy:
            generic_font = Platform::GenericFont::Fantasy;
            break;
        case Keyword::SansSerif:
            generic_font = Platform::GenericFont::SansSerif;
            break;
        case Keyword::Cursive:
            generic_font = Platform::GenericFont::Cursive;
            break;
        case Keyword::UiSerif:
            generic_font = Platform::GenericFont::UiSerif;
            break;
        case Keyword::UiSansSerif:
            generic_font = Platform::GenericFont::UiSansSerif;
            break;
        case Keyword::UiRounded:
            generic_font = Platform::GenericFont::UiRounded;
            break;
        default:
            return {};
        }
        return find_font(Platform::FontPlugin::the().generic_font_name(generic_font, weight, slope));
    };

    auto font_list = Gfx::FontCascadeList::create();

    for (auto const& family : font_family.as_value_list().values()) {
        RefPtr<Gfx::FontCascadeList const> other_font_list;
        if (family->is_keyword()) {
            other_font_list = find_generic_font(family->to_keyword());
        } else {
            other_font_list = find_font(string_from_style_value(family));
        }

        if (other_font_list)
            font_list->extend(*other_font_list);
    }

    // NB: @font-feature-values can't apply to the default font since it's not loaded from CSS
    auto default_font = Platform::FontPlugin::the().default_font(font_size_in_pt, variation, font_feature_data.to_shape_features({}));
    if (font_list->is_empty()) {
        // This is needed to make sure we check default font before reaching to emojis.
        font_list->add(*default_font);
    }

    // Add emoji and symbol fonts
    for (auto font_name : Platform::FontPlugin::the().symbol_font_names()) {
        if (auto other_font_list = find_font(font_name)) {
            font_list->extend(*other_font_list);
        }
    }

    // The default font is already included in the font list, but we explicitly set it
    // as the last-resort font. This ensures that if none of the specified fonts contain
    // the requested code point, there is still a font available to provide a fallback glyph.
    font_list->set_last_resort_font(*default_font);

    if (!Platform::FontPlugin::the().is_layout_test_mode()) {
        font_list->set_system_font_fallback_callback([](u32 code_point, Gfx::Font const& reference_font) -> RefPtr<Gfx::Font const> {
            return Gfx::FontDatabase::the().get_font_for_code_point(
                code_point,
                reference_font.point_size(),
                reference_font.weight(),
                reference_font.typeface().width(),
                reference_font.slope());
        });
    }

    return font_list;
}

Gfx::Font const& FontComputer::initial_font() const
{
    // FIXME: This is not correct.
    static auto font = ComputedProperties::font_fallback(false, false, 12);
    return font;
}

static bool style_value_references_font_family(StyleValue const& font_family_value, FlyString const& family_name)
{
    if (!font_family_value.is_value_list())
        return false;

    for (auto const& item : font_family_value.as_value_list().values()) {
        if (item->is_keyword())
            continue; // Skip generic keywords (monospace, serif, etc.)

        FlyString item_family_name = string_from_style_value(*item);

        if (item_family_name.equals_ignoring_ascii_case(family_name))
            return true;
    }
    return false;
}

void FontComputer::clear_computed_font_cache(FlyString const& family_name)
{
    // Only clear cache entries that reference the loaded font family.
    m_computed_font_cache.remove_all_matching([&](auto const& key, auto const&) {
        return style_value_references_font_family(key.font_family, family_name);
    });

    auto element_uses_font_family = [&](DOM::Element const& element) {
        // Check the element's own font-family.
        if (auto style = element.computed_properties()) {
            if (style_value_references_font_family(style->property(PropertyID::FontFamily), family_name))
                return true;
        }

        // Check pseudo-elements, which may use a different font-family than the element itself.
        for (size_t i = 0; i < to_underlying(PseudoElement::KnownPseudoElementCount); ++i) {
            if (auto style = element.computed_properties(static_cast<PseudoElement>(i))) {
                if (style_value_references_font_family(style->property(PropertyID::FontFamily), family_name))
                    return true;
            }
        }

        return false;
    };

    if (document().needs_full_style_update())
        return;

    // Walk the DOM tree (including shadow trees) and invalidate elements that use this font family.
    document().for_each_shadow_including_inclusive_descendant([&](DOM::Node& node) {
        auto* element = as_if<DOM::Element>(node);
        if (!element)
            return TraversalDecision::Continue;

        // If this element's subtree is already marked for style update, skip the entire subtree.
        if (element->entire_subtree_needs_style_update())
            return TraversalDecision::SkipChildrenAndContinue;

        // If this element already needs a style update, check descendants but don't re-check this element.
        if (element->needs_style_update())
            return TraversalDecision::Continue;

        if (element_uses_font_family(*element)) {
            element->set_needs_style_update(true);
            return TraversalDecision::Continue;
        }

        return TraversalDecision::Continue;
    });
}

void FontComputer::clear_font_feature_values_cache(FlyString const& family_name)
{
    m_font_feature_values_cache.remove(family_name);
}

void FontComputer::did_load_font(FlyString const& family_name)
{
    clear_computed_font_cache(family_name);
}

void FontComputer::register_font_face(GC::Ref<FontFace> face)
{
    VERIFY(face->should_be_registered_with_font_computer());

    FontFaceKey key {
        .family_name = FlyString(face->family()),
        .weight = face->declared_weight_range(),
        .slope = face->declared_slope(),
    };
    auto& faces = m_font_faces.ensure(key);
    if (!faces.contains_slow(face))
        faces.append(face);
    did_load_font(key.family_name);
}

void FontComputer::unregister_font_face(GC::Ref<FontFace> face)
{
    VERIFY(face->should_be_registered_with_font_computer());

    FontFaceKey key {
        .family_name = FlyString(face->family()),
        .weight = face->declared_weight_range(),
        .slope = face->declared_slope(),
    };
    if (auto it = m_font_faces.find(key); it != m_font_faces.end()) {
        it->value.remove_all_matching([&](auto const& entry) { return entry == face; });
        if (it->value.is_empty())
            m_font_faces.remove(it);
    }
    did_load_font(key.family_name);
}

GC::Ptr<FontLoader> FontComputer::load_font_face(ParsedFontFace const& font_face, GC::Ptr<GC::Function<void(RefPtr<Gfx::Typeface const>)>> on_load)
{
    if (font_face.sources().is_empty()) {
        if (on_load)
            on_load->function()({});
        return {};
    }

    // FIXME: Handle local() font sources.
    Vector<URL> urls;
    for (auto const& source : font_face.sources()) {
        if (source.local_or_url.has<URL>())
            urls.append(source.local_or_url.get<URL>());
    }

    if (urls.is_empty()) {
        if (on_load)
            on_load->function()({});
        return {};
    }

    RuleOrDeclaration rule_or_declaration {
        .environment_settings_object = document().relevant_settings_object(),
        .value = RuleOrDeclaration::Rule {
            .parent_style_sheet = font_face.parent_rule()->parent_style_sheet(),
        }
    };

    return heap().allocate<FontLoader>(*this, rule_or_declaration, font_face.font_family(), font_face.unicode_ranges(), move(urls), move(on_load));
}

void FontComputer::load_fonts_from_sheet(CSSStyleSheet& sheet)
{
    // FIXME: Handle @font-face and @font-feature-values within grouping rules (@media, @supports, etc)
    for (auto const& rule : sheet.rules()) {
        if (auto* font_face_rule = as_if<CSSFontFaceRule>(*rule)) {
            if (!font_face_rule->is_valid())
                continue;

            auto font_face = FontFace::create_css_connected(document().realm(), *font_face_rule);
            document().fonts()->add_css_connected_font(font_face);

            // NB: Load via FontFace::load(), to satisfy this requirement:
            // https://drafts.csswg.org/css-font-loading/#font-face-load
            // User agents can initiate font loads on their own, whenever they determine that a given font face is
            // necessary to render something on the page. When this happens, they must act as if they had called the
            // corresponding FontFace’s load() method described here.
            font_face->load();
        }

        if (auto* font_feature_values_rule = as_if<CSSFontFeatureValuesRule>(*rule))
            font_feature_values_rule->clear_caches();
    }
}

void FontComputer::unload_fonts_from_sheet(CSSStyleSheet& sheet)
{
    // FIXME: Handle @font-face and @font-feature-values within grouping rules (@media, @supports, etc)
    // https://drafts.csswg.org/css-font-loading/#font-face-css-connection
    // If a @font-face rule is removed from the document, its connected FontFace object is no longer CSS-connected.
    for (auto const& rule : sheet.rules()) {
        if (auto* font_face_rule = as_if<CSSFontFaceRule>(*rule)) {
            if (auto font_face = font_face_rule->css_connected_font_face())
                unregister_font_face(*font_face);
            font_face_rule->disconnect_font_face();
        }

        if (auto* font_feature_values_rule = as_if<CSSFontFeatureValuesRule>(*rule))
            font_feature_values_rule->clear_caches();
    }
}

}