ladybird/Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateNamedCharacterReferences.cpp

/*
 * Copyright (c) 2024, the SerenityOS developers.
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include "GeneratorUtil.h"
#include <AK/Array.h>
#include <AK/CharacterTypes.h>
#include <AK/FixedArray.h>
#include <AK/SourceGenerator.h>
#include <AK/StringBuilder.h>
#include <LibCore/ArgsParser.h>
#include <LibMain/Main.h>

// The goal is to encode the necessary data compactly while still allowing for fast matching of
// named character references, and taking full advantage of the note in the spec[1] that:
//
// > This list [of named character references] is static and will not be expanded or changed in the future.
//
// An overview of the approach taken (see [2] for more background/context):
//
// First, a deterministic acyclic finite state automaton (DAFSA) [3] is constructed from the set of
// named character references. The nodes in the DAFSA are populated with a "number" field that
// represents the count of all possible valid words from that node. This "number" field allows for
// minimal perfect hashing, where each word in the set corresponds to a unique index. The unique
// index of a word in the set is calculated during traversal/search of the DAFSA:
// - For any non-matching node that is iterated when searching a list of children, add their number
//   to the unique index
// - For nodes that match the current character, if the node is a valid end-of-word, add 1 to the
//   unique index
// Note that "searching a list of children" is assumed to use a linear scan, so, for example, if
// a list of children contained 'a', 'b', 'c', and 'd' (in that order), and the character 'c' was
// being searched for, then the "number" of both 'a' and 'b' would get added to the unique index,
// and then 1 would be added after matching 'c' (this minimal perfect hashing strategy comes from [4]).
//
// Something worth noting is that a DAFSA can be used with the set of named character references
// (with minimal perfect hashing) while keeping the nodes of the DAFSA <= 32-bits. This is a property
// that really matters, since any increase over 32-bits would immediately double the size of the data
// due to padding bits when storing the nodes in a contiguous array.
//
// There are also a few modifications made to the DAFSA to increase performance:
// - The 'first layer' of nodes is extracted out and replaced with a lookup table. This turns
//   the search for the first character from O(n) to O(1), and doesn't increase the data size because
//   all first characters in the set of named character references have the values 'a'-'z'/'A'-'Z',
//   so a lookup array of exactly 52 elements can be used. The lookup table stores the cumulative
//   "number" fields that would be calculated by a linear scan that matches a given node, thus allowing
//   the unique index to be built-up as normal with a O(1) search instead of a linear scan.
// - The 'second layer' of nodes is also extracted out and searches of the second layer are done
//   using a bit field of 52 bits (the set bits of the bit field depend on the first character's value),
//   where each set bit corresponds to one of 'a'-'z'/'A'-'Z' (similar to the first layer, the second
//   layer can only contain ASCII alphabetic characters). The bit field is then re-used (along with
//   an offset) to get the index into the array of second layer nodes. This technique ultimately
//   allows for storing the minimum number of nodes in the second layer, and therefore only increasing the
//   size of the data by the size of the 'first to second layer link' info which is 52 * 8 = 416 bytes.
// - After the second layer, the rest of the data is stored using a mostly-normal DAFSA, but there
//   are still a few differences:
//    - The "number" field is cumulative, in the same way that the first/second layer store a
//      cumulative "number" field. This cuts down slightly on the amount of work done during
//      the search of a list of children, and we can get away with it because the cumulative
//      "number" fields of the remaining nodes in the DAFSA (after the first and second layer
//      nodes were extracted out) happens to require few enough bits that we can store the
//      cumulative version while staying under our 32-bit budget.
//    - Instead of storing a 'last sibling' flag to denote the end of a list of children, the
//      length of each node's list of children is stored. Again, this is mostly done just because
//      there are enough bits available to do so while keeping the DAFSA node within 32 bits.
//    - Note: Together, these modifications open up the possibility of using a binary search instead
//      of a linear search over the children, but due to the consistently small lengths of the lists
//      of children in the remaining DAFSA, a linear search actually seems to be the better option.
//
// [1]: https://html.spec.whatwg.org/multipage/named-characters.html#named-character-references
// [2]: https://www.ryanliptak.com/blog/better-named-character-reference-tokenization/
// [3]: https://en.wikipedia.org/wiki/Deterministic_acyclic_finite_state_automaton
// [4]: Applications of finite automata representing large vocabularies (Cláudio L. Lucchesi,
//      Tomasz Kowaltowski, 1993) https://doi.org/10.1002/spe.4380230103

ErrorOr<void> generate_header_file(Core::File& file);
ErrorOr<void> generate_implementation_file(JsonObject& named_character_reference_data, Core::File& file);

ErrorOr<int> ladybird_main(Main::Arguments arguments)
{
    StringView generated_header_path;
    StringView generated_implementation_path;
    StringView json_path;

    Core::ArgsParser args_parser;
    args_parser.add_option(generated_header_path, "Path to the Entities header file to generate", "generated-header-path", 'h', "generated-header-path");
    args_parser.add_option(generated_implementation_path, "Path to the Entities implementation file to generate", "generated-implementation-path", 'c', "generated-implementation-path");
    args_parser.add_option(json_path, "Path to the JSON file to read from", "json-path", 'j', "json-path");
    args_parser.parse(arguments);

    auto json = TRY(read_entire_file_as_json(json_path));
    VERIFY(json.is_object());
    auto named_character_reference_data = json.as_object();

    auto generated_header_file = TRY(Core::File::open(generated_header_path, Core::File::OpenMode::Write));
    auto generated_implementation_file = TRY(Core::File::open(generated_implementation_path, Core::File::OpenMode::Write));

    TRY(generate_header_file(*generated_header_file));
    TRY(generate_implementation_file(named_character_reference_data, *generated_implementation_file));

    return 0;
}

struct Codepoints {
    u32 first;
    u32 second;
};

inline static StringView get_second_codepoint_enum_name(u32 codepoint)
{
    switch (codepoint) {
    case 0x0338:
        return "CombiningLongSolidusOverlay"sv;
    case 0x20D2:
        return "CombiningLongVerticalLineOverlay"sv;
    case 0x200A:
        return "HairSpace"sv;
    case 0x0333:
        return "CombiningDoubleLowLine"sv;
    case 0x20E5:
        return "CombiningReverseSolidusOverlay"sv;
    case 0xFE00:
        return "VariationSelector1"sv;
    case 0x006A:
        return "LatinSmallLetterJ"sv;
    case 0x0331:
        return "CombiningMacronBelow"sv;
    default:
        return "None"sv;
    }
}

ErrorOr<void> generate_header_file(Core::File& file)
{
    StringBuilder builder;
    SourceGenerator generator { builder };
    generator.append(R"~~~(
#pragma once

#include <AK/Optional.h>
#include <AK/Types.h>

namespace Web::HTML {

// Uses u32 to match the `first` field of NamedCharacterReferenceCodepoints for bit-field packing purposes.
enum class NamedCharacterReferenceSecondCodepoint : u32 {
    None,
    CombiningLongSolidusOverlay, // U+0338
    CombiningLongVerticalLineOverlay, // U+20D2
    HairSpace, // U+200A
    CombiningDoubleLowLine, // U+0333
    CombiningReverseSolidusOverlay, // U+20E5
    VariationSelector1, // U+FE00
    LatinSmallLetterJ, // U+006A
    CombiningMacronBelow, // U+0331
};

inline Optional<u16> named_character_reference_second_codepoint_value(NamedCharacterReferenceSecondCodepoint codepoint)
{
    switch (codepoint) {
    case NamedCharacterReferenceSecondCodepoint::None:
        return {};
    case NamedCharacterReferenceSecondCodepoint::CombiningLongSolidusOverlay:
        return 0x0338;
    case NamedCharacterReferenceSecondCodepoint::CombiningLongVerticalLineOverlay:
        return 0x20D2;
    case NamedCharacterReferenceSecondCodepoint::HairSpace:
        return 0x200A;
    case NamedCharacterReferenceSecondCodepoint::CombiningDoubleLowLine:
        return 0x0333;
    case NamedCharacterReferenceSecondCodepoint::CombiningReverseSolidusOverlay:
        return 0x20E5;
    case NamedCharacterReferenceSecondCodepoint::VariationSelector1:
        return 0xFE00;
    case NamedCharacterReferenceSecondCodepoint::LatinSmallLetterJ:
        return 0x006A;
    case NamedCharacterReferenceSecondCodepoint::CombiningMacronBelow:
        return 0x0331;
    default:
        VERIFY_NOT_REACHED();
    }
}

// Note: The first codepoint could fit in 17 bits, and the second could fit in 4 (if unsigned).
// However, to get any benefit from minimizing the struct size, it would need to be accompanied by
// bit-packing the g_named_character_reference_codepoints_lookup array.
struct NamedCharacterReferenceCodepoints {
    u32 first : 24; // Largest value is U+1D56B
    NamedCharacterReferenceSecondCodepoint second : 8;
};
static_assert(sizeof(NamedCharacterReferenceCodepoints) == 4);

struct NamedCharacterReferenceFirstLayerNode {
    // Really only needs 12 bits.
    u16 number;
};
static_assert(sizeof(NamedCharacterReferenceFirstLayerNode) == 2);

struct NamedCharacterReferenceFirstToSecondLayerLink {
    u64 mask : 52;
    u64 second_layer_offset : 12;
};
static_assert(sizeof(NamedCharacterReferenceFirstToSecondLayerLink) == 8);

// Note: It is possible to fit this information within 24 bits, which could then allow for tightly
// bit-packing the second layer array. This would reduce the size of the array by 630 bytes.
struct NamedCharacterReferenceSecondLayerNode {
    // Could be 10 bits
    u16 child_index;
    u8 number;
    // Could be 4 bits
    u8 children_len : 7;
    bool end_of_word : 1;
};
static_assert(sizeof(NamedCharacterReferenceSecondLayerNode) == 4);

struct NamedCharacterReferenceNode {
    // The actual alphabet of characters used in the list of named character references only
    // includes 61 unique characters ('1'...'8', ';', 'a'...'z', 'A'...'Z').
    u8 character;
    // Typically, nodes are numbered with "an integer which gives the number of words that
    // would be accepted by the automaton starting from that state." This numbering
    // allows calculating "a one-to-one correspondence between the integers 1 to L
    // (L is the number of words accepted by the automaton) and the words themselves."
    //
    // This allows us to have a minimal perfect hashing scheme such that it's possible to store
    // and lookup the codepoint transformations of each named character reference using a separate
    // array.
    //
    // This uses that idea, but instead of storing a per-node number that gets built up while
    // searching a list of children, the cumulative number that would result from adding together
    // the numbers of all the previous sibling nodes is stored instead. This cuts down on a bit
    // of work done while searching while keeping the minimal perfect hashing strategy intact.
    //
    // Empirically, the largest number in our DAFSA is 51, so all number values could fit in a u6.
    u8 number : 7;
    bool end_of_word : 1;
    // Index of the first child of this node.
    // There are 3190 nodes in our DAFSA after the first and second layers were extracted out, so
    // all indexes can fit in a u12 (there would be 3872 nodes with the first/second layers
    // included, so still a u12).
    u16 child_index : 12;
    u16 children_len : 4;
};
static_assert(sizeof(NamedCharacterReferenceNode) == 4);

extern NamedCharacterReferenceNode g_named_character_reference_nodes[];
extern NamedCharacterReferenceFirstLayerNode g_named_character_reference_first_layer[];
extern NamedCharacterReferenceFirstToSecondLayerLink g_named_character_reference_first_to_second_layer[];
extern NamedCharacterReferenceSecondLayerNode g_named_character_reference_second_layer[];

Optional<NamedCharacterReferenceCodepoints> named_character_reference_codepoints_from_unique_index(u16 unique_index);

} // namespace Web::HTML

)~~~");

    TRY(file.write_until_depleted(generator.as_string_view().bytes()));
    return {};
}

static u8 ascii_alphabetic_to_index(u8 c)
{
    ASSERT(AK::is_ascii_alpha(c));
    return c <= 'Z' ? (c - 'A') : (c - 'a' + 26);
}

class Node final : public RefCounted<Node> {
private:
    struct NonnullRefPtrNodeTraits {
        static unsigned hash(NonnullRefPtr<Node> const& node)
        {
            u32 hash = 0;
            for (int i = 0; i < 128; i++) {
                hash ^= ptr_hash(node->m_children[i].ptr());
            }
            hash ^= int_hash(static_cast<u32>(node->m_is_terminal));
            return hash;
        }
        static bool equals(NonnullRefPtr<Node> const& a, NonnullRefPtr<Node> const& b)
        {
            if (a->m_is_terminal != b->m_is_terminal)
                return false;
            for (int i = 0; i < 128; i++) {
                if (a->m_children[i] != b->m_children[i])
                    return false;
            }
            return true;
        }
    };

public:
    static NonnullRefPtr<Node> create()
    {
        return adopt_ref(*new (nothrow) Node());
    }

    using NodeTableType = HashTable<NonnullRefPtr<Node>, NonnullRefPtrNodeTraits, false>;

    void calc_numbers()
    {
        m_number = static_cast<u16>(m_is_terminal);
        for (int i = 0; i < 128; i++) {
            if (m_children[i] == nullptr)
                continue;
            m_children[i]->calc_numbers();
            m_number += m_children[i]->m_number;
        }
    }

    u8 num_direct_children()
    {
        u8 num = 0;
        for (int i = 0; i < 128; i++) {
            if (m_children[i] != nullptr)
                num += 1;
        }
        return num;
    }

    u64 get_ascii_alphabetic_bit_mask()
    {
        u64 mask = 0;
        for (int i = 0; i < 128; i++) {
            if (m_children[i] == nullptr)
                continue;
            mask |= ((u64)1) << ascii_alphabetic_to_index(i);
        }
        return mask;
    }

    Array<RefPtr<Node>, 128>& children() { return m_children; }

    void set_as_terminal() { m_is_terminal = true; }

    bool is_terminal() const { return m_is_terminal; }

    u16 number() const { return m_number; }

private:
    Node() = default;

    Array<RefPtr<Node>, 128> m_children { 0 };
    bool m_is_terminal { false };
    u16 m_number { 0 };
};

struct UncheckedNode {
    RefPtr<Node> parent;
    char character;
    RefPtr<Node> child;
};

class DafsaBuilder {
    AK_MAKE_NONCOPYABLE(DafsaBuilder);

public:
    using MappingType = HashMap<StringView, String>;

    DafsaBuilder()
        : m_root(Node::create())
    {
    }

    void insert(StringView str)
    {
        // Must be inserted in sorted order
        VERIFY(str > m_previous_word);

        size_t common_prefix_len = 0;
        for (size_t i = 0; i < min(str.length(), m_previous_word.length()); i++) {
            if (str[i] != m_previous_word[i])
                break;
            common_prefix_len++;
        }

        minimize(common_prefix_len);

        RefPtr<Node> node;
        if (m_unchecked_nodes.size() == 0)
            node = m_root;
        else
            node = m_unchecked_nodes.last().child;

        auto remaining = str.substring_view(common_prefix_len);
        for (char const c : remaining) {
            VERIFY(node->children().at(c) == nullptr);

            auto child = Node::create();
            node->children().at(c) = child;
            m_unchecked_nodes.append(UncheckedNode { node, c, child });
            node = child;
        }
        node->set_as_terminal();

        bool fits = str.copy_characters_to_buffer(m_previous_word_buf, sizeof(m_previous_word_buf));
        // It's guaranteed that m_previous_word_buf is large enough to hold the longest named character reference
        VERIFY(fits);
        m_previous_word = StringView(m_previous_word_buf, str.length());
    }

    void minimize(size_t down_to)
    {
        if (m_unchecked_nodes.size() == 0)
            return;
        while (m_unchecked_nodes.size() > down_to) {
            auto unchecked_node = m_unchecked_nodes.take_last();
            auto child = unchecked_node.child.release_nonnull();
            auto it = m_minimized_nodes.find(child);
            if (it != m_minimized_nodes.end()) {
                unchecked_node.parent->children().at(unchecked_node.character) = *it;
            } else {
                m_minimized_nodes.set(child);
            }
        }
    }

    void calc_numbers()
    {
        m_root->calc_numbers();
    }

    Optional<size_t> get_unique_index(StringView str)
    {
        size_t index = 0;
        Node* node = m_root.ptr();

        for (char const c : str) {
            if (node->children().at(c) == nullptr)
                return {};
            for (int sibling_c = 0; sibling_c < 128; sibling_c++) {
                if (node->children().at(sibling_c) == nullptr)
                    continue;
                if (sibling_c < c) {
                    index += node->children().at(sibling_c)->number();
                }
            }
            node = node->children().at(c);
            if (node->is_terminal())
                index += 1;
        }

        return index;
    }

    NonnullRefPtr<Node> root()
    {
        return m_root;
    }

private:
    NonnullRefPtr<Node> m_root;
    Node::NodeTableType m_minimized_nodes;
    Vector<UncheckedNode> m_unchecked_nodes;
    char m_previous_word_buf[64];
    StringView m_previous_word = { m_previous_word_buf, 0 };
};

struct NodeData {
    u8 character;
    u8 number;
    bool end_of_word;
    u16 child_index;
    u8 children_len;
};

static u16 queue_children(NonnullRefPtr<Node> const& node, Vector<NonnullRefPtr<Node>>& queue, HashMap<Node*, u16>& child_indexes, u16 first_available_index)
{
    auto current_available_index = first_available_index;
    for (u8 c = 0; c < 128; c++) {
        if (node->children().at(c) == nullptr)
            continue;
        auto child = NonnullRefPtr(*node->children().at(c));

        if (!child_indexes.contains(child.ptr())) {
            auto child_num_children = child->num_direct_children();
            if (child_num_children > 0) {
                child_indexes.set(child, current_available_index);
                current_available_index += child_num_children;
            }
            queue.append(child);
        }
    }
    return current_available_index;
}

static u16 write_children_data(NonnullRefPtr<Node> const& node, Vector<NodeData>& node_data, Vector<NonnullRefPtr<Node>>& queue, HashMap<Node*, u16>& child_indexes, u16 first_available_index)
{
    auto current_available_index = first_available_index;
    u8 unique_index_tally = 0;
    for (u8 c = 0; c < 128; c++) {
        if (node->children().at(c) == nullptr)
            continue;
        auto child = NonnullRefPtr(*node->children().at(c));
        auto child_num_children = child->num_direct_children();

        if (!child_indexes.contains(child.ptr())) {
            if (child_num_children > 0) {
                child_indexes.set(child, current_available_index);
                current_available_index += child_num_children;
            }
            queue.append(child);
        }

        node_data.append({ c, unique_index_tally, child->is_terminal(), child_indexes.get(child).value_or(0), child_num_children });

        unique_index_tally += child->number();
    }
    return current_available_index;
}

// Does not include the root node
static void write_node_data(DafsaBuilder& dafsa_builder, Vector<NodeData>& node_data, HashMap<Node*, u16>& child_indexes)
{
    Vector<NonnullRefPtr<Node>> queue;

    u16 first_available_index = 1;
    first_available_index = queue_children(dafsa_builder.root(), queue, child_indexes, first_available_index);

    child_indexes.clear_with_capacity();
    first_available_index = 1;
    auto second_layer_length = queue.size();
    for (size_t i = 0; i < second_layer_length; i++) {
        auto node = queue.take_first();
        first_available_index = queue_children(node, queue, child_indexes, first_available_index);
    }

    while (queue.size() > 0) {
        auto node = queue.take_first();
        first_available_index = write_children_data(node, node_data, queue, child_indexes, first_available_index);
    }
}

ErrorOr<void> generate_implementation_file(JsonObject& named_character_reference_data, Core::File& file)
{
    StringBuilder builder;
    SourceGenerator generator { builder };
    DafsaBuilder dafsa_builder;

    named_character_reference_data.for_each_member([&](auto& key, auto&) {
        dafsa_builder.insert(key.bytes_as_string_view().substring_view(1));
    });
    dafsa_builder.minimize(0);
    dafsa_builder.calc_numbers();

    // As a sanity check, confirm that the minimal perfect hashing doesn't
    // have any collisions
    {
        HashTable<size_t> index_set;

        named_character_reference_data.for_each_member([&](auto& key, auto&) {
            auto index = dafsa_builder.get_unique_index(key.bytes_as_string_view().substring_view(1)).value();
            VERIFY(!index_set.contains(index));
            index_set.set(index);
        });
        VERIFY(named_character_reference_data.size() == index_set.size());
    }

    auto index_to_codepoints = MUST(FixedArray<Codepoints>::create(named_character_reference_data.size()));

    named_character_reference_data.for_each_member([&](auto& key, auto& value) {
        auto codepoints = value.as_object().get_array("codepoints"sv).value();
        auto unique_index = dafsa_builder.get_unique_index(key.bytes_as_string_view().substring_view(1)).value();
        auto array_index = unique_index - 1;
        u32 second_codepoint = 0;
        if (codepoints.size() == 2) {
            second_codepoint = codepoints[1].template as_integer<u32>();
        }
        index_to_codepoints[array_index] = Codepoints { codepoints[0].template as_integer<u32>(), second_codepoint };
    });

    generator.append(R"~~~(
#include <LibWeb/HTML/Parser/Entities.h>

namespace Web::HTML {

static NamedCharacterReferenceCodepoints g_named_character_reference_codepoints_lookup[] = {
)~~~");

    for (auto codepoints : index_to_codepoints) {
        auto member_generator = generator.fork();
        member_generator.set("first_codepoint", MUST(String::formatted("0x{:X}", codepoints.first)));
        member_generator.set("second_codepoint_name", get_second_codepoint_enum_name(codepoints.second));
        member_generator.append(R"~~~(    {@first_codepoint@, NamedCharacterReferenceSecondCodepoint::@second_codepoint_name@},
)~~~");
    }

    Vector<NodeData> node_data;
    HashMap<Node*, u16> child_indexes;
    write_node_data(dafsa_builder, node_data, child_indexes);

    generator.append(R"~~~(};

NamedCharacterReferenceNode g_named_character_reference_nodes[] = {
    { 0, 0, false, 0, 0 },
)~~~");

    for (auto data : node_data) {
        auto member_generator = generator.fork();
        member_generator.set("char", StringView(&data.character, 1));
        member_generator.set("number", String::number(data.number));
        member_generator.set("end_of_word", MUST(String::formatted("{}", data.end_of_word)));
        member_generator.set("child_index", String::number(data.child_index));
        member_generator.set("children_len", String::number(data.children_len));
        member_generator.append(R"~~~(    { '@char@', @number@, @end_of_word@, @child_index@, @children_len@ },
)~~~");
    }

    generator.append(R"~~~(};

NamedCharacterReferenceFirstLayerNode g_named_character_reference_first_layer[] = {
)~~~");

    auto num_children = dafsa_builder.root()->num_direct_children();
    VERIFY(num_children == 52); // A-Z, a-z exactly
    u16 unique_index_tally = 0;
    for (u8 c = 0; c < 128; c++) {
        if (dafsa_builder.root()->children().at(c) == nullptr)
            continue;
        VERIFY(AK::is_ascii_alpha(c));
        auto child = dafsa_builder.root()->children().at(c);

        auto member_generator = generator.fork();
        member_generator.set("number", String::number(unique_index_tally));
        member_generator.append(R"~~~(    { @number@ },
)~~~");

        unique_index_tally += child->number();
    }

    generator.append(R"~~~(};

NamedCharacterReferenceFirstToSecondLayerLink g_named_character_reference_first_to_second_layer[] = {
)~~~");

    u16 second_layer_offset = 0;
    for (u8 c = 0; c < 128; c++) {
        if (dafsa_builder.root()->children().at(c) == nullptr)
            continue;
        VERIFY(AK::is_ascii_alpha(c));
        auto child = dafsa_builder.root()->children().at(c);
        auto bit_mask = child->get_ascii_alphabetic_bit_mask();

        auto member_generator = generator.fork();
        member_generator.set("bit_mask", String::number(bit_mask));
        member_generator.set("second_layer_offset", String::number(second_layer_offset));
        member_generator.append(R"~~~(    { @bit_mask@ull, @second_layer_offset@ },
)~~~");

        second_layer_offset += child->num_direct_children();
    }

    generator.append(R"~~~(};

NamedCharacterReferenceSecondLayerNode g_named_character_reference_second_layer[] = {
)~~~");

    for (u8 c = 0; c < 128; c++) {
        if (dafsa_builder.root()->children().at(c) == nullptr)
            continue;
        VERIFY(AK::is_ascii_alpha(c));
        auto first_layer_node = dafsa_builder.root()->children().at(c);

        u8 unique_index_tally = 0;
        for (u8 child_c = 0; child_c < 128; child_c++) {
            if (first_layer_node->children().at(child_c) == nullptr)
                continue;
            VERIFY(AK::is_ascii_alpha(child_c));
            auto second_layer_node = first_layer_node->children().at(child_c);
            auto child_num_children = second_layer_node->num_direct_children();
            auto child_index = child_indexes.get(second_layer_node).value_or(0);

            auto member_generator = generator.fork();
            member_generator.set("child_index", String::number(child_index));
            member_generator.set("number", String::number(unique_index_tally));
            member_generator.set("children_len", String::number(child_num_children));
            member_generator.set("end_of_word", MUST(String::formatted("{}", second_layer_node->is_terminal())));
            member_generator.append(R"~~~(    { @child_index@, @number@, @children_len@, @end_of_word@ },
)~~~");
            unique_index_tally += second_layer_node->number();
        }
    }

    generator.append(R"~~~(};

// Note: The unique index is 1-based.
Optional<NamedCharacterReferenceCodepoints> named_character_reference_codepoints_from_unique_index(u16 unique_index) {
    if (unique_index == 0) return {};
    return g_named_character_reference_codepoints_lookup[unique_index - 1];
}

} // namespace Web::HTML
)~~~");

    TRY(file.write_until_depleted(generator.as_string_view().bytes()));
    return {};
}