ladybird/Libraries/LibWeb/CSS/Parser/SyntaxParsing.cpp

/*
 * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/GenericShorthands.h>
#include <LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.h>
#include <LibWeb/CSS/Parser/Parser.h>
#include <LibWeb/CSS/Parser/Syntax.h>
#include <LibWeb/CSS/Parser/SyntaxParsing.h>
#include <LibWeb/CSS/Parser/TokenStream.h>
#include <LibWeb/CSS/StyleValues/CSSKeywordValue.h>
#include <LibWeb/CSS/StyleValues/CustomIdentStyleValue.h>
#include <LibWeb/CSS/StyleValues/GuaranteedInvalidStyleValue.h>
#include <LibWeb/CSS/StyleValues/StyleValueList.h>
#include <LibWeb/CSS/StyleValues/UnresolvedStyleValue.h>

namespace Web::CSS::Parser {

static OwnPtr<SyntaxNode> parse_syntax_single_component(TokenStream<ComponentValue>& tokens)
{
    // <syntax-single-component> = '<' <syntax-type-name> '>' | <ident>
    // <syntax-type-name> = angle | color | custom-ident | image | integer
    //                    | length | length-percentage | number
    //                    | percentage | resolution | string | time
    //                    | url | transform-function

    auto transaction = tokens.begin_transaction();
    tokens.discard_whitespace();

    // <ident>
    if (tokens.next_token().is(Token::Type::Ident)) {
        auto ident = tokens.consume_a_token().token().ident();
        transaction.commit();
        return IdentSyntaxNode::create(move(ident));
    }

    // '<' <syntax-type-name> '>'
    if (tokens.next_token().is_delim('<')) {
        tokens.discard_a_token(); // '<'
        auto const& type_name = tokens.consume_a_token();
        auto const& end_token = tokens.consume_a_token();

        if (end_token.is_delim('>')
            && type_name.is(Token::Type::Ident)
            && first_is_one_of(type_name.token().ident(), "angle"sv,
                "color"sv,
                "custom-ident"sv,
                "image"sv,
                "integer"sv,
                "length"sv,
                "length-percentage"sv,
                "number"sv,
                "percentage"sv,
                "resolution"sv,
                "string"sv,
                "time"sv,
                "url"sv,
                "transform-function"sv)) {
            transaction.commit();
            return TypeSyntaxNode::create(type_name.token().ident());
        }
    }

    return nullptr;
}

static Optional<char> parse_syntax_multiplier(TokenStream<ComponentValue>& tokens)
{
    // <syntax-multiplier> = [ '#' | '+' ]
    auto transaction = tokens.begin_transaction();

    auto delim = tokens.consume_a_token();
    if (delim.is_delim('#') || delim.is_delim('+')) {
        transaction.commit();
        return delim.token().delim();
    }

    return {};
}

static OwnPtr<SyntaxNode> parse_syntax_component(TokenStream<ComponentValue>& tokens)
{
    // <syntax-component> = <syntax-single-component> <syntax-multiplier>?
    //                    | '<' transform-list '>'

    auto transaction = tokens.begin_transaction();

    tokens.discard_whitespace();

    // '<' transform-list '>'
    if (tokens.next_token().is_delim('<')) {
        auto transform_list_transaction = transaction.create_child();
        tokens.discard_a_token(); // '<'
        auto& ident_token = tokens.consume_a_token();
        auto& end_token = tokens.consume_a_token();

        if (ident_token.is_ident("transform-list"sv) && end_token.is_delim('>')) {
            transform_list_transaction.commit();
            return TypeSyntaxNode::create("transform-list"_fly_string);
        }
    }

    // <syntax-single-component> <syntax-multiplier>?
    auto syntax_single_component = parse_syntax_single_component(tokens);
    if (!syntax_single_component)
        return nullptr;

    auto multiplier = parse_syntax_multiplier(tokens);
    if (!multiplier.has_value()) {
        transaction.commit();
        return syntax_single_component.release_nonnull();
    }

    switch (multiplier.value()) {
    case '#':
        transaction.commit();
        return CommaSeparatedMultiplierSyntaxNode::create(syntax_single_component.release_nonnull());
    case '+':
        transaction.commit();
        return MultiplierSyntaxNode::create(syntax_single_component.release_nonnull());
    default:
        return nullptr;
    }
}

static Optional<char> parse_syntax_combinator(TokenStream<ComponentValue>& tokens)
{
    // <syntax-combinator> = '|'
    auto transaction = tokens.begin_transaction();
    tokens.discard_whitespace();

    auto delim = tokens.consume_a_token();
    if (delim.is_delim('|')) {
        transaction.commit();
        return delim.token().delim();
    }

    return {};
}

// https://drafts.csswg.org/css-values-5/#typedef-syntax
OwnPtr<SyntaxNode> parse_as_syntax(Vector<ComponentValue> const& component_values)
{
    // <syntax> = '*' | <syntax-component> [ <syntax-combinator> <syntax-component> ]* | <syntax-string>
    // <syntax-component> = <syntax-single-component> <syntax-multiplier>?
    //                    | '<' transform-list '>'
    // <syntax-single-component> = '<' <syntax-type-name> '>' | <ident>
    // <syntax-type-name> = angle | color | custom-ident | image | integer
    //                    | length | length-percentage | number
    //                    | percentage | resolution | string | time
    //                    | url | transform-function
    // <syntax-combinator> = '|'
    // <syntax-multiplier> = [ '#' | '+' ]
    //
    // <syntax-string> = <string>
    // FIXME: Eventually, extend this to also parse *any* CSS grammar, not just for the <syntax> type.

    TokenStream tokens { component_values };
    tokens.discard_whitespace();

    // '*'
    if (tokens.next_token().is_delim('*')) {
        tokens.discard_a_token(); // '*'
        tokens.discard_whitespace();
        if (tokens.has_next_token())
            return nullptr;
        return UniversalSyntaxNode::create();
    }

    // <syntax-string> = <string>
    // A <syntax-string> is a <string> whose value successfully parses as a <syntax>, and represents the same value as
    // that <syntax> would.
    // NB: For now, this is the only time a string is allowed in a <syntax>.
    if (tokens.next_token().is(Token::Type::String)) {
        auto string = tokens.consume_a_token().token().string();
        tokens.discard_whitespace();
        if (tokens.has_next_token())
            return nullptr;

        auto child_component_values = Parser::create(ParsingParams {}, string).parse_as_list_of_component_values();
        return parse_as_syntax(child_component_values);
    }

    // <syntax-component> [ <syntax-combinator> <syntax-component> ]*
    auto first = parse_syntax_component(tokens);
    if (!first)
        return nullptr;
    Vector<NonnullOwnPtr<SyntaxNode>> syntax_components;
    syntax_components.append(first.release_nonnull());

    tokens.discard_whitespace();
    while (tokens.has_next_token()) {
        auto combinator = parse_syntax_combinator(tokens);
        tokens.discard_whitespace();
        auto component = parse_syntax_component(tokens);
        tokens.discard_whitespace();
        if (!combinator.has_value() || !component) {
            dbgln("Failed parsing syntax portion, combinator = `{}`, component = `{}`", combinator, component);
            return nullptr;
        }

        // FIXME: Make this logic smarter once we have more than one type of combinator.
        // For now, assume we're always making an AlternativesSyntaxNode.
        VERIFY(combinator == '|');

        syntax_components.append(component.release_nonnull());
    }

    if (syntax_components.size() == 1)
        return syntax_components.take_first();
    return AlternativesSyntaxNode::create(move(syntax_components));
}

NonnullRefPtr<CSSStyleValue const> parse_with_a_syntax(ParsingParams const& parsing_params, Vector<ComponentValue> const& input, SyntaxNode const& syntax, Optional<DOM::AbstractElement> const& element)
{
    return Parser::create(parsing_params, ""sv).parse_with_a_syntax(input, syntax, element);
}

RefPtr<CSSStyleValue const> Parser::parse_according_to_syntax_node(TokenStream<ComponentValue>& tokens, SyntaxNode const& syntax_node, Optional<DOM::AbstractElement> const& element)
{
    auto transaction = tokens.begin_transaction();

    switch (syntax_node.type()) {
    case SyntaxNode::NodeType::Universal:
        if (auto declaration_value = parse_declaration_value(tokens); declaration_value.has_value()) {
            transaction.commit();
            return UnresolvedStyleValue::create(declaration_value.release_value());
        }
        return nullptr;
    case SyntaxNode::NodeType::Ident: {
        auto const& ident_node = as<IdentSyntaxNode>(syntax_node);
        tokens.discard_whitespace();
        if (tokens.consume_a_token().is_ident(ident_node.ident())) {
            transaction.commit();
            if (auto keyword = keyword_from_string(ident_node.ident()); keyword.has_value())
                return CSSKeywordValue::create(keyword.release_value());
            return CustomIdentStyleValue::create(ident_node.ident());
        }
        return nullptr;
    }
    case SyntaxNode::NodeType::Type: {
        auto const& type_node = as<TypeSyntaxNode>(syntax_node);
        auto const& type_name = type_node.type_name();
        if (auto value_type = value_type_from_string(type_name); value_type.has_value()) {
            if (auto result = parse_value(*value_type, tokens)) {
                transaction.commit();
                return result.release_nonnull();
            }
            return nullptr;
        }

        dbgln_if(CSS_PARSER_DEBUG, "Couldn't parse `<{}>` because we don't know what it is.", type_name);
        return nullptr;
    }
    case SyntaxNode::NodeType::Multiplier: {
        auto const& multiplier_node = as<MultiplierSyntaxNode>(syntax_node);
        StyleValueVector values;
        tokens.discard_whitespace();
        while (tokens.has_next_token()) {
            auto parsed_child = parse_according_to_syntax_node(tokens, multiplier_node.child(), element);
            if (!parsed_child)
                break;
            values.append(parsed_child.release_nonnull());
            tokens.discard_whitespace();
        }
        if (values.is_empty())
            return nullptr;
        transaction.commit();
        return StyleValueList::create(move(values), StyleValueList::Separator::Space);
    }
    case SyntaxNode::NodeType::CommaSeparatedMultiplier: {
        auto const& multiplier_node = as<CommaSeparatedMultiplierSyntaxNode>(syntax_node);
        auto result = parse_comma_separated_value_list(tokens, [&](auto& tokens) {
            return parse_according_to_syntax_node(tokens, multiplier_node.child(), element);
        });
        if (!result)
            return nullptr;
        transaction.commit();
        return result.release_nonnull();
    }
    case SyntaxNode::NodeType::Alternatives: {
        auto const& alternatives_node = as<AlternativesSyntaxNode>(syntax_node);
        for (auto const& child : alternatives_node.children()) {
            if (auto result = parse_according_to_syntax_node(tokens, *child, element)) {
                transaction.commit();
                return result.release_nonnull();
            }
        }
        return nullptr;
    }
    }

    VERIFY_NOT_REACHED();
}

// https://drafts.csswg.org/css-values-5/#parse-with-a-syntax
NonnullRefPtr<CSSStyleValue const> Parser::parse_with_a_syntax(Vector<ComponentValue> const& input, SyntaxNode const& syntax, Optional<DOM::AbstractElement> const& element)
{
    // 1. Parse a list of component values from values, and let raw parse be the result.
    // NB: Already done before this point.

    // FIXME: 2. If el was given, substitute arbitrary substitution functions in raw parse, and set raw parse to that result.
    // NB: This is currently a no-op because our only caller already substitutes ASFs in the input before calling us.
    // FIXME: Move substitute_arbitrary_substitution_functions() into the Parser, and keep the guarded contexts there,
    //        so we don't have this awkward situation of needing to pass that to random other functions.

    // 3. parse values according to syntax, with a * value treated as <declaration-value>?, and let parsed result be
    //    the result.
    //    If syntax used a | combinator, let parsed result be the parse result from the first matching clause.
    TokenStream tokens { input };
    auto parsed_result = parse_according_to_syntax_node(tokens, syntax, element);
    tokens.discard_whitespace();

    // 4. If parsed result is failure, return the guaranteed-invalid value.
    if (!parsed_result || tokens.has_next_token())
        return GuaranteedInvalidStyleValue::create();

    // 5. Assert: parsed result is now a well-defined list of one or more CSS values, since each branch of a <syntax>
    //    defines an unambiguous parse result (or the * syntax is unambiguous on its own).
    // NB: Nothing to do.

    // 6. Return parsed result.
    return parsed_result.release_nonnull();
}

}