/*
 * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
 * Copyright (c) 2020-2021, the SerenityOS developers.
 * Copyright (c) 2021, Sam Atkins <atkinssj@gmail.com>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Debug.h>
#include <AK/NonnullRefPtrVector.h>
#include <AK/SourceLocation.h>
#include <LibWeb/CSS/CSSStyleDeclaration.h>
#include <LibWeb/CSS/CSSStyleRule.h>
#include <LibWeb/CSS/CSSStyleSheet.h>
#include <LibWeb/CSS/Parser/DeclarationOrAtRule.h>
#include <LibWeb/CSS/Parser/Parser.h>
#include <LibWeb/CSS/Parser/StyleBlockRule.h>
#include <LibWeb/CSS/Parser/StyleComponentValueRule.h>
#include <LibWeb/CSS/Parser/StyleFunctionRule.h>
#include <LibWeb/CSS/Parser/StyleRule.h>
#include <LibWeb/CSS/Selector.h>
#include <LibWeb/DOM/Document.h>
#include <LibWeb/Dump.h>

static void log_parse_error(const SourceLocation& location = SourceLocation::current())
{
    dbgln_if(CSS_PARSER_DEBUG, "Parse error (CSS) {}", location);
}

namespace Web::CSS {

ParsingContext::ParsingContext()
{
}

ParsingContext::ParsingContext(DOM::Document& document)
    : m_document(&document)
{
}

ParsingContext::ParsingContext(DOM::ParentNode& parent_node)
    : m_document(&parent_node.document())
{
}

bool ParsingContext::in_quirks_mode() const
{
    return m_document ? m_document->in_quirks_mode() : false;
}

URL ParsingContext::complete_url(String const& addr) const
{
    return m_document ? m_document->url().complete_url(addr) : URL::create_with_url_or_path(addr);
}

template<typename T>
TokenStream<T>::TokenStream(Vector<T> const& tokens)
    : m_tokens(tokens)
    , m_eof(make_eof())
{
}

template<typename T>
TokenStream<T>::~TokenStream()
{
}

template<typename T>
bool TokenStream<T>::has_next_token()
{
    return (size_t)(m_iterator_offset + 1) < m_tokens.size();
}

template<typename T>
T const& TokenStream<T>::peek_token()
{
    if (!has_next_token())
        return m_eof;

    return m_tokens.at(m_iterator_offset + 1);
}

template<typename T>
T const& TokenStream<T>::next_token()
{
    if (!has_next_token())
        return m_eof;

    ++m_iterator_offset;

    return m_tokens.at(m_iterator_offset);
}

template<typename T>
T const& TokenStream<T>::current_token()
{
    if ((size_t)m_iterator_offset >= m_tokens.size())
        return m_eof;

    return m_tokens.at(m_iterator_offset);
}

template<typename T>
void TokenStream<T>::reconsume_current_input_token()
{
    VERIFY(m_iterator_offset >= 0);
    --m_iterator_offset;
}

template<typename T>
void TokenStream<T>::skip_whitespace()
{
    while (peek_token().is(Token::Type::Whitespace))
        next_token();
}

template<>
Token TokenStream<Token>::make_eof()
{
    return Tokenizer::create_eof_token();
}

template<>
StyleComponentValueRule TokenStream<StyleComponentValueRule>::make_eof()
{
    return StyleComponentValueRule(Tokenizer::create_eof_token());
}

template<typename T>
void TokenStream<T>::dump_all_tokens()
{
    dbgln("Dumping all tokens:");
    for (size_t i = 0; i < m_tokens.size(); ++i) {
        auto& token = m_tokens[i];
        if ((i - 1) == (size_t)m_iterator_offset)
            dbgln("-> {}", token.to_debug_string());
        else
            dbgln("   {}", token.to_debug_string());
    }
}

Parser::Parser(ParsingContext const& context, StringView const& input, String const& encoding)
    : m_context(context)
    , m_tokenizer(input, encoding)
    , m_tokens(m_tokenizer.parse())
    , m_token_stream(TokenStream(m_tokens))
{
}

Parser::~Parser()
{
}

NonnullRefPtr<CSSStyleSheet> Parser::parse_as_stylesheet()
{
    return parse_as_stylesheet(m_token_stream);
}

template<typename T>
NonnullRefPtr<CSSStyleSheet> Parser::parse_as_stylesheet(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_as_stylesheet");

    auto parser_rules = consume_a_list_of_rules(tokens, true);
    NonnullRefPtrVector<CSSRule> rules;

    for (auto& raw_rule : parser_rules) {
        auto rule = convert_to_rule(raw_rule);
        if (rule)
            rules.append(*rule);
    }

    auto stylesheet = CSSStyleSheet::create(rules);
    dump_sheet(stylesheet);
    return stylesheet;
}

Optional<SelectorList> Parser::parse_a_selector()
{
    return parse_a_selector(m_token_stream);
}

template<typename T>
Optional<SelectorList> Parser::parse_a_selector(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_a_selector");

    auto selector_list = parse_a_selector_list(tokens);
    if (selector_list.has_value())
        return selector_list;

    return {};
}

Optional<SelectorList> Parser::parse_a_relative_selector()
{
    return parse_a_relative_selector(m_token_stream);
}

template<typename T>
Optional<SelectorList> Parser::parse_a_relative_selector(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_a_relative_selector");

    auto selector_list = parse_a_relative_selector_list(tokens);
    if (selector_list.has_value())
        return selector_list;

    return {};
}

template<typename T>
Optional<SelectorList> Parser::parse_a_selector_list(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_a_selector_list");

    auto comma_separated_lists = parse_as_comma_separated_list_of_component_values(tokens);

    NonnullRefPtrVector<Selector> selectors;
    for (auto& selector_parts : comma_separated_lists) {
        auto stream = TokenStream(selector_parts);
        auto selector = parse_complex_selector(stream, false);
        if (selector)
            selectors.append(selector.release_nonnull());
        else
            return {};
    }

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

    return selectors;
}

template<typename T>
Optional<SelectorList> Parser::parse_a_relative_selector_list(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_a_relative_selector_list");

    auto comma_separated_lists = parse_as_comma_separated_list_of_component_values(tokens);

    NonnullRefPtrVector<Selector> selectors;
    for (auto& selector_parts : comma_separated_lists) {
        auto stream = TokenStream(selector_parts);
        auto selector = parse_complex_selector(stream, true);
        if (selector)
            selectors.append(selector.release_nonnull());
        else
            return {};
    }

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

    return selectors;
}

RefPtr<Selector> Parser::parse_complex_selector(TokenStream<StyleComponentValueRule>& tokens, bool allow_starting_combinator)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_complex_selector");

    Vector<Selector::CompoundSelector> compound_selectors;

    auto first_selector = parse_compound_selector(tokens);
    if (first_selector.is_error())
        return {};
    if (!allow_starting_combinator) {
        if (first_selector.value().combinator != Selector::Combinator::Descendant)
            return {};
        first_selector.value().combinator = Selector::Combinator::None;
    }
    compound_selectors.append(first_selector.value());

    while (tokens.has_next_token()) {
        auto compound_selector = parse_compound_selector(tokens);
        if (compound_selector.is_error()) {
            if (compound_selector.error() == SelectorParsingResult::Done)
                break;
            else
                return {};
        }
        compound_selectors.append(compound_selector.value());
    }

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

    return Selector::create(move(compound_selectors));
}

Result<Selector::CompoundSelector, Parser::SelectorParsingResult> Parser::parse_compound_selector(TokenStream<StyleComponentValueRule>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_compound_selector");

    tokens.skip_whitespace();

    auto combinator = parse_selector_combinator(tokens).value_or(Selector::Combinator::Descendant);

    tokens.skip_whitespace();

    Vector<Selector::SimpleSelector> simple_selectors;

    while (tokens.has_next_token()) {
        auto component = parse_simple_selector(tokens);
        if (component.is_error()) {
            if (component.error() == SelectorParsingResult::Done)
                break;
            else
                return component.error();
        }

        simple_selectors.append(component.value());
    }

    if (simple_selectors.is_empty())
        return SelectorParsingResult::Done;

    return Selector::CompoundSelector { combinator, move(simple_selectors) };
}

Optional<Selector::Combinator> Parser::parse_selector_combinator(TokenStream<StyleComponentValueRule>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_selector_combinator");

    auto& current_value = tokens.next_token();
    if (current_value.is(Token::Type::Delim)) {
        auto delim = current_value.token().delim();
        if (delim == ">"sv) {
            return Selector::Combinator::ImmediateChild;
        } else if (delim == "+"sv) {
            return Selector::Combinator::NextSibling;
        } else if (delim == "~"sv) {
            return Selector::Combinator::SubsequentSibling;
        } else if (delim == "|"sv) {
            auto& next = tokens.peek_token();
            if (next.is(Token::Type::EndOfFile))
                return {};

            if (next.is(Token::Type::Delim) && next.token().delim() == "|"sv) {
                tokens.next_token();
                return Selector::Combinator::Column;
            }
        }
    }

    tokens.reconsume_current_input_token();
    return {};
}

Result<Selector::SimpleSelector, Parser::SelectorParsingResult> Parser::parse_simple_selector(TokenStream<StyleComponentValueRule>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_simple_selector");

    auto peek_token_ends_selector = [&]() -> bool {
        auto& value = tokens.peek_token();
        return (value.is(Token::Type::EndOfFile) || value.is(Token::Type::Whitespace) || value.is(Token::Type::Comma));
    };

    if (peek_token_ends_selector())
        return SelectorParsingResult::Done;

    auto& first_value = tokens.next_token();

    if (first_value.is(Token::Type::Delim) && first_value.token().delim() == "*"sv) {
        return Selector::SimpleSelector {
            .type = Selector::SimpleSelector::Type::Universal
        };

    } else if (first_value.is(Token::Type::Hash)) {
        if (first_value.token().hash_type() != Token::HashType::Id) {
            dbgln_if(CSS_PARSER_DEBUG, "Selector contains hash token that is not an id: {}", first_value.to_debug_string());
            return SelectorParsingResult::SyntaxError;
        }
        return Selector::SimpleSelector {
            .type = Selector::SimpleSelector::Type::Id,
            .value = first_value.token().hash_value()
        };

    } else if (first_value.is(Token::Type::Delim) && first_value.token().delim() == "."sv) {
        if (peek_token_ends_selector())
            return SelectorParsingResult::SyntaxError;

        auto& class_name_value = tokens.next_token();
        if (!class_name_value.is(Token::Type::Ident)) {
            dbgln_if(CSS_PARSER_DEBUG, "Expected an ident after '.', got: {}", class_name_value.to_debug_string());
            return SelectorParsingResult::SyntaxError;
        }
        return Selector::SimpleSelector {
            .type = Selector::SimpleSelector::Type::Class,
            .value = class_name_value.token().ident()
        };

    } else if (first_value.is(Token::Type::Ident)) {
        return Selector::SimpleSelector {
            .type = Selector::SimpleSelector::Type::TagName,
            .value = first_value.token().ident()
        };

    } else if (first_value.is_block() && first_value.block().is_square()) {
        auto& attribute_parts = first_value.block().values();

        if (attribute_parts.is_empty()) {
            dbgln_if(CSS_PARSER_DEBUG, "CSS attribute selector is empty!");
            return SelectorParsingResult::SyntaxError;
        }

        // FIXME: Handle namespace prefix for attribute name.
        auto& attribute_part = attribute_parts.first();
        if (!attribute_part.is(Token::Type::Ident)) {
            dbgln_if(CSS_PARSER_DEBUG, "Expected ident for attribute name, got: '{}'", attribute_part.to_debug_string());
            return SelectorParsingResult::SyntaxError;
        }

        Selector::SimpleSelector simple_selector {
            .type = Selector::SimpleSelector::Type::Attribute,
            .attribute = {
                .match_type = Selector::SimpleSelector::Attribute::MatchType::HasAttribute,
                .name = attribute_part.token().ident(),
            }
        };

        if (attribute_parts.size() == 1)
            return simple_selector;

        size_t attribute_index = 1;
        auto& delim_part = attribute_parts.at(attribute_index);
        if (!delim_part.is(Token::Type::Delim)) {
            dbgln_if(CSS_PARSER_DEBUG, "Expected a delim for attribute comparison, got: '{}'", delim_part.to_debug_string());
            return SelectorParsingResult::SyntaxError;
        }

        if (delim_part.token().delim() == "="sv) {
            simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ExactValueMatch;
            attribute_index++;
        } else {
            attribute_index++;
            if (attribute_index >= attribute_parts.size()) {
                dbgln_if(CSS_PARSER_DEBUG, "Attribute selector ended part way through a match type.");
                return SelectorParsingResult::SyntaxError;
            }

            auto& delim_second_part = attribute_parts.at(attribute_index);
            if (!(delim_second_part.is(Token::Type::Delim) && delim_second_part.token().delim() == "=")) {
                dbgln_if(CSS_PARSER_DEBUG, "Expected a double delim for attribute comparison, got: '{}{}'", delim_part.to_debug_string(), delim_second_part.to_debug_string());
                return SelectorParsingResult::SyntaxError;
            }

            if (delim_part.token().delim() == "~"sv) {
                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ContainsWord;
                attribute_index++;
            } else if (delim_part.token().delim() == "*"sv) {
                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ContainsString;
                attribute_index++;
            } else if (delim_part.token().delim() == "|"sv) {
                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::StartsWithSegment;
                attribute_index++;
            } else if (delim_part.token().delim() == "^"sv) {
                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::StartsWithString;
                attribute_index++;
            } else if (delim_part.token().delim() == "$"sv) {
                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::EndsWithString;
                attribute_index++;
            }
        }

        if (attribute_index >= attribute_parts.size()) {
            dbgln_if(CSS_PARSER_DEBUG, "Attribute selector ended without a value to match.");
            return SelectorParsingResult::SyntaxError;
        }

        auto& value_part = attribute_parts.at(attribute_index);
        if (!value_part.is(Token::Type::Ident) && !value_part.is(Token::Type::String)) {
            dbgln_if(CSS_PARSER_DEBUG, "Expected a string or ident for the value to match attribute against, got: '{}'", value_part.to_debug_string());
            return SelectorParsingResult::SyntaxError;
        }
        simple_selector.attribute.value = value_part.token().is(Token::Type::Ident) ? value_part.token().ident() : value_part.token().string();

        // FIXME: Handle case-sensitivity suffixes. https://www.w3.org/TR/selectors-4/#attribute-case
        return simple_selector;

    } else if (first_value.is(Token::Type::Colon)) {
        if (peek_token_ends_selector())
            return SelectorParsingResult::SyntaxError;

        bool is_pseudo = false;
        if (tokens.peek_token().is(Token::Type::Colon)) {
            is_pseudo = true;
            tokens.next_token();
            if (peek_token_ends_selector())
                return SelectorParsingResult::SyntaxError;
        }

        if (is_pseudo) {
            Selector::SimpleSelector simple_selector {
                .type = Selector::SimpleSelector::Type::PseudoElement
            };

            auto& name_token = tokens.next_token();
            if (!name_token.is(Token::Type::Ident)) {
                dbgln_if(CSS_PARSER_DEBUG, "Expected an ident for pseudo-element, got: '{}'", name_token.to_debug_string());
                return SelectorParsingResult::SyntaxError;
            }

            auto pseudo_name = name_token.token().ident();

            if (pseudo_name.equals_ignoring_case("after")) {
                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::After;
            } else if (pseudo_name.equals_ignoring_case("before")) {
                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::Before;
            } else if (pseudo_name.equals_ignoring_case("first-letter")) {
                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLetter;
            } else if (pseudo_name.equals_ignoring_case("first-line")) {
                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLine;
            } else {
                dbgln_if(CSS_PARSER_DEBUG, "Unrecognized pseudo-element: '{}'", pseudo_name);
                return SelectorParsingResult::SyntaxError;
            }

            return simple_selector;
        }

        if (peek_token_ends_selector())
            return SelectorParsingResult::SyntaxError;

        auto& pseudo_class_token = tokens.next_token();
        Selector::SimpleSelector simple_selector {
            .type = Selector::SimpleSelector::Type::PseudoClass
        };

        if (pseudo_class_token.is(Token::Type::Ident)) {
            auto pseudo_name = pseudo_class_token.token().ident();
            if (pseudo_name.equals_ignoring_case("active")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Active;
            } else if (pseudo_name.equals_ignoring_case("checked")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Checked;
            } else if (pseudo_name.equals_ignoring_case("disabled")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Disabled;
            } else if (pseudo_name.equals_ignoring_case("empty")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Empty;
            } else if (pseudo_name.equals_ignoring_case("enabled")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Enabled;
            } else if (pseudo_name.equals_ignoring_case("first-child")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::FirstChild;
            } else if (pseudo_name.equals_ignoring_case("first-of-type")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::FirstOfType;
            } else if (pseudo_name.equals_ignoring_case("focus")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Focus;
            } else if (pseudo_name.equals_ignoring_case("hover")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Hover;
            } else if (pseudo_name.equals_ignoring_case("last-child")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::LastChild;
            } else if (pseudo_name.equals_ignoring_case("last-of-type")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::LastOfType;
            } else if (pseudo_name.equals_ignoring_case("link")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Link;
            } else if (pseudo_name.equals_ignoring_case("only-child")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::OnlyChild;
            } else if (pseudo_name.equals_ignoring_case("root")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Root;
            } else if (pseudo_name.equals_ignoring_case("visited")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Visited;

            } else if (pseudo_name.equals_ignoring_case("after")) {
                // Single-colon syntax allowed for compatibility. https://www.w3.org/TR/selectors/#pseudo-element-syntax
                simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::After;
            } else if (pseudo_name.equals_ignoring_case("before")) {
                // See :after
                simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::Before;
            } else if (pseudo_name.equals_ignoring_case("first-letter")) {
                // See :after
                simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLetter;
            } else if (pseudo_name.equals_ignoring_case("first-line")) {
                // See :after
                simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLine;
            } else {
                dbgln_if(CSS_PARSER_DEBUG, "Unknown pseudo class: '{}'", pseudo_name);
                return SelectorParsingResult::SyntaxError;
            }

            return simple_selector;

        } else if (pseudo_class_token.is_function()) {

            auto& pseudo_function = pseudo_class_token.function();
            if (pseudo_function.name().equals_ignoring_case("not")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Not;
                auto function_token_stream = TokenStream(pseudo_function.values());
                auto not_selector = parse_a_selector(function_token_stream);
                if (!not_selector.has_value()) {
                    dbgln_if(CSS_PARSER_DEBUG, "Invalid selector in :not() clause");
                    return SelectorParsingResult::SyntaxError;
                }
                simple_selector.pseudo_class.not_selector = not_selector.value();
            } else if (pseudo_function.name().equals_ignoring_case("nth-child")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthChild;
                auto function_values = TokenStream<StyleComponentValueRule>(pseudo_function.values());
                auto nth_child_pattern = parse_a_n_plus_b_pattern(function_values);
                if (nth_child_pattern.has_value()) {
                    simple_selector.pseudo_class.nth_child_pattern = nth_child_pattern.value();
                } else {
                    dbgln_if(CSS_PARSER_DEBUG, "!!! Invalid nth-child format");
                    function_values.dump_all_tokens();
                    return SelectorParsingResult::SyntaxError;
                }
            } else if (pseudo_function.name().equals_ignoring_case("nth-last-child")) {
                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthLastChild;
                auto function_values = TokenStream<StyleComponentValueRule>(pseudo_function.values());
                auto nth_child_pattern = parse_a_n_plus_b_pattern(function_values);
                if (nth_child_pattern.has_value()) {
                    dbgln("+++ Got An+B pattern: '{}'", nth_child_pattern.value().to_string());
                    simple_selector.pseudo_class.nth_child_pattern = nth_child_pattern.value();
                } else {
                    dbgln_if(CSS_PARSER_DEBUG, "!!! Invalid nth-child format");
                    function_values.dump_all_tokens();
                    return SelectorParsingResult::SyntaxError;
                }
            } else {
                dbgln_if(CSS_PARSER_DEBUG, "Unknown pseudo class: '{}'()", pseudo_function.name());
                return SelectorParsingResult::SyntaxError;
            }

            return simple_selector;

        } else {
            dbgln_if(CSS_PARSER_DEBUG, "Unexpected Block in pseudo-class name, expected a function or identifier. '{}'", pseudo_class_token.to_debug_string());
            return SelectorParsingResult::SyntaxError;
        }
    }

    dbgln_if(CSS_PARSER_DEBUG, "!!! Invalid simple selector!");
    return SelectorParsingResult::SyntaxError;
}

NonnullRefPtrVector<StyleRule> Parser::consume_a_list_of_rules(bool top_level)
{
    return consume_a_list_of_rules(m_token_stream, top_level);
}

template<typename T>
NonnullRefPtrVector<StyleRule> Parser::consume_a_list_of_rules(TokenStream<T>& tokens, bool top_level)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_a_list_of_rules");

    NonnullRefPtrVector<StyleRule> rules;

    for (;;) {
        auto& token = tokens.next_token();

        if (token.is(Token::Type::Whitespace)) {
            continue;
        }

        if (token.is(Token::Type::EndOfFile)) {
            break;
        }

        if (token.is(Token::Type::CDO) || token.is(Token::Type::CDC)) {
            if (top_level) {
                continue;
            }

            tokens.reconsume_current_input_token();
            auto maybe_qualified = consume_a_qualified_rule(tokens);
            if (maybe_qualified) {
                rules.append(maybe_qualified.release_nonnull());
            }

            continue;
        }

        if (token.is(Token::Type::AtKeyword)) {
            tokens.reconsume_current_input_token();
            rules.append(consume_an_at_rule(tokens));
            continue;
        }

        tokens.reconsume_current_input_token();
        auto maybe_qualified = consume_a_qualified_rule(tokens);
        if (maybe_qualified) {
            rules.append(maybe_qualified.release_nonnull());
        }
    }

    return rules;
}

NonnullRefPtr<StyleRule> Parser::consume_an_at_rule()
{
    return consume_an_at_rule(m_token_stream);
}

template<typename T>
NonnullRefPtr<StyleRule> Parser::consume_an_at_rule(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_an_at_rule");

    auto& name_ident = tokens.next_token();
    VERIFY(name_ident.is(Token::Type::AtKeyword));

    NonnullRefPtr<StyleRule> rule = create<StyleRule>(StyleRule::Type::At);
    rule->m_name = ((Token)name_ident).at_keyword();

    for (;;) {
        auto& token = tokens.next_token();
        if (token.is(Token::Type::Semicolon)) {
            return rule;
        }

        if (token.is(Token::Type::EndOfFile)) {
            log_parse_error();
            return rule;
        }

        if (token.is(Token::Type::OpenCurly)) {
            rule->m_block = consume_a_simple_block(tokens);
            return rule;
        }

        // how is "simple block with an associated token of <{-token>" a valid token?

        tokens.reconsume_current_input_token();
        auto value = consume_a_component_value(tokens);
        rule->m_prelude.append(value);
    }
}

RefPtr<StyleRule> Parser::consume_a_qualified_rule()
{
    return consume_a_qualified_rule(m_token_stream);
}

template<typename T>
RefPtr<StyleRule> Parser::consume_a_qualified_rule(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_a_qualified_rule");

    NonnullRefPtr<StyleRule> rule = create<StyleRule>(StyleRule::Type::Qualified);

    for (;;) {
        auto& token = tokens.next_token();

        if (token.is(Token::Type::EndOfFile)) {
            log_parse_error();
            return {};
        }

        if (token.is(Token::Type::OpenCurly)) {
            rule->m_block = consume_a_simple_block(tokens);
            return rule;
        }

        // how is "simple block with an associated token of <{-token>" a valid token?

        tokens.reconsume_current_input_token();
        auto value = consume_a_component_value(tokens);
        rule->m_prelude.append(value);
    }

    return rule;
}

template<>
StyleComponentValueRule Parser::consume_a_component_value(TokenStream<StyleComponentValueRule>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_a_component_value - shortcut: '{}'", tokens.peek_token().to_debug_string());

    return tokens.next_token();
}

template<typename T>
StyleComponentValueRule Parser::consume_a_component_value(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_a_component_value");

    auto& token = tokens.next_token();

    if (token.is(Token::Type::OpenCurly) || token.is(Token::Type::OpenSquare) || token.is(Token::Type::OpenParen))
        return StyleComponentValueRule(consume_a_simple_block(tokens));

    if (token.is(Token::Type::Function))
        return StyleComponentValueRule(consume_a_function(tokens));

    return StyleComponentValueRule(token);
}

StyleComponentValueRule Parser::consume_a_component_value()
{
    return consume_a_component_value(m_token_stream);
}

NonnullRefPtr<StyleBlockRule> Parser::consume_a_simple_block()
{
    return consume_a_simple_block(m_token_stream);
}

template<typename T>
NonnullRefPtr<StyleBlockRule> Parser::consume_a_simple_block(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_a_simple_block");

    auto ending_token = ((Token)tokens.current_token()).mirror_variant();

    NonnullRefPtr<StyleBlockRule> block = create<StyleBlockRule>();
    block->m_token = tokens.current_token();

    for (;;) {
        auto& token = tokens.next_token();

        if (token.is(ending_token)) {
            return block;
        }

        if (token.is(Token::Type::EndOfFile)) {
            log_parse_error();
            return block;
        }

        tokens.reconsume_current_input_token();
        auto value = consume_a_component_value(tokens);
        block->m_values.append(value);
    }
}

NonnullRefPtr<StyleFunctionRule> Parser::consume_a_function()
{
    return consume_a_function(m_token_stream);
}

template<typename T>
NonnullRefPtr<StyleFunctionRule> Parser::consume_a_function(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_a_function");

    auto name_ident = tokens.current_token();
    VERIFY(name_ident.is(Token::Type::Function));
    NonnullRefPtr<StyleFunctionRule> function = create<StyleFunctionRule>(((Token)name_ident).m_value.to_string());

    for (;;) {
        auto& token = tokens.next_token();
        if (token.is(Token::Type::CloseParen)) {
            return function;
        }

        if (token.is(Token::Type::EndOfFile)) {
            log_parse_error();
            return function;
        }

        tokens.reconsume_current_input_token();
        auto value = consume_a_component_value(tokens);
        function->m_values.append(value);
    }

    return function;
}

Optional<StyleDeclarationRule> Parser::consume_a_declaration()
{
    return consume_a_declaration(m_token_stream);
}

template<typename T>
Optional<StyleDeclarationRule> Parser::consume_a_declaration(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_a_declaration");

    auto& token = tokens.next_token();

    StyleDeclarationRule declaration;
    VERIFY(token.is(Token::Type::Ident));
    declaration.m_name = ((Token)token).ident();

    tokens.skip_whitespace();

    auto& maybe_colon = tokens.next_token();
    if (!maybe_colon.is(Token::Type::Colon)) {
        log_parse_error();
        return {};
    }

    tokens.skip_whitespace();

    for (;;) {
        if (tokens.peek_token().is(Token::Type::EndOfFile)) {
            break;
        }
        declaration.m_values.append(consume_a_component_value(tokens));
    }

    if (declaration.m_values.size() >= 2) {
        auto second_last = declaration.m_values.at(declaration.m_values.size() - 2);
        auto last = declaration.m_values.at(declaration.m_values.size() - 1);

        if (second_last.m_type == StyleComponentValueRule::ComponentType::Token && last.m_type == StyleComponentValueRule::ComponentType::Token) {
            auto last_token = last.m_token;
            auto second_last_token = second_last.m_token;

            if (second_last_token.is(Token::Type::Delim) && second_last_token.m_value.to_string().equals_ignoring_case("!")) {
                if (last_token.is(Token::Type::Ident) && last_token.m_value.to_string().equals_ignoring_case("important")) {
                    declaration.m_values.remove(declaration.m_values.size() - 2);
                    declaration.m_values.remove(declaration.m_values.size() - 1);
                    declaration.m_important = true;
                }
            }
        }
    }

    while (!declaration.m_values.is_empty()) {
        auto maybe_whitespace = declaration.m_values.last();
        if (!(maybe_whitespace.is(Token::Type::Whitespace))) {
            break;
        }
        declaration.m_values.take_last();
    }

    return declaration;
}

Vector<DeclarationOrAtRule> Parser::consume_a_list_of_declarations()
{
    return consume_a_list_of_declarations(m_token_stream);
}

template<typename T>
Vector<DeclarationOrAtRule> Parser::consume_a_list_of_declarations(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::consume_a_list_of_declarations");

    Vector<DeclarationOrAtRule> list;

    for (;;) {
        auto& token = tokens.next_token();
        if (token.is(Token::Type::Whitespace) || token.is(Token::Type::Semicolon)) {
            continue;
        }

        if (token.is(Token::Type::EndOfFile)) {
            return list;
        }

        if (token.is(Token::Type::AtKeyword)) {
            tokens.reconsume_current_input_token();
            list.append(DeclarationOrAtRule(consume_an_at_rule(tokens)));
            continue;
        }

        if (token.is(Token::Type::Ident)) {
            Vector<StyleComponentValueRule> temp;
            temp.append(token);

            for (;;) {
                auto& peek = tokens.peek_token();
                if (peek.is(Token::Type::Semicolon) || peek.is(Token::Type::EndOfFile)) {
                    break;
                }
                temp.append(consume_a_component_value(tokens));
            }

            auto token_stream = TokenStream(temp);
            auto maybe_declaration = consume_a_declaration(token_stream);
            if (maybe_declaration.has_value()) {
                list.append(DeclarationOrAtRule(maybe_declaration.value()));
            }
            continue;
        }

        log_parse_error();
        tokens.reconsume_current_input_token();

        for (;;) {
            auto& peek = tokens.peek_token();
            if (peek.is(Token::Type::Semicolon) || peek.is(Token::Type::EndOfFile))
                break;
            dbgln("Discarding token: '{}'", peek.to_debug_string());
            (void)consume_a_component_value(tokens);
        }
    }

    return list;
}

RefPtr<CSSRule> Parser::parse_as_rule()
{
    return parse_as_rule(m_token_stream);
}

template<typename T>
RefPtr<CSSRule> Parser::parse_as_rule(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_as_rule");

    RefPtr<CSSRule> rule;

    tokens.skip_whitespace();

    auto& token = tokens.peek_token();

    if (token.is(Token::Type::EndOfFile)) {
        return {};
    } else if (token.is(Token::Type::AtKeyword)) {
        auto at_rule = consume_an_at_rule();
        rule = convert_to_rule(at_rule);
    } else {
        auto qualified_rule = consume_a_qualified_rule(tokens);
        if (!qualified_rule)
            return {};

        rule = convert_to_rule(*qualified_rule);
    }

    tokens.skip_whitespace();

    auto& maybe_eof = tokens.peek_token();
    if (maybe_eof.is(Token::Type::EndOfFile)) {
        return rule;
    }

    return {};
}

NonnullRefPtrVector<CSSRule> Parser::parse_as_list_of_rules()
{
    return parse_as_list_of_rules(m_token_stream);
}

template<typename T>
NonnullRefPtrVector<CSSRule> Parser::parse_as_list_of_rules(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_as_list_of_rules");

    auto parsed_rules = consume_a_list_of_rules(tokens, false);
    NonnullRefPtrVector<CSSRule> rules;

    for (auto& rule : parsed_rules) {
        auto converted_rule = convert_to_rule(rule);
        if (converted_rule)
            rules.append(*converted_rule);
    }

    return rules;
}

Optional<StyleProperty> Parser::parse_as_declaration()
{
    return parse_as_declaration(m_token_stream);
}

template<typename T>
Optional<StyleProperty> Parser::parse_as_declaration(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_as_declaration");

    tokens.skip_whitespace();

    auto& token = tokens.peek_token();

    if (!token.is(Token::Type::Ident)) {
        return {};
    }

    auto declaration = consume_a_declaration(tokens);
    if (declaration.has_value())
        return convert_to_style_property(declaration.value());

    return {};
}

RefPtr<CSSStyleDeclaration> Parser::parse_as_list_of_declarations()
{
    return parse_as_list_of_declarations(m_token_stream);
}

template<typename T>
RefPtr<CSSStyleDeclaration> Parser::parse_as_list_of_declarations(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_as_list_of_declarations");

    auto declarations_and_at_rules = consume_a_list_of_declarations(tokens);

    Vector<StyleProperty> properties;
    HashMap<String, StyleProperty> custom_properties;

    for (auto& declaration_or_at_rule : declarations_and_at_rules) {
        if (declaration_or_at_rule.is_at_rule()) {
            dbgln("Parser::parse_as_list_of_declarations(): At-rule is not allowed here!");
            continue;
        }

        auto& declaration = declaration_or_at_rule.m_declaration;

        auto maybe_property = convert_to_style_property(declaration);
        if (maybe_property.has_value()) {
            auto property = maybe_property.value();
            if (property.property_id == PropertyID::Custom) {
                custom_properties.set(property.custom_name, property);
            } else {
                properties.append(property);
            }
        }
    }

    return CSSStyleDeclaration::create(move(properties), move(custom_properties));
}

Optional<StyleComponentValueRule> Parser::parse_as_component_value()
{
    return parse_as_component_value(m_token_stream);
}

template<typename T>
Optional<StyleComponentValueRule> Parser::parse_as_component_value(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_as_component_value");

    tokens.skip_whitespace();

    auto& token = tokens.peek_token();

    if (token.is(Token::Type::EndOfFile)) {
        return {};
    }

    auto value = consume_a_component_value(tokens);

    tokens.skip_whitespace();

    auto& maybe_eof = tokens.peek_token();
    if (maybe_eof.is(Token::Type::EndOfFile)) {
        return value;
    }

    return {};
}

Vector<StyleComponentValueRule> Parser::parse_as_list_of_component_values()
{
    return parse_as_list_of_component_values(m_token_stream);
}

template<typename T>
Vector<StyleComponentValueRule> Parser::parse_as_list_of_component_values(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_as_list_of_component_values");

    Vector<StyleComponentValueRule> rules;

    for (;;) {
        if (tokens.peek_token().is(Token::Type::EndOfFile)) {
            break;
        }

        rules.append(consume_a_component_value(tokens));
    }

    return rules;
}

Vector<Vector<StyleComponentValueRule>> Parser::parse_as_comma_separated_list_of_component_values()
{
    return parse_as_comma_separated_list_of_component_values(m_token_stream);
}

template<typename T>
Vector<Vector<StyleComponentValueRule>> Parser::parse_as_comma_separated_list_of_component_values(TokenStream<T>& tokens)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_as_comma_separated_list_of_component_values");

    Vector<Vector<StyleComponentValueRule>> lists;
    lists.append({});

    for (;;) {
        auto& next = tokens.next_token();

        if (next.is(Token::Type::Comma)) {
            lists.append({});
            continue;
        } else if (next.is(Token::Type::EndOfFile)) {
            break;
        }

        tokens.reconsume_current_input_token();
        auto component_value = consume_a_component_value(tokens);
        lists.last().append(component_value);
    }

    return lists;
}

Optional<URL> Parser::parse_url_function(ParsingContext const& context, StyleComponentValueRule const& component_value)
{
    // FIXME: Handle list of media queries. https://www.w3.org/TR/css-cascade-3/#conditional-import

    if (component_value.is(Token::Type::Url))
        return context.complete_url(component_value.token().url());
    if (component_value.is_function() && component_value.function().name().equals_ignoring_case("url")) {
        auto& function_values = component_value.function().values();
        // FIXME: Handle url-modifiers. https://www.w3.org/TR/css-values-4/#url-modifiers
        for (size_t i = 0; i < function_values.size(); ++i) {
            auto& value = function_values[i];
            if (value.is(Token::Type::Whitespace))
                continue;
            if (value.is(Token::Type::String)) {
                // FIXME: RFC2397
                if (value.token().string().starts_with("data:"))
                    break;
                return context.complete_url(value.token().string());
            }
        }
    }

    return {};
}

RefPtr<CSSRule> Parser::convert_to_rule(NonnullRefPtr<StyleRule> rule)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::convert_to_rule");

    if (rule->m_type == StyleRule::Type::At) {
        if (rule->m_name.equals_ignoring_case("import"sv) && !rule->prelude().is_empty()) {

            Optional<URL> url;
            for (auto& token : rule->prelude()) {
                if (token.is(Token::Type::Whitespace))
                    continue;

                if (token.is(Token::Type::String)) {
                    url = m_context.complete_url(token.token().string());
                } else {
                    url = parse_url_function(m_context, token);
                }

                // FIXME: Handle list of media queries. https://www.w3.org/TR/css-cascade-3/#conditional-import
                if (url.has_value())
                    break;
            }

            if (url.has_value())
                return CSSImportRule::create(url.value());
            else
                dbgln("Unable to parse url from @import rule");
        } else {
            dbgln("Unrecognized CSS at-rule: {}", rule->m_name);
        }

        // FIXME: More at rules!

    } else {
        auto prelude_stream = TokenStream(rule->m_prelude);
        auto selectors = parse_a_selector(prelude_stream);
        if (!selectors.has_value() || selectors.value().is_empty()) {
            dbgln("CSSParser: style rule selectors invalid; discarding.");
            prelude_stream.dump_all_tokens();
            return {};
        }

        auto declaration = convert_to_declaration(*rule->m_block);
        if (!declaration) {
            dbgln("CSSParser: style rule declaration invalid; discarding.");
            return {};
        }

        return CSSStyleRule::create(move(selectors.value()), move(*declaration));
    }

    return {};
}

RefPtr<CSSStyleDeclaration> Parser::convert_to_declaration(NonnullRefPtr<StyleBlockRule> block)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::convert_to_declaration");

    if (!block->is_curly())
        return {};

    auto stream = TokenStream(block->m_values);
    return parse_as_list_of_declarations(stream);
}

Optional<StyleProperty> Parser::convert_to_style_property(StyleDeclarationRule& declaration)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::convert_to_style_property");

    auto& property_name = declaration.m_name;
    auto property_id = property_id_from_string(property_name);
    if (property_id == PropertyID::Invalid && property_name.starts_with("--"))
        property_id = PropertyID::Custom;

    if (property_id == PropertyID::Invalid && !property_name.starts_with("-")) {
        dbgln("Parser::convert_to_style_property(): Unrecognized property '{}'", property_name);
        return {};
    }

    auto value_token_stream = TokenStream(declaration.m_values);
    auto value = parse_css_value(property_id, value_token_stream);
    if (!value) {
        dbgln("Parser::convert_to_style_property(): Property '{}' has no value.", property_name);
        return {};
    }

    if (property_id == PropertyID::Custom) {
        return StyleProperty { property_id, value.release_nonnull(), declaration.m_name, declaration.m_important };
    } else {
        return StyleProperty { property_id, value.release_nonnull(), {}, declaration.m_important };
    }
}

Optional<float> Parser::try_parse_float(StringView string)
{
    // FIXME: This is copied from DeprecatedCSSParser, so may not be to spec.

    const char* str = string.characters_without_null_termination();
    size_t len = string.length();
    size_t weight = 1;
    int exp_val = 0;
    float value = 0.0f;
    float fraction = 0.0f;
    bool has_sign = false;
    bool is_negative = false;
    bool is_fractional = false;
    bool is_scientific = false;

    if (str[0] == '-') {
        is_negative = true;
        has_sign = true;
    }
    if (str[0] == '+') {
        has_sign = true;
    }

    for (size_t i = has_sign; i < len; i++) {

        // Looks like we're about to start working on the fractional part
        if (str[i] == '.') {
            is_fractional = true;
            continue;
        }

        if (str[i] == 'e' || str[i] == 'E') {
            if (str[i + 1] == '-' || str[i + 1] == '+')
                exp_val = atoi(str + i + 2);
            else
                exp_val = atoi(str + i + 1);

            is_scientific = true;
            continue;
        }

        if (str[i] < '0' || str[i] > '9' || exp_val != 0) {
            return {};
            continue;
        }

        if (is_fractional) {
            fraction *= 10;
            fraction += str[i] - '0';
            weight *= 10;
        } else {
            value = value * 10;
            value += str[i] - '0';
        }
    }

    fraction /= weight;
    value += fraction;

    if (is_scientific) {
        bool divide = exp_val < 0;
        if (divide)
            exp_val *= -1;

        for (int i = 0; i < exp_val; i++) {
            if (divide)
                value /= 10;
            else
                value *= 10;
        }
    }

    return is_negative ? -value : value;
}

RefPtr<StyleValue> Parser::parse_keyword_or_custom_value(ParsingContext const&, StyleComponentValueRule const& component_value)
{
    if (component_value.is(Token::Type::Ident)) {
        auto ident = component_value.token().ident();
        if (ident.equals_ignoring_case("inherit"))
            return InheritStyleValue::create();
        if (ident.equals_ignoring_case("initial"))
            return InitialStyleValue::create();
        if (ident.equals_ignoring_case("auto"))
            return LengthStyleValue::create(Length::make_auto());
        // FIXME: Implement `unset` keyword
    }

    if (component_value.is_function() && component_value.function().name().equals_ignoring_case("var")) {
        // FIXME: Handle fallback value as second parameter
        // https://www.w3.org/TR/css-variables-1/#using-variables
        if (!component_value.function().values().is_empty()) {
            auto& property_name_token = component_value.function().values().first();
            if (property_name_token.is(Token::Type::Ident))
                return CustomStyleValue::create(property_name_token.token().ident());
            else
                dbgln("First argument to var() function was not an ident: '{}'", property_name_token.to_debug_string());
        }
    }

    return {};
}

RefPtr<StyleValue> Parser::parse_length_value(ParsingContext const& context, StyleComponentValueRule const& component_value)
{
    auto parse_length = [&]() -> Optional<Length> {
        Length::Type type = Length::Type::Undefined;
        Optional<float> numeric_value;

        if (component_value.is(Token::Type::Dimension)) {
            auto length_string = component_value.token().m_value.string_view();
            auto unit_string = component_value.token().m_unit.string_view();

            if (unit_string.equals_ignoring_case("%")) {
                type = Length::Type::Percentage;
            } else if (unit_string.equals_ignoring_case("px")) {
                type = Length::Type::Px;
            } else if (unit_string.equals_ignoring_case("pt")) {
                type = Length::Type::Pt;
            } else if (unit_string.equals_ignoring_case("pc")) {
                type = Length::Type::Pc;
            } else if (unit_string.equals_ignoring_case("mm")) {
                type = Length::Type::Mm;
            } else if (unit_string.equals_ignoring_case("rem")) {
                type = Length::Type::Rem;
            } else if (unit_string.equals_ignoring_case("em")) {
                type = Length::Type::Em;
            } else if (unit_string.equals_ignoring_case("ex")) {
                type = Length::Type::Ex;
            } else if (unit_string.equals_ignoring_case("vw")) {
                type = Length::Type::Vw;
            } else if (unit_string.equals_ignoring_case("vh")) {
                type = Length::Type::Vh;
            } else if (unit_string.equals_ignoring_case("vmax")) {
                type = Length::Type::Vmax;
            } else if (unit_string.equals_ignoring_case("vmin")) {
                type = Length::Type::Vmin;
            } else if (unit_string.equals_ignoring_case("cm")) {
                type = Length::Type::Cm;
            } else if (unit_string.equals_ignoring_case("in")) {
                type = Length::Type::In;
            } else if (unit_string.equals_ignoring_case("Q")) {
                type = Length::Type::Q;
            } else if (context.in_quirks_mode()) {
                type = Length::Type::Px;
            }

            numeric_value = try_parse_float(length_string);
        } else if (component_value.is(Token::Type::Number)) {
            auto value_string = component_value.token().m_value.string_view();
            if (value_string == "0") {
                type = Length::Type::Px;
                numeric_value = 0;
            } else if (context.in_quirks_mode()) {
                type = Length::Type::Px;
                numeric_value = try_parse_float(value_string);
            }
        } else if (component_value.is(Token::Type::Percentage)) {
            type = Length::Type::Percentage;
            auto value_string = component_value.token().m_value.string_view();
            numeric_value = try_parse_float(value_string);
        }

        if (!numeric_value.has_value())
            return {};

        return Length(numeric_value.value(), type);
    };

    if (component_value.is(Token::Type::Dimension) || component_value.is(Token::Type::Number) || component_value.is(Token::Type::Percentage)) {
        auto length = parse_length();
        if (length.has_value())
            return LengthStyleValue::create(length.value());
    }

    return {};
}

RefPtr<StyleValue> Parser::parse_numeric_value(ParsingContext const&, StyleComponentValueRule const& component_value)
{
    if (component_value.is(Token::Type::Number)) {
        auto number = component_value.token();
        if (number.m_number_type == Token::NumberType::Integer) {
            return NumericStyleValue::create(number.integer());
        } else {
            auto float_value = try_parse_float(number.m_value.string_view());
            if (float_value.has_value())
                return NumericStyleValue::create(float_value.value());
        }
    }

    return {};
}

RefPtr<StyleValue> Parser::parse_identifier_value(ParsingContext const&, StyleComponentValueRule const& component_value)
{
    if (component_value.is(Token::Type::Ident)) {
        auto value_id = value_id_from_string(component_value.token().ident());
        if (value_id != ValueID::Invalid)
            return IdentifierStyleValue::create(value_id);
    }

    return {};
}

RefPtr<StyleValue> Parser::parse_color_value(ParsingContext const&, StyleComponentValueRule const& component_value)
{
    // https://www.w3.org/TR/css-color-3/
    auto parse_css_color = [&]() -> Optional<Color> {
        if (component_value.is(Token::Type::Ident)) {
            auto ident = component_value.token().ident();
            if (ident.equals_ignoring_case("transparent"))
                return Color::from_rgba(0x00000000);

            auto color = Color::from_string(ident.to_string().to_lowercase());
            if (color.has_value())
                return color;

        } else if (component_value.is(Token::Type::Hash)) {
            // FIXME: Read it directly
            auto color = Color::from_string(String::formatted("#{}", component_value.token().m_value.to_string().to_lowercase()));
            if (color.has_value())
                return color;

        } else if (component_value.is_function()) {
            auto& function = component_value.function();
            auto& values = function.values();

            Vector<Token> params;
            for (size_t i = 0; i < values.size(); ++i) {
                auto& value = values.at(i);
                if (value.is(Token::Type::Whitespace))
                    continue;

                if (value.is(Token::Type::Percentage) || value.is(Token::Type::Number)) {
                    params.append(value.token());
                    // Eat following comma and whitespace
                    while ((i + 1) < values.size()) {
                        auto& next = values.at(i + 1);
                        if (next.is(Token::Type::Whitespace))
                            i++;
                        else if (next.is(Token::Type::Comma))
                            break;

                        return {};
                    }
                }
            }

            if (function.name().equals_ignoring_case("rgb")) {
                if (params.size() != 3)
                    return {};

                auto r_val = params[0];
                auto g_val = params[1];
                auto b_val = params[2];

                if (r_val.is(Token::NumberType::Integer)
                    && g_val.is(Token::NumberType::Integer)
                    && b_val.is(Token::NumberType::Integer)) {

                    auto maybe_r = r_val.m_value.string_view().to_uint<u8>();
                    auto maybe_g = g_val.m_value.string_view().to_uint<u8>();
                    auto maybe_b = b_val.m_value.string_view().to_uint<u8>();
                    if (maybe_r.has_value() && maybe_g.has_value() && maybe_b.has_value())
                        return Color(maybe_r.value(), maybe_g.value(), maybe_b.value());

                } else if (r_val.is(Token::Type::Percentage)
                    && g_val.is(Token::Type::Percentage)
                    && b_val.is(Token::Type::Percentage)) {

                    auto maybe_r = try_parse_float(r_val.m_value.string_view());
                    auto maybe_g = try_parse_float(g_val.m_value.string_view());
                    auto maybe_b = try_parse_float(b_val.m_value.string_view());
                    if (maybe_r.has_value() && maybe_g.has_value() && maybe_b.has_value()) {
                        u8 r = clamp(lroundf(maybe_r.value() * 2.55f), 0, 255);
                        u8 g = clamp(lroundf(maybe_g.value() * 2.55f), 0, 255);
                        u8 b = clamp(lroundf(maybe_b.value() * 2.55f), 0, 255);
                        return Color(r, g, b);
                    }
                }
            } else if (function.name().equals_ignoring_case("rgba")) {
                if (params.size() != 4)
                    return {};

                auto r_val = params[0];
                auto g_val = params[1];
                auto b_val = params[2];
                auto a_val = params[3];

                if (r_val.is(Token::NumberType::Integer)
                    && g_val.is(Token::NumberType::Integer)
                    && b_val.is(Token::NumberType::Integer)
                    && a_val.is(Token::Type::Number)) {

                    auto maybe_r = r_val.m_value.string_view().to_uint<u8>();
                    auto maybe_g = g_val.m_value.string_view().to_uint<u8>();
                    auto maybe_b = b_val.m_value.string_view().to_uint<u8>();
                    auto maybe_a = try_parse_float(a_val.m_value.string_view());
                    if (maybe_r.has_value() && maybe_g.has_value() && maybe_b.has_value() && maybe_a.has_value()) {
                        u8 a = clamp(lroundf(maybe_a.value() * 255.0f), 0, 255);
                        return Color(maybe_r.value(), maybe_g.value(), maybe_b.value(), a);
                    }

                } else if (r_val.is(Token::Type::Percentage)
                    && g_val.is(Token::Type::Percentage)
                    && b_val.is(Token::Type::Percentage)
                    && a_val.is(Token::Type::Number)) {

                    auto maybe_r = try_parse_float(r_val.m_value.string_view());
                    auto maybe_g = try_parse_float(g_val.m_value.string_view());
                    auto maybe_b = try_parse_float(b_val.m_value.string_view());
                    auto maybe_a = try_parse_float(a_val.m_value.string_view());
                    if (maybe_r.has_value() && maybe_g.has_value() && maybe_b.has_value() && maybe_a.has_value()) {
                        u8 r = clamp(lroundf(maybe_r.value() * 2.55f), 0, 255);
                        u8 g = clamp(lroundf(maybe_g.value() * 2.55f), 0, 255);
                        u8 b = clamp(lroundf(maybe_b.value() * 2.55f), 0, 255);
                        u8 a = clamp(lroundf(maybe_a.value() * 255.0f), 0, 255);
                        return Color(r, g, b, a);
                    }
                }
            } else if (function.name().equals_ignoring_case("hsl")) {
                if (params.size() != 3)
                    return {};

                auto h_val = params[0];
                auto s_val = params[1];
                auto l_val = params[2];

                if (h_val.is(Token::Type::Number)
                    && s_val.is(Token::Type::Percentage)
                    && l_val.is(Token::Type::Percentage)) {

                    auto maybe_h = try_parse_float(h_val.m_value.string_view());
                    auto maybe_s = try_parse_float(s_val.m_value.string_view());
                    auto maybe_l = try_parse_float(l_val.m_value.string_view());
                    if (maybe_h.has_value() && maybe_s.has_value() && maybe_l.has_value()) {
                        float h = maybe_h.value();
                        float s = maybe_s.value() / 100.0f;
                        float l = maybe_l.value() / 100.0f;
                        return Color::from_hsl(h, s, l);
                    }
                }
            } else if (function.name().equals_ignoring_case("hsla")) {
                if (params.size() != 4)
                    return {};

                auto h_val = params[0];
                auto s_val = params[1];
                auto l_val = params[2];
                auto a_val = params[3];

                if (h_val.is(Token::Type::Number)
                    && s_val.is(Token::Type::Percentage)
                    && l_val.is(Token::Type::Percentage)
                    && a_val.is(Token::Type::Number)) {

                    auto maybe_h = try_parse_float(h_val.m_value.string_view());
                    auto maybe_s = try_parse_float(s_val.m_value.string_view());
                    auto maybe_l = try_parse_float(l_val.m_value.string_view());
                    auto maybe_a = try_parse_float(a_val.m_value.string_view());
                    if (maybe_h.has_value() && maybe_s.has_value() && maybe_l.has_value() && maybe_a.has_value()) {
                        float h = maybe_h.value();
                        float s = maybe_s.value() / 100.0f;
                        float l = maybe_l.value() / 100.0f;
                        float a = maybe_a.value();
                        return Color::from_hsla(h, s, l, a);
                    }
                }
            }
            return {};
        }

        return {};
    };

    auto color = parse_css_color();
    if (color.has_value())
        return ColorStyleValue::create(color.value());

    return {};
}

RefPtr<StyleValue> Parser::parse_string_value(ParsingContext const&, StyleComponentValueRule const& component_value)
{
    if (component_value.is(Token::Type::String))
        return StringStyleValue::create(component_value.token().string());

    return {};
}

RefPtr<StyleValue> Parser::parse_image_value(ParsingContext const& context, StyleComponentValueRule const& component_value)
{
    auto url = parse_url_function(context, component_value);
    if (url.has_value())
        return ImageStyleValue::create(url.value(), *context.document());
    // FIXME: Handle gradients.

    return {};
}

RefPtr<StyleValue> Parser::parse_css_value(PropertyID property_id, TokenStream<StyleComponentValueRule>& tokens)
{
    Vector<StyleComponentValueRule> component_values;

    while (tokens.has_next_token()) {
        auto& token = tokens.next_token();

        if (token.is(Token::Type::Semicolon)) {
            tokens.reconsume_current_input_token();
            break;
        }

        if (token.is(Token::Type::Whitespace))
            continue;

        component_values.append(token);
    }

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

    if (component_values.size() == 1)
        return parse_css_value(m_context, property_id, component_values.first());

    return ValueListStyleValue::create(move(component_values));
}

RefPtr<StyleValue> Parser::parse_css_value(ParsingContext const& context, PropertyID property_id, StyleComponentValueRule const& component_value)
{
    // FIXME: Figure out if we still need takes_integer_value, and if so, move this information
    // into Properties.json.
    auto takes_integer_value = [](PropertyID property_id) -> bool {
        return property_id == PropertyID::ZIndex
            || property_id == PropertyID::FontWeight
            || property_id == PropertyID::Custom;
    };
    if (takes_integer_value(property_id) && component_value.is(Token::Type::Number)) {
        auto number = component_value.token();
        if (number.m_number_type == Token::NumberType::Integer) {
            return LengthStyleValue::create(Length::make_px(number.integer()));
        }
    }

    if (auto keyword_or_custom = parse_keyword_or_custom_value(context, component_value))
        return keyword_or_custom;

    if (auto length = parse_length_value(context, component_value))
        return length;

    if (auto numeric = parse_numeric_value(context, component_value))
        return numeric;

    if (auto identifier = parse_identifier_value(context, component_value))
        return identifier;

    if (auto color = parse_color_value(context, component_value))
        return color;

    if (auto string = parse_string_value(context, component_value))
        return string;

    if (auto image = parse_image_value(context, component_value))
        return image;

    return {};
}

Optional<Selector::SimpleSelector::NthChildPattern> Parser::parse_nth_child_pattern(TokenStream<StyleComponentValueRule>& values)
{
    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_nth_child_pattern");

    Selector::SimpleSelector::NthChildPattern pattern;

    auto current_value = values.next_token();
    if (current_value.is(Token::Type::Ident)) {
        auto ident = current_value.token().ident();
        if (ident.equals_ignoring_case("odd")) {
            pattern.step_size = 2;
            pattern.offset = 1;
            return pattern;

        } else if (ident.equals_ignoring_case("even")) {
            pattern.step_size = 2;
            return pattern;
        }
    }

    // Try to match any of following patterns:
    // 1. An+B
    // 2. An
    // 3. B
    // ...where "A" is "step_size", "B" is "offset" and rest are literals.
    // "A" can be omitted, in that case "A" = 1.
    // "A" may have "+" or "-" sign, "B" always must be predated by sign for pattern (1).

    auto is_n = [](StyleComponentValueRule value) -> bool {
        return value.is(Token::Type::Ident) && value.token().ident().equals_ignoring_case("n");
    };

    auto is_delim = [](StyleComponentValueRule value, StringView delim) -> bool {
        return value.is(Token::Type::Delim) && value.token().delim().equals_ignoring_case(delim);
    };

    int step_size_or_offset = 0;

    // "When a=1, or a=-1, the 1 may be omitted from the rule."
    if (is_n(current_value)) {
        step_size_or_offset = +1;
    } else if (is_delim(current_value, "+"sv) && is_n(values.peek_token())) {
        step_size_or_offset = +1;
        values.next_token();
    } else if (is_delim(current_value, "-"sv) && is_n(values.peek_token())) {
        step_size_or_offset = -1;
        values.next_token();
    } else if (current_value.is(Token::Type::Number)) {
        step_size_or_offset = current_value.token().integer();
    } else {
        values.reconsume_current_input_token();
    }

    current_value = values.next_token();

    if (is_n(current_value)) {
        values.skip_whitespace();

        auto next_value = values.peek_token();
        if (is_delim(next_value, "+") || is_delim(next_value, "-")) {
            const auto sign = is_delim(next_value, "+") ? 1 : -1;
            values.next_token();

            values.skip_whitespace();

            // "An+B" pattern
            auto number = values.next_token();
            if (!number.is(Token::Type::Number))
                return {};

            pattern.step_size = step_size_or_offset;
            pattern.offset = sign * number.token().integer();
        } else {
            // "An" pattern
            pattern.step_size = step_size_or_offset;
        }
    } else {
        // "B" pattern
        pattern.offset = step_size_or_offset;
    }

    if (values.has_next_token())
        return {};

    return pattern;
}
}