/*
 * Copyright (c) 2021, Itamar S. <itamar8910@gmail.com>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include "Parser.h"
#include "AST.h"
#include <AK/Debug.h>
#include <AK/ScopeGuard.h>
#include <AK/ScopeLogger.h>
#include <LibCpp/Lexer.h>

#define LOG_SCOPE() ScopeLogger<CPP_DEBUG> logger(String::formatted("'{}' - {} ({})", peek().text(), peek().type_as_string(), m_state.token_index))

namespace Cpp {

Parser::Parser(Vector<Token> tokens, String const& filename)
    : m_filename(filename)
    , m_tokens(move(tokens))
{
    if constexpr (CPP_DEBUG) {
        dbgln("Tokens:");
        for (size_t i = 0; i < m_tokens.size(); ++i) {
            dbgln("{}- {}", i, m_tokens[i].to_string());
        }
    }
}

NonnullRefPtr<TranslationUnit> Parser::parse()
{
    LOG_SCOPE();
    if (m_tokens.is_empty())
        return create_root_ast_node({}, {});
    auto unit = create_root_ast_node(m_tokens.first().start(), m_tokens.last().end());
    unit->set_declarations(parse_declarations_in_translation_unit(*unit));
    return unit;
}

NonnullRefPtrVector<Declaration> Parser::parse_declarations_in_translation_unit(ASTNode& parent)
{
    NonnullRefPtrVector<Declaration> declarations;
    while (!eof()) {
        auto declaration = parse_single_declaration_in_translation_unit(parent);
        if (declaration) {
            declarations.append(declaration.release_nonnull());
        } else {
            error("unexpected token"sv);
            consume();
        }
    }
    return declarations;
}

RefPtr<Declaration> Parser::parse_single_declaration_in_translation_unit(ASTNode& parent)
{
    while (!eof()) {
        if (match_comment()) {
            consume(Token::Type::Comment);
            continue;
        }

        if (match_preprocessor()) {
            consume_preprocessor();
            continue;
        }

        auto declaration = match_declaration_in_translation_unit();
        if (declaration.has_value()) {
            return parse_declaration(parent, declaration.value());
        }
        return {};
    }
    return {};
}

NonnullRefPtr<Declaration> Parser::parse_declaration(ASTNode& parent, DeclarationType declaration_type)
{
    switch (declaration_type) {
    case DeclarationType::Function:
        return parse_function_declaration(parent);
    case DeclarationType::Variable:
        return parse_variable_declaration(parent);
    case DeclarationType::Enum:
        return parse_enum_declaration(parent);
    case DeclarationType::Class:
        return parse_class_declaration(parent);
    case DeclarationType::Namespace:
        return parse_namespace_declaration(parent);
    case DeclarationType::Constructor:
        return parse_constructor(parent);
    case DeclarationType::Destructor:
        return parse_destructor(parent);
    default:
        error("unexpected declaration type"sv);
        return create_ast_node<InvalidDeclaration>(parent, position(), position());
    }
}

NonnullRefPtr<FunctionDeclaration> Parser::parse_function_declaration(ASTNode& parent)
{
    auto func = create_ast_node<FunctionDeclaration>(parent, position(), {});

    func->set_qualifiers(parse_function_qualifiers());
    func->set_return_type(parse_type(*func));

    func->set_name(parse_name(*func));

    consume(Token::Type::LeftParen);
    auto parameters = parse_parameter_list(*func);
    if (parameters.has_value())
        func->set_parameters(parameters.value());

    consume(Token::Type::RightParen);

    while (match_keyword("const") || match_keyword("override")) {
        consume();
        // FIXME: Note that this function is supposed to be a class member, and `this` has to be const, somehow.
    }

    RefPtr<FunctionDefinition> body;
    Position func_end {};
    if (peek(Token::Type::LeftCurly).has_value()) {
        body = parse_function_definition(*func);
        func_end = body->end();
    } else {
        func_end = position();
        if (match_attribute_specification())
            consume_attribute_specification(); // we don't use the value of __attribute__
        consume(Token::Type::Semicolon);
    }

    func->set_definition(move(body));
    func->set_end(func_end);
    return func;
}

NonnullRefPtr<FunctionDefinition> Parser::parse_function_definition(ASTNode& parent)
{
    LOG_SCOPE();
    auto func = create_ast_node<FunctionDefinition>(parent, position(), {});
    consume(Token::Type::LeftCurly);
    while (!eof() && peek().type() != Token::Type::RightCurly) {
        func->add_statement(parse_statement(func));
    }
    func->set_end(position());
    if (!eof())
        consume(Token::Type::RightCurly);
    return func;
}

NonnullRefPtr<Statement> Parser::parse_statement(ASTNode& parent)
{
    LOG_SCOPE();
    ArmedScopeGuard consume_semicolon([this]() {
        consume(Token::Type::Semicolon);
    });

    if (match_block_statement()) {
        consume_semicolon.disarm();
        return parse_block_statement(parent);
    }
    if (match_comment()) {
        consume_semicolon.disarm();
        return parse_comment(parent);
    }
    if (match_variable_declaration()) {
        return parse_variable_declaration(parent, false);
    }
    if (match_expression()) {
        return parse_expression(parent);
    }
    if (match_keyword("return")) {
        return parse_return_statement(parent);
    }
    if (match_keyword("for")) {
        consume_semicolon.disarm();
        return parse_for_statement(parent);
    }
    if (match_keyword("if")) {
        consume_semicolon.disarm();
        return parse_if_statement(parent);
    } else {
        error("unexpected statement type"sv);
        consume_semicolon.disarm();
        consume();
        return create_ast_node<InvalidStatement>(parent, position(), position());
    }
}

NonnullRefPtr<Comment> Parser::parse_comment(ASTNode& parent)
{
    auto comment = create_ast_node<Comment>(parent, position(), {});
    consume(Token::Type::Comment);
    comment->set_end(position());
    return comment;
}

bool Parser::match_block_statement()
{
    return peek().type() == Token::Type::LeftCurly;
}

NonnullRefPtr<BlockStatement> Parser::parse_block_statement(ASTNode& parent)
{
    LOG_SCOPE();
    auto block_statement = create_ast_node<BlockStatement>(parent, position(), {});
    consume(Token::Type::LeftCurly);
    while (!eof() && peek().type() != Token::Type::RightCurly) {
        block_statement->add_statement(parse_statement(*block_statement));
    }
    consume(Token::Type::RightCurly);
    block_statement->set_end(position());
    return block_statement;
}

bool Parser::match_type()
{
    return match_named_type();
}

bool Parser::match_named_type()
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    parse_type_qualifiers();
    if (match_keyword("auto")) {
        return true;
    }

    if (match_keyword("struct")) {
        consume(Token::Type::Keyword); // Consume struct prefix
    }

    if (!match_name())
        return false;

    return true;
}

bool Parser::match_template_arguments()
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    if (!peek(Token::Type::Less).has_value())
        return false;
    consume();

    while (!eof() && peek().type() != Token::Type::Greater) {
        if (!match_named_type())
            return false;
        (void)parse_type(get_dummy_node());
    }

    return peek().type() == Token::Type::Greater;
}

NonnullRefPtrVector<Type> Parser::parse_template_arguments(ASTNode& parent)
{
    LOG_SCOPE();

    consume(Token::Type::Less);

    NonnullRefPtrVector<Type> template_arguments;
    while (!eof() && peek().type() != Token::Type::Greater) {
        template_arguments.append(parse_type(parent));
    }

    consume(Token::Type::Greater);

    return template_arguments;
}

bool Parser::match_variable_declaration()
{
    LOG_SCOPE();
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    if (!match_type()) {
        return false;
    }

    VERIFY(m_root_node);
    (void)parse_type(get_dummy_node());

    // Identifier
    if (!match_name())
        return false;

    (void)parse_name(get_dummy_node());

    if (match(Token::Type::Equals)) {
        consume(Token::Type::Equals);
        if (!match_expression()) {
            error("initial value of variable is not an expression"sv);
            return false;
        }
        return true;
    }

    if (match_braced_init_list())
        (void)parse_braced_init_list(get_dummy_node());

    return match(Token::Type::Semicolon);
}

NonnullRefPtr<VariableDeclaration> Parser::parse_variable_declaration(ASTNode& parent, bool expect_semicolon)
{
    LOG_SCOPE();
    auto var = create_ast_node<VariableDeclaration>(parent, position(), {});
    if (!match_variable_declaration()) {
        error("unexpected token for variable type"sv);
        var->set_end(position());
        return var;
    }
    var->set_type(parse_type(var));
    auto name = parse_name(*var);
    RefPtr<Expression> initial_value;

    if (match(Token::Type::Equals)) {
        consume(Token::Type::Equals);
        initial_value = parse_expression(var);
    }

    if (match_braced_init_list()) {
        initial_value = parse_braced_init_list(var);
    }

    if (expect_semicolon)
        consume(Token::Type::Semicolon);

    var->set_end(position());
    var->set_name(name);
    var->set_initial_value(move(initial_value));

    return var;
}

NonnullRefPtr<Expression> Parser::parse_expression(ASTNode& parent)
{
    LOG_SCOPE();
    auto expression = parse_primary_expression(parent);
    // TODO: remove eof() logic, should still work without it
    if (eof() || match(Token::Type::Semicolon)) {
        return expression;
    }

    NonnullRefPtrVector<Expression> secondary_expressions;

    while (match_secondary_expression()) {
        // FIXME: Handle operator precedence
        expression = parse_secondary_expression(parent, expression);
        secondary_expressions.append(expression);
    }

    for (size_t i = 0; secondary_expressions.size() != 0 && i < secondary_expressions.size() - 1; ++i) {
        secondary_expressions[i].set_parent(secondary_expressions[i + 1]);
    }

    return expression;
}

bool Parser::match_secondary_expression()
{
    auto type = peek().type();
    return type == Token::Type::Plus
        || type == Token::Type::PlusEquals
        || type == Token::Type::Minus
        || type == Token::Type::MinusEquals
        || type == Token::Type::Asterisk
        || type == Token::Type::AsteriskEquals
        || type == Token::Type::Percent
        || type == Token::Type::PercentEquals
        || type == Token::Type::Equals
        || type == Token::Type::Greater
        || type == Token::Type::GreaterEquals
        || type == Token::Type::Less
        || type == Token::Type::LessEquals
        || type == Token::Type::Dot
        || type == Token::Type::PlusPlus
        || type == Token::Type::MinusMinus
        || type == Token::Type::And
        || type == Token::Type::AndEquals
        || type == Token::Type::Pipe
        || type == Token::Type::PipeEquals
        || type == Token::Type::Caret
        || type == Token::Type::CaretEquals
        || type == Token::Type::LessLess
        || type == Token::Type::LessLessEquals
        || type == Token::Type::GreaterGreater
        || type == Token::Type::GreaterGreaterEquals
        || type == Token::Type::EqualsEquals
        || type == Token::Type::AndAnd
        || type == Token::Type::PipePipe
        || type == Token::Type::ExclamationMarkEquals
        || type == Token::Type::Arrow
        || type == Token::Type::LeftParen;
}

NonnullRefPtr<Expression> Parser::parse_primary_expression(ASTNode& parent)
{
    LOG_SCOPE();
    // TODO: remove eof() logic, should still work without it
    if (eof()) {
        auto node = create_ast_node<Identifier>(parent, position(), position());
        return node;
    }

    if (match_unary_expression())
        return parse_unary_expression(parent);

    if (match_literal()) {
        return parse_literal(parent);
    }

    if (match_cpp_cast_expression())
        return parse_cpp_cast_expression(parent);

    if (match_c_style_cast_expression())
        return parse_c_style_cast_expression(parent);

    if (match_sizeof_expression())
        return parse_sizeof_expression(parent);

    if (match_braced_init_list())
        return parse_braced_init_list(parent);

    if (match_name()) {
        return parse_name(parent);
    }

    error("could not parse primary expression"sv);
    auto token = consume();
    return create_ast_node<InvalidExpression>(parent, token.start(), token.end());
}

bool Parser::match_literal()
{
    switch (peek().type()) {
    case Token::Type::Integer:
        return true;
    case Token::Type::SingleQuotedString:
        return true;
    case Token::Type::DoubleQuotedString:
        return true;
    case Token::Type::Float:
        return true;
    case Token::Type::Keyword: {
        return match_boolean_literal() || peek().text() == "nullptr";
    }
    default:
        return false;
    }
}

bool Parser::match_unary_expression()
{
    auto type = peek().type();
    return type == Token::Type::PlusPlus
        || type == Token::Type::MinusMinus
        || type == Token::Type::ExclamationMark
        || type == Token::Type::Tilde
        || type == Token::Type::Plus
        || type == Token::Type::Minus
        || type == Token::Type::And;
}

NonnullRefPtr<UnaryExpression> Parser::parse_unary_expression(ASTNode& parent)
{
    auto unary_exp = create_ast_node<UnaryExpression>(parent, position(), {});
    auto op_token = consume();
    UnaryOp op { UnaryOp::Invalid };
    switch (op_token.type()) {
    case Token::Type::Minus:
        op = UnaryOp::Minus;
        break;
    case Token::Type::Plus:
        op = UnaryOp::Plus;
        break;
    case Token::Type::ExclamationMark:
        op = UnaryOp::Not;
        break;
    case Token::Type::Tilde:
        op = UnaryOp::BitwiseNot;
        break;
    case Token::Type::PlusPlus:
        op = UnaryOp::PlusPlus;
        break;
    case Token::Type::And:
        op = UnaryOp::Address;
        break;
    default:
        break;
    }
    unary_exp->set_op(op);
    auto lhs = parse_expression(*unary_exp);
    unary_exp->set_lhs(lhs);
    unary_exp->set_end(lhs->end());
    return unary_exp;
}

NonnullRefPtr<Expression> Parser::parse_literal(ASTNode& parent)
{
    switch (peek().type()) {
    case Token::Type::Integer: {
        auto token = consume();
        return create_ast_node<NumericLiteral>(parent, token.start(), token.end(), text_of_token(token));
    }
    case Token::Type::SingleQuotedString:
        [[fallthrough]];
    case Token::Type::DoubleQuotedString:
        return parse_string_literal(parent);
    case Token::Type::Keyword: {
        if (match_boolean_literal())
            return parse_boolean_literal(parent);
        if (peek().text() == "nullptr") {
            auto token = consume();
            return create_ast_node<NullPointerLiteral>(parent, token.start(), token.end());
        }
        [[fallthrough]];
    }
    default: {
        error("could not parse literal"sv);
        auto token = consume();
        return create_ast_node<InvalidExpression>(parent, token.start(), token.end());
    }
    }
}

NonnullRefPtr<Expression> Parser::parse_secondary_expression(ASTNode& parent, NonnullRefPtr<Expression> lhs)
{
    LOG_SCOPE();
    switch (peek().type()) {
    case Token::Type::Plus:
        return parse_binary_expression(parent, lhs, BinaryOp::Addition);
    case Token::Type::Less:
        return parse_binary_expression(parent, lhs, BinaryOp::LessThan);
    case Token::Type::EqualsEquals:
        return parse_binary_expression(parent, lhs, BinaryOp::EqualsEquals);
    case Token::Type::ExclamationMarkEquals:
        return parse_binary_expression(parent, lhs, BinaryOp::NotEqual);
    case Token::Type::And:
        return parse_binary_expression(parent, lhs, BinaryOp::BitwiseAnd);
    case Token::Type::AndAnd:
        return parse_binary_expression(parent, lhs, BinaryOp::LogicalAnd);
    case Token::Type::Pipe:
        return parse_binary_expression(parent, lhs, BinaryOp::BitwiseOr);
    case Token::Type::PipePipe:
        return parse_binary_expression(parent, lhs, BinaryOp::LogicalOr);
    case Token::Type::Arrow:
        return parse_binary_expression(parent, lhs, BinaryOp::Arrow);
    case Token::Type::Equals:
        return parse_assignment_expression(parent, lhs, AssignmentOp::Assignment);
    case Token::Type::Dot: {
        consume();
        auto exp = create_ast_node<MemberExpression>(parent, lhs->start(), {});
        lhs->set_parent(*exp);
        exp->set_object(move(lhs));
        auto identifier_token = consume(Token::Type::Identifier);
        exp->set_property(create_ast_node<Identifier>(*exp, identifier_token.start(), identifier_token.end(), identifier_token.text()));
        exp->set_end(position());
        return exp;
    }
    case Token::Type::LeftParen: {
        consume();
        auto func = create_ast_node<FunctionCall>(parent, lhs->start(), {});
        lhs->set_parent(*func);
        func->set_callee(move(lhs));
        while (peek().type() != Token::Type::RightParen && !eof()) {
            func->add_argument(parse_expression(*func));
            if (peek().type() == Token::Type::Comma)
                consume(Token::Type::Comma);
        }
        consume(Token::Type::RightParen);
        func->set_end(position());
        return func;
    }
    default: {
        error(String::formatted("unexpected operator for expression. operator: {}", peek().to_string()));
        auto token = consume();
        return create_ast_node<InvalidExpression>(parent, token.start(), token.end());
    }
    }
}

NonnullRefPtr<BinaryExpression> Parser::parse_binary_expression(ASTNode& parent, NonnullRefPtr<Expression> lhs, BinaryOp op)
{
    consume(); // Operator
    auto exp = create_ast_node<BinaryExpression>(parent, lhs->start(), {});
    lhs->set_parent(*exp);
    exp->set_op(op);
    exp->set_lhs(move(lhs));
    auto rhs = parse_expression(exp);
    exp->set_end(rhs->end());
    exp->set_rhs(move(rhs));
    return exp;
}

NonnullRefPtr<AssignmentExpression> Parser::parse_assignment_expression(ASTNode& parent, NonnullRefPtr<Expression> lhs, AssignmentOp op)
{
    consume(); // Operator
    auto exp = create_ast_node<AssignmentExpression>(parent, lhs->start(), {});
    lhs->set_parent(*exp);
    exp->set_op(op);
    exp->set_lhs(move(lhs));
    auto rhs = parse_expression(exp);
    exp->set_end(rhs->end());
    exp->set_rhs(move(rhs));
    return exp;
}

Optional<Parser::DeclarationType> Parser::match_declaration_in_translation_unit()
{
    if (match_function_declaration())
        return DeclarationType::Function;
    if (match_enum_declaration())
        return DeclarationType::Enum;
    if (match_class_declaration())
        return DeclarationType::Class;
    if (match_namespace_declaration())
        return DeclarationType::Namespace;
    if (match_variable_declaration())
        return DeclarationType::Variable;
    return {};
}

Optional<Parser::DeclarationType> Parser::match_class_member(StringView class_name)
{
    if (match_function_declaration())
        return DeclarationType::Function;
    if (match_enum_declaration())
        return DeclarationType::Enum;
    if (match_class_declaration())
        return DeclarationType::Class;
    if (match_variable_declaration())
        return DeclarationType::Variable;
    if (match_constructor(class_name))
        return DeclarationType::Constructor;
    if (match_destructor(class_name))
        return DeclarationType::Destructor;
    return {};
}

bool Parser::match_enum_declaration()
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    if (!match_keyword("enum"))
        return false;

    consume(Token::Type::Keyword);

    if (match_keyword("class"))
        consume(Token::Type::Keyword);

    if (!match(Token::Type::Identifier))
        return false;

    consume(Token::Type::Identifier);

    return match(Token::Type::LeftCurly);
}

bool Parser::match_class_declaration()
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    if (!match_keyword("struct") && !match_keyword("class"))
        return false;

    consume(Token::Type::Keyword);

    if (!match(Token::Type::Identifier))
        return false;

    consume(Token::Type::Identifier);

    auto has_final = match_keyword("final");

    if (peek(has_final ? 1 : 0).type() == Token::Type::Colon) {
        if (has_final)
            consume();

        do {
            consume();

            while (match_keyword("private") || match_keyword("public") || match_keyword("protected") || match_keyword("virtual"))
                consume();

            if (!match_name())
                return false;
            (void)parse_name(get_dummy_node());
        } while (peek().type() == Token::Type::Comma);
    }

    return match(Token::Type::LeftCurly);
}

bool Parser::match_namespace_declaration()
{
    return match_keyword("namespace");
}

bool Parser::match_function_declaration()
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    parse_function_qualifiers();

    if (!match_type())
        return false;

    VERIFY(m_root_node);
    (void)parse_type(get_dummy_node());

    if (!match_name())
        return false;

    (void)parse_name(get_dummy_node());

    if (!peek(Token::Type::LeftParen).has_value())
        return false;
    consume();

    while (consume().type() != Token::Type::RightParen && !eof()) { };

    while (match_keyword("const") || match_keyword("override"))
        consume();

    if (peek(Token::Type::Semicolon).has_value() || peek(Token::Type::LeftCurly).has_value())
        return true;

    if (match_attribute_specification()) {
        consume_attribute_specification();
        return peek(Token::Type::Semicolon).has_value();
    }

    return false;
}

Optional<NonnullRefPtrVector<Parameter>> Parser::parse_parameter_list(ASTNode& parent)
{
    LOG_SCOPE();
    NonnullRefPtrVector<Parameter> parameters;
    while (peek().type() != Token::Type::RightParen && !eof()) {
        if (match_ellipsis()) {
            auto param = create_ast_node<Parameter>(parent, position(), {}, RefPtr<Name> {});
            consume(Token::Type::Dot);
            consume(Token::Type::Dot);
            auto last_dot = consume(Token::Type::Dot);
            param->set_ellipsis(true);
            param->set_end(last_dot.end());
            parameters.append(move(param));
        } else {
            auto type = parse_type(parent);

            RefPtr<Name> name;
            if (match_name()) {
                name = parse_name(parent);
            }

            auto param = create_ast_node<Parameter>(parent, type->start(), !name.is_null() ? name->end() : type->end(), name);
            type->set_parent(*param.ptr());

            param->set_type(move(type));
            parameters.append(move(param));
        }

        if (peek(Token::Type::Comma).has_value())
            consume(Token::Type::Comma);
    }
    return parameters;
}

bool Parser::match_comment()
{
    return match(Token::Type::Comment);
}

bool Parser::match_whitespace()
{
    return match(Token::Type::Whitespace);
}

bool Parser::match_preprocessor()
{
    return match(Token::Type::PreprocessorStatement) || match(Token::Type::IncludeStatement);
}

void Parser::consume_preprocessor()
{
    LOG_SCOPE();
    switch (peek().type()) {
    case Token::Type::PreprocessorStatement:
        consume();
        break;
    case Token::Type::IncludeStatement:
        consume();
        consume(Token::Type::IncludePath);
        break;
    default:
        error("unexpected token while parsing preprocessor statement"sv);
        consume();
    }
}

Optional<Token> Parser::consume_whitespace()
{
    LOG_SCOPE();
    return consume(Token::Type::Whitespace);
}

Token Parser::consume(Token::Type type)
{
    auto token = consume();
    if (token.type() != type)
        error(String::formatted("expected {} at {}:{}, found: {}", Token::type_to_string(type), token.start().line, token.start().column, Token::type_to_string(token.type())));
    return token;
}

bool Parser::match(Token::Type type)
{
    return peek().type() == type;
}

Token Parser::consume()
{
    if (eof()) {
        error("C++ Parser: out of tokens"sv);
        return { Token::Type::EOF_TOKEN, position(), position(), {} };
    }
    return m_tokens[m_state.token_index++];
}

Token Parser::peek(size_t offset) const
{
    if (m_state.token_index + offset >= m_tokens.size())
        return { Token::Type::EOF_TOKEN, position(), position(), {} };
    return m_tokens[m_state.token_index + offset];
}

Optional<Token> Parser::peek(Token::Type type) const
{
    auto token = peek();
    if (token.type() == type)
        return token;
    return {};
}

void Parser::save_state()
{
    m_saved_states.append(m_state);
    m_state.state_nodes.clear();
}

void Parser::load_state()
{
    m_state = m_saved_states.take_last();
}

StringView Parser::text_of_token(Cpp::Token const& token) const
{
    return token.text();
}

String Parser::text_of_node(ASTNode const& node) const
{
    return text_in_range(node.start(), node.end());
}

String Parser::text_in_range(Position start, Position end) const
{
    StringBuilder builder;
    for (auto token : tokens_in_range(start, end)) {
        builder.append(token.text());
    }
    return builder.to_string();
}

Vector<Token> Parser::tokens_in_range(Position start, Position end) const
{
    auto start_token_index = index_of_token_at(start);
    auto end_node_index = index_of_token_at(end);
    VERIFY(start_token_index.has_value());
    VERIFY(end_node_index.has_value());

    Vector<Token> tokens;
    for (size_t i = start_token_index.value(); i <= end_node_index.value(); ++i) {
        tokens.append(m_tokens[i]);
    }
    return tokens;
}

void Parser::error(StringView message)
{
    LOG_SCOPE();

    if (!m_saved_states.is_empty())
        return;

    if (message.is_null() || message.is_empty())
        message = "<empty>"sv;
    String formatted_message;
    if (m_state.token_index >= m_tokens.size()) {
        formatted_message = String::formatted("C++ Parsed error on EOF.{}", message);
    } else {
        formatted_message = String::formatted("C++ Parser error: {}. token: {} ({}:{})",
            message,
            m_state.token_index < m_tokens.size() ? text_of_token(m_tokens[m_state.token_index]) : "EOF"sv,
            m_tokens[m_state.token_index].start().line,
            m_tokens[m_state.token_index].start().column);
    }

    m_errors.append(formatted_message);
}

bool Parser::match_expression()
{
    return match_literal()
        || match_name()
        || match_unary_expression()
        || match_cpp_cast_expression()
        || match_c_style_cast_expression()
        || match_sizeof_expression()
        || match_braced_init_list();
}

bool Parser::eof() const
{
    return m_state.token_index >= m_tokens.size();
}

Position Parser::position() const
{
    if (m_tokens.is_empty())
        return {};

    if (eof())
        return m_tokens.last().end();

    return peek().start();
}

Position Parser::previous_token_end() const
{
    if (m_state.token_index < 1)
        return {};
    return m_tokens[m_state.token_index - 1].end();
}

RefPtr<ASTNode> Parser::node_at(Position pos) const
{
    VERIFY(m_saved_states.is_empty());
    auto index = index_of_node_at(pos);
    if (!index.has_value())
        return nullptr;
    return m_nodes[index.value()];
}

Optional<size_t> Parser::index_of_node_at(Position pos) const
{
    VERIFY(!m_tokens.is_empty());
    VERIFY(m_saved_states.is_empty());
    Optional<size_t> match_node_index;

    auto node_span = [](ASTNode const& node) {
        VERIFY(node.end().line >= node.start().line);
        VERIFY((node.end().line > node.start().line) || (node.end().column >= node.start().column));
        return Position { node.end().line - node.start().line, node.start().line != node.end().line ? 0 : node.end().column - node.start().column };
    };

    for (size_t node_index = 0; node_index < m_nodes.size(); ++node_index) {
        auto& node = m_nodes[node_index];
        if (node.start() > pos || node.end() < pos)
            continue;

        if (!match_node_index.has_value() || (node_span(node) <= node_span(m_nodes[match_node_index.value()])))
            match_node_index = node_index;
    }
    return match_node_index;
}

Optional<Token> Parser::token_at(Position pos) const
{
    auto index = index_of_token_at(pos);
    if (!index.has_value())
        return {};
    return m_tokens[index.value()];
}

Optional<size_t> Parser::index_of_token_at(Position pos) const
{
    for (size_t token_index = 0; token_index < m_tokens.size(); ++token_index) {
        auto token = m_tokens[token_index];
        if (token.start() > pos || token.end() < pos)
            continue;
        return token_index;
    }
    return {};
}

void Parser::print_tokens() const
{
    for (auto& token : m_tokens) {
        outln("{}", token.to_string());
    }
}

Vector<CodeComprehension::TodoEntry> Parser::get_todo_entries() const
{
    Vector<CodeComprehension::TodoEntry> ret;
    for (auto& token : m_tokens) {
        if (token.type() == Token::Type::Comment) {
            if (token.text().contains("TODO"sv) || token.text().contains("FIXME"sv)) {
                ret.append({ token.text(), m_filename, token.start().line, token.start().column });
            }
        }
    }
    return ret;
}

NonnullRefPtr<StringLiteral> Parser::parse_string_literal(ASTNode& parent)
{
    LOG_SCOPE();
    Optional<size_t> start_token_index;
    Optional<size_t> end_token_index;
    while (!eof()) {
        auto token = peek();
        if (token.type() != Token::Type::DoubleQuotedString && token.type() != Token::Type::SingleQuotedString && token.type() != Token::Type::EscapeSequence) {
            VERIFY(start_token_index.has_value());
            end_token_index = m_state.token_index - 1;
            break;
        }
        if (!start_token_index.has_value())
            start_token_index = m_state.token_index;
        consume();
    }

    // String was not terminated
    if (!end_token_index.has_value()) {
        end_token_index = m_tokens.size() - 1;
    }

    VERIFY(start_token_index.has_value());
    VERIFY(end_token_index.has_value());

    Token start_token = m_tokens[start_token_index.value()];
    Token end_token = m_tokens[end_token_index.value()];

    auto text = text_in_range(start_token.start(), end_token.end());
    auto string_literal = create_ast_node<StringLiteral>(parent, start_token.start(), end_token.end());
    string_literal->set_value(move(text));
    return string_literal;
}

NonnullRefPtr<ReturnStatement> Parser::parse_return_statement(ASTNode& parent)
{
    LOG_SCOPE();
    auto return_statement = create_ast_node<ReturnStatement>(parent, position(), {});
    consume(Token::Type::Keyword);
    if (!peek(Token::Type::Semicolon).has_value()) {
        return_statement->set_value(parse_expression(*return_statement));
    }
    return_statement->set_end(position());
    return return_statement;
}

NonnullRefPtr<EnumDeclaration> Parser::parse_enum_declaration(ASTNode& parent)
{
    LOG_SCOPE();
    auto enum_decl = create_ast_node<EnumDeclaration>(parent, position(), {});
    consume_keyword("enum");

    if (match_keyword("class")) {
        consume(Token::Type::Keyword);
        enum_decl->set_type(EnumDeclaration::Type::EnumClass);
    } else {
        enum_decl->set_type(EnumDeclaration::Type::RegularEnum);
    }

    auto name = parse_name(*enum_decl);
    enum_decl->set_name(name);
    consume(Token::Type::LeftCurly);
    while (!eof() && peek().type() != Token::Type::RightCurly) {
        auto name = text_of_token(consume(Token::Type::Identifier));
        RefPtr<Expression> value;
        if (peek().type() == Token::Type::Equals) {
            consume();
            value = parse_expression(enum_decl);
        }
        enum_decl->add_entry(name, move(value));
        if (peek().type() != Token::Type::Comma) {
            break;
        }
        consume(Token::Type::Comma);
    }
    consume(Token::Type::RightCurly);
    consume(Token::Type::Semicolon);
    enum_decl->set_end(position());
    return enum_decl;
}

Token Parser::consume_keyword(String const& keyword)
{
    auto token = consume();
    if (token.type() != Token::Type::Keyword) {
        error(String::formatted("unexpected token: {}, expected Keyword", token.to_string()));
        return token;
    }
    if (text_of_token(token) != keyword) {
        error(String::formatted("unexpected keyword: {}, expected {}", text_of_token(token), keyword));
        return token;
    }
    return token;
}

bool Parser::match_keyword(String const& keyword)
{
    auto token = peek();
    if (token.type() != Token::Type::Keyword) {
        return false;
    }
    if (text_of_token(token) != keyword) {
        return false;
    }
    return true;
}

NonnullRefPtr<StructOrClassDeclaration> Parser::parse_class_declaration(ASTNode& parent)
{
    LOG_SCOPE();

    auto type_token = consume(Token::Type::Keyword);
    StructOrClassDeclaration::Type type {};

    if (type_token.text() == "struct")
        type = StructOrClassDeclaration::Type::Struct;
    if (type_token.text() == "class")
        type = StructOrClassDeclaration::Type::Class;

    auto decl = create_ast_node<StructOrClassDeclaration>(parent, position(), {}, type);

    auto name = parse_name(*decl);
    decl->set_name(name);

    auto has_final = match_keyword("final");

    NonnullRefPtrVector<Name> baseclasses;

    // FIXME: Don't ignore this.
    if (peek(has_final ? 1 : 0).type() == Token::Type::Colon) {
        if (has_final)
            consume();

        do {
            consume();

            while (match_keyword("private") || match_keyword("public") || match_keyword("protected") || match_keyword("virtual"))
                consume();

            baseclasses.append(parse_name(*decl));
        } while (peek().type() == Token::Type::Comma);
    }

    decl->set_baseclasses(move(baseclasses));

    consume(Token::Type::LeftCurly);

    while (!eof() && peek().type() != Token::Type::RightCurly) {
        decl->set_members(parse_class_members(*decl));
    }

    consume(Token::Type::RightCurly);
    consume(Token::Type::Semicolon);
    decl->set_end(position());
    return decl;
}

NonnullRefPtr<BooleanLiteral> Parser::parse_boolean_literal(ASTNode& parent)
{
    LOG_SCOPE();
    auto token = consume(Token::Type::Keyword);
    auto text = text_of_token(token);
    // text == "true" || text == "false";
    bool value = (text == "true");
    return create_ast_node<BooleanLiteral>(parent, token.start(), token.end(), value);
}

bool Parser::match_boolean_literal()
{
    auto token = peek();
    if (token.type() != Token::Type::Keyword)
        return false;
    auto text = text_of_token(token);
    return text == "true" || text == "false";
}

NonnullRefPtr<Type> Parser::parse_type(ASTNode& parent)
{
    LOG_SCOPE();

    if (!match_named_type()) {
        error("expected named named_type"sv);
        auto token = consume();
        return create_ast_node<NamedType>(parent, token.start(), token.end());
    }

    auto named_type = create_ast_node<NamedType>(parent, position(), {});

    auto qualifiers = parse_type_qualifiers();
    named_type->set_qualifiers(move(qualifiers));

    if (match_keyword("auto")) {
        consume(Token::Type::Keyword);
        named_type->set_auto(true);
        auto original_qualifiers = named_type->qualifiers();
        original_qualifiers.extend(parse_type_qualifiers());
        named_type->set_qualifiers(move(original_qualifiers));
        named_type->set_end(position());
        return named_type;
    }

    if (match_keyword("struct")) {
        consume(Token::Type::Keyword); // Consume struct prefix
    }

    if (!match_name()) {
        named_type->set_end(position());
        error(String::formatted("expected name instead of: {}", peek().text()));
        return named_type;
    }
    named_type->set_name(parse_name(*named_type));

    auto original_qualifiers = named_type->qualifiers();
    original_qualifiers.extend(parse_type_qualifiers());
    named_type->set_qualifiers(move(original_qualifiers));

    NonnullRefPtr<Type> type = named_type;
    while (!eof() && peek().type() == Token::Type::Asterisk) {
        type->set_end(position());
        auto asterisk = consume();
        auto ptr = create_ast_node<Pointer>(parent, type->start(), asterisk.end());
        type->set_parent(*ptr);
        ptr->set_pointee(type);
        ptr->set_qualifiers(parse_type_qualifiers());
        ptr->set_end(position());
        type = ptr;
    }

    if (!eof() && (peek().type() == Token::Type::And || peek().type() == Token::Type::AndAnd)) {
        type->set_end(position());
        auto ref_token = consume();
        auto ref = create_ast_node<Reference>(parent, type->start(), ref_token.end(), ref_token.type() == Token::Type::And ? Reference::Kind::Lvalue : Reference::Kind::Rvalue);
        type->set_parent(*ref);
        ref->set_referenced_type(type);
        ref->set_end(position());
        type = ref;
    }

    if (peek().type() == Token::Type::LeftParen) {
        type->set_end(previous_token_end());
        consume();
        auto fn_type = create_ast_node<FunctionType>(parent, type->start(), position());
        fn_type->set_return_type(*type);
        type->set_parent(*fn_type);
        if (auto parameters = parse_parameter_list(*type); parameters.has_value())
            fn_type->set_parameters(parameters.release_value());
        consume(Token::Type::RightParen);
        type = fn_type;
    }

    type->set_end(previous_token_end());

    return type;
}

NonnullRefPtr<ForStatement> Parser::parse_for_statement(ASTNode& parent)
{
    LOG_SCOPE();
    auto for_statement = create_ast_node<ForStatement>(parent, position(), {});
    consume(Token::Type::Keyword);
    consume(Token::Type::LeftParen);
    if (peek().type() != Token::Type::Semicolon)
        for_statement->set_init(parse_variable_declaration(*for_statement, false));
    consume(Token::Type::Semicolon);

    if (peek().type() != Token::Type::Semicolon)
        for_statement->set_test(parse_expression(*for_statement));
    consume(Token::Type::Semicolon);

    if (peek().type() != Token::Type::RightParen)
        for_statement->set_update(parse_expression(*for_statement));
    consume(Token::Type::RightParen);

    for_statement->set_body(parse_statement(*for_statement));

    for_statement->set_end(for_statement->body()->end());
    return for_statement;
}

NonnullRefPtr<IfStatement> Parser::parse_if_statement(ASTNode& parent)
{
    LOG_SCOPE();
    auto if_statement = create_ast_node<IfStatement>(parent, position(), {});
    consume(Token::Type::Keyword);
    consume(Token::Type::LeftParen);
    if_statement->set_predicate(parse_expression(*if_statement));
    consume(Token::Type::RightParen);
    if_statement->set_then_statement(parse_statement(*if_statement));
    if (match_keyword("else")) {
        consume(Token::Type::Keyword);
        if_statement->set_else_statement(parse_statement(*if_statement));
        if_statement->set_end(if_statement->else_statement()->end());
    } else {
        if_statement->set_end(if_statement->then_statement()->end());
    }
    return if_statement;
}
Vector<StringView> Parser::parse_type_qualifiers()
{
    LOG_SCOPE();
    Vector<StringView> qualifiers;
    while (!eof()) {
        auto token = peek();
        if (token.type() != Token::Type::Keyword)
            break;
        auto text = text_of_token(token);
        if (text == "static" || text == "const" || text == "extern") {
            qualifiers.append(text);
            consume();
        } else {
            break;
        }
    }
    return qualifiers;
}

Vector<StringView> Parser::parse_function_qualifiers()
{
    LOG_SCOPE();
    Vector<StringView> qualifiers;
    while (!eof()) {
        auto token = peek();
        if (token.type() != Token::Type::Keyword)
            break;
        auto text = text_of_token(token);
        if (text == "static" || text == "inline" || text == "extern" || text == "virtual") {
            qualifiers.append(text);
            consume();
        } else {
            break;
        }
    }
    return qualifiers;
}

bool Parser::match_attribute_specification()
{
    return text_of_token(peek()) == "__attribute__";
}
void Parser::consume_attribute_specification()
{
    consume(); // __attribute__
    consume(Token::Type::LeftParen);
    size_t left_count = 1;
    while (!eof()) {
        auto token = consume();
        if (token.type() == Token::Type::LeftParen) {
            ++left_count;
        }
        if (token.type() == Token::Type::RightParen) {
            --left_count;
        }
        if (left_count == 0)
            return;
    }
}

bool Parser::match_ellipsis()
{
    if (m_state.token_index > m_tokens.size() - 3)
        return false;
    return peek().type() == Token::Type::Dot && peek(1).type() == Token::Type::Dot && peek(2).type() == Token::Type::Dot;
}

NonnullRefPtr<NamespaceDeclaration> Parser::parse_namespace_declaration(ASTNode& parent, bool is_nested_namespace)
{
    auto namespace_decl = create_ast_node<NamespaceDeclaration>(parent, position(), {});

    if (!is_nested_namespace)
        consume(Token::Type::Keyword);

    auto name = parse_name(*namespace_decl);
    namespace_decl->set_name(name);

    if (peek().type() == Token::Type::ColonColon) {
        consume(Token::Type::ColonColon);
        namespace_decl->add_declaration(parse_namespace_declaration(*namespace_decl, true));
        namespace_decl->set_end(position());
        return namespace_decl;
    }

    consume(Token::Type::LeftCurly);
    while (!eof() && peek().type() != Token::Type::RightCurly) {
        auto declaration = parse_single_declaration_in_translation_unit(*namespace_decl);
        if (declaration) {
            namespace_decl->add_declaration(declaration.release_nonnull());
        } else {
            error("unexpected token"sv);
            consume();
        }
    }
    consume(Token::Type::RightCurly);
    namespace_decl->set_end(position());
    return namespace_decl;
}

bool Parser::match_name()
{
    auto type = peek().type();
    return type == Token::Type::Identifier || type == Token::Type::KnownType;
}

NonnullRefPtr<Name> Parser::parse_name(ASTNode& parent)
{
    LOG_SCOPE();
    NonnullRefPtr<Name> name_node = create_ast_node<Name>(parent, position(), {});
    while (!eof() && (peek().type() == Token::Type::Identifier || peek().type() == Token::Type::KnownType) && peek(1).type() == Token::Type::ColonColon) {
        auto token = consume();
        name_node->add_to_scope(create_ast_node<Identifier>(*name_node, token.start(), token.end(), token.text()));
        consume(Token::Type::ColonColon);
    }

    if (peek().type() == Token::Type::Identifier || peek().type() == Token::Type::KnownType) {
        auto token = consume();
        name_node->set_name(create_ast_node<Identifier>(*name_node, token.start(), token.end(), token.text()));
    } else {
        name_node->set_end(position());
        return name_node;
    }

    if (match_template_arguments()) {
        consume(Token::Type::Less);
        NonnullRefPtr<TemplatizedName> templatized_name = create_ast_node<TemplatizedName>(parent, name_node->start(), {});
        templatized_name->set_name(name_node->name());
        templatized_name->set_scope(name_node->scope());
        name_node->set_end(position());
        name_node = templatized_name;
        while (peek().type() != Token::Type::Greater && !eof()) {
            templatized_name->add_template_argument(parse_type(*templatized_name));
            if (peek().type() == Token::Type::Comma)
                consume(Token::Type::Comma);
        }
        consume(Token::Type::Greater);
    }

    name_node->set_end(previous_token_end());
    return name_node;
}

bool Parser::match_cpp_cast_expression()
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    auto token = consume();
    if (token.type() != Token::Type::Keyword)
        return false;

    auto text = token.text();
    if (text == "static_cast" || text == "reinterpret_cast" || text == "dynamic_cast" || text == "const_cast")
        return true;
    return false;
}

bool Parser::match_c_style_cast_expression()
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    if (consume().type() != Token::Type::LeftParen)
        return false;

    if (!match_type())
        return false;
    (void)parse_type(get_dummy_node());

    if (consume().type() != Token::Type::RightParen)
        return false;

    if (!match_expression())
        return false;

    return true;
}

NonnullRefPtr<CStyleCastExpression> Parser::parse_c_style_cast_expression(ASTNode& parent)
{
    auto parse_exp = create_ast_node<CStyleCastExpression>(parent, position(), {});

    consume(Token::Type::LeftParen);
    parse_exp->set_type(parse_type(*parse_exp));
    consume(Token::Type::RightParen);
    parse_exp->set_expression(parse_expression(*parse_exp));
    parse_exp->set_end(position());

    return parse_exp;
}

NonnullRefPtr<CppCastExpression> Parser::parse_cpp_cast_expression(ASTNode& parent)
{
    auto cast_expression = create_ast_node<CppCastExpression>(parent, position(), {});

    cast_expression->set_cast_type(consume(Token::Type::Keyword).text());

    consume(Token::Type::Less);
    cast_expression->set_type(parse_type(*cast_expression));
    consume(Token::Type::Greater);

    consume(Token::Type::LeftParen);
    cast_expression->set_expression(parse_expression(*cast_expression));
    consume(Token::Type::RightParen);

    cast_expression->set_end(position());

    return cast_expression;
}

bool Parser::match_sizeof_expression()
{
    return match_keyword("sizeof");
}

NonnullRefPtr<SizeofExpression> Parser::parse_sizeof_expression(ASTNode& parent)
{
    auto exp = create_ast_node<SizeofExpression>(parent, position(), {});
    consume(Token::Type::Keyword);
    consume(Token::Type::LeftParen);
    exp->set_type(parse_type(parent));
    consume(Token::Type::RightParen);
    exp->set_end(position());
    return exp;
}

bool Parser::match_braced_init_list()
{
    return match(Token::Type::LeftCurly);
}

NonnullRefPtr<BracedInitList> Parser::parse_braced_init_list(ASTNode& parent)
{
    auto init_list = create_ast_node<BracedInitList>(parent, position(), {});

    consume(Token::Type::LeftCurly);
    while (!eof() && peek().type() != Token::Type::RightCurly) {
        init_list->add_expression(parse_expression(*init_list));
    }
    consume(Token::Type::RightCurly);
    init_list->set_end(position());
    return init_list;
}
NonnullRefPtrVector<Declaration> Parser::parse_class_members(StructOrClassDeclaration& parent)
{
    auto class_name = parent.full_name();

    NonnullRefPtrVector<Declaration> members;
    while (!eof() && peek().type() != Token::Type::RightCurly) {
        if (match_access_specifier())
            consume_access_specifier(); // FIXME: Do not ignore access specifiers
        auto member_type = match_class_member(class_name);
        if (member_type.has_value()) {
            members.append(parse_declaration(parent, member_type.value()));
        } else {
            error("Expected class member"sv);
            consume();
        }
    }
    return members;
}

bool Parser::match_access_specifier()
{
    if (peek(1).type() != Token::Type::Colon)
        return false;

    return match_keyword("private") || match_keyword("protected") || match_keyword("public");
}

void Parser::consume_access_specifier()
{
    consume(Token::Type::Keyword);
    consume(Token::Type::Colon);
}

bool Parser::match_constructor(StringView class_name)
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    auto token = consume();
    if (token.text() != class_name)
        return false;

    if (!peek(Token::Type::LeftParen).has_value())
        return false;
    consume();

    while (consume().type() != Token::Type::RightParen && !eof()) { };

    return (peek(Token::Type::Semicolon).has_value() || peek(Token::Type::LeftCurly).has_value());
}

bool Parser::match_destructor(StringView class_name)
{
    save_state();
    ScopeGuard state_guard = [this] { load_state(); };

    if (match_keyword("virtual"))
        consume();

    if (!match(Token::Type::Tilde))
        return false;
    consume();

    auto token = peek();

    if (token.text() != class_name)
        return false;
    consume();

    if (!peek(Token::Type::LeftParen).has_value())
        return false;
    consume();

    while (consume().type() != Token::Type::RightParen && !eof()) { };

    if (match_keyword("override"))
        consume();

    return (peek(Token::Type::Semicolon).has_value() || peek(Token::Type::LeftCurly).has_value());
}

void Parser::parse_constructor_or_destructor_impl(FunctionDeclaration& func, CtorOrDtor type)
{
    if (type == CtorOrDtor::Dtor) {
        if (match_keyword("virtual"))
            func.set_qualifiers({ consume().text() });
        consume(Token::Type::Tilde);
    }

    auto name = parse_name(func);
    func.set_name(name);

    consume(Token::Type::LeftParen);
    auto parameters = parse_parameter_list(func);
    if (parameters.has_value()) {
        if (type == CtorOrDtor::Dtor && !parameters->is_empty())
            error("Destructor declaration that takes parameters"sv);
        else
            func.set_parameters(parameters.value());
    }

    consume(Token::Type::RightParen);

    if (type == CtorOrDtor::Dtor && match_keyword("override"))
        consume();

    // TODO: Parse =default, =delete.

    RefPtr<FunctionDefinition> body;
    Position ctor_end {};
    if (peek(Token::Type::LeftCurly).has_value()) {
        body = parse_function_definition(func);
        ctor_end = body->end();
    } else {
        ctor_end = position();
        if (match_attribute_specification())
            consume_attribute_specification(); // we don't use the value of __attribute__
        consume(Token::Type::Semicolon);
    }

    func.set_definition(move(body));
    func.set_end(ctor_end);
}

NonnullRefPtr<Constructor> Parser::parse_constructor(ASTNode& parent)
{
    auto ctor = create_ast_node<Constructor>(parent, position(), {});
    parse_constructor_or_destructor_impl(*ctor, CtorOrDtor::Ctor);
    return ctor;
}

NonnullRefPtr<Destructor> Parser::parse_destructor(ASTNode& parent)
{
    auto ctor = create_ast_node<Destructor>(parent, position(), {});
    parse_constructor_or_destructor_impl(*ctor, CtorOrDtor::Dtor);
    return ctor;
}

}