using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using Nub.Lang.Diagnostics;
using Nub.Lang.Frontend.Lexing;
using Nub.Lang.Frontend.Parsing.Definitions;
using Nub.Lang.Frontend.Parsing.Expressions;
using Nub.Lang.Frontend.Parsing.Statements;
using Nub.Lang.Frontend.Typing;

namespace Nub.Lang.Frontend.Parsing;

public class Parser
{
    private List<Diagnostic> _diagnostics = [];
    private List<Token> _tokens = [];
    private int _index;
    private string _namespace = string.Empty;

    public DiagnosticsResult<SourceFile?> ParseModule(List<Token> tokens)
    {
        _diagnostics = [];
        _tokens = tokens;
        _index = 0;
        _namespace = string.Empty;

        try
        {
            ExpectSymbol(Symbol.Namespace);
            _namespace = ExpectIdentifier().Value;

            List<DefinitionNode> definitions = [];

            while (Peek().HasValue)
            {
                definitions.Add(ParseDefinition());
            }

            return new DiagnosticsResult<SourceFile?>(_diagnostics, new SourceFile(_namespace, definitions));
        }
        catch (ParseException ex)
        {
            _diagnostics.Add(ex.Diagnostic);
            RecoverToNextDefinition();
        }

        return new DiagnosticsResult<SourceFile?>(_diagnostics, null);
    }

    private DefinitionNode ParseDefinition()
    {
        var startIndex = _index;
        List<ModifierToken> modifiers = [];

        List<string> documentationParts = [];
        while (_index < _tokens.Count && _tokens[_index] is DocumentationToken commentToken)
        {
            documentationParts.Add(commentToken.Documentation);
            _index++;
        }

        var documentation = documentationParts.Count == 0 ? null : string.Join('\n', documentationParts);

        while (TryExpectModifier(out var modifier))
        {
            modifiers.Add(modifier);
        }

        var keyword = ExpectSymbol();
        return keyword.Symbol switch
        {
            Symbol.Func => ParseFuncDefinition(startIndex, modifiers, Optional.OfNullable(documentation)),
            Symbol.Struct => ParseStruct(startIndex, modifiers, Optional.OfNullable(documentation)),
            _ => throw new ParseException(Diagnostic
                .Error($"Expected 'func' or 'struct', but found '{keyword.Symbol}'")
                .WithHelp("Valid definition keywords are 'func' and 'struct'")
                .At(keyword)
                .Build())
        };
    }

    private DefinitionNode ParseFuncDefinition(int startIndex, List<ModifierToken> modifiers, Optional<string> documentation)
    {
        var name = ExpectIdentifier();
        List<FuncParameter> parameters = [];

        ExpectSymbol(Symbol.OpenParen);

        while (!TryExpectSymbol(Symbol.CloseParen))
        {
            parameters.Add(ParseFuncParameter());

            if (!TryExpectSymbol(Symbol.Comma) && Peek().TryGetValue(out var token) && token is not SymbolToken { Symbol: Symbol.CloseParen })
            {
                _diagnostics.Add(Diagnostic
                    .Warning("Missing comma between function parameters")
                    .WithHelp("Add a ',' to separate parameters")
                    .At(token)
                    .Build());
            }
        }

        var returnType = TryExpectSymbol(Symbol.Colon) ? ParseType() : new NubVoidType();

        var isExtern = modifiers.RemoveAll(x => x.Modifier == Modifier.Extern) > 0;
        if (isExtern)
        {
            if (modifiers.Count != 0)
            {
                throw new ParseException(Diagnostic
                    .Error($"Invalid modifier for extern function: {modifiers[0].Modifier}")
                    .WithHelp($"Extern functions cannot use the '{modifiers[0].Modifier}' modifier")
                    .At(modifiers[0])
                    .Build());
            }

            var callName = name.Value;
            
            if (TryExpectSymbol(Symbol.Calls))
            {
                callName = ExpectIdentifier().Value;
            }

            return new ExternFuncDefinitionNode(GetTokensForNode(startIndex), documentation, name.Value, _namespace, callName, parameters, returnType);
        }

        var body = ParseBlock();
        var exported = modifiers.RemoveAll(x => x.Modifier == Modifier.Export) > 0;

        if (modifiers.Count != 0)
        {
            throw new ParseException(Diagnostic
                .Error($"Invalid modifiers for function: {modifiers[0].Modifier}")
                .WithHelp($"Functions cannot use the '{modifiers[0].Modifier}' modifier")
                .At(modifiers[0])
                .Build());
        }

        return new LocalFuncDefinitionNode(GetTokensForNode(startIndex), documentation, name.Value, _namespace, parameters, body, returnType, exported);
    }

    private StructDefinitionNode ParseStruct(int startIndex, List<ModifierToken> _, Optional<string> documentation)
    {
        var name = ExpectIdentifier().Value;

        ExpectSymbol(Symbol.OpenBrace);

        List<StructField> variables = [];

        while (!TryExpectSymbol(Symbol.CloseBrace))
        {
            var variableName = ExpectIdentifier().Value;
            ExpectSymbol(Symbol.Colon);
            var variableType = ParseType();

            var variableValue = Optional<ExpressionNode>.Empty();

            if (TryExpectSymbol(Symbol.Assign))
            {
                variableValue = ParseExpression();
            }

            variables.Add(new StructField(variableName, variableType, variableValue));
        }

        return new StructDefinitionNode(GetTokensForNode(startIndex), documentation, name, _namespace, variables);
    }

    private FuncParameter ParseFuncParameter()
    {
        var variadic = false;
        if (TryExpectSymbol(Symbol.Period))
        {
            ExpectSymbol(Symbol.Period);
            ExpectSymbol(Symbol.Period);
            variadic = true;
        }

        var name = ExpectIdentifier();
        ExpectSymbol(Symbol.Colon);
        var type = ParseType();

        return new FuncParameter(name.Value, type, variadic);
    }

    private StatementNode ParseStatement()
    {
        var startIndex = _index;
        if (!Peek().TryGetValue(out var token))
        {
            throw new ParseException(Diagnostic
                .Error("Unexpected end of file while parsing statement")
                .At(_tokens.Last())
                .Build());
        }

        if (token is SymbolToken symbol)
        {
            switch (symbol.Symbol)
            {
                case Symbol.Return:
                    return ParseReturn(startIndex);
                case Symbol.If:
                    return ParseIf(startIndex);
                case Symbol.While:
                    return ParseWhile(startIndex);
                case Symbol.Let:
                    return ParseVariableDeclaration(startIndex);
                case Symbol.Break:
                    return ParseBreak(startIndex);
                case Symbol.Continue:
                    return ParseContinue(startIndex);
            }
        }

        return ParseStatementExpression(startIndex);
    }

    private StatementNode ParseStatementExpression(int startIndex)
    {
        var expr = ParseExpression();

        if (Peek().TryGetValue(out var token))
        {
            if (token is SymbolToken symbol)
            {
                switch (symbol.Symbol)
                {
                    case Symbol.Assign:
                    {
                        switch (expr)
                        {
                            case MemberAccessNode memberAccess:
                            {
                                Next();
                                var value = ParseExpression();
                                return new MemberAssignmentNode(GetTokensForNode(startIndex), memberAccess, value);
                            }
                            case ArrayIndexAccessNode arrayIndexAccess:
                            {
                                Next();
                                var value = ParseExpression();
                                return new ArrayIndexAssignmentNode(GetTokensForNode(startIndex), arrayIndexAccess, value);
                            }
                            case IdentifierNode identifier:
                            {
                                Next();
                                var value = ParseExpression();
                                return new VariableAssignmentNode(GetTokensForNode(startIndex), identifier, value);
                            }
                            case DereferenceNode dereference:
                            {
                                Next();
                                var value = ParseExpression();
                                return new DereferenceAssignmentNode(GetTokensForNode(startIndex), dereference, value);
                            }
                        }

                        break;
                    }
                }
            }
        }

        return new StatementExpressionNode(GetTokensForNode(startIndex), expr);
    }

    private VariableDeclarationNode ParseVariableDeclaration(int startIndex)
    {
        ExpectSymbol(Symbol.Let);
        var name = ExpectIdentifier().Value;
        var type = Optional<NubType>.Empty();
        if (TryExpectSymbol(Symbol.Colon))
        {
            type = ParseType();
        }

        var value = Optional<ExpressionNode>.Empty();
        if (TryExpectSymbol(Symbol.Assign))
        {
            value = ParseExpression();
        }

        return new VariableDeclarationNode(GetTokensForNode(startIndex), name, type, value);
    }

    private StatementNode ParseBreak(int startIndex)
    {
        ExpectSymbol(Symbol.Break);
        Next();
        return new BreakNode(GetTokensForNode(startIndex));
    }

    private StatementNode ParseContinue(int startIndex)
    {
        ExpectSymbol(Symbol.Continue);
        return new ContinueNode(GetTokensForNode(startIndex));
    }

    private ReturnNode ParseReturn(int startIndex)
    {
        ExpectSymbol(Symbol.Return);
        var value = Optional<ExpressionNode>.Empty();
        if (!TryExpectSymbol(Symbol.Semicolon))
        {
            value = ParseExpression();
        }

        return new ReturnNode(GetTokensForNode(startIndex), value);
    }

    private IfNode ParseIf(int startIndex)
    {
        ExpectSymbol(Symbol.If);
        var condition = ParseExpression();
        var body = ParseBlock();

        var elseStatement = Optional<Variant<IfNode, BlockNode>>.Empty();
        if (TryExpectSymbol(Symbol.Else))
        {
            var newStartIndex = _index;
            elseStatement = TryExpectSymbol(Symbol.If)
                ? (Variant<IfNode, BlockNode>)ParseIf(newStartIndex)
                : (Variant<IfNode, BlockNode>)ParseBlock();
        }

        return new IfNode(GetTokensForNode(startIndex), condition, body, elseStatement);
    }

    private WhileNode ParseWhile(int startIndex)
    {
        ExpectSymbol(Symbol.While);
        var condition = ParseExpression();
        var body = ParseBlock();
        return new WhileNode(GetTokensForNode(startIndex), condition, body);
    }

    private ExpressionNode ParseExpression(int precedence = 0)
    {
        var startIndex = _index;
        var left = ParsePrimaryExpression();

        while (true)
        {
            var token = Peek();
            if (!token.HasValue || token.Value is not SymbolToken symbolToken || !TryGetBinaryOperator(symbolToken.Symbol, out var op) ||
                GetBinaryOperatorPrecedence(op.Value) < precedence)
            {
                break;
            }

            Next();
            var right = ParseExpression(GetBinaryOperatorPrecedence(op.Value) + 1);

            left = new BinaryExpressionNode(GetTokensForNode(startIndex), left, op.Value, right);
        }

        return left;
    }

    private static int GetBinaryOperatorPrecedence(BinaryExpressionOperator binaryExpressionOperator)
    {
        return binaryExpressionOperator switch
        {
            BinaryExpressionOperator.Multiply => 3,
            BinaryExpressionOperator.Divide => 3,
            BinaryExpressionOperator.Plus => 2,
            BinaryExpressionOperator.Minus => 2,
            BinaryExpressionOperator.GreaterThan => 1,
            BinaryExpressionOperator.GreaterThanOrEqual => 1,
            BinaryExpressionOperator.LessThan => 1,
            BinaryExpressionOperator.LessThanOrEqual => 1,
            BinaryExpressionOperator.Equal => 0,
            BinaryExpressionOperator.NotEqual => 0,
            _ => throw new ArgumentOutOfRangeException(nameof(binaryExpressionOperator), binaryExpressionOperator, null)
        };
    }

    private static bool TryGetBinaryOperator(Symbol symbol, [NotNullWhen(true)] out BinaryExpressionOperator? binaryExpressionOperator)
    {
        switch (symbol)
        {
            case Symbol.Equal:
                binaryExpressionOperator = BinaryExpressionOperator.Equal;
                return true;
            case Symbol.NotEqual:
                binaryExpressionOperator = BinaryExpressionOperator.NotEqual;
                return true;
            case Symbol.LessThan:
                binaryExpressionOperator = BinaryExpressionOperator.LessThan;
                return true;
            case Symbol.LessThanOrEqual:
                binaryExpressionOperator = BinaryExpressionOperator.LessThanOrEqual;
                return true;
            case Symbol.GreaterThan:
                binaryExpressionOperator = BinaryExpressionOperator.GreaterThan;
                return true;
            case Symbol.GreaterThanOrEqual:
                binaryExpressionOperator = BinaryExpressionOperator.GreaterThanOrEqual;
                return true;
            case Symbol.Plus:
                binaryExpressionOperator = BinaryExpressionOperator.Plus;
                return true;
            case Symbol.Minus:
                binaryExpressionOperator = BinaryExpressionOperator.Minus;
                return true;
            case Symbol.Star:
                binaryExpressionOperator = BinaryExpressionOperator.Multiply;
                return true;
            case Symbol.ForwardSlash:
                binaryExpressionOperator = BinaryExpressionOperator.Divide;
                return true;
            default:
                binaryExpressionOperator = null;
                return false;
        }
    }

    private ExpressionNode ParsePrimaryExpression()
    {
        var startIndex = _index;
        ExpressionNode expr;

        var token = ExpectToken();
        switch (token)
        {
            case LiteralToken literal:
            {
                expr = new LiteralNode(GetTokensForNode(startIndex), literal.Value, literal.Kind);
                break;
            }
            case IdentifierToken identifier:
            {
                var next = Peek();
                switch (next.Value)
                {
                    case SymbolToken { Symbol: Symbol.DoubleColon }:
                    {
                        Next();
                        var name = ExpectIdentifier();
                        ExpectSymbol(Symbol.OpenParen);
                        var parameters = new List<ExpressionNode>();
                        while (!TryExpectSymbol(Symbol.CloseParen))
                        {
                            parameters.Add(ParseExpression());
                            if (!TryExpectSymbol(Symbol.Comma) && Peek().TryGetValue(out var nextToken) && nextToken is not SymbolToken { Symbol: Symbol.CloseParen })
                            {
                                _diagnostics.Add(Diagnostic
                                    .Warning("Missing comma between function arguments")
                                    .WithHelp("Add a ',' to separate arguments")
                                    .At(nextToken)
                                    .Build());
                            }
                        }

                        expr = new FuncCallNode(GetTokensForNode(startIndex), identifier.Value, name.Value, parameters);
                        break;
                    }
                    case SymbolToken { Symbol: Symbol.OpenParen }:
                    {
                        Next();
                        var parameters = new List<ExpressionNode>();
                        while (!TryExpectSymbol(Symbol.CloseParen))
                        {
                            parameters.Add(ParseExpression());
                            if (!TryExpectSymbol(Symbol.Comma) && Peek().TryGetValue(out var nextToken) && nextToken is not SymbolToken { Symbol: Symbol.CloseParen })
                            {
                                _diagnostics.Add(Diagnostic
                                    .Warning("Missing comma between function arguments")
                                    .WithHelp("Add a ',' to separate arguments")
                                    .At(nextToken)
                                    .Build());
                            }
                        }

                        expr = new FuncCallNode(GetTokensForNode(startIndex), _namespace, identifier.Value, parameters);
                        break;
                    }
                    default:
                    {
                        expr = new IdentifierNode(GetTokensForNode(startIndex), identifier.Value);
                        break;
                    }
                }

                break;
            }
            case SymbolToken symbolToken:
            {
                switch (symbolToken.Symbol)
                {
                    case Symbol.OpenParen:
                    {
                        var expression = ParseExpression();
                        ExpectSymbol(Symbol.CloseParen);
                        expr = expression;
                        break;
                    }
                    case Symbol.Ampersand:
                    {
                        var expression = ParsePrimaryExpression();
                        if (expression is not LValueNode lValue)
                        {
                            throw new ParseException(Diagnostic
                                .Error("& symbol can only be used on lvalues")
                                .At(expression)
                                .Build());
                        }
                        
                        expr = new AddressOfNode(GetTokensForNode(startIndex), lValue);
                        break;
                    }
                    case Symbol.Minus:
                    {
                        var expression = ParsePrimaryExpression();
                        expr = new UnaryExpressionNode(GetTokensForNode(startIndex), UnaryExpressionOperator.Negate, expression);
                        break;
                    }
                    case Symbol.Bang:
                    {
                        var expression = ParsePrimaryExpression();
                        expr = new UnaryExpressionNode(GetTokensForNode(startIndex), UnaryExpressionOperator.Invert, expression);
                        break;
                    }
                    case Symbol.OpenBracket:
                    {
                        if (Peek().TryGetValue(out var capacityToken) && capacityToken is LiteralToken { Kind: LiteralKind.Integer } literalToken)
                        {
                            var capacity = int.Parse(literalToken.Value);
                            Next();
                            ExpectSymbol(Symbol.CloseBracket);
                            var elementType = ParseType();
                        
                            if (capacity > 0)
                            {
                                expr = new FixedArrayInitializerNode(GetTokensForNode(startIndex), elementType, capacity);
                            }
                            else
                            {
                                throw new ParseException(Diagnostic
                                    .Error("Fixed array size must be a positive integer")
                                    .WithHelp("Use a positive integer literal for the array size")
                                    .At(literalToken)
                                    .Build());
                            }
                        }
                        else
                        {
                            var capacity = ParseExpression();
                            ExpectSymbol(Symbol.CloseBracket);
                            var type = ParseType();

                            expr = new ArrayInitializerNode(GetTokensForNode(startIndex), capacity, type);
                        }
                        
                        break;
                    }
                    case Symbol.Alloc:
                    {
                        var type = ParseType();
                        Dictionary<string, ExpressionNode> initializers = [];
                        ExpectSymbol(Symbol.OpenBrace);
                        while (!TryExpectSymbol(Symbol.CloseBrace))
                        {
                            var name = ExpectIdentifier().Value;
                            ExpectSymbol(Symbol.Assign);
                            var value = ParseExpression();
                            initializers.Add(name, value);
                        }

                        if (type is not NubStructType structType)
                        {
                            throw new ParseException(Diagnostic
                                .Error($"Cannot alloc type '{type}'")
                                .At(symbolToken)
                                .Build());
                        }

                        expr = new StructInitializerNode(GetTokensForNode(startIndex), structType, initializers);
                        break;
                    }
                    default:
                    {
                        throw new ParseException(Diagnostic
                            .Error($"Unexpected symbol '{symbolToken.Symbol}' in expression")
                            .WithHelp("Expected literal, identifier, or '(' to start expression")
                            .At(symbolToken)
                            .Build());
                    }
                }

                break;
            }
            default:
            {
                throw new ParseException(Diagnostic
                    .Error($"Unexpected token '{token.GetType().Name}' in expression")
                    .WithHelp("Expected literal, identifier, or parenthesized expression")
                    .At(token)
                    .Build());
            }
        }

        return ParsePostfixOperators(startIndex, expr);
    }

    private ExpressionNode ParsePostfixOperators(int startIndex, ExpressionNode expr)
    {
        while (true)
        {
            if (TryExpectSymbol(Symbol.Caret))
            {
                expr = new DereferenceNode(GetTokensForNode(startIndex), expr);
                continue;
            }

            if (TryExpectSymbol(Symbol.Period))
            {
                var structMember = ExpectIdentifier().Value;
                expr = new MemberAccessNode(GetTokensForNode(startIndex), expr, structMember);
                continue;
            }

            if (TryExpectSymbol(Symbol.OpenBracket))
            {
                var index = ParseExpression();
                ExpectSymbol(Symbol.CloseBracket);
                expr = new ArrayIndexAccessNode(GetTokensForNode(startIndex), expr, index);
                continue;
            }

            break;
        }

        return expr;
    }

    private BlockNode ParseBlock()
    {
        var startIndex = _index;
        ExpectSymbol(Symbol.OpenBrace);
        List<StatementNode> statements = [];
        while (Peek().HasValue && !TryExpectSymbol(Symbol.CloseBrace))
        {
            try
            {
                statements.Add(ParseStatement());
            }
            catch (ParseException ex)
            {
                _diagnostics.Add(ex.Diagnostic);
                RecoverToNextStatement();
            }
        }

        return new BlockNode(GetTokensForNode(startIndex), statements);
    }

    private NubType ParseType()
    {
        if (TryExpectIdentifier(out var name))
        {
            if (name == "any")
            {
                return new NubAnyType();
            }

            if (name == "void")
            {
                return new NubVoidType();
            }

            if (NubPrimitiveType.TryParse(name, out var primitiveTypeKind))
            {
                return new NubPrimitiveType(primitiveTypeKind.Value);
            }

            var @namespace = _namespace;
            if (TryExpectSymbol(Symbol.DoubleColon))
            {
                @namespace = ExpectIdentifier().Value;
            }

            return new NubStructType(@namespace, name);
        }

        if (TryExpectSymbol(Symbol.Caret))
        {
            var baseType = ParseType();
            return new NubPointerType(baseType);
        }

        if (TryExpectSymbol(Symbol.OpenBracket))
        {
            if (Peek().TryGetValue(out var token) && token is LiteralToken { Kind: LiteralKind.Integer, Value: var sizeValue })
            {
                Next();
                ExpectSymbol(Symbol.CloseBracket);
                var baseType = ParseType();

                var size = int.Parse(sizeValue);
                
                if (size > 0)
                {
                    return new NubFixedArrayType(baseType, size);
                }
                else
                {
                    throw new UnreachableException();
                }
            }
            else
            {
                ExpectSymbol(Symbol.CloseBracket);
                var baseType = ParseType();
                return new NubArrayType(baseType);
            }
        }

        if (!Peek().TryGetValue(out var peekToken))
        {
            throw new ParseException(Diagnostic
                .Error("Unexpected end of file while parsing type")
                .WithHelp("Expected a type name")
                .At(_tokens.Last())
                .Build());
        }

        throw new ParseException(Diagnostic
            .Error("Invalid type syntax")
            .WithHelp("Expected type name, '^' for pointer, or '[]' for array")
            .At(peekToken)
            .Build());
}

    private Token ExpectToken()
    {
        if (!Peek().TryGetValue(out var token))
        {
            throw new ParseException(Diagnostic
                .Error("Unexpected end of file")
                .WithHelp("Expected more tokens to complete the syntax")
                .At(_tokens.Last())
                .Build());
        }

        Next();
        return token;
    }

    private SymbolToken ExpectSymbol()
    {
        var token = ExpectToken();
        if (token is not SymbolToken symbol)
        {
            throw new ParseException(Diagnostic
                .Error($"Expected symbol, but found {token.GetType().Name}")
                .WithHelp("This position requires a symbol like '(', ')', '{', '}', etc.")
                .At(token)
                .Build());
        }

        return symbol;
    }

    private void ExpectSymbol(Symbol expectedSymbol)
    {
        var token = ExpectSymbol();
        if (token.Symbol != expectedSymbol)
        {
            throw new ParseException(Diagnostic
                .Error($"Expected '{expectedSymbol}', but found '{token.Symbol}'")
                .WithHelp($"Insert '{expectedSymbol}' here")
                .At(token)
                .Build());
        }
    }

    private bool TryExpectSymbol(Symbol symbol)
    {
        if (Peek() is { Value: SymbolToken symbolToken } && symbolToken.Symbol == symbol)
        {
            Next();
            return true;
        }

        return false;
    }

    private bool TryExpectModifier([NotNullWhen(true)] out ModifierToken? modifier)
    {
        if (Peek() is { Value: ModifierToken modifierToken })
        {
            modifier = modifierToken;
            Next();
            return true;
        }

        modifier = null;
        return false;
    }

    private bool TryExpectIdentifier([NotNullWhen(true)] out string? identifier)
    {
        if (Peek() is { Value: IdentifierToken identifierToken })
        {
            identifier = identifierToken.Value;
            Next();
            return true;
        }

        identifier = null;
        return false;
    }

    private IdentifierToken ExpectIdentifier()
    {
        var token = ExpectToken();
        if (token is not IdentifierToken identifier)
        {
            throw new ParseException(Diagnostic
                .Error($"Expected identifier, but found {token.GetType().Name}")
                .WithHelp("Provide a valid identifier name here")
                .At(token)
                .Build());
        }

        return identifier;
    }

    private void RecoverToNextDefinition()
    {
        while (Peek().HasValue)
        {
            var token = Peek().Value;
            if (token is SymbolToken { Symbol: Symbol.Func or Symbol.Struct })
            {
                break;
            }

            Next();
        }
    }

    private void RecoverToNextStatement()
    {
        while (Peek().TryGetValue(out var token))
        {
            if (token is SymbolToken { Symbol: Symbol.CloseBrace } or IdentifierToken or SymbolToken
                {
                    Symbol: Symbol.Return or Symbol.If or Symbol.While or Symbol.Let or Symbol.Break or Symbol.Continue
                })
            {
                break;
            }

            Next();
        }
    }

    private Optional<Token> Peek(int offset = 0)
    {
        var peekIndex = _index + offset;
        while (peekIndex < _tokens.Count && _tokens[peekIndex] is DocumentationToken)
        {
            peekIndex++;
        }

        if (peekIndex < _tokens.Count)
        {
            return _tokens[peekIndex];
        }

        return Optional<Token>.Empty();
    }

    private void Next()
    {
        while (_index < _tokens.Count && _tokens[_index] is DocumentationToken)
        {
            _index++;
        }

        _index++;
    }

    private IReadOnlyList<Token> GetTokensForNode(int startIndex)
    {
        return _tokens[startIndex..Math.Min(_index, _tokens.Count - 1)];
    }
}

public class ParseException : Exception
{
    public Diagnostic Diagnostic { get; }

    public ParseException(Diagnostic diagnostic) : base(diagnostic.Message)
    {
        Diagnostic = diagnostic;
    }
}