using System.Diagnostics.CodeAnalysis;
using NubLang.Diagnostics;
using NubLang.Parsing.Syntax;
using NubLang.Tokenization;

namespace NubLang.Parsing;

public sealed class Parser
{
    private readonly List<Diagnostic> _diagnostics = [];
    private IReadOnlyList<Token> _tokens = [];
    private int _tokenIndex;
    private string _moduleName = string.Empty;

    private Token? CurrentToken => _tokenIndex < _tokens.Count ? _tokens[_tokenIndex] : null;
    private bool HasToken => CurrentToken != null;

    public IReadOnlyList<Diagnostic> GetDiagnostics()
    {
        return _diagnostics;
    }

    public SyntaxTree Parse(IReadOnlyList<Token> tokens)
    {
        _diagnostics.Clear();
        _tokens = tokens;
        _tokenIndex = 0;
        _moduleName = string.Empty;

        var metadata = ParseMetadata();
        var definitions = ParseDefinitions();

        return new SyntaxTree(definitions, metadata);
    }

    private SyntaxTreeMetadata ParseMetadata()
    {
        var imports = new List<string>();

        try
        {
            ExpectSymbol(Symbol.Module);
            _moduleName = ExpectLiteral(LiteralKind.String).Value;

            while (TryExpectSymbol(Symbol.Import))
            {
                imports.Add(ExpectIdentifier().Value);
            }
        }
        catch (ParseException e)
        {
            _diagnostics.Add(e.Diagnostic);
            while (HasToken)
            {
                if (CurrentToken is SymbolToken { Symbol: Symbol.Module or Symbol.Import })
                {
                    break;
                }

                Next();
            }
        }

        return new SyntaxTreeMetadata(_moduleName, imports);
    }

    private List<DefinitionSyntax> ParseDefinitions()
    {
        var definitions = new List<DefinitionSyntax>();

        while (HasToken)
        {
            try
            {
                var startIndex = _tokenIndex;
                var exported = TryExpectSymbol(Symbol.Export);

                if (TryExpectSymbol(Symbol.Extern))
                {
                    var externSymbol = ExpectLiteral(LiteralKind.String).Value;
                    ExpectSymbol(Symbol.Func);
                    definitions.Add(ParseFunc(startIndex, exported, externSymbol));
                    continue;
                }

                var keyword = ExpectSymbol();
                var definition = keyword.Symbol switch
                {
                    Symbol.Func => ParseFunc(startIndex, exported, null),
                    Symbol.Struct => ParseStruct(startIndex, exported),
                    Symbol.Interface => ParseInterface(startIndex, exported),
                    _ => throw new ParseException(Diagnostic
                        .Error($"Expected 'func', 'struct' or 'interface' but found '{keyword.Symbol}'")
                        .WithHelp("Valid definition keywords are 'func', 'struct' and 'interface'")
                        .At(keyword)
                        .Build())
                };

                definitions.Add(definition);
            }
            catch (ParseException e)
            {
                _diagnostics.Add(e.Diagnostic);
                while (HasToken)
                {
                    if (CurrentToken is SymbolToken { Symbol: Symbol.Extern or Symbol.Func or Symbol.Struct or Symbol.Interface })
                    {
                        break;
                    }

                    Next();
                }
            }
        }

        return definitions;
    }

    private FuncSignatureSyntax ParseFuncSignature()
    {
        var startIndex = _tokenIndex;
        List<FuncParameterSyntax> parameters = [];

        ExpectSymbol(Symbol.OpenParen);

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

            if (!TryExpectSymbol(Symbol.Comma))
            {
                ExpectSymbol(Symbol.CloseParen);
                break;
            }
        }

        var returnType = TryExpectSymbol(Symbol.Colon) ? ParseType() : new VoidTypeSyntax([]);

        return new FuncSignatureSyntax(GetTokens(startIndex), parameters, returnType);
    }

    private FuncParameterSyntax ParseFuncParameter()
    {
        var startIndex = _tokenIndex;
        var name = ExpectIdentifier();
        ExpectSymbol(Symbol.Colon);
        var type = ParseType();

        return new FuncParameterSyntax(GetTokens(startIndex), name.Value, type);
    }

    private FuncSyntax ParseFunc(int startIndex, bool exported, string? externSymbol)
    {
        var name = ExpectIdentifier();
        var signature = ParseFuncSignature();

        BlockSyntax? body = null;
        if (CurrentToken is SymbolToken { Symbol: Symbol.OpenBrace })
        {
            Next();
            body = ParseBlock();
        }

        return new FuncSyntax(GetTokens(startIndex), name.Value, exported, externSymbol, signature, body);
    }

    private DefinitionSyntax ParseStruct(int startIndex, bool exported)
    {
        var name = ExpectIdentifier();
        var interfaceImplementations = new List<TypeSyntax>();

        if (TryExpectSymbol(Symbol.Colon))
        {
            do
            {
                var interfaceType = ParseType();
                interfaceImplementations.Add(interfaceType);
            } while (TryExpectSymbol(Symbol.Comma));
        }

        ExpectSymbol(Symbol.OpenBrace);

        List<StructFieldSyntax> fields = [];
        List<StructFuncSyntax> funcs = [];

        var fieldIndex = 0;

        while (!TryExpectSymbol(Symbol.CloseBrace))
        {
            var memberStartIndex = _tokenIndex;

            if (TryExpectSymbol(Symbol.Func))
            {
                var funcName = ExpectIdentifier().Value;
                var funcSignature = ParseFuncSignature();
                var funcBody = ParseBlock();

                funcs.Add(new StructFuncSyntax(GetTokens(memberStartIndex), funcName, funcSignature, funcBody));
            }
            else
            {
                var fieldName = ExpectIdentifier().Value;
                ExpectSymbol(Symbol.Colon);
                var fieldType = ParseType();

                var fieldValue = Optional<ExpressionSyntax>.Empty();

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

                fields.Add(new StructFieldSyntax(GetTokens(memberStartIndex), fieldIndex++, fieldName, fieldType, fieldValue));
            }
        }

        return new StructSyntax(GetTokens(startIndex), name.Value, exported, fields, funcs, interfaceImplementations);
    }

    private InterfaceSyntax ParseInterface(int startIndex, bool exported)
    {
        var name = ExpectIdentifier();

        ExpectSymbol(Symbol.OpenBrace);

        List<InterfaceFuncSyntax> functions = [];

        while (!TryExpectSymbol(Symbol.CloseBrace))
        {
            var funcStartIndex = _tokenIndex;

            ExpectSymbol(Symbol.Func);

            var funcName = ExpectIdentifier().Value;
            var signature = ParseFuncSignature();

            functions.Add(new InterfaceFuncSyntax(GetTokens(funcStartIndex), funcName, signature));
        }

        return new InterfaceSyntax(GetTokens(startIndex), name.Value, exported, functions);
    }

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

        return ParseStatementExpression();
    }

    private StatementSyntax ParseStatementExpression()
    {
        var startIndex = _tokenIndex;
        var expr = ParseExpression();

        if (TryExpectSymbol(Symbol.Assign))
        {
            var value = ParseExpression();
            return new AssignmentSyntax(GetTokens(startIndex), expr, value);
        }

        return new StatementExpressionSyntax(GetTokens(startIndex), expr);
    }

    private VariableDeclarationSyntax ParseVariableDeclaration()
    {
        var startIndex = _tokenIndex;
        ExpectSymbol(Symbol.Let);
        var name = ExpectIdentifier().Value;

        var explicitType = Optional<TypeSyntax>.Empty();
        if (TryExpectSymbol(Symbol.Colon))
        {
            explicitType = ParseType();
        }

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

        return new VariableDeclarationSyntax(GetTokens(startIndex), name, explicitType, assignment);
    }

    private StatementSyntax ParseBreak()
    {
        var startIndex = _tokenIndex;
        ExpectSymbol(Symbol.Break);
        return new BreakSyntax(GetTokens(startIndex));
    }

    private StatementSyntax ParseContinue()
    {
        var startIndex = _tokenIndex;
        ExpectSymbol(Symbol.Continue);
        return new ContinueSyntax(GetTokens(startIndex));
    }

    private ReturnSyntax ParseReturn()
    {
        var startIndex = _tokenIndex;
        ExpectSymbol(Symbol.Return);

        var value = Optional<ExpressionSyntax>.Empty();

        if (!TryExpectSymbol(Symbol.Semi))
        {
            value = ParseExpression();
        }

        return new ReturnSyntax(GetTokens(startIndex), value);
    }

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

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

        return new IfSyntax(GetTokens(startIndex), condition, body, elseStatement);
    }

    private WhileSyntax ParseWhile()
    {
        var startIndex = _tokenIndex;
        ExpectSymbol(Symbol.While);
        var condition = ParseExpression();
        var body = ParseBlock();
        return new WhileSyntax(GetTokens(startIndex), condition, body);
    }

    private ExpressionSyntax ParseExpression(int precedence = 0)
    {
        var startIndex = _tokenIndex;
        var left = ParsePrimaryExpression();

        while (CurrentToken is SymbolToken symbolToken && TryGetBinaryOperator(symbolToken.Symbol, out var op) && GetBinaryOperatorPrecedence(op.Value) >= precedence)
        {
            Next();
            var right = ParseExpression(GetBinaryOperatorPrecedence(op.Value) + 1);
            left = new BinaryExpressionSyntax(GetTokens(startIndex), left, op.Value, right);
        }

        return left;
    }

    private static int GetBinaryOperatorPrecedence(BinaryOperatorSyntax operatorSyntax)
    {
        return operatorSyntax switch
        {
            BinaryOperatorSyntax.Multiply => 10,
            BinaryOperatorSyntax.Divide => 10,
            BinaryOperatorSyntax.Modulo => 10,

            BinaryOperatorSyntax.Plus => 9,
            BinaryOperatorSyntax.Minus => 9,

            BinaryOperatorSyntax.LeftShift => 8,
            BinaryOperatorSyntax.RightShift => 8,

            BinaryOperatorSyntax.GreaterThan => 7,
            BinaryOperatorSyntax.GreaterThanOrEqual => 7,
            BinaryOperatorSyntax.LessThan => 7,
            BinaryOperatorSyntax.LessThanOrEqual => 7,

            BinaryOperatorSyntax.Equal => 7,
            BinaryOperatorSyntax.NotEqual => 7,

            BinaryOperatorSyntax.BitwiseAnd => 6,
            BinaryOperatorSyntax.BitwiseXor => 5,
            BinaryOperatorSyntax.BitwiseOr => 4,

            BinaryOperatorSyntax.LogicalAnd => 3,
            BinaryOperatorSyntax.LogicalOr => 2,

            _ => throw new ArgumentOutOfRangeException(nameof(operatorSyntax), operatorSyntax, null)
        };
    }

    private bool TryGetBinaryOperator(Symbol symbol, [NotNullWhen(true)] out BinaryOperatorSyntax? binaryExpressionOperator)
    {
        switch (symbol)
        {
            case Symbol.Equal:
                binaryExpressionOperator = BinaryOperatorSyntax.Equal;
                return true;
            case Symbol.NotEqual:
                binaryExpressionOperator = BinaryOperatorSyntax.NotEqual;
                return true;
            case Symbol.LessThan:
                binaryExpressionOperator = BinaryOperatorSyntax.LessThan;
                return true;
            case Symbol.LessThanOrEqual:
                binaryExpressionOperator = BinaryOperatorSyntax.LessThanOrEqual;
                return true;
            case Symbol.GreaterThan:
                binaryExpressionOperator = BinaryOperatorSyntax.GreaterThan;
                return true;
            case Symbol.GreaterThanOrEqual:
                binaryExpressionOperator = BinaryOperatorSyntax.GreaterThanOrEqual;
                return true;
            case Symbol.And:
                binaryExpressionOperator = BinaryOperatorSyntax.LogicalAnd;
                return true;
            case Symbol.Or:
                binaryExpressionOperator = BinaryOperatorSyntax.LogicalOr;
                return true;
            case Symbol.Plus:
                binaryExpressionOperator = BinaryOperatorSyntax.Plus;
                return true;
            case Symbol.Minus:
                binaryExpressionOperator = BinaryOperatorSyntax.Minus;
                return true;
            case Symbol.Star:
                binaryExpressionOperator = BinaryOperatorSyntax.Multiply;
                return true;
            case Symbol.ForwardSlash:
                binaryExpressionOperator = BinaryOperatorSyntax.Divide;
                return true;
            case Symbol.Percent:
                binaryExpressionOperator = BinaryOperatorSyntax.Modulo;
                return true;
            case Symbol.LeftShift:
                binaryExpressionOperator = BinaryOperatorSyntax.LeftShift;
                return true;
            case Symbol.RightShift:
                binaryExpressionOperator = BinaryOperatorSyntax.RightShift;
                return true;
            case Symbol.Ampersand:
                binaryExpressionOperator = BinaryOperatorSyntax.BitwiseAnd;
                return true;
            case Symbol.Pipe:
                binaryExpressionOperator = BinaryOperatorSyntax.BitwiseOr;
                return true;
            case Symbol.Caret:
                binaryExpressionOperator = BinaryOperatorSyntax.BitwiseXor;
                return true;
            default:
                binaryExpressionOperator = null;
                return false;
        }
    }

    private ExpressionSyntax ParsePrimaryExpression()
    {
        var startIndex = _tokenIndex;
        var token = ExpectToken();
        var expr = token switch
        {
            LiteralToken literal => new LiteralSyntax(GetTokens(startIndex), literal.Value, literal.Kind),
            IdentifierToken identifier => new IdentifierSyntax(GetTokens(startIndex), Optional<string>.Empty(), identifier.Value),
            SymbolToken symbolToken => symbolToken.Symbol switch
            {
                Symbol.OpenParen => ParseParenthesizedExpression(),
                Symbol.Minus => new UnaryExpressionSyntax(GetTokens(startIndex), UnaryOperatorSyntax.Negate, ParsePrimaryExpression()),
                Symbol.Bang => new UnaryExpressionSyntax(GetTokens(startIndex), UnaryOperatorSyntax.Invert, ParsePrimaryExpression()),
                Symbol.OpenBracket => ParseArrayInitializer(startIndex),
                Symbol.OpenBrace => new StructInitializerSyntax(GetTokens(startIndex), Optional<TypeSyntax>.Empty(), ParseStructInitializerBody()),
                Symbol.Struct => ParseStructInitializer(startIndex),
                _ => throw new ParseException(Diagnostic
                    .Error($"Unexpected symbol '{symbolToken.Symbol}' in expression")
                    .WithHelp("Expected '(', '-', '!', '[' or '{'")
                    .At(symbolToken)
                    .Build())
            },
            _ => throw new ParseException(Diagnostic
                .Error($"Unexpected token '{token.GetType().Name}' in expression")
                .WithHelp("Expected literal, identifier, or parenthesized expression")
                .At(token)
                .Build())
        };

        return ParsePostfixOperators(expr);
    }

    private ExpressionSyntax ParseParenthesizedExpression()
    {
        var expression = ParseExpression();
        ExpectSymbol(Symbol.CloseParen);
        return expression;
    }

    private ExpressionSyntax ParsePostfixOperators(ExpressionSyntax expr)
    {
        var startIndex = _tokenIndex;
        while (HasToken)
        {
            if (TryExpectSymbol(Symbol.Ampersand))
            {
                expr = new AddressOfSyntax(GetTokens(startIndex), expr);
                continue;
            }

            if (TryExpectSymbol(Symbol.Caret))
            {
                expr = new DereferenceSyntax(GetTokens(startIndex), expr);
                continue;
            }

            if (TryExpectSymbol(Symbol.Period))
            {
                var member = ExpectIdentifier().Value;
                if (TryExpectSymbol(Symbol.OpenParen))
                {
                    var parameters = new List<ExpressionSyntax>();

                    while (!TryExpectSymbol(Symbol.CloseParen))
                    {
                        parameters.Add(ParseExpression());
                        if (!TryExpectSymbol(Symbol.Comma))
                        {
                            ExpectSymbol(Symbol.CloseParen);
                            break;
                        }
                    }

                    expr = new DotFuncCallSyntax(GetTokens(startIndex), member, expr, parameters);
                    continue;
                }

                expr = new StructFieldAccessSyntax(GetTokens(startIndex), expr, member);
                continue;
            }

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

            if (TryExpectSymbol(Symbol.OpenParen))
            {
                var parameters = new List<ExpressionSyntax>();

                while (!TryExpectSymbol(Symbol.CloseParen))
                {
                    parameters.Add(ParseExpression());
                    if (!TryExpectSymbol(Symbol.Comma))
                    {
                        ExpectSymbol(Symbol.CloseParen);
                        break;
                    }
                }

                expr = new FuncCallSyntax(GetTokens(startIndex), expr, parameters);
                continue;
            }

            break;
        }

        return expr;
    }

    private ArrayInitializerSyntax ParseArrayInitializer(int startIndex)
    {
        var capacity = ParseExpression();
        ExpectSymbol(Symbol.CloseBracket);
        var type = ParseType();
        return new ArrayInitializerSyntax(GetTokens(startIndex), capacity, type);
    }

    private StructInitializerSyntax ParseStructInitializer(int startIndex)
    {
        var type = Optional.Empty<TypeSyntax>();
        if (!TryExpectSymbol(Symbol.OpenBrace))
        {
            type = ParseType();
            ExpectSymbol(Symbol.OpenBrace);
        }

        var initializers = ParseStructInitializerBody();

        return new StructInitializerSyntax(GetTokens(startIndex), type, initializers);
    }

    private Dictionary<string, ExpressionSyntax> ParseStructInitializerBody()
    {
        Dictionary<string, ExpressionSyntax> initializers = [];
        while (!TryExpectSymbol(Symbol.CloseBrace))
        {
            var name = ExpectIdentifier().Value;
            ExpectSymbol(Symbol.Assign);
            var value = ParseExpression();
            initializers.Add(name, value);
        }

        return initializers;
    }

    private BlockSyntax ParseBlock()
    {
        var startIndex = _tokenIndex;
        List<StatementSyntax> statements = [];
        while (!TryExpectSymbol(Symbol.CloseBrace))
        {
            try
            {
                statements.Add(ParseStatement());
            }
            catch (ParseException ex)
            {
                _diagnostics.Add(ex.Diagnostic);
                Next();
            }
        }

        return new BlockSyntax(GetTokens(startIndex), statements);
    }

    private TypeSyntax ParseType()
    {
        var startIndex = _tokenIndex;
        if (TryExpectIdentifier(out var name))
        {
            if (name.Value[0] == 'u' && int.TryParse(name.Value[1..], out var size))
            {
                if (size is not 8 and not 16 and not 32 and not 64)
                {
                    throw new ParseException(Diagnostic
                        .Error("Arbitrary uint size is not supported")
                        .WithHelp("Use u8, u16, u32 or u64")
                        .At(name)
                        .Build());
                }

                return new IntTypeSyntax(GetTokens(startIndex), false, size);
            }

            if (name.Value[0] == 'i' && int.TryParse(name.Value[1..], out size))
            {
                if (size is not 8 and not 16 and not 32 and not 64)
                {
                    throw new ParseException(Diagnostic
                        .Error("Arbitrary int size is not supported")
                        .WithHelp("Use i8, i16, i32 or i64")
                        .At(name)
                        .Build());
                }

                return new IntTypeSyntax(GetTokens(startIndex), true, size);
            }

            if (name.Value[0] == 'f' && int.TryParse(name.Value[1..], out size))
            {
                if (size is not 32 and not 64)
                {
                    throw new ParseException(Diagnostic
                        .Error("Arbitrary float size is not supported")
                        .WithHelp("Use f32 or f64")
                        .At(name)
                        .Build());
                }

                return new FloatTypeSyntax(GetTokens(startIndex), size);
            }

            return name.Value switch
            {
                "void" => new VoidTypeSyntax(GetTokens(startIndex)),
                "string" => new StringTypeSyntax(GetTokens(startIndex)),
                "cstring" => new CStringTypeSyntax(GetTokens(startIndex)),
                "bool" => new BoolTypeSyntax(GetTokens(startIndex)),
                _ => new CustomTypeSyntax(GetTokens(startIndex), _moduleName, name.Value)
            };
        }

        if (TryExpectSymbol(Symbol.Caret))
        {
            var baseType = ParseType();
            return new PointerTypeSyntax(GetTokens(startIndex), baseType);
        }

        if (TryExpectSymbol(Symbol.Func))
        {
            ExpectSymbol(Symbol.OpenParen);

            List<TypeSyntax> parameters = [];
            while (!TryExpectSymbol(Symbol.CloseParen))
            {
                parameters.Add(ParseType());
                if (!TryExpectSymbol(Symbol.Comma))
                {
                    ExpectSymbol(Symbol.CloseParen);
                    break;
                }
            }

            var returnType = TryExpectSymbol(Symbol.Colon)
                ? ParseType()
                : new VoidTypeSyntax([]);

            return new FuncTypeSyntax(GetTokens(startIndex), parameters, returnType);
        }

        if (TryExpectSymbol(Symbol.OpenBracket))
        {
            ExpectSymbol(Symbol.CloseBracket);
            var baseType = ParseType();
            return new ArrayTypeSyntax(GetTokens(startIndex), baseType);
        }

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

    private Token ExpectToken()
    {
        if (!HasToken)
        {
            throw new ParseException(Diagnostic
                .Error("Unexpected end of file")
                .WithHelp("Expected more tokens to complete the syntax")
                .At(_tokens[^1])
                .Build());
        }

        var token = CurrentToken!;
        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 (CurrentToken is SymbolToken symbolToken && symbolToken.Symbol == symbol)
        {
            Next();
            return true;
        }

        return false;
    }

    private bool TryExpectIdentifier([NotNullWhen(true)] out IdentifierToken? identifier)
    {
        if (CurrentToken is IdentifierToken identifierToken)
        {
            identifier = identifierToken;
            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 LiteralToken ExpectLiteral()
    {
        var token = ExpectToken();
        if (token is not LiteralToken identifier)
        {
            throw new ParseException(Diagnostic
                .Error($"Expected literal, but found {token.GetType().Name}")
                .WithHelp("Provide a valid literal name here")
                .At(token)
                .Build());
        }

        return identifier;
    }

    private LiteralToken ExpectLiteral(LiteralKind kind)
    {
        var literal = ExpectLiteral();
        if (literal.Kind != kind)
        {
            throw new ParseException(Diagnostic
                .Error($"Expected {kind} literal, but found {literal.Kind}")
                .WithHelp($"Provide a {kind} literal name here")
                .At(literal)
                .Build());
        }

        return literal;
    }

    private bool TryExpectLiteral(LiteralKind kind, [NotNullWhen(true)] out LiteralToken? literal)
    {
        if (CurrentToken is LiteralToken identifierToken)
        {
            literal = identifierToken;
            Next();
            return true;
        }

        literal = null;
        return false;
    }

    private void Next()
    {
        _tokenIndex++;
    }

    private IEnumerable<Token> GetTokens(int tokenStartIndex)
    {
        return _tokens.Skip(tokenStartIndex).Take(_tokenIndex - tokenStartIndex);
    }
}

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

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