20
\$\begingroup\$

As we have our Community Oriented Development Extravaganza, Requiring Extreme Vigor Inciting Extraordinary Winners 2017 Challenge. Finally I had some time to write a tokenizer for my own language. It is the most complex tokenizer I have written so far. The language consists of 32 keywords and supports most of the operators. Beyond that we have numeric literals, string literals, address literals (virtual memory pointer address) and character literals. I read a lot before writing this tokenizer, as many similiar questions were asked here on Code Review. And because it is my first attempt in implementing a programming language in C#, I did C before, I would like to know what I can improve or change, if there are any exceptions I did not notice.

The tokenizer generates tokens which I will use to generate an AST during parsing. I did not use Regex or any lexer generators as that was not the point for me.


In the tokens sfunc and svar, the s stands for static.

I didn't include the preprocessor here, as it is already fairly big chunk of code.

CharStream.cs

using System;
using System.Collections.Generic;
using System.Text;
namespace Compiler.Lexer
{
    public class CharStream
    {
        private readonly IList<string> Lines;
        public bool EOF { get; private set; }
        public int CurrentLineNumber { get; set; }
        public int CurrentPosition { get; set; }

        public CharStream(string source)
        {
            if (!string.IsNullOrEmpty(source))
            {
                Lines = Preprocessor.Utilities.GenerateList(source);
            }
            else
            {
                EOF = true;
                return;
            }

            int line = 0;
            while (string.IsNullOrEmpty(Lines[line]))
            {
                line++;
            }
            EOF = false;
            CurrentLineNumber = line;
            CurrentPosition = 0;

        }

        public char? Peek()
        {
            if (EOF)
            {
                return null;
            }

            char? c = Get();
            Unget();
            return c;
        }

        public string Peek(int count)
        {
            string peek = Get(count);
            Unget(count);
            return peek;
        }

        public char? Get()
        {
            if (EOF) 
            {
                return null;    
            }

            while (String.IsNullOrEmpty(Lines[CurrentLineNumber]) 
                   && CurrentLineNumber < Lines.Count)
            {
                CurrentLineNumber++;
            }

            char c = Lines[CurrentLineNumber][CurrentPosition];

            if (CurrentPosition + 1 < Lines[CurrentLineNumber].Length)
            {
                CurrentPosition++;
            }
            else 
            {

                if (CurrentLineNumber + 1 < Lines.Count) 
                {

                    CurrentLineNumber++;
                    CurrentPosition = 0;
                }
                else
                {
                    EOF = true;
                }
            }

            return c;
        }

        public string Get(int count)
        {
            var stringBuilder = new StringBuilder();
            for (int i = 0; i < count; i++)
            {
                char? c = Get();
                if (c == null)
                {
                    return null;
                }
                else
                {
                    stringBuilder.Append(c);
                }

            }
            return stringBuilder.ToString();
        }

        private void Unget()
        {
            if (EOF)
            {
                EOF = false;
            }
            else
            {
                if (CurrentPosition > 0)
                {
                    CurrentPosition--;
                }
                else if (CurrentLineNumber > 0)
                {
                    while (string.IsNullOrEmpty(Lines[--CurrentLineNumber])) ;
                    CurrentPosition = Lines[CurrentLineNumber].Length - 1;
                }

            }
        }

        private void Unget(int count)
        {
            for (int i = 0; i < count; i++)
            {
                Unget();
            }
        }

    }
}

TokenKind.cs

using System;
namespace Compiler.Lexer
{
    // do not change token numbers
    public enum TokenKind : ushort
    {
        UnknownToken = 0,

        AndKeyword = 1,
        BreakKeyword = 2,
        CaseKeyword = 3, 
        CatchKeyword = 4,
        ClassKeyword = 5,
        ConstKeyword = 6, 
        ContinueKeyword = 7,
        DefaultKeyword = 8,
        DoKeyword = 9,
        ElseKeyword = 10,
        EnumKeyword = 11,
        FalseKeyword = 12, 
        ForKeyword = 13,
        FuncKeyword = 14,
        IfKeyword = 15,
        InKeyword = 16,
        IsKeyword = 17,
        NewKeyword = 18,
        NorKeyword = 19,
        NullKeyword = 20,
        ObjectKeyword = 21,
        OrKeyword = 22,
        PackageKeyword = 23,
        ReturnKeyword = 24,
        SfuncKeyword = 25,
        SvarKeyword = 26,
        SwitchKeyword = 27,
        ThisKeyword = 28,
        TrueKeyword = 29,
        TryKeyword = 30,
        VarKeyword = 31,
        WhileKeyword = 32,


        Identifier = 100,

        CharacterLiteral = 101,
        StringLiteral = 102,
        IntegerLiteral = 103,
        RealLiteral = 104,
        AddressLiteral = 105,

        Assignment = 200,
        Addition = 201,
        Subtraction = 202,
        UnaryPlus = 203,
        UnaryMinus = 204,
        Multiplication = 205,
        Division = 206,
        Modulo = 207,

        EqualTo = 208,
        NotEqualTo = 209,
        GeaterThan = 210,
        LessThan = 211,
        GreaterThanOrEqualTo = 212,
        LessThanOrEqualTo = 213,

        LogicalNOT = 214,
        LogicalAND = 215,
        LogicalOR = 216,

        PostfixPlus = 217,
        PostfixMinus = 218,
        PrefixPlus = 219,
        PrefixMinus = 220,

        BitwiseNOT = 300,
        BitwiseAND = 301,
        BitwiseOR = 302,
        BitwiseXOR = 303,
        BitwiseLeftShift = 304,
        BitwiseRightShift = 305,

        AdditionAssignment = 400,
        SubtractionAssignment = 401,
        MultiplicationAssignment = 402,
        DivisionAssignment = 403,
        ModuloAssignment = 404,
        BitwiseANDAssignment = 405,
        BitwiseORAssignment = 406,
        BitwiseXORAssignment = 407,
        BitwiseLeftShiftAssignment = 408,
        BitwiseRightShiftAssignment = 409,

        OpenParenthesis = 500,
        CloseParenthesis = 501,
        OpenBrace = 502,
        CloseBrace = 503,
        OpenBracket = 504,
        CloseBracket = 505,
        Colon = 506,
        Semicolon = 507,
        Comma = 508,
        Dot = 509,
        Question = 510,

    }
}

Token.cs

using System;

namespace Compiler.Lexer
{
    public sealed class Token
    {
        public TokenKind Kind { get; }
        public string Lexeme { get; }
        public int LineNumber { get; }
        public int Position { get; }

        public Token(TokenKind kind, string lexeme, int lineNumber, int position)
        {
            Kind = kind;
            Lexeme = lexeme;
            LineNumber = lineNumber;
            Position = position;
        }
    }
}

Tokenizer.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Compiler.Lexer
{
    public partial class Tokenizer
    {
        private CharStream Stream;
        private Token LastToken;

        public Tokenizer(string source)
        {
            Stream = new CharStream(source);
            LastToken = new Token
                (TokenKind.UnknownToken, string.Empty, 0, 0);
        }

        public Token Get()
        {
            Token get;
            int lineNumber = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;

            do
            {
                if (Stream.EOF)
                {
                    return null;
                }
            } while (IsWhiteSpace());

            get = IsSymbol();
            if (get != null) 
            {
                return get;
            }

            get = IsOperator();
            if (get != null)
            {
                return get;
            }

            get = IsIdentifier();
            if (get != null)
            {
                return get;
            }

            get = IsCharacterLiteral();
            if (get != null)
            {
                return get;
            }

            get = IsStringLiteral();
            if (get != null)
            {
                return get;
            }

            get = IsNumericLiteral();
            if (get != null)
            {
                return get;
            }

            get = IsAddressLiteral();
            if (get != null)
            {
                return get;
            }

            get = new Token(TokenKind.UnknownToken, Stream.Get().ToString(),
                        lineNumber, position);
            return get;

        }

        public Token Peek()
        {
            int line = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;
            Token previousToken = LastToken;
            Token peek = Get();
            Stream.CurrentLineNumber = line;
            Stream.CurrentPosition = position;
            LastToken = previousToken;
            return peek;
        }

        private bool IsWhiteSpace()
        {
            bool isWhiteSpace = false;
            while (Stream.Peek() != null && char.IsWhiteSpace((char)Stream.Peek()))
            {
                isWhiteSpace = true;
                Stream.Get();
            }
            return isWhiteSpace;
        }

        private Token IsSymbol()
        {
            int lineNumber = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;
            string lexeme = Stream.Peek().ToString();           
            TokenKind tokenKind;

            if (SymbolToTokenKind.TryGetValue(lexeme, out tokenKind))
            {
                Stream.Get();
                Token token = new Token(tokenKind, lexeme, lineNumber, position);
                LastToken = token;
                return token;
            }


            return null;
        }

        private Token IsOperator()
        {
            int lineNumber = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;
            var lexeme = new StringBuilder();

            char? firstChar = null;
            if (Stream.Peek() == '+' || Stream.Peek() == '-')
            {
                firstChar = Stream.Peek();
            }

            while (Stream.Peek() != null
                   && lineNumber == Stream.CurrentLineNumber
                   && OperatorToTokenKind.ContainsKey(Stream.Peek().ToString()) 
                   || (Stream.Peek() == '+' && firstChar == '+') 
                   || (Stream.Peek() == '-' && firstChar == '-'))
            {
                lexeme.Append(Stream.Get());
            }

            if ((lexeme.ToString() == "+" || lexeme.ToString() == "-")
                & (ushort)LastToken.Kind > 99 && (ushort)LastToken.Kind < 200)
            {
                //A+B or A-B
                lexeme.Insert(0, 'A');
                lexeme.Insert(2, 'B');
            }
            else if (lexeme.ToString() == "+" || lexeme.ToString() == "-")
            {
                //+A or -A
                lexeme.Insert(1, 'A');
            }

            if ((lexeme.ToString() == "++" || lexeme.ToString() == "--")
                & (ushort)LastToken.Kind > 99 && (ushort)LastToken.Kind < 200)
            {
                //A++ or A--
                lexeme.Insert(0, 'A');
            } 
            else if (lexeme.ToString() == "++" || lexeme.ToString() == "--")
            {
                //++A or --A
                lexeme.Insert(2, 'A');
            }

            TokenKind tokenKind;
            if (OperatorToTokenKind.TryGetValue(lexeme.ToString(), out tokenKind))
            {
                Token token = new Token
                    (tokenKind, lexeme.ToString(), lineNumber, position);
                LastToken = token;
                return token;
            }


            return null;
        }

        private Token IsIdentifier()
        {
            int lineNumber = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;
            var lexeme = new StringBuilder();
            TokenKind tokenKind;
            Token token;

            if (Stream.Peek() == null 
                || !(char.IsLetter((char)Stream.Peek()) || Stream.Peek() == '_'))
            {
                return null;
            }

            lexeme.Append(Stream.Get());

            int count = 0;
            while (Stream.Peek() != null
                    && lineNumber == Stream.CurrentLineNumber
                    && (char.IsLetter((char)Stream.Peek()) 
                    || char.IsDigit((char)Stream.Peek()) 
                    || Stream.Peek() == '_'))
            {
                count++;
                lexeme.Append(Stream.Get());
            }


            if (KeywordToTokenKind.TryGetValue(lexeme.ToString(), out tokenKind))
            {
                token = new Token
                    (tokenKind, lexeme.ToString(), lineNumber, position);
                LastToken = token;
                return token;
            }

            token = new Token
                (TokenKind.Identifier, lexeme.ToString(), lineNumber, position);
            LastToken = token;
            return token;


        }

        private Token IsCharacterLiteral()
        {
            int lineNumber = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;
            var lexeme = new StringBuilder();
            Token token;

            if (Stream.Peek() != '\'')
            {
                return null;
            }

            int singleQuoteCount = 2;
            while (Stream.Peek() != null 
                   && singleQuoteCount > 0 
                   && lineNumber == Stream.CurrentLineNumber)
            {
                char c = (char)Stream.Get();

                if (c == '\\' && Stream.Peek() == '\'')
                {
                    singleQuoteCount++;
                }

                if (c == '\'')
                {
                    singleQuoteCount--;
                }

                lexeme.Append(c);
            }

            if (singleQuoteCount != 0)
            {
                return null;
            }

            token = new Token
                (TokenKind.CharacterLiteral, lexeme.ToString(), lineNumber, position);
            LastToken = token;
            return token;

        }

        private Token IsStringLiteral()
        {
            int lineNumber = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;
            var lexeme = new StringBuilder();
            Token token;

            if (Stream.Peek() != '\"')
            {
                return null;
            }

            int doubleQuoteCount = 2;
            while (Stream.Peek() != null && doubleQuoteCount > 0)
            {
                char c = (char)Stream.Get();

                if (c == '\\' && Stream.Peek() == '\"')
                {
                    doubleQuoteCount++;
                }

                if (c == '\"')
                {
                    doubleQuoteCount--;
                }

                lexeme.Append(c);
            }

            if (doubleQuoteCount != 0)
            {
                return null;
            }

            token = new Token
                (TokenKind.StringLiteral, lexeme.ToString(), lineNumber, position);
            LastToken = token;
            return token;

        }

        private Token IsNumericLiteral()
        {
            int lineNumber = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;
            var lexeme = new StringBuilder();
            Token token;

            bool isReal = false;

            if (Stream.Peek() == null || !char.IsDigit((char)Stream.Peek()))
            {
                return null;
            }

            while (Stream.Peek() != null 
                   && lineNumber == Stream.CurrentLineNumber
                   && (char.IsDigit((char)Stream.Peek()) 
                   || Stream.Peek() == '.'
                   || Stream.Peek() == 'M'))
            {
                if (Stream.Peek() == '.' && (!char.IsDigit(Stream.Peek(2)[1]) || isReal))
                {
                    break;
                }

                if (Stream.Peek() == '.') 
                {
                    isReal = true;    
                }

                lexeme.Append(Stream.Get());
            }

            if (isReal)
            {
                token = new Token
                    (TokenKind.RealLiteral, lexeme.ToString(), lineNumber, position);
            }
            else
            {
                token = new Token
                    (TokenKind.IntegerLiteral, lexeme.ToString(), lineNumber, position);
            }


            LastToken = token;
            return token;

        }

        private Token IsAddressLiteral()
        {
            int lineNumber = Stream.CurrentLineNumber;
            int position = Stream.CurrentPosition;
            var lexeme = new StringBuilder();
            Token token;

            bool isAddress = false;

            if (Stream.Peek() == null || Stream.Peek() != '@')
            {
                return null;
            }

            while (Stream.Peek() != null
                   && lineNumber == Stream.CurrentLineNumber
                   && (char.IsDigit((char)Stream.Peek())
                   || Stream.Peek() == '@'))
            {
                if (Stream.Peek() == '@' && (!char.IsDigit(Stream.Peek(2)[1]) || isAddress))
                {
                    break;
                }

                if (Stream.Peek() == '@')
                {
                    isAddress = true;
                }

                lexeme.Append(Stream.Get());
            }

            token = new Token
                    (TokenKind.AddressLiteral, lexeme.ToString(), lineNumber, position);
            LastToken = token;
            return token;

        }

    }
}

Keywords.cs

using System;
using System.Collections.Generic;

namespace Compiler.Lexer
{
    public partial class Tokenizer
    {
        private static readonly Dictionary<string, TokenKind> KeywordToTokenKind =
        new Dictionary<string, TokenKind>(StringComparer.OrdinalIgnoreCase)
        {
            ["and"] = TokenKind.AndKeyword,
            ["break"] = TokenKind.BreakKeyword,
            ["case"] = TokenKind.CaseKeyword,
            ["catch"] = TokenKind.CatchKeyword,
            ["class"] = TokenKind.ClassKeyword,
            ["const"] = TokenKind.ConstKeyword,
            ["continue"] = TokenKind.ContinueKeyword,
            ["default"] = TokenKind.DefaultKeyword,
            ["do"] = TokenKind.DoKeyword,
            ["else"] = TokenKind.ElseKeyword,
            ["enum"] = TokenKind.EnumKeyword,
            ["false"] = TokenKind.FalseKeyword,
            ["for"] = TokenKind.ForKeyword,
            ["func"] = TokenKind.FuncKeyword,
            ["if"] = TokenKind.IfKeyword,
            ["in"] = TokenKind.InKeyword,
            ["is"] = TokenKind.IsKeyword,
            ["new"] = TokenKind.NewKeyword,
            ["nor"] = TokenKind.NorKeyword,
            ["null"] = TokenKind.NullKeyword,
            ["object"] = TokenKind.ObjectKeyword,
            ["or"] = TokenKind.OrKeyword,
            ["package"] = TokenKind.PackageKeyword,
            ["return"] = TokenKind.ReturnKeyword,
            ["sfunc"] = TokenKind.SfuncKeyword,
            ["svar"] = TokenKind.SvarKeyword,
            ["switch"] = TokenKind.SwitchKeyword,
            ["this"] = TokenKind.ThisKeyword,
            ["true"] = TokenKind.TrueKeyword,
            ["try"] = TokenKind.TryKeyword,
            ["var"] = TokenKind.VarKeyword,
            ["while"] = TokenKind.WhileKeyword,
        };

    }
}

Operators.cs

using System;
using System.Collections.Generic;

namespace Compiler.Lexer
{
    public partial class Tokenizer
    {
        private static readonly Dictionary<string, TokenKind> OperatorToTokenKind =
        new Dictionary<string, TokenKind>(StringComparer.OrdinalIgnoreCase)
        {
            ["="] = TokenKind.Assignment,
            ["A+B"] = TokenKind.Addition,
            ["A-B"] = TokenKind.Subtraction,
            ["+A"] = TokenKind.UnaryPlus,
            ["-A"] = TokenKind.UnaryMinus,
            ["*"] = TokenKind.Multiplication,
            ["/"] = TokenKind.Division,
            ["%"] = TokenKind.Modulo,
            ["=="] = TokenKind.EqualTo,
            ["!="] = TokenKind.NotEqualTo,
            [">"] = TokenKind.GeaterThan,
            ["<"] = TokenKind.LessThan,
            [">="] = TokenKind.GreaterThanOrEqualTo,
            ["<="] = TokenKind.LessThanOrEqualTo,
            ["!"] = TokenKind.LogicalNOT,
            ["&&"] = TokenKind.LogicalAND,
            ["||"] = TokenKind.LogicalOR,
            ["A++"] = TokenKind.PostfixPlus,
            ["A--"] = TokenKind.PostfixMinus,
            ["++A"] = TokenKind.PrefixPlus,
            ["--A"] = TokenKind.PrefixMinus,
            ["~"] = TokenKind.BitwiseNOT,
            ["&"] = TokenKind.BitwiseAND,
            ["|"] = TokenKind.BitwiseOR,
            ["^"] = TokenKind.BitwiseXOR,
            ["<<"] = TokenKind.BitwiseLeftShift,
            [">>"] = TokenKind.BitwiseRightShift,
            ["+="] = TokenKind.AdditionAssignment,
            ["-="] = TokenKind.SubtractionAssignment,
            ["*="] = TokenKind.MultiplicationAssignment,
            ["/="] = TokenKind.DivisionAssignment,
            ["%="] = TokenKind.ModuloAssignment,
            ["&="] = TokenKind.BitwiseANDAssignment,
            ["|="] = TokenKind.BitwiseORAssignment,
            ["^="] = TokenKind.BitwiseXORAssignment,
            ["<<="] = TokenKind.BitwiseLeftShiftAssignment,
            [">>="] = TokenKind.BitwiseRightShiftAssignment,
        };

    }
}

Symbols.cs

using System;
using System.Collections.Generic;

namespace Compiler.Lexer
{
    public partial class Tokenizer
    {
        private static readonly Dictionary<string, TokenKind> SymbolToTokenKind =
        new Dictionary<string, TokenKind>(StringComparer.OrdinalIgnoreCase)
        {
            ["("] = TokenKind.OpenParenthesis,
            [")"] = TokenKind.CloseParenthesis,
            ["{"] = TokenKind.OpenBrace,
            ["}"] = TokenKind.CloseBrace,
            ["["] = TokenKind.OpenBracket,
            ["]"] = TokenKind.CloseBracket,
            [":"] = TokenKind.Colon,
            [";"] = TokenKind.Semicolon,
            [","] = TokenKind.Comma,
            ["."] = TokenKind.Dot,
            ["?"] = TokenKind.Question,
        };

    }
}

Utilities.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
namespace Compiler.Preprocessor
{
    public static class Utilities
    {
        public static List<string> GenerateList(string source)
        {
            return source.Split(new string[] { "\r\n", "\n" },
                                StringSplitOptions.None).ToList();
        }
    }
}

Test script

 Tokenizer tokenizer = new Tokenizer(source);

 Token token;
 while ((token = tokenizer.Peek()) != null)
 {
    Console.WriteLine(token.Kind + " LEX: " + token.Lexeme + " POS: " + token.Position + " LN: " + token.LineNumber);
    tokenizer.Get();
 }

Samples

Hello World

func Main():
    print("Hello World")

Generated tokens:

  1. FuncKeyword LEX: func POS: 0 LN: 0
  2. Identifier LEX: Main POS: 5 LN: 0
  3. OpenParenthesis LEX: ( POS: 9 LN: 0
  4. CloseParenthesis LEX: ) POS: 10 LN: 0
  5. Colon LEX: : POS: 11 LN: 0
  6. Identifier LEX: print POS: 4 LN: 1
  7. OpenParenthesis LEX: ( POS: 9 LN: 1
  8. StringLiteral LEX: "Hello World" POS: 10 LN: 1
  9. CloseParenthesis LEX: ) POS: 23 LN: 1

Fibonacci

func Fib(n):
    if n == 1 or n == 2:
        return 1
    return fib(n - 1) + fib(n-1)

func Main():
    print(fib(5))

Generated tokens:

  1. FuncKeyword LEX: func POS: 0 LN: 0
  2. Identifier LEX: Fib POS: 5 LN: 0
  3. OpenParenthesis LEX: ( POS: 8 LN: 0
  4. Identifier LEX: n POS: 9 LN: 0
  5. CloseParenthesis LEX: ) POS: 10 LN: 0
  6. Colon LEX: : POS: 11 LN: 0
  7. IfKeyword LEX: if POS: 4 LN: 1
  8. Identifier LEX: n POS: 7 LN: 1
  9. EqualTo LEX: == POS: 9 LN: 1
  10. IntegerLiteral LEX: 1 POS: 12 LN: 1
  11. OrKeyword LEX: or POS: 14 LN: 1
  12. Identifier LEX: n POS: 17 LN: 1
  13. EqualTo LEX: == POS: 19 LN: 1
  14. IntegerLiteral LEX: 2 POS: 22 LN: 1
  15. Colon LEX: : POS: 23 LN: 1
  16. ReturnKeyword LEX: return POS: 8 LN: 2
  17. IntegerLiteral LEX: 1 POS: 15 LN: 2
  18. ReturnKeyword LEX: return POS: 4 LN: 3
  19. Identifier LEX: fib POS: 11 LN: 3
  20. OpenParenthesis LEX: ( POS: 14 LN: 3
  21. Identifier LEX: n POS: 15 LN: 3
  22. Subtraction LEX: A-B POS: 17 LN: 3
  23. IntegerLiteral LEX: 1 POS: 19 LN: 3
  24. CloseParenthesis LEX: ) POS: 20 LN: 3
  25. Addition LEX: A+B POS: 22 LN: 3
  26. Identifier LEX: fib POS: 24 LN: 3
  27. OpenParenthesis LEX: ( POS: 27 LN: 3
  28. Identifier LEX: n POS: 28 LN: 3
  29. Subtraction LEX: A-B POS: 29 LN: 3
  30. IntegerLiteral LEX: 1 POS: 30 LN: 3
  31. CloseParenthesis LEX: ) POS: 31 LN: 3
  32. FuncKeyword LEX: func POS: 0 LN: 5
  33. Identifier LEX: Main POS: 5 LN: 5
  34. OpenParenthesis LEX: ( POS: 9 LN: 5
  35. CloseParenthesis LEX: ) POS: 10 LN: 5
  36. Colon LEX: : POS: 11 LN: 5
  37. Identifier LEX: print POS: 4 LN: 6
  38. OpenParenthesis LEX: ( POS: 9 LN: 6
  39. Identifier LEX: fib POS: 10 LN: 6
  40. OpenParenthesis LEX: ( POS: 13 LN: 6
  41. IntegerLiteral LEX: 5 POS: 14 LN: 6
  42. CloseParenthesis LEX: ) POS: 15 LN: 6
  43. CloseParenthesis LEX: ) POS: 16 LN: 6
\$\endgroup\$
  • \$\begingroup\$ Looks good to me! \$\endgroup\$ – J_H Aug 13 '17 at 6:02
  • \$\begingroup\$ OUCH! Looks great. \$\endgroup\$ – paparazzo Aug 13 '17 at 6:15
  • 1
    \$\begingroup\$ I'd be great if you could add a sample fo the language you're tokenizing... \$\endgroup\$ – t3chb0t Aug 13 '17 at 13:41
  • 1
    \$\begingroup\$ Added samples and the output tokens \$\endgroup\$ – Michał Paszkowski Aug 13 '17 at 14:50
  • \$\begingroup\$ Prefix and Postfix plus should be called increment, instead of plus. \$\endgroup\$ – theonlygusti Aug 13 '17 at 16:42
10
\$\begingroup\$

This looks well structured, with responsibilities nicely separated, but as they say: 'the devil is in the details'. Let's have a look:

CharStream

  • This class contains some surprising behavior:
    • Peek(int count) returns null if there are less than count characters left, instead of returning only the remaining characters. This is something that I expect to be documented. The same goes for Get.
    • Newline characters are completely ignored. This makes things more complicated for the tokenizer: instead of only having to check characters (which it has to do anyway), it now also has to compare line numbers (which is easy to forget).
  • Both Peek(int count) and Get(int count) do not correctly (or at all) reset the stream position if they return null. Instead of an Unget method, you could simply store the previous location in local variables, and reset them before returning. Reset them in a finally block to make sure an exception won't mess things up.
  • Line number and position have public setters. You should probably make those private. If you want them to be public, they should guard against invalid positions.
  • The empty-line skipping loop in the constructor fails on input that consists of only empty lines (ArgumentOutOfRangeException). Personally I wouldn't ignore newline characters and skip empty lines at this level - the tokenizer can easily deal with it when it skips whitespace.
  • The Get() logic fails on input that ends with empty lines. Empty lines should be skipped as part of the stream position advancing code - after you fetch a character, and before you determine whether the end of the stream is reached.
  • Unget contains a while loop with an empty body. While it works as it should, it does make the code harder to read.

TokenKind

  • There's a comment here that says that token numbers should not be changed, but it does not explain why.
  • Personally I would add a category name comment above each group of token types, to improve readability.

Tokenizer

  • Using partial classes strikes me as rather odd. partial is mostly used when generated and hand-written code need to be combined. I would probably use regions for this kind of separation, if at all, but I think t3chb0t's suggestion is much nicer: annotated enum values combined with a little initialization code.
  • Turning IsWhitespace into a SkipWhitespace method would reduce clutter in your Get method.
  • Due to CharStream ignoring newlines, your tokenizer has to compare line numbers, but it doesn't do so consistently: identifiers, numbers and some operators are split up by a newline, while strings and some other operators (such as ++) are not.
  • The use of LastToken is questionable:
    • Every token-parsing method has to set it, which is easy to get wrong. It'd be better if Get did so, perhaps in a finally block.
    • It's only used by IsOperator, to distinguish between unary and binary +/- and prefix and postfix ++/--. But the tokenizer does not have sufficient information to (reliably) do so: take (a) + b and a() + b for example. Another example would be < and > in C#: are they less-than and greater-than operators, or the start and end of a generic parameter list? Don't try to do this in the tokenizer, leave it for the parser.
  • There are a few Stream.Peek().ToString() calls without a null-check, and there are certain inputs where that causes problems (such as 4. and @).
  • A lot of the Is* methods contain an early-out check, but they all do a little bit of work even before that check. That work is wasted if the check fails, so it should be done after the check.
  • IsCharacterLiteral and IsStringLiteral can be simplified by skipping the first ' or " (which was peeked already) and by creating a helper method for parsing escape sequences (such as \', \", \n and various others). This allows you to simplify these methods and to get consistent escape sequence behavior across characters and strings.
  • IsCharacterLiteral does not limit character literals to a single character.
  • Why is 4MMM4 a valid numeric literal? What does the M stand for?
  • IsAddressLiteral gets stuck on inputs like @a - the tokenizer continues to return empty address tokens.

Utilities

  • The name GenerateList doesn't really describe what the method does. Given how simple the method is, I'm not sure you even need this. I'm also not sure why it returns a list instead of an array - CharStream doesn't seem to need it to be a list.

General

  • Some documentation, and a few explanation comments in the more complex parsing methods, would be helpful.
  • Some names could be a little clearer: NextChar and NextToken are a bit more obvious than simply Get, and SplitLines is more descriptive than GenerateList.
  • Private fields are normally written in camelCase, not PascalCase. It's also common to prefix them with a _.
  • Declaring variables at the top of a method is very C-like. I usually declare variables as close to where they're actually used as possible.
  • I don't know if you've got automated tests for this, but if not, it's probably a good idea to create some. It may also be useful to do some 'fuzzing': feeding randomly generated strings into your tokenizer may help you find other problems.
\$\endgroup\$
8
\$\begingroup\$

Looks pretty tidy, a few things:

  1. Consider reversing the check for an empty source in CharStream:

    if (string.IsNullOrEmpty(source))
    {
        EOF = true;
        return;
    }
    Lines = Preprocessor.Utilities.GenerateList(source);
    

    This deals with that special condition immediately and doesn't disrupt the logic flow (as opposed to - do something if valid, then handle empty case and then continue with normal case)

  2. I don't know which language this is for but you should name you namespaces slightly less generic than Compiler - there are many compilers out there and your namespace should be more distinguishable.

  3. I'm not convinced that the implementation of Peek is ideal with the Get and Unget logic. I think it would be better to use temporary variables to update current positions and then discard them for Peek or set them as current positions for Get. Same as you've done in the Peek method in Tokenizer.

  4. In the Get method of Tokenizer the local variable should be named token or currentToken. get is an action and doesn't describe what the variable is meant to contain.

  5. The null coalescing operator (??) is quite useful in situations like in the Tokenizer. With its help Get can be shortened to:

    get = IsSymbol() ??         
          IsOperator() ??
          IsIdentifier() ?? 
          IsCharacterLiteral() ??
          IsStringLiteral() ??
          IsNumericLiteral() ??
          IsAddressLiteral();
    
\$\endgroup\$
4
\$\begingroup\$
private static readonly Dictionary<string, TokenKind> KeywordToTokenKind =
        new Dictionary<string, TokenKind>(StringComparer.OrdinalIgnoreCase)
        {
            ["and"] = TokenKind.AndKeyword,
            ..
        };

I would create this and other similar dictionaries automatically with reflection by decorating the enums with some attribute and drop the suffixes (if possible):

public enum TokenKind : ushort
{
    [Keyword("and")]
    And = 1,

    ..

    [Operator("A+B")]
    Addition = 201,

    ..
}

You could get all values with a certain attribute and put them in a dictionary:

private static readonly Dictionary<string, TokenKind> KeywordToTokenKind =
    Enum
        .GetNames(typeof(TokenKind))
        .Select(name => (Name: name, Keyword: typeof(TokenKind).GetField(name).GetCustomAttribute<KeywordAttribute>()?.ToString()))
        .Where(t => t.Keyword != null)
        .ToDictionary(t => t.Keyword, t => Enum.Parse(typeof(TokenKind), t.Name));

where:

public class KeywordAttribute : Attribute
{
    private readonly string _value;
    public KeywordAttribute(string value) => _value = value;
    public override string ToString() => _value;
}
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  • 1
    \$\begingroup\$ +1, this is a great (and fun) suggestion, though I abhor the use of lambda syntax for non-returning (and non-trivial) methods/constructors (KeywordAttribute constructor)! \$\endgroup\$ – VisualMelon Aug 13 '17 at 9:06

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