nub-lang/lang/Nub.Lang/Frontend/Lexing/Lexer.cs

namespace Nub.Lang.Frontend.Lexing;

public class Lexer
{
    private static readonly Dictionary<string, Symbol> Keywords = new()
    { 
        ["func"] = Symbol.Func,
        ["extern"] = Symbol.Extern,
        ["import"] = Symbol.Import,
        ["let"] = Symbol.Let,
        ["if"] = Symbol.If,
        ["else"] = Symbol.Else,
        ["while"] = Symbol.While,
        ["break"] = Symbol.Break,
        ["continue"] = Symbol.Continue,
        ["return"] = Symbol.Return,
        ["new"] = Symbol.New,
        ["struct"] = Symbol.Struct,
    };

    private static readonly Dictionary<char[], Symbol> Chians = new()
    {
        [['=', '=']] = Symbol.Equal,
        [['!', '=']] = Symbol.NotEqual,
        [['<', '=']] = Symbol.LessThanOrEqual,
        [['>', '=']] = Symbol.GreaterThanOrEqual,
    };
    
    private static readonly Dictionary<char, Symbol> Chars = new()
    {
        [';'] = Symbol.Semicolon,
        [':'] = Symbol.Colon,
        ['('] = Symbol.OpenParen,
        [')'] = Symbol.CloseParen,
        ['{'] = Symbol.OpenBrace,
        ['}'] = Symbol.CloseBrace,
        ['['] = Symbol.OpenBracket,
        [']'] = Symbol.CloseBracket,
        [','] = Symbol.Comma,
        ['.'] = Symbol.Period,
        ['='] = Symbol.Assign,
        ['<'] = Symbol.LessThan,
        ['>'] = Symbol.GreaterThan,
        ['+'] = Symbol.Plus,
        ['-'] = Symbol.Minus,
        ['*'] = Symbol.Star,
        ['/'] = Symbol.ForwardSlash,
        ['!'] = Symbol.Bang,
    };
    
    private string _src = string.Empty;
    private int _index;

    public List<Token> Lex(string src)
    {
        _src = src;
        _index = 0;
        
        List<Token> tokens = [];
        while (Peek().HasValue)
        {
            tokens.Add(ParseToken());
        }
        return tokens;
    }

    private Token ParseToken()
    {
        var current = Peek();
        
        if (char.IsLetter(current.Value) || current.Value == '_')
        {
            var buffer = string.Empty;
                
            while (current.HasValue && (char.IsLetterOrDigit(current.Value) || current.Value == '_'))
            {
                buffer += current.Value;
                Next();
                current = Peek();
            }

            if (Keywords.TryGetValue(buffer, out var keywordSymbol))
            {
                return new SymbolToken(keywordSymbol);
            }
            
            if (buffer is "true" or "false")
            {
                return new LiteralToken(NubType.Bool, buffer);
            }

            return new IdentifierToken(buffer);
        }

        if (char.IsDigit(current.Value))
        {
            var buffer = string.Empty;
                
            while (current.HasValue && char.IsDigit(current.Value))
            {
                buffer += current.Value;
                Next();
                current = Peek();
            }

            return new LiteralToken(NubType.Int64, buffer);
        }

        // TODO: Revisit this
        foreach (var chain in Chians)
        {
            if (current.Value != chain.Key[0]) continue;
            
            for (var i = 1; i < chain.Key.Length; i++)
            {
                var c = Peek(i);
                if (!c.HasValue || c.Value != chain.Key[i]) break;

                if (i == chain.Key.Length - 1)
                {
                    for (var j = 0; j <= i; j++)
                    {
                        Next();
                    }

                    return new SymbolToken(chain.Value);
                }
            }
        }

        if (Chars.TryGetValue(current.Value, out var charSymbol))
        {
            Next();
            return new SymbolToken(charSymbol);
        }

        if (current.Value == '"')
        {
            Next();
            var buffer = string.Empty;
            
            while (true)
            {
                current = Peek();
                Next();
                if (!current.HasValue) throw new Exception("Unclosed string literal");
                if (current.Value == '"') break;
                buffer += current.Value;
            }
            
            return new LiteralToken(NubType.String, buffer);
        }

        if (char.IsWhiteSpace(current.Value))
        {
            Next();
            return new SymbolToken(Symbol.Whitespace);
        }

        throw new Exception($"Unknown character {current.Value}");
    }

    private Optional<char> Peek(int offset = 0)
    {
        if (_index + offset < _src.Length)
        {
            return _src[_index + offset];
        }
        
        return Optional<char>.Empty();
    }

    private void Next()
    {
        _index++;
    }
}