Amogulator/Suslang/Parser.cs

namespace Amogulator.Suslang;

public class Parser {
    private string text;
    private Lexer lexer;
    private Token currentToken;

    public Parser(string text, Lexer lexer) {
        this.text = text;
        this.lexer = lexer;
        this.currentToken = this.lexer.next();
    }

    public Expr parseExpr() {
        return parsePrec11();
    }

    public Expr parsePrec11() {
        var left = parsePrec12();
        if (!done() && current().type == TokenType.Plus) {
            step();
            var right = parsePrec11();
            return new BinaryExpr() {
                type = BinaryType.Add,
                left = left,
                right = right,
            };
        } else if (!done() && current().type == TokenType.Minus) {
            step();
            var right = parsePrec11();
            return new BinaryExpr() {
                type = BinaryType.Subtract,
                left = left,
                right = right,
            };
        } else {
            return left;
        }
    }

    public Expr parsePrec12() {
        var left = parsePrec13();
        if (!done() && current().type == TokenType.Asterisk) {
            step();
            var right = parsePrec12();
            return new BinaryExpr() {
                type = BinaryType.Multiply,
                left = left,
                right = right,
            };
        } else if (!done() && current().type == TokenType.Slash) {
            step();
            var right = parsePrec12();
            return new BinaryExpr() {
                type = BinaryType.Divide,
                left = left,
                right = right,
            };
        } else if (!done() && current().type == TokenType.Percent) {
            step();
            var right = parsePrec12();
            return new BinaryExpr() {
                type = BinaryType.Modulo,
                left = left,
                right = right,
            };
        } else {
            return left;
        }
    }

    public Expr parsePrec13() {
        return parsePrec18();
    }

    public Expr parsePrec18() {
        if (!done() && current().type == TokenType.LParen) {
            step();
            var expr = parseExpr();
            if (done() || current().type != TokenType.RParen) {
                var pos = position();
                step();
                return new ErrorExpr() {
                    position = pos,
                    message = "expected ')'"
                };
            }
            step();
            return expr;
        } else {
            return parseOperand();
        }
    }

    public Expr parseOperand() {
        var start = position();
        if (done()) {
            step();
            return new ErrorExpr() {
                position = start,
                message = "expected value, got Eof",
            };
        } else if (current().type == TokenType.Int) {
            var intToken = current();
            step();
            if (!done() && current().type == TokenType.Decimal) {
                var decimalToken = current();
                step();
                return new FloatExpr() {
                    value = double.Parse(this.text.Substring(start.index, intToken.length + decimalToken.length).Replace(".", ",")),
                };
            } else {
                return new IntExpr() {
                    value = int.Parse(this.text.Substring(start.index, intToken.length)),
                };
            }
        } else if (current().type == TokenType.Decimal) {
            var decimalToken = current();
            step();
            return new FloatExpr() {
                value = double.Parse("0" + this.text.Substring(start.index, decimalToken.length).Replace(".", ",")),
            };
        } else if (current().type == TokenType.String) {
            var stringToken = current();
            step();
            return new StringExpr() {
                value = this.text.Substring(start.index, stringToken.length),
            };
        } else if (current().type == TokenType.False) {
            step();
            return new BoolExpr() {
                value = false,
            };
        } else if (current().type == TokenType.True) {
            step();
            return new BoolExpr() {
                value = true,
            };
        } else {
            step();
            return new ErrorExpr() {
                position = start,
                message = "expected value",
            };
        }
    }

    private Position position() => current().position;
    private Token current() => this.currentToken;
    private bool done() => this.currentToken.type == TokenType.Eof;

    private void step() {
        this.currentToken = this.lexer.next();
    }
}