oden/oden-script/src/parser.rs

use crate::tokens::{Token, TokenKind, Tokens};
use std::fmt;

#[derive(PartialEq, Eq)]
pub struct SyntaxError {
    pub line: usize,
    pub column: usize,
    pub message: String,
}

impl SyntaxError {
    pub fn new(line: usize, column: usize, message: String) -> Self {
        SyntaxError {
            line,
            column,
            message,
        }
    }
}

impl fmt::Debug for SyntaxError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}:{}: {}", self.line, self.column, self.message)
    }
}

impl fmt::Display for SyntaxError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}:{}: {}", self.line, self.column, self.message)
    }
}

pub enum Literal {
    Float64(f64),
}

pub enum UnaryOp {
    Negate,
}

pub enum BinaryOp {
    Add,
    Subtract,
    Mutiply,
    Divide,
    And,
    Or,
}

pub enum Expr {
    Literal(Literal),
    Unary(UnaryOp, ExprRef),
    Binary(BinaryOp, ExprRef, ExprRef),
}

pub struct ExprRef(Option<usize>);

impl ExprRef {
    pub fn error() -> Self {
        ExprRef(None)
    }
}

pub struct SyntaxTree {
    pub errors: Vec<SyntaxError>,
    expressions: Vec<Expr>,
}

impl SyntaxTree {
    pub fn new() -> Self {
        SyntaxTree {
            errors: Vec::new(),
            expressions: Vec::new(),
        }
    }

    pub fn add_error(&mut self, error: SyntaxError) {
        self.errors.push(error);
    }

    pub fn add_expr(&mut self, expr: Expr) -> ExprRef {
        let index = self.expressions.len();
        self.expressions.push(expr);
        ExprRef(Some(index))
    }

    pub fn dump_expr(&self, expr: &ExprRef) -> String {
        match expr.0 {
            Some(idx) => {
                let expr = &self.expressions[idx];
                match expr {
                    Expr::Literal(lit) => match lit {
                        Literal::Float64(f) => f.to_string(),
                    },
                    Expr::Unary(op, e) => {
                        let op = match op {
                            UnaryOp::Negate => "-",
                        };
                        format!("({op} {})", self.dump_expr(e))
                    }
                    Expr::Binary(op, l, r) => {
                        let op = match op {
                            BinaryOp::Add => "+",
                            BinaryOp::Subtract => "-",
                            BinaryOp::Mutiply => "*",
                            BinaryOp::Divide => "/",
                            BinaryOp::And => "and",
                            BinaryOp::Or => "or",
                        };
                        format!("({op} {} {})", self.dump_expr(l), self.dump_expr(r))
                    }
                }
            }
            None => "<|EOF|>".to_string(),
        }
    }
}

// BINDING POWERS. When parsing expressions we only accept expressions that
// meet a minimum binding power. (This is like "precedence" but I just super
// don't like that terminology.)
const ASSIGNMENT_POWER: u8 = 0; // =
const OR_POWER: u8 = 1; // or
const AND_POWER: u8 = 2; // and
const EQUALITY_POWER: u8 = 3; // == !=
const COMPARISON_POWER: u8 = 4; // < > <= >=
const TERM_POWER: u8 = 5; // + -
const FACTOR_POWER: u8 = 6; // * /
const UNARY_POWER: u8 = 7; // ! -

// const CALL_POWER: u8 = 8; // . ()
// const PRIMARY_POWER: u8 = 9;

fn token_power<'a>(token: &Option<Token<'a>>) -> Option<u8> {
    let token = match token {
        Some(t) => t,
        None => return None,
    };

    match token.kind() {
        TokenKind::Equal => Some(ASSIGNMENT_POWER),
        TokenKind::Or => Some(OR_POWER),
        TokenKind::And => Some(AND_POWER),
        TokenKind::EqualEqual | TokenKind::BangEqual => Some(EQUALITY_POWER),
        TokenKind::Less | TokenKind::Greater | TokenKind::GreaterEqual | TokenKind::LessEqual => {
            Some(COMPARISON_POWER)
        }
        TokenKind::Plus | TokenKind::Minus => Some(TERM_POWER),
        TokenKind::Star | TokenKind::Slash => Some(FACTOR_POWER),
        _ => None,
    }
}

pub struct Parser<'a> {
    tokens: Tokens<'a>,
    tree: SyntaxTree,
    current: Option<Token<'a>>,
    previous: Option<Token<'a>>,

    panic_mode: bool,
}

impl<'a> Parser<'a> {
    pub fn new(source: &'a str) -> Self {
        let mut parser = Parser {
            tokens: Tokens::new(source),
            tree: SyntaxTree::new(),
            current: None,
            previous: None,
            panic_mode: false,
        };
        parser.advance();
        parser
    }

    pub fn parse(mut self) -> (SyntaxTree, ExprRef) {
        let expr = self.expression();
        self.consume(None, "expected end of expression");
        (self.tree, expr)
    }

    fn expression(&mut self) -> ExprRef {
        self.expression_with_power(0)
    }

    fn expression_with_power(&mut self, minimum_power: u8) -> ExprRef {
        self.advance();
        let mut expr = self.prefix_expression();
        loop {
            let power = match token_power(&self.current) {
                Some(p) => p,
                None => break, // EOF, end of expression?
            };

            if power < minimum_power {
                break;
            }

            self.advance();
            expr = self.infix_expression(power, expr);
        }
        expr
    }

    fn prefix_expression(&mut self) -> ExprRef {
        let token = self.previous.as_ref();
        match token {
            Some(token) => match token.kind() {
                TokenKind::LeftParen => self.grouping(),
                TokenKind::Number => self.number(),
                TokenKind::Minus => self.unary(),
                _ => {
                    self.error("expected an expression");
                    ExprRef::error()
                }
            },
            None => {
                self.error("expected an expression");
                ExprRef::error()
            }
        }
    }

    fn infix_expression(&mut self, power: u8, left: ExprRef) -> ExprRef {
        let kind = self.previous.as_ref().unwrap().kind();
        match kind {
            TokenKind::Plus | TokenKind::Minus | TokenKind::Star | TokenKind::Slash => {
                self.binary(power, left)
            }
            _ => panic!("Unknown infix operator, dispatch error?"),
        }
    }

    fn number(&mut self) -> ExprRef {
        let token = self.previous.as_ref().unwrap();
        // What kind is it? For now let's just ... make it good.

        match token.as_str().parse::<f64>() {
            Ok(v) => self.tree.add_expr(Expr::Literal(Literal::Float64(v))),
            Err(e) => {
                self.error(format!("invalid f64: {e}"));
                ExprRef::error()
            }
        }
    }

    fn grouping(&mut self) -> ExprRef {
        let result = self.number();
        self.consume(
            Some(TokenKind::RightParen),
            "expected ')' after an expression",
        );
        result
    }

    fn unary(&mut self) -> ExprRef {
        let kind = self.previous.as_ref().unwrap().kind();
        let expr = self.expression_with_power(UNARY_POWER);
        let op = match kind {
            TokenKind::Minus => UnaryOp::Negate,
            _ => panic!("unsuitable unary: {:?}: no op", kind),
        };
        self.tree.add_expr(Expr::Unary(op, expr))
    }

    fn binary(&mut self, power: u8, left: ExprRef) -> ExprRef {
        let right = self.expression_with_power(power + 1);
        let op = match self.previous.as_ref().unwrap().kind() {
            TokenKind::Plus => BinaryOp::Add,
            TokenKind::Minus => BinaryOp::Subtract,
            TokenKind::Star => BinaryOp::Mutiply,
            TokenKind::Slash => BinaryOp::Divide,
            TokenKind::And => BinaryOp::And,
            TokenKind::Or => BinaryOp::Or,
            _ => panic!("unsuitable binary: {:?}: no op", self.previous),
        };

        self.tree.add_expr(Expr::Binary(op, left, right))
    }

    fn advance(&mut self) {
        self.previous = self.current.take();
        loop {
            self.current = self.tokens.next();
            match &self.current {
                Some(token) if token.kind() == TokenKind::Error => {
                    self.error_at_current(token.clone())
                }
                _ => break,
            }
        }
    }

    fn consume(&mut self, kind: Option<TokenKind>, error: &str) {
        match (&self.current, kind) {
            (Some(token), Some(kind)) if token.kind() == kind => self.advance(),
            (None, None) => (),
            _ => {
                self.error_at_current(error);
            }
        }
    }

    fn error<T>(&mut self, message: T)
    where
        T: Into<String>,
    {
        self.error_at(self.previous.clone(), message)
    }

    fn error_at_current<T>(&mut self, message: T)
    where
        T: Into<String>,
    {
        self.error_at(self.current.clone(), message)
    }

    fn error_at<T>(&mut self, token: Option<Token<'a>>, message: T)
    where
        T: Into<String>,
    {
        if self.panic_mode {
            return;
        }
        self.panic_mode = true;

        let message: String = message.into();
        let (line, column) = self.tokens.token_position(&token);
        let mut final_message = "Error ".to_string();
        match token {
            None => final_message.push_str("at end"),
            Some(t) => {
                if t.kind() != TokenKind::Error {
                    final_message.push_str("at '");
                    final_message.push_str(t.as_str());
                    final_message.push_str("'");
                }
            }
        }
        final_message.push_str(": ");
        final_message.push_str(&message);

        self.tree
            .add_error(SyntaxError::new(line, column, final_message));
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use pretty_assertions::assert_eq;

    #[test]
    pub fn number_expressions() {
        // How am I going to test this?
        let (tree, expr) = Parser::new("23.5").parse();
        assert_eq!(Vec::<SyntaxError>::new(), tree.errors);
        assert_eq!("23.5", tree.dump_expr(&expr));
    }
}