scrabble-with-numbers/board-shared/src/parser.rs

use crate::lexer::Token;
use crate::tile::Operator;
use std::fmt;
use std::iter::Peekable;

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Expression {
    Digit(i64),
    Parentheses(Box<Expression>),
    Unary(Operator, Box<Expression>),
    Binary(Operator, Box<Expression>, Box<Expression>),
}

impl fmt::Display for Expression {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Expression::Digit(value) => write!(f, "{value}"),
            Expression::Parentheses(expr) => write!(f, "({expr})"),
            Expression::Unary(operator, expr) => write!(f, "{operator}{expr}"),
            Expression::Binary(operator, left, right) => {
                write!(f, "{left} {operator} {right}")
            }
        }
    }
}

pub type Expressions = Vec<Expression>;

pub fn parse(tokens: &[Token]) -> Result<Expressions, ()> {
    let mut tokens = tokens.iter().peekable();
    let mut expressions = Vec::new();
    while tokens.peek().is_some() {
        expressions.push(parse_expression(&mut tokens)?);
        tokens.next();
    }
    Ok(expressions)
}

fn parse_expression<'a>(
    tokens: &mut Peekable<impl Iterator<Item = &'a Token>>,
) -> Result<Expression, ()> {
    let mut left = parse_term(tokens)?;
    while let Some(Token::Operator(operator)) = tokens.peek() {
        let operator = *operator;
        tokens.next();
        let right = parse_term(tokens)?;
        left = Expression::Binary(operator, Box::new(left), Box::new(right));
    }
    Ok(left)
}

fn parse_term<'a>(
    tokens: &mut Peekable<impl Iterator<Item = &'a Token>>,
) -> Result<Expression, ()> {
    let mut left = parse_unary(tokens)?;
    while let Some(Token::Operator(operator)) = tokens.peek() {
        let operator = *operator;
        tokens.next();
        let right = parse_unary(tokens)?;
        left = Expression::Binary(operator, Box::new(left), Box::new(right));
    }
    Ok(left)
}

fn parse_factor<'a>(
    tokens: &mut Peekable<impl Iterator<Item = &'a Token>>,
) -> Result<Expression, ()> {
    match tokens.next() {
        Some(Token::NumberLiteral(value)) => Ok(Expression::Digit(*value)),
        Some(Token::LeftParen) => {
            let expression = parse_expression(tokens)?;
            if let Some(Token::RightParen) = tokens.next() {
                Ok(Expression::Parentheses(Box::new(expression)))
            } else {
                Err(())
            }
        }
        _ => Err(()),
    }
}

fn parse_unary<'a>(
    tokens: &mut Peekable<impl Iterator<Item = &'a Token>>,
) -> Result<Expression, ()> {
    if let Some(Token::Operator(Operator::Subtract)) = tokens.peek() {
        tokens.next();
        let expression = parse_unary(tokens)?;
        Ok(Expression::Unary(Operator::Subtract, Box::new(expression)))
    } else {
        parse_factor(tokens)
    }
}

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

    #[test]
    fn test_parse() {
        let tokens = vec![
            Token::NumberLiteral(1),
            Token::Operator(Operator::Add),
            Token::NumberLiteral(2),
            Token::Operator(Operator::Multiply),
            Token::NumberLiteral(3),
        ];
        let expression = parse(&tokens).unwrap();
        assert_eq!(
            expression,
            vec![Expression::Binary(
                Operator::Multiply,
                Box::new(Expression::Binary(
                    Operator::Add,
                    Box::new(Expression::Digit(1)),
                    Box::new(Expression::Digit(2)),
                )),
                Box::new(Expression::Digit(3)),
            )],
        );
    }

    #[test]
    fn test_parse_parentheses() {
        let tokens = vec![
            Token::LeftParen,
            Token::NumberLiteral(1),
            Token::Operator(Operator::Add),
            Token::NumberLiteral(2),
            Token::RightParen,
            Token::Operator(Operator::Multiply),
            Token::NumberLiteral(3),
        ];
        let expression = parse(&tokens).unwrap();
        assert_eq!(
            expression,
            vec![Expression::Binary(
                Operator::Multiply,
                Box::new(Expression::Parentheses(Box::new(Expression::Binary(
                    Operator::Add,
                    Box::new(Expression::Digit(1)),
                    Box::new(Expression::Digit(2)),
                )))),
                Box::new(Expression::Digit(3)),
            )],
        );
    }

    #[test]
    fn test_parse_equals() {
        let tokens = vec![
            Token::NumberLiteral(1),
            Token::Equals,
            Token::NumberLiteral(2),
            Token::Equals,
            Token::NumberLiteral(3),
        ];
        let expression = parse(&tokens).unwrap();
        assert_eq!(
            expression,
            vec![
                Expression::Digit(1),
                Expression::Digit(2),
                Expression::Digit(3),
            ],
        );
    }

    #[test]
    fn test_parse_unary_and_binary_minus() {
        let tokens = vec![
            Token::Operator(Operator::Subtract),
            Token::NumberLiteral(1),
            Token::Operator(Operator::Subtract),
            Token::NumberLiteral(2),
        ];
        let expression = parse(&tokens).unwrap();
        assert_eq!(
            expression,
            vec![Expression::Binary(
                Operator::Subtract,
                Box::new(Expression::Unary(
                    Operator::Subtract,
                    Box::new(Expression::Digit(1)),
                )),
                Box::new(Expression::Digit(2)),
            )],
        );
    }

    #[test]
    fn test_parse_unary_before_parenthesis() {
        let tokens = vec![
            Token::Operator(Operator::Subtract),
            Token::LeftParen,
            Token::NumberLiteral(9),
            Token::Operator(Operator::Multiply),
            Token::NumberLiteral(3),
            Token::RightParen,
        ];
        let expression = parse(&tokens).unwrap();
        assert_eq!(
            expression,
            vec![Expression::Unary(
                Operator::Subtract,
                Box::new(Expression::Parentheses(Box::new(Expression::Binary(
                    Operator::Multiply,
                    Box::new(Expression::Digit(9)),
                    Box::new(Expression::Digit(3)),
                )))),
            )],
        );
    }

    #[test]
    fn test_double_unary() {
        let tokens = vec![
            Token::Operator(Operator::Subtract),
            Token::Operator(Operator::Subtract),
            Token::NumberLiteral(7),
        ];
        let expression = parse(&tokens).unwrap();
        assert_eq!(
            expression,
            vec![Expression::Unary(
                Operator::Subtract,
                Box::new(Expression::Unary(
                    Operator::Subtract,
                    Box::new(Expression::Digit(7))
                )),
            )],
        );
    }
}