STLCPlayground/src/parse/sexpr.rs

use std::iter::Peekable;
use std::ops::{Deref, RangeInclusive};
use std::usize;
use std::vec::IntoIter;

use super::ParseError;

#[derive(Debug, PartialEq, Clone)]
pub enum Token {
    LeftParen,
    RightParen,
    Symbol(String),
}

#[derive(Debug, PartialEq, Clone)]
pub enum Sexpr {
    Symbol(String),
    List(Vec<Sexpr>),
}

impl Sexpr {
    pub fn symbol(self) -> Option<String> {
        match self {
            Sexpr::Symbol(item) => Some(item),
            _ => None,
        }
    }

    pub fn list(self) -> Option<Vec<Sexpr>> {
        match self {
            Sexpr::List(item) => Some(item),
            _ => None,
        }
    }
}

fn tokenize(input: &str) -> Vec<Token> {
    let mut tokens = Vec::new();
    // let mut chars = input.chars().peekable();
    let mut chars = input.chars().peekable();
    while let Some(c) = chars.next() {
        match c {
            '(' => tokens.push(Token::LeftParen),
            ')' => tokens.push(Token::RightParen),
            _ if c.is_whitespace() => (),
            _ => {
                let mut symbol = c.to_string();
                while let Some(c) = chars.peek() {
                    if c.is_whitespace() || *c == '(' || *c == ')' {
                        break;
                    }
                    symbol.push(*c);
                    chars.next();
                }
                tokens.push(Token::Symbol(symbol));
            }
        }
    }
    tokens
}

fn parse_expr(tokens: &mut Peekable<IntoIter<Token>>) -> Result<Sexpr, ParseError> {
    match tokens.next() {
        Some(Token::LeftParen) => {
            let mut list = Vec::new();
            while !matches!(tokens.peek(), Some(Token::RightParen,)) {
                list.push(parse_expr(tokens)?);
            }
            let Some(Token::RightParen) = tokens.next() else {
                unreachable!()
            };
            Ok(Sexpr::List(list))
        }
        Some(Token::RightParen) => Err(ParseError::UnexpectedParenClose),
        Some(Token::Symbol(s)) => Ok(Sexpr::Symbol(s)),
        None => Err(ParseError::UnexpectedEof),
    }
}

fn parse(tokens: Vec<Token>) -> Result<Sexpr, ParseError> {
    let mut tokens = tokens.into_iter().peekable();
    let ast = parse_expr(&mut tokens)?;
    if tokens.peek().is_some() {
        return Err(ParseError::TrailingTokens);
    };
    Ok(ast)
}

pub fn parse_string(src: &str) -> Result<Sexpr, ParseError> {
    let tokens = tokenize(src);
    parse(tokens)
}