nom 4.2.0

#[macro_use]
extern crate nom;

use std::fmt;
use std::fmt::{Debug, Display, Formatter};

use std::str::FromStr;

use nom::{digit, multispace};
use nom::types::CompleteStr;

pub enum Expr {
  Value(i64),
  Add(Box<Expr>, Box<Expr>),
  Sub(Box<Expr>, Box<Expr>),
  Mul(Box<Expr>, Box<Expr>),
  Div(Box<Expr>, Box<Expr>),
  Paren(Box<Expr>),
}

#[derive(Debug)]
pub enum Oper {
  Add,
  Sub,
  Mul,
  Div,
}

impl Display for Expr {
  fn fmt(&self, format: &mut Formatter) -> fmt::Result {
    use self::Expr::*;
    match *self {
      Value(val) => write!(format, "{}", val),
      Add(ref left, ref right) => write!(format, "{} + {}", left, right),
      Sub(ref left, ref right) => write!(format, "{} - {}", left, right),
      Mul(ref left, ref right) => write!(format, "{} * {}", left, right),
      Div(ref left, ref right) => write!(format, "{} / {}", left, right),
      Paren(ref expr) => write!(format, "({})", expr),
    }
  }
}

impl Debug for Expr {
  fn fmt(&self, format: &mut Formatter) -> fmt::Result {
    use self::Expr::*;
    match *self {
      Value(val) => write!(format, "{}", val),
      Add(ref left, ref right) => write!(format, "({:?} + {:?})", left, right),
      Sub(ref left, ref right) => write!(format, "({:?} - {:?})", left, right),
      Mul(ref left, ref right) => write!(format, "({:?} * {:?})", left, right),
      Div(ref left, ref right) => write!(format, "({:?} / {:?})", left, right),
      Paren(ref expr) => write!(format, "[{:?}]", expr),
    }
  }
}

named!(parens< CompleteStr, Expr >, delimited!(
    delimited!(opt!(multispace), tag!("("), opt!(multispace)),
    map!(map!(expr, Box::new), Expr::Paren),
    delimited!(opt!(multispace), tag!(")"), opt!(multispace))
  )
);

named!(factor< CompleteStr, Expr >, alt_complete!(
    map!(
      map_res!(
        delimited!(opt!(multispace), digit, opt!(multispace)),
        |s: CompleteStr| { FromStr::from_str(s.0) }
      ),
      Expr::Value
    )
  | parens
  )
);

fn fold_exprs(initial: Expr, remainder: Vec<(Oper, Expr)>) -> Expr {
  remainder.into_iter().fold(initial, |acc, pair| {
    let (oper, expr) = pair;
    match oper {
      Oper::Add => Expr::Add(Box::new(acc), Box::new(expr)),
      Oper::Sub => Expr::Sub(Box::new(acc), Box::new(expr)),
      Oper::Mul => Expr::Mul(Box::new(acc), Box::new(expr)),
      Oper::Div => Expr::Div(Box::new(acc), Box::new(expr)),
    }
  })
}

named!(term< CompleteStr, Expr >, do_parse!(
    initial: factor >>
    remainder: many0!(
           alt!(
             do_parse!(tag!("*") >> mul: factor >> (Oper::Mul, mul)) |
             do_parse!(tag!("/") >> div: factor >> (Oper::Div, div))
           )
         ) >>
    (fold_exprs(initial, remainder))
));

named!(expr< CompleteStr, Expr >, do_parse!(
    initial: term >>
    remainder: many0!(
           alt!(
             do_parse!(tag!("+") >> add: term >> (Oper::Add, add)) |
             do_parse!(tag!("-") >> sub: term >> (Oper::Sub, sub))
           )
         ) >>
    (fold_exprs(initial, remainder))
));

#[test]
fn factor_test() {
  assert_eq!(
    factor(CompleteStr("  3  ")).map(|(i, x)| (i, format!("{:?}", x))),
    Ok((CompleteStr(""), String::from("3")))
  );
}

#[test]
fn term_test() {
  assert_eq!(
    term(CompleteStr(" 3 *  5   ")).map(|(i, x)| (i, format!("{:?}", x))),
    Ok((CompleteStr(""), String::from("(3 * 5)")))
  );
}

#[test]
fn expr_test() {
  assert_eq!(
    expr(CompleteStr(" 1 + 2 *  3 ")).map(|(i, x)| (i, format!("{:?}", x))),
    Ok((CompleteStr(""), String::from("(1 + (2 * 3))")))
  );
  assert_eq!(
    expr(CompleteStr(" 1 + 2 *  3 / 4 - 5 ")).map(|(i, x)| (i, format!("{:?}", x))),
    Ok((CompleteStr(""), String::from("((1 + ((2 * 3) / 4)) - 5)")))
  );
  assert_eq!(
    expr(CompleteStr(" 72 / 2 / 3 ")).map(|(i, x)| (i, format!("{:?}", x))),
    Ok((CompleteStr(""), String::from("((72 / 2) / 3)")))
  );
}

#[test]
fn parens_test() {
  assert_eq!(
    expr(CompleteStr(" ( 1 + 2 ) *  3 ")).map(|(i, x)| (i, format!("{:?}", x))),
    Ok((CompleteStr(""), String::from("([(1 + 2)] * 3)")))
  );
}