nom 2.2.1

A byte-oriented, zero-copy, parser combinators library
Documentation 
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#[macro_use]
extern crate nom;

use nom::{IResult,digit};

// Parser definition

use std::str;
use std::str::FromStr;

// We parse any expr surrounded by parens, ignoring all whitespaces around those
named!(parens<i64>, ws!(delimited!( tag!("("), expr, tag!(")") )) );

// We transform an integer string into a i64, ignoring surrounding whitespaces
// We look for a digit suite, and try to convert it.
// If either str::from_utf8 or FromStr::from_str fail,
// we fallback to the parens parser defined above
named!(factor<i64>, alt!(
    map_res!(
      map_res!(
        ws!(digit),
        str::from_utf8
      ),
      FromStr::from_str
    )
  | parens
  )
);

// We read an initial factor and for each time we find
// a * or / operator followed by another factor, we do
// the math by folding everything
named!(term <i64>, do_parse!(
    init: factor >>
    res:  fold_many0!(
        pair!(alt!(tag!("*") | tag!("/")), factor),
        init,
        |acc, (op, val): (&[u8], i64)| {
            if (op[0] as char) == '*' { acc * val } else { acc / val }
        }
    ) >>
    (res)
  )
);

named!(expr <i64>, do_parse!(
    init: term >>
    res:  fold_many0!(
        pair!(alt!(tag!("+") | tag!("-")), term),
        init,
        |acc, (op, val): (&[u8], i64)| {
            if (op[0] as char) == '+' { acc + val } else { acc - val }
        }
    ) >>
    (res)
  )
);

#[test]
fn factor_test() {
  assert_eq!(factor(&b"3"[..]), IResult::Done(&b""[..], 3));
  assert_eq!(factor(&b" 12"[..]), IResult::Done(&b""[..], 12));
  assert_eq!(factor(&b"537  "[..]), IResult::Done(&b""[..], 537));
  assert_eq!(factor(&b"  24   "[..]), IResult::Done(&b""[..], 24));
}


#[test]
fn term_test() {
  assert_eq!(term(&b" 12 *2 /  3"[..]), IResult::Done(&b""[..], 8));
  assert_eq!(term(&b" 2* 3  *2 *2 /  3"[..]), IResult::Done(&b""[..], 8));
  assert_eq!(term(&b" 48 /  3/2"[..]), IResult::Done(&b""[..], 8));
}

#[test]
fn expr_test() {
  assert_eq!(expr(&b" 1 +  2 "[..]), IResult::Done(&b""[..], 3));
  assert_eq!(expr(&b" 12 + 6 - 4+  3"[..]), IResult::Done(&b""[..], 17));
  assert_eq!(expr(&b" 1 + 2*3 + 4"[..]), IResult::Done(&b""[..], 11));
}

#[test]
fn parens_test() {
  assert_eq!(expr(&b" (  2 )"[..]), IResult::Done(&b""[..], 2));
  assert_eq!(expr(&b" 2* (  3 + 4 ) "[..]), IResult::Done(&b""[..], 14));
  assert_eq!(expr(&b"  2*2 / ( 5 - 1) + 3"[..]), IResult::Done(&b""[..], 4));
}