nom 0.4.0

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

use nom::{IResult,digit, multispace};

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

named!(parens<i64>, delimited!(
    delimited!(opt!(multispace), tag!("("), opt!(multispace)),
    expr,
    delimited!(opt!(multispace), tag!(")"), opt!(multispace))
  )
);

named!(factor<i64>,
  alt!(
    map_res!(
      map_res!(
        delimited!(opt!(multispace), digit, opt!(multispace)),
        str::from_utf8
      ),
      FromStr::from_str
    )
  | parens
  )
);

named!(term <i64>,
  chain!(
    mut acc: factor  ~
             many0!(
               alt!(
                 tap!(mul: preceded!(tag!("*"), factor) => acc = acc * mul) |
                 tap!(div: preceded!(tag!("/"), factor) => acc = acc / div)
               )
             ),
    || { return acc }
  )
);

named!(expr <i64>,
  chain!(
    mut acc: term  ~
             many0!(
               alt!(
                 tap!(add: preceded!(tag!("+"), term) => acc = acc + add) |
                 tap!(sub: preceded!(tag!("-"), term) => acc = acc - sub)
               )
             ),
    || { return acc }
  )
);

#[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));
}