nom 4.1.0

#[macro_use]
extern crate nom;

use nom::digit;
use nom::types::CompleteByteSlice;

// Parser definition

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

use self::Operator::*;

enum Operator {
  Slash,
  Star,
}

impl Operator {
  fn to_str(&self) -> &'static str {
    match *self {
      Slash => "/",
      Star => "*",
    }
  }
}

// Parse the specified `Operator`.
named_args!(operator(op: Operator) <CompleteByteSlice, CompleteByteSlice>,
    tag!(op.to_str())
);

// We parse any expr surrounded by the tags `open_tag` and `close_tag`, ignoring all whitespaces around those
named_args!(brackets<'a>(open_tag: &str, close_tag: &str) <CompleteByteSlice<'a>, i64>,
  ws!(delimited!( tag!(open_tag), expr, tag!(close_tag) )) );

fn complete_byte_slice_to_str<'a>(s: CompleteByteSlice<'a>) -> Result<&'a str, str::Utf8Error> {
  str::from_utf8(s.0)
}

// 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 brackets parser defined above
named!(factor<CompleteByteSlice, i64>, alt!(
    map_res!(
      map_res!(
        ws!(digit),
        complete_byte_slice_to_str
      ),
      FromStr::from_str
    )
  | call!(brackets, "(", ")")
  )
);

// 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 <CompleteByteSlice, i64>, do_parse!(
    init: factor >>
    res:  fold_many0!(
        pair!(alt!(call!(operator, Star) | call!(operator, Slash)), factor),
        init,
        |acc, (op, val): (CompleteByteSlice, i64)| {
            if ((op.0)[0] as char) == '*' { acc * val } else { acc / val }
        }
    ) >>
    (res)
  )
);

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

#[test]
fn factor_test() {
  assert_eq!(
    factor(CompleteByteSlice(b"3")),
    Ok((CompleteByteSlice(b""), 3))
  );
  assert_eq!(
    factor(CompleteByteSlice(b" 12")),
    Ok((CompleteByteSlice(b""), 12))
  );
  assert_eq!(
    factor(CompleteByteSlice(b"537  ")),
    Ok((CompleteByteSlice(b""), 537))
  );
  assert_eq!(
    factor(CompleteByteSlice(b"  24   ")),
    Ok((CompleteByteSlice(b""), 24))
  );
}

#[test]
fn term_test() {
  assert_eq!(
    term(CompleteByteSlice(b" 12 *2 /  3")),
    Ok((CompleteByteSlice(b""), 8))
  );
  assert_eq!(
    term(CompleteByteSlice(b" 2* 3  *2 *2 /  3")),
    Ok((CompleteByteSlice(b""), 8))
  );
  assert_eq!(
    term(CompleteByteSlice(b" 48 /  3/2")),
    Ok((CompleteByteSlice(b""), 8))
  );
}

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

#[test]
fn parens_test() {
  assert_eq!(
    expr(CompleteByteSlice(b" (  2 )")),
    Ok((CompleteByteSlice(b""), 2))
  );
  assert_eq!(
    expr(CompleteByteSlice(b" 2* (  3 + 4 ) ")),
    Ok((CompleteByteSlice(b""), 14))
  );
  assert_eq!(
    expr(CompleteByteSlice(b"  2*2 / ( 5 - 1) + 3")),
    Ok((CompleteByteSlice(b""), 4))
  );
}