nom 5.0.1

extern crate nom;


use nom::{
  IResult,
  branch::alt,
  combinator::map_res,
  character::complete::char,
  bytes::complete::tag,
  character::complete::{digit1 as digit, space0 as space},
  multi::fold_many0,
  sequence::{delimited, pair}
};

// Parser definition

use std::str::FromStr;

// We parse any expr surrounded by parens, ignoring all whitespaces around those
fn parens(i: &str) -> IResult<&str, i64> {
  delimited(
    space,
    delimited(
      tag("("),
      expr,
      tag(")")
    ),
    space
  )(i)
}

// 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
fn factor(i: &str) -> IResult<&str, i64> {
  alt((
    map_res(delimited(space, digit, space), FromStr::from_str),
    parens
  ))(i)
}

// 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
fn term(i: &str) -> IResult<&str, i64> {
  let (i, init) = factor(i)?;

  fold_many0(
    pair(alt((char('*'), char('/'))), factor),
    init,
    |acc, (op, val): (char, i64)| {
        if op  == '*' { acc * val } else { acc / val }
    }
  )(i)
}

fn expr(i: &str) -> IResult<&str, i64> {
  let (i, init) = term(i)?;

  fold_many0(
    pair(alt((char('+'), char('-'))), term),
    init,
    |acc, (op, val): (char, i64)| {
        if op  == '+' { acc + val } else { acc - val }
    }
  )(i)
}

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

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

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

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