pub struct Shebang<'a> {
  pub interpreter: &'a str,
  pub argument:    Option<&'a str>,
}

impl<'a> Shebang<'a> {
  pub fn new(text: &'a str) -> Option<Shebang<'a>> {
    if !text.starts_with("#!") {
      return None;
    }

    let mut pieces = text[2..]
      .lines()
      .nth(0)
      .unwrap_or("")
      .trim()
      .splitn(2, |c| c == ' ' || c == '\t');

    let interpreter = pieces.next().unwrap_or("");
    let argument    = pieces.next();

    if interpreter == "" {
      return None;
    }

    Some(Shebang{interpreter, argument})
  }
}

#[cfg(test)]
mod test {
  use super::Shebang;

  #[test]
  fn split_shebang() {
    fn check(text: &str, expected_split: Option<(&str, Option<&str>)>) {
      let shebang = Shebang::new(text);
      assert_eq!(shebang.map(|shebang| (shebang.interpreter, shebang.argument)), expected_split);
    }

    check("#!    ",                         None                                       );
    check("#!",                             None                                       );
    check("#!/bin/bash",                    Some(("/bin/bash",    None               )));
    check("#!/bin/bash    ",                Some(("/bin/bash",    None               )));
    check("#!/usr/bin/env python",          Some(("/usr/bin/env", Some("python"     ))));
    check("#!/usr/bin/env python   ",       Some(("/usr/bin/env", Some("python"     ))));
    check("#!/usr/bin/env python -x",       Some(("/usr/bin/env", Some("python -x"  ))));
    check("#!/usr/bin/env python   -x",     Some(("/usr/bin/env", Some("python   -x"))));
    check("#!/usr/bin/env python \t-x\t",   Some(("/usr/bin/env", Some("python \t-x"))));
    check("#/usr/bin/env python \t-x\t",    None                                       );
    check("#!  /bin/bash",                  Some(("/bin/bash",    None               )));
    check("#!\t\t/bin/bash    ",            Some(("/bin/bash",    None               )));
    check("#!  \t\t/usr/bin/env python",    Some(("/usr/bin/env", Some("python"     ))));
    check("#!  /usr/bin/env python   ",     Some(("/usr/bin/env", Some("python"     ))));
    check("#!  /usr/bin/env python -x",     Some(("/usr/bin/env", Some("python -x"  ))));
    check("#!  /usr/bin/env python   -x",   Some(("/usr/bin/env", Some("python   -x"))));
    check("#!  /usr/bin/env python \t-x\t", Some(("/usr/bin/env", Some("python \t-x"))));
    check("#  /usr/bin/env python \t-x\t",  None                                       );
  }
}