fn main() {
   println!("{:?}", encrypt("Hello, world!".to_string(), "password".to_string()));
}

struct KeyIter {
   key: Vec<u8>,
   index: usize,
}

impl KeyIter {
   pub fn new(key: Vec<u8>) -> KeyIter {
      KeyIter {
         key,
         index: 0,
     }
  }
   pub fn next(&mut self) -> u8 {
      let k = self.key[self.index];
      self.index += 1;
      if self.index == self.key.len() {
         self.index = 0;
      }
      k
  }
}

fn encrypt(msg: String, key: String) -> Vec<u8> {
   let msg: Vec<u8> = msg.into_bytes();
   let key: Vec<u8> = key.into_bytes();
   let mut encrypted: Vec<u8> = vec!();

   let mut key_iter: KeyIter = KeyIter::new(key);

   for b in msg.iter() {
      let k = key_iter.next();
      if k > *b {
         encrypted.push(*b^k); // b xor k
      } else {
         encrypted.push(b-k);
      }
   }

   encrypted
}