//! A Rust library for decoding [UTF-7](https://datatracker.ietf.org/doc/html/rfc2152) string as defined by the [IMAP](https://datatracker.ietf.org/doc/html/rfc3501) standard in [RFC 3501 (#5.1.3)](https://datatracker.ietf.org/doc/html/rfc3501#section-5.1.3).
//!
//! Since this library is currently experimental, only decode is supported.
//!
//! Idea is based on Python [mutf7](https://github.com/cheshire-mouse/mutf7) library.

extern crate base64;
extern crate encoding_rs;
extern crate regex;

use encoding_rs::UTF_16BE;
use regex::Regex;

/// Decode UTF-7 IMAP mailbox name
///
/// <https://datatracker.ietf.org/doc/html/rfc3501#section-5.1.3>
///
/// # Usage:
///
/// ```
/// use utf7_imap::decode_utf7_imap;
///
/// let test_string = String::from("&BB4EQgQ,BEAEMAQyBDsENQQ9BD0ESwQ1-");
/// assert_eq!(decode_utf7_imap(test_string), "Отправленные");
/// ```
pub fn decode_utf7_imap(text: String) -> String {
  let re = Regex::new(r"&[^&-]*-").unwrap();
  let mut result = text.clone();

  for cap in re.captures_iter(&text) {
    let encoded_text = cap.get(0).map_or("", |m| m.as_str());
    let decoded_text = decode_utf7_part(String::from(encoded_text));

    result = result.replace(&encoded_text, &decoded_text);
  }

  return result;
}

fn decode_utf7_part(text: String) -> String {
  if text == "&-" {
    return String::from("&");
  }

  let text_mb64 = &text[1..text.len() - 1];
  let mut text_b64 = text_mb64.replace(",", "/");

  while (text_b64.len() % 4) != 0 {
    text_b64 += "=";
  }

  let text_u16 = base64::decode(text_b64).unwrap();
  let (cow, _encoding_used, _had_errors) = UTF_16BE.decode(&text_u16);
  let result = cow.as_ref();

  return String::from(result);
}

#[cfg(test)]
mod tests {
  use super::*;
  #[test]
  fn decode_test() {
    let test_string = String::from("&BB4EQgQ,BEAEMAQyBDsENQQ9BD0ESwQ1-");
    assert_eq!(decode_utf7_imap(test_string), "Отправленные");
  }
  #[test]
  fn decode_test_split() {
    // input string with utf7 encoded bits being separated by ascii
    let test_string = String::from("&AWA-iuk&AWE-liad&ARcBfgEX-");
    assert_eq!(decode_utf7_imap(test_string), "Šiukšliadėžė")
  }
}