type t = {
    localpart : string option;
    domainpart : string;
    resourcepart : string option;
  }

exception InvalidUTF8

type uchars = Uchar.t list

let uchars_of_string s : uchars =
  let open Uchar in
  let len = String.length s in
  let rec loop acc i =
    if i >= len then
      List.rev acc
    else
      let c = String.get_utf_8_uchar s i in
      if not (utf_decode_is_valid c)
      then raise InvalidUTF8
      else
        let k = utf_decode_length c in
        let u = utf_decode_uchar c in
        loop (u :: acc) (i + k)
  in loop [] (String.length s)

let of_string (jid : string) : t =
  let open Uchar in
  let len = String.length jid in
  let rec to_localpart i =
    if i <= 0 then
      None
    else
      let c = String.get_utf_8_uchar jid i in
      if not (utf_decode_is_valid c)
      then raise InvalidUTF8
      else
        let k = utf_decode_length c in
        match utf_decode_uchar c |> to_char with
        | '@' -> Some (i + 1)
        | _ -> to_localpart (i - k)
  and to_resourcepart i =
    if i >= len then
      None
    else
      let c = String.get_utf_8_uchar jid i in
      if not (utf_decode_is_valid c)
      then raise InvalidUTF8
      else
        let k = utf_decode_length c in
        match utf_decode_uchar c |> to_char with
        | '/' -> Some i
        | _ -> to_resourcepart (i + k)
  in let r = to_resourcepart 0 in
     let rv = Option.value ~default:len r in
     let l = to_localpart (rv-1) in
     let lv = Option.value ~default:0 l in
     {
       localpart = Option.map (fun i -> String.sub jid 0 (i-1)) l;
       resourcepart = Option.map (fun i -> String.sub jid (i+1) (len-i-1)) r;
       domainpart = String.sub jid lv (rv-lv);
     }

let to_string ({ localpart; domainpart; resourcepart } : t) =
  let local = Option.fold ~none:"" ~some:(fun l -> l ^ "@") localpart
  and resource = Option.fold ~none:"" ~some:(fun r -> "/" ^ r) resourcepart
  in local ^ domainpart ^ resource