exception ClosedStream

type auth_mechanism =
  | PLAIN
  | Unknown of string [@@deriving show { with_path = false }]

let parse_auth_mechanism = function
  | "PLAIN" -> PLAIN
  | other -> Unknown other

type features = {
    mechanisms : auth_mechanism list;
    starttls : [`Required | `Optional | `None];
    unknown : Xml.element list;
  }

(** [parse_features el] is a [features] record with all the features of the
    [<stream:features>] stanza contained in [el]. *)
let parse_features (el : Xml.element) : features =
  let open Xml in
  let open Either in
  let parse_mechanism_stanza = function
    | Left {local_name = "mechanism"; children = [Right mechanism]; _} ->
       Some (parse_auth_mechanism mechanism)
    | _ -> None
  in
  let parse_feature (acc : features) (feature : Xml.element) : features =
    let parse_mechanisms ch =
      List.filter_map parse_mechanism_stanza ch
    and parse_starttls = function
      | [Left {local_name="required"; _}] -> `Required
      | [] -> `Optional
      | _ -> raise (InvalidStanza (element_to_string el))
    in match feature.local_name with
       | "mechanisms" -> {acc with mechanisms=parse_mechanisms feature.children}
       | "starttls" ->  {acc with starttls=parse_starttls feature.children}
       | _ -> {acc with unknown = feature :: acc.unknown}
  in List.fold_left
       parse_feature
       {mechanisms=[]; starttls=`None; unknown=[]}
       (List.filter_map find_left el.children)