open Utils

type week_first_day = [ `Sunday | `Monday ]

type simple_weekly = {
  count_or_until : Icalendar.count_or_until option;
  interval : int option;  (** Optional interval for weekly recurrence, default is 1 *)
  byday : Icalendar.weekday list;
  week_start : week_first_day option;  (** First day of the week for weekly recurrence *)
}
(** A simple weekly REM command *)

type rem = {
  source : string;  (** Source file or identifier for the reminder *)
  original_uuid : string;  (** Original UID from the iCalendar event *)
  summary : string;  (** Summary or title of the reminder *)
  date : Timedesc.Date.t;  (** Date specification (day, month, year) *)
  end_date : Timedesc.Date.t option;  (** Optional end date for a date range *)
  time : Timedesc.Time.t option;  (** Optional time specification (hour, minute) *)
  duration : Timedesc.Span.t option;  (** Optional duration for timed events *)
  yearly : (int * int) option;  (** Optional simple yearly recurrence (month, day) *)
  weekly : simple_weekly option;  (** Optional simple weekly recurrence *)
  recurring : Icalendar.event list;
      (** List of events that are part of the same recurring series: these are only the overrides, not the master event
      *)
  exdate : Icalendar.date_or_datetime list;  (** List of excluded dates for recurring events *)
}
(** A complete REM command *)

let empty =
  {
    source = "";
    original_uuid = "";
    summary = "";
    date = Timedesc.Date.Ymd.make_exn ~year:1970 ~month:1 ~day:1;
    end_date = None;
    time = None;
    duration = None;
    yearly = None;
    weekly = None;
    recurring = [];
    exdate = [];
  }

(* ── buffer primitives ────────────────────────────────────────── *)

let add_rem b = Buffer.add_string b "REM "
let add_info b uuid = Buffer.add_string b (spf "INFO \"UID: %s\" " uuid)
let add_date b date = Buffer.add_string b (Timedesc.Date.to_rfc3339 date)
let add_weekday b wd = Buffer.add_string b (spf "%s " (string_of_weekday wd))

let add_interval b w =
  let n = Option.value ~default:1 w.interval in
  Buffer.add_string b (spf "*%d " (n * 7))

let add_until b rem w =
  match w.count_or_until with
  | None -> ()
  | Some (`Until d) ->
      let ts = timedesc_of_utc_or_timestamp_local d in
      Buffer.add_string b (spf "UNTIL %s " (Timedesc.Date.to_rfc3339 (Timedesc.date ts)))
  | Some (`Count count) ->
      let wd = Timedesc.Date.weekday rem.date in
      let wd_int = Timedesc.Utils.tm_int_of_weekday wd in
      let sub =
        match w.week_start with
        | Some `Monday -> wd_int - 1
        | _ -> wd_int
      in
      let iv = Option.value ~default:1 w.interval in
      let until = Timedesc.Date.add ~days:((count * 7 * iv) - sub) rem.date in
      Buffer.add_string b (spf "UNTIL %s " (Timedesc.Date.to_rfc3339 until))

let add_at b = function
  | Some t -> Buffer.add_string b (spf " AT %s" (string_of_time t))
  | None -> ()

let add_duration b = function
  | Some d -> Buffer.add_string b (spf " DURATION %s" (string_of_span d))
  | None -> ()

let add_through b = function
  | Some d -> Buffer.add_string b (spf " THROUGH %s" (Timedesc.Date.to_rfc3339 d))
  | None -> ()

let add_msg b summary = Buffer.add_string b (spf " MSG %s\n" summary)

(* ── rendering ────────────────────────────────────────────────── *)

let render_weekly rem w =
  let b = Buffer.create 256 in
  List.iter
    (fun wd ->
      add_rem b;
      add_info b rem.original_uuid;
      add_weekday b wd;
      add_date b rem.date;
      Buffer.add_char b ' ';
      add_interval b w;
      add_until b rem w;
      add_at b rem.time;
      add_duration b rem.duration;
      add_msg b rem.summary)
    w.byday;
  Buffer.contents b

let render_single rem =
  let b = Buffer.create 256 in
  add_rem b;
  add_info b rem.original_uuid;
  add_date b rem.date;
  add_at b rem.time;
  add_duration b rem.duration;
  add_through b rem.end_date;
  add_msg b rem.summary;
  Buffer.contents b

let render_yearly month day summary =
  let b = Buffer.create 64 in
  add_rem b;
  Buffer.add_string b (spf "%s %d" (month_of_int month |> string_of_month) day);
  add_msg b summary;
  Buffer.contents b

(* ── dispatcher ───────────────────────────────────────────────── *)

let string_of_rem rem =
  match rem.weekly with
  | Some w -> render_weekly rem w
  | None -> (
      match rem.yearly with
      | Some (month, day) -> render_yearly month day rem.summary
      | None -> render_single rem)