open Sexplib0.Sexp_conv

(* How a sanctoral entry expresses its date. Plan 2 ships the only form the EF
   sanctoral needs -- a fixed calendar date -- because EF movable feasts come
   from the rite's temporal code, not from data. Sunday-relative and
   Easter-relative forms arrive with the OF sanctoral. *)
module Repr = struct
  type t = Fixed of { month : int; day : int } [@@deriving sexp]
end

type t = Repr.t = Fixed of { month : int; day : int }

(* Leap-year maximum, so 29 February is constructible; it simply does not
   resolve in a common year. *)
let max_day = function
  | 1 | 3 | 5 | 7 | 8 | 10 | 12 -> 31
  | 4 | 6 | 9 | 11 -> 30
  | 2 -> 29
  | _ -> 0

let fixed ~month ~day =
  if month < 1 || month > 12 then Error (Printf.sprintf "date_spec: month %d out of range 1..12" month)
  else if day < 1 || day > max_day month then
    Error (Printf.sprintf "date_spec: day %d out of range for month %d" day month)
  else Ok (Fixed { month; day })

let resolve t ~year =
  match t with
  | Fixed { month; day } -> Result.to_option (Date.make ~year ~month ~day)

let sexp_of_t = Repr.sexp_of_t

(* Validating parser, matching Slug and Lang: [ppx_sexp_conv]'s derived
   [t_of_sexp] (relocated to [Repr] above) accepts any in-range int pair, so
   [(Fixed (month 13) (day 40))] would otherwise deserialise into a spec that
   silently never resolves -- a saint quietly vanishing with no diagnostic.
   Re-running the value through [fixed] closes that gap the same way loaders
   already close it for slugs and language codes. *)
let t_of_sexp sexp =
  let (Fixed { month; day }) = Repr.t_of_sexp sexp in
  match fixed ~month ~day with
  | Ok t -> t
  | Error msg -> Sexplib0.Sexp_conv.of_sexp_error msg sexp