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(** A sanctoral entry's date expression.
[Fixed] is a civil (month, day) that recurs every year. The other two move:
[Easter_offset] is a signed day count from the rite's own Easter, and
[Nth_weekday] is the nth (or nth-from-last) given weekday of a month.
Sunday-relative forms ("the Sunday on or after 2 November") are
deliberately absent: nothing in this repository needs one yet, and the same
mechanism admits them when something does. *)
type t =
| Fixed of { month : int; day : int }
| Easter_offset of int
| Nth_weekday of { month : int; nth : int; weekday : Date.weekday }
[@@deriving sexp]
(** Validates against the leap-year maximum, so 29 February is constructible. *)
val fixed : month:int -> day:int -> (t, string) result
(** Rejects an offset further than a year from Easter in either direction --
past that, a spec is a data error rather than a calendar. Deliberately
generous: Septuagesima is Easter-63 and Time after Pentecost runs well past
Easter+180. *)
val easter_offset : int -> (t, string) result
(** [nth] is 1-based forward, or negative from the end ([-1] is the last).
Rejects [nth = 0] (meaningless), [|nth| > 5] (no month has six of any
weekday), and a month outside 1..12. *)
val nth_weekday : month:int -> nth:int -> weekday:Date.weekday -> (t, string) result
(** [resolve spec ~year ~easter] is the civil date [spec] denotes in [year],
where [easter] is that year's Easter as the RITE reckons it -- supplied by
the caller, never computed here, since {!Computus} ships both Gregorian and
Julian and choosing one in the kernel would be silently wrong for a
Julian-reckoning rite.
[None] means "this spec does not occur in that year", per variant:
- [Fixed]: 29 February in a common year, or a date outside 1583..9999.
- [Easter_offset]: the resolved date falls outside 1583..9999. This is
reachable at the domain edges -- {!Date.add_days} is unbounded total
arithmetic and will happily denote a date outside the domain.
- [Nth_weekday]: the month has no such occurrence (a fifth Sunday it
lacks).
Total: never raises on any in-range year, for any constructible spec. *)
val resolve : t -> year:int -> easter:Date.t -> Date.t option
|
