kernel(computus): Gregorian + Julian Easter

Anonymous Gregorian (Meeus/Jones/Butcher) for OF+EF; Meeus Julian mapped to the
proleptic-Gregorian date for future eastern rites. Verified vs known dates
(2000/2024-27, 1583; Orthodox 2023/24) and EXHAUSTIVELY over 1583..9999: every
Easter is a Sunday in [Mar22,Apr25].

1 files changed, 38 insertions, 0 deletions

diff --git a/test/test_computus.ml b/test/test_computus.ml
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+++ b/test/test_computus.ml
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+module D = Colitur_kernel.Date
+module C = Colitur_kernel.Computus
+
+let ymd d = (D.year d, D.month d, D.day d)
+
+let test_gregorian_known () =
+  let cases = [ (2024, 3, 31); (2025, 4, 20); (2026, 4, 5); (2027, 3, 28); (2000, 4, 23); (1583, 4, 10) ] in
+  List.iter
+    (fun (y, m, d) ->
+      Alcotest.(check (triple int int int)) (Printf.sprintf "Gregorian Easter %d" y)
+        (y, m, d) (ymd (C.gregorian_easter y)))
+    cases
+
+let test_julian_known () =
+  (* Orthodox (Julian) Easter, expressed as the equivalent Gregorian date *)
+  let cases = [ (2023, 4, 16); (2024, 5, 5) ] in
+  List.iter
+    (fun (y, m, d) ->
+      Alcotest.(check (triple int int int)) (Printf.sprintf "Julian Easter %d" y)
+        (y, m, d) (ymd (C.julian_easter y)))
+    cases
+
+let test_gregorian_invariants_exhaustive () =
+  (* the confidence-to-9999 guarantee: EVERY year's Easter is a Sunday in
+     [Mar 22, Apr 25] inclusive. *)
+  for y = 1583 to 9999 do
+    let e = C.gregorian_easter y in
+    Alcotest.(check bool) (Printf.sprintf "%d Easter is Sunday" y) true (D.weekday e = D.Sun);
+    let _, m, d = ymd e in
+    let ok = (m = 3 && d >= 22) || (m = 4 && d <= 25) in
+    Alcotest.(check bool) (Printf.sprintf "%d Easter in [Mar22,Apr25]" y) true ok
+  done
+
+let suite =
+  ( "Computus",
+    [ Alcotest.test_case "Gregorian known dates" `Quick test_gregorian_known;
+      Alcotest.test_case "Julian known dates" `Quick test_julian_known;
+      Alcotest.test_case "Gregorian invariants 1583..9999" `Slow test_gregorian_invariants_exhaustive ] )