kernel(computus): Easter-relative movable-feast anchors

Ash Wednesday/Palm Sunday/Ascension/Pentecost/Corpus Christi as Easter+/-N.
Verified vs 2026 dates; exhaustive weekday invariants 1583..9999 (Ash Wed=Wed,
Ascension/Corpus Christi=Thu, Palm/Pentecost=Sun).

3 files changed, 36 insertions, 1 deletions

diff --git a/lib/kernel/computus.ml b/lib/kernel/computus.ml
index 1f60fb7..ba803d2 100644
--- a/lib/kernel/computus.ml
+++ b/lib/kernel/computus.ml
@@ -18,6 +18,13 @@ let gregorian_easter y =
   let day = ((h + l - (7 * m) + 114) mod 31) + 1 in
   mk y month day
 
+let anchor y offset = Date.add_days (gregorian_easter y) offset
+let ash_wednesday y = anchor y (-46)
+let palm_sunday y = anchor y (-7)
+let ascension y = anchor y 39
+let pentecost y = anchor y 49
+let corpus_christi y = anchor y 60
+
 (* Meeus Julian Easter: gives the JULIAN-calendar month/day; convert to the same
    physical day expressed in the proleptic Gregorian calendar by adding the
    Julian->Gregorian offset (days = y/100 - y/400 - 2). *)
diff --git a/lib/kernel/computus.mli b/lib/kernel/computus.mli
index dc8f215..0e227dc 100644
--- a/lib/kernel/computus.mli
+++ b/lib/kernel/computus.mli
@@ -4,6 +4,16 @@
 (** Easter Sunday by the Gregorian reckoning (OF + EF). *)
 val gregorian_easter : int -> Date.t
 
+(** Easter-relative movable-feast anchors (register section 0), Gregorian
+    reckoning: Ash Wednesday = Easter-46 (Wed), Palm Sunday = Easter-7,
+    Ascension = Easter+39 (Thu), Pentecost = Easter+49, Corpus Christi =
+    Easter+60 (Thu, EF). *)
+val ash_wednesday : int -> Date.t
+val palm_sunday : int -> Date.t
+val ascension : int -> Date.t
+val pentecost : int -> Date.t
+val corpus_christi : int -> Date.t
+
 (** Easter Sunday by the Julian reckoning, returned as the equivalent proleptic
     Gregorian date (for future eastern rites; EF/OF use {!gregorian_easter}). *)
 val julian_easter : int -> Date.t
diff --git a/test/test_computus.ml b/test/test_computus.ml
index 1502ad2..cbb8cab 100644
--- a/test/test_computus.ml
+++ b/test/test_computus.ml
@@ -31,8 +31,26 @@ let test_gregorian_invariants_exhaustive () =
     Alcotest.(check bool) (Printf.sprintf "%d Easter in [Mar22,Apr25]" y) true ok
   done
 
+let test_anchors_2026 () =
+  (* Easter 2026 = Apr 5 *)
+  Alcotest.(check (triple int int int)) "Ash Wednesday 2026" (2026, 2, 18) (ymd (C.ash_wednesday 2026));
+  Alcotest.(check (triple int int int)) "Palm Sunday 2026" (2026, 3, 29) (ymd (C.palm_sunday 2026));
+  Alcotest.(check (triple int int int)) "Ascension 2026" (2026, 5, 14) (ymd (C.ascension 2026));
+  Alcotest.(check (triple int int int)) "Pentecost 2026" (2026, 5, 24) (ymd (C.pentecost 2026))
+
+let test_anchor_weekdays_exhaustive () =
+  for y = 1583 to 9999 do
+    Alcotest.(check bool) (Printf.sprintf "%d Ash Wed = Wed" y) true (D.weekday (C.ash_wednesday y) = D.Wed);
+    Alcotest.(check bool) (Printf.sprintf "%d Palm Sun = Sun" y) true (D.weekday (C.palm_sunday y) = D.Sun);
+    Alcotest.(check bool) (Printf.sprintf "%d Ascension = Thu" y) true (D.weekday (C.ascension y) = D.Thu);
+    Alcotest.(check bool) (Printf.sprintf "%d Pentecost = Sun" y) true (D.weekday (C.pentecost y) = D.Sun);
+    Alcotest.(check bool) (Printf.sprintf "%d Corpus Christi = Thu" y) true (D.weekday (C.corpus_christi y) = D.Thu)
+  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 ] )
+      Alcotest.test_case "Gregorian invariants 1583..9999" `Slow test_gregorian_invariants_exhaustive;
+      Alcotest.test_case "anchors 2026" `Quick test_anchors_2026;
+      Alcotest.test_case "anchor weekdays 1583..9999" `Slow test_anchor_weekdays_exhaustive ] )