rite(ef): pin down floor_div and week/sunday_slug agreement

Task 12 review: add a permanent regression test for floor_div's negative
branch (Ash Wednesday, 4 days before the Lent I origin -- the only date in
the system where it fires), and stop sunday_slug from independently
recomputing the Pentecost-relative week number that week already computes.
A cross-check test pins the two together before the refactor and continues
to guard against future duplication.

1 files changed, 18 insertions, 14 deletions

diff --git a/lib/rites/rite_ef/temporal_ef.ml b/lib/rites/rite_ef/temporal_ef.ml
index 9e8bcc4..6786cd2 100644
--- a/lib/rites/rite_ef/temporal_ef.ml
+++ b/lib/rites/rite_ef/temporal_ef.ml
@@ -165,20 +165,24 @@ let sunday_slug d =
           Some "ef-holy-name-sunday"
         else None
     | Time_after_pentecost -> (
-        let easter = Computus.gregorian_easter y in
-        let pentecost = Date.add_days easter 49 in
-        let last_sunday = Date.add_days (advent_start y) (-7) in
-        let n = days_between pentecost d / 7 in
-        if same d last_sunday then
-          (* The last Sunday before Advent always keeps the 24th (Last) Mass. *)
-          Some "ef-time-after-pentecost-sunday-24"
-        else if n > 23 then
-          (* Surplus Sundays resume the Sundays after Epiphany that Septuagesima
-             cut short -- the highest-numbered ones, so the 6th sits just before
-             the Last. *)
-          let total = days_between pentecost last_sunday / 7 in
-          Some (Printf.sprintf "ef-time-after-epiphany-sunday-%d" (n - total + 7))
-        else Some (Printf.sprintf "ef-time-after-pentecost-sunday-%d" n))
+        (* Reuse [week] rather than recomputing the Pentecost-relative week
+           number locally, so the two can never drift apart (see
+           test_week_sunday_slug_agree). Only the last Sunday and the resumed
+           tail are genuinely special. *)
+        match week d with
+        | None -> None
+        | Some n ->
+            let last_sunday = Date.add_days (advent_start y) (-7) in
+            if same d last_sunday then
+              (* The last Sunday before Advent always keeps the 24th (Last) Mass. *)
+              Some "ef-time-after-pentecost-sunday-24"
+            else if n > 23 then
+              (* Surplus Sundays resume the Sundays after Epiphany that Septuagesima
+                 cut short -- the highest-numbered ones, so the 6th sits just before
+                 the Last. *)
+              let total = match week last_sunday with Some t -> t | None -> n in
+              Some (Printf.sprintf "ef-time-after-epiphany-sunday-%d" (n - total + 7))
+            else Some (Printf.sprintf "ef-time-after-pentecost-sunday-%d" n))
     | _ -> (
         match week d with
         | Some n -> Some (Printf.sprintf "ef-%s-sunday-%d" (season_slug_word s) n)