Fix Vespers concurrentia to compare today's rank against tomorrow's directly

keepsOwnSecondVespers used a fixed duplex-2-classis floor on today's rank
alone, so a lower-ranked but still-real sanctoral winner (e.g. Simplex)
tomorrow could still steal First Vespers from a higher-ranked-but-below-the-
floor winner today (semiduplex St. Raymond Nonnatus losing to simplex St.
Giles on 2026-08-31). The real "De Concurrentia Officii" rubric compares the
two days' ranks directly, with ties going to tomorrow — replace the floor
with that comparison when both sides have a real sanctoral winner, keeping
the floor only as a fallback against a plain Sunday.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014QEe9oTbn1UjQVAYcCoW1a

src/calendar/vespers.ts

@@ -12,7 +12,7 @@ import type { Commemoration, DayWinner, LiturgicalDay } from './types';
 import { resolveDay } from './index';
 import { addDays } from './date-math';
 import { easterOffsetOf } from './temporal';
-import { isAtLeast } from './commemorations';
+import { compareFeastClass, isAtLeast } from './commemorations';
 import { getSaintRecord } from './feasts';
 import { getTemporalFeastRecord } from './temporal-feasts';
 
@@ -53,18 +53,27 @@ function hasFirstVespers(day: LiturgicalDay): boolean {
   return day.winner.kind === 'sanctoral';
 }
 
-/** Does this day keep its own Second Vespers regardless of what tomorrow is? */
+/** Does this day keep its own Second Vespers regardless of what tomorrow is,
+ * independent of any comparison against tomorrow's rank? Covers only the
+ * *absolute* cases — Sunday, a privileged feria, or a major fixed feast of
+ * the Lord. The ordinary sanctoral-vs-sanctoral case is decided separately,
+ * by direct rank comparison (see `resolveEveningDay`) rather than a fixed
+ * floor here: this function used to also require the winner be at least
+ * `duplex-2-classis`, but that's the wrong rule — the real "De Concurrentia
+ * Officii" rubric (divinum-officium-reference's rubrics.txt, Titulus XI)
+ * compares today's rank against tomorrow's directly, with ties going to
+ * tomorrow, not a fixed threshold on today alone. That floor produced a
+ * wrong result for 2026-08-31 -> 09-01: St. Raymond Nonnatus (Semiduplex,
+ * below the floor) lost his own Second Vespers to St. Giles (Simplex) even
+ * though Raymond outranks Giles. */
 function keepsOwnSecondVespers(day: LiturgicalDay): boolean {
-  if (
+  return (
     day.weekday === 'sunday' ||
     day.temporalCategory === 'privileged-feria-minor' ||
     day.temporalCategory === 'privileged-feria' ||
     day.temporalCategory === 'privileged-feria-major' ||
     isMajorFixedFeastOfTheLord(day.date)
-  ) {
-    return true;
-  }
-  return day.winner.kind === 'sanctoral' && isAtLeast(day.winner.rank, 'duplex-2-classis');
+  );
 }
 
 /**
@@ -200,6 +209,23 @@ function anticipated(today: LiturgicalDay, tomorrow: LiturgicalDay): LiturgicalD
  * would likewise displace an actual Sunday if one ever landed on Dec 24 or
  * an Ember Saturday. This is why it's a short, explicit list rather than a
  * threshold: these particular days are understood to be *absolute*.
+ *
+ * When neither side is covered by an absolute case and both today and
+ * tomorrow have a real sanctoral winner, the rubric's actual rule applies:
+ * direct rank comparison, with today keeping its own Second Vespers only if
+ * it *strictly* outranks tomorrow — an exact tie goes to tomorrow ("a
+ * Capitulo fit de sequenti cum commemoratione praecedentis"). Note this
+ * checks tomorrow's actual *winner*, not merely whether tomorrow's calendar
+ * weekday is Sunday — a plain Duplex can itself outright win an ordinary
+ * Sunday's daytime office (see commemorations.ts's header comment, e.g. St.
+ * Anne beating an ordinary Sunday on 2026-07-26), in which case it's that
+ * Duplex's own rank being compared, not some notion of "Sunday's rank".
+ * When tomorrow's winner is genuinely the Sunday itself (`kind !==
+ * 'sanctoral'`), today's sufficiently high sanctoral rank
+ * (`duplex-2-classis`+) still claims the evening outright per the rubric's
+ * "Duplex always keeps both Vespers" rule — handled by the
+ * `isAtLeast`-style fallback below, same floor as before, just no longer
+ * applied when tomorrow also has a real (comparable) sanctoral winner.
  */
 export function resolveEveningDay(isoDate: string): LiturgicalDay {
   const today = resolveDay(isoDate);
@@ -208,7 +234,19 @@ export function resolveEveningDay(isoDate: string): LiturgicalDay {
   if (isMajorFixedFeastOfTheLord(tomorrow.date)) {
     return tagVespersFrom(tomorrow);
   }
+
+  let todayWins: boolean;
   if (keepsOwnSecondVespers(today)) {
+    todayWins = true;
+  } else if (today.winner.kind === 'sanctoral' && tomorrow.winner.kind === 'sanctoral') {
+    todayWins = compareFeastClass(today.winner.rank, tomorrow.winner.rank) > 0;
+  } else if (today.winner.kind === 'sanctoral' && isAtLeast(today.winner.rank, 'duplex-2-classis')) {
+    todayWins = true;
+  } else {
+    todayWins = !hasFirstVespers(tomorrow);
+  }
+
+  if (todayWins) {
     // Today wins outright, but tomorrow is still commemorated here as
     // long as it's a real sanctoral winner — see
     // commemorationOfDisplacedSuccessor's own doc comment for why this
@@ -216,8 +254,5 @@ export function resolveEveningDay(isoDate: string): LiturgicalDay {
     const commemoration = commemorationOfDisplacedSuccessor(today.winner, tomorrow.winner);
     return commemoration ? { ...today, commemorations: [...today.commemorations, commemoration] } : today;
   }
-  if (!hasFirstVespers(tomorrow)) {
-    return today;
-  }
   return anticipated(today, tomorrow);
 }

tests/calendar/vespers.test.ts

@@ -90,4 +90,29 @@ describe('resolveEveningDay', () => {
     expect(day.date).toBe('2026-08-24');
     expect(day.commemorations).toContainEqual(expect.objectContaining({ kind: 'sanctoral', id: 'st-louis' }));
   });
+
+  it('a higher-ranked today keeps its own Second Vespers against a lower-ranked tomorrow, even below the old duplex-2-classis floor', () => {
+    // Aug 31, 2026: St. Raymond Nonnatus (Semiduplex) vs. Sep 1's St. Giles
+    // (Simplex). Raymond outranks Giles, so Raymond should keep his own
+    // evening even though Semiduplex never cleared the old fixed
+    // duplex-2-classis floor -- the real "De Concurrentia Officii" rule is a
+    // direct comparison against tomorrow's rank, not a fixed threshold on
+    // today alone. Reported by the user 2026-08-31.
+    const day = resolveEveningDay('2026-08-31');
+    expect(day.date).toBe('2026-08-31');
+    expect(day.winner).toMatchObject({ kind: 'sanctoral', id: 'st-raymond-nonnatus' });
+    expect(day.commemorations).toContainEqual(expect.objectContaining({ kind: 'sanctoral', id: 'st-giles', vespersNote: 'tomorrow' }));
+  });
+
+  it('an exact rank tie goes to tomorrow, per the rubric\'s tie-break', () => {
+    // Aug 25, 2026: St. Louis (Simplex) vs. Aug 26's St. Zephyrinus (also
+    // Simplex) -- live-verified (see hasFirstVespers's own doc comment):
+    // "Vespera de sequenti; nihil de præcedenti." Tomorrow wins on the tie,
+    // and Louis (Simplex) is excluded from commemoration, same as any other
+    // Simplex predecessor.
+    const day = resolveEveningDay('2026-08-25');
+    expect(day.date).toBe('2026-08-26');
+    expect(day.winner).toMatchObject({ kind: 'sanctoral', id: 'st-zephyrinus' });
+    expect(day.commemorations.some((c) => c.kind === 'sanctoral' && c.id === 'st-louis')).toBe(false);
+  });
 });