will
67037469fc
resolveDay() no longer hardcodes season: 'trinitytide'. Easter is computed via the Anonymous Gregorian algorithm (verified against 16 known reference dates spanning 1583-2100); Advent/Christmastide/ Epiphanytide follow the Christmas-anchored rules (including Advent's "Sunday nearest Nov 30"); Septuagesima through Trinitytide come from easter-offsets.yml, reshaped from single named anchor points into real ranges plus single-day overrides for Corpus Christi and Sacred Heart. occurring feasts (calendar/feasts.ts) are still a stub — the sanctoral calendar and Double-vs-not ranking are a separate, larger project.
30 lines
1.2 KiB
TypeScript
30 lines
1.2 KiB
TypeScript
// Computus (the date of Easter, and everything the temporal cycle hangs off
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// of it — Septuagesima, Ash Wednesday, Ascension, Pentecost, Trinity Sunday).
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//
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// This is the Anonymous Gregorian algorithm (a.k.a. Meeus/Jones/Butcher),
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// valid for the whole Gregorian era (1583 onward). Hand-rolled rather than
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// pulling in a dependency: it's ~15 lines of pure integer arithmetic with a
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// long history of independent verification (see calendar/easter.test.ts),
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// not something that benefits from a library's abstraction or maintenance
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// surface for an app with zero runtime dependencies otherwise.
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import type { CalendarDate } from './date-math';
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export function easterSunday(year: number): CalendarDate {
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const a = year % 19;
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const b = Math.floor(year / 100);
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const c = year % 100;
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const d = Math.floor(b / 4);
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const e = b % 4;
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const f = Math.floor((b + 8) / 25);
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const g = Math.floor((b - f + 1) / 3);
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const h = (19 * a + b - d - g + 15) % 30;
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const i = Math.floor(c / 4);
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const k = c % 4;
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const l = (32 + 2 * e + 2 * i - h - k) % 7;
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const m = Math.floor((a + 11 * h + 22 * l) / 451);
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const n = h + l - 7 * m + 114;
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const month = Math.floor(n / 31);
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const day = (n % 31) + 1;
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return { year, month, day };
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}
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