Implement Long Count date calculation

src/dates.test.ts

@@ -1,4 +1,4 @@
-import {dateName, frJDN, gregorianJDN, jdnGregorian, monthName} from './dates';
+import {dateName, frJDN, gregorianJDN, jdnGregorian, jdnLongCount, monthName} from './dates';
 
 describe('gregorianJDN', () => {
     it('works', () => {
@@ -51,6 +51,67 @@ describe('jdnGregorian', () => {
     });
 });
 
+describe('jdnLongCount', () => {
+    it('works for normal dates', () => {
+        expect(jdnLongCount(2456282)).toEqual('12.19.19.17.19');
+        expect(jdnLongCount(2459413)).toEqual('13.0.8.12.10');
+        expect(jdnLongCount(1708616)).toEqual('7.16.3.2.13');
+        expect(jdnLongCount(1787816)).toEqual('8.7.3.2.13');
+        expect(jdnLongCount(1709981)).toEqual('7.16.6.16.18');
+        expect(jdnLongCount(1725275)).toEqual('7.18.9.7.12');
+        expect(jdnLongCount(1727095)).toEqual('7.18.14.8.12');
+        expect(jdnLongCount(1731775)).toEqual('7.19.7.8.12');
+        expect(jdnLongCount(1734635)).toEqual('7.19.15.7.12');
+        expect(jdnLongCount(1751618)).toEqual('8.2.2.10.15');
+        expect(jdnLongCount(1758818)).toEqual('8.3.2.10.15');
+        expect(jdnLongCount(1767234)).toEqual('8.4.5.17.11');
+        expect(jdnLongCount(1773428)).toEqual('8.5.3.3.5');
+        expect(jdnLongCount(1778230)).toEqual('8.5.16.9.7');
+        expect(jdnLongCount(1780300)).toEqual('8.6.2.4.17');
+        expect(jdnLongCount(2283483)).toEqual('11.16.0.0.0');
+        expect(jdnLongCount(584283)).toEqual('0.0.0.0.0');
+        expect(jdnLongCount(728283)).toEqual('1.0.0.0.0');
+        expect(jdnLongCount(872283)).toEqual('2.0.0.0.0');
+        expect(jdnLongCount(1016283)).toEqual('3.0.0.0.0');
+        expect(jdnLongCount(1160283)).toEqual('4.0.0.0.0');
+        expect(jdnLongCount(1304283)).toEqual('5.0.0.0.0');
+        expect(jdnLongCount(1448283)).toEqual('6.0.0.0.0');
+        expect(jdnLongCount(1592283)).toEqual('7.0.0.0.0');
+        expect(jdnLongCount(1736283)).toEqual('8.0.0.0.0');
+        expect(jdnLongCount(1880283)).toEqual('9.0.0.0.0');
+        expect(jdnLongCount(2024283)).toEqual('10.0.0.0.0');
+        expect(jdnLongCount(2168283)).toEqual('11.0.0.0.0');
+        expect(jdnLongCount(2312283)).toEqual('12.0.0.0.0');
+        expect(jdnLongCount(2456283)).toEqual('13.0.0.0.0');
+        expect(jdnLongCount(2600283)).toEqual('14.0.0.0.0');
+        expect(jdnLongCount(2744283)).toEqual('15.0.0.0.0');
+        expect(jdnLongCount(2888283)).toEqual('16.0.0.0.0');
+        expect(jdnLongCount(3032283)).toEqual('17.0.0.0.0');
+        expect(jdnLongCount(3176283)).toEqual('18.0.0.0.0');
+        expect(jdnLongCount(3320283)).toEqual('19.0.0.0.0');
+        expect(jdnLongCount(3464283)).toEqual('1.0.0.0.0.0');
+        expect(jdnLongCount(1941383)).toEqual('9.8.9.13.0');
+    });
+
+    it('works for insane dates in the future', () => {
+        expect(jdnLongCount(591279564516)).toEqual('1.5.13.5.5.4.0.11.13.13');
+        expect(jdnLongCount(570988471138)).toEqual('1.4.15.12.19.13.13.3.8.15');
+        expect(jdnLongCount(166410754861)).toEqual('7.4.9.1.6.3.13.14.18');
+        expect(jdnLongCount(176632006419)).toEqual('7.13.6.10.7.1.19.4.16');
+        expect(jdnLongCount(652557304645)).toEqual('1.8.6.9.2.3.17.16.10.2');
+        expect(jdnLongCount(140305417242)).toEqual('6.1.15.16.19.2.7.1.19');
+        expect(jdnLongCount(805888002058)).toEqual('1.14.19.11.2.0.8.0.8.15');
+        expect(jdnLongCount(176433890202)).toEqual('7.13.3.1.11.5.16.7.19');
+        expect(jdnLongCount(331888546361)).toEqual('14.8.1.18.17.10.5.13.18');
+        expect(jdnLongCount(657363764536)).toEqual('1.8.10.12.11.2.1.14.0.13');
+    });
+
+    it('returns null for dates before inception of calendar', () => {
+        expect(jdnLongCount(0)).toBeNull();
+        expect(jdnLongCount(584282)).toBeNull();
+    });
+})
+
 describe('monthName', () => {
     it('works', () => {
         expect(monthName(1)).toBe('Vendémiaire');

src/dates.ts

@@ -96,6 +96,25 @@ export function jdnGregorian(jdn: number): Date {
     return new Date(year, month, day);
 }
 
+export function jdnLongCount(jdn: number): string | null {
+    let z = jdn - 584283;
+    if (z < 0)
+        return null;
+
+    const parts = [z % 20, Math.floor(z / 20) % 18];
+    z = Math.floor(z / 360);
+    while (z > 0) {
+        parts.push(z % 20);
+        z = Math.floor(z / 20);
+    }
+
+    while (parts.length < 5) {
+        parts.push(0);
+    }
+
+    return parts.reverse().join('.');
+}
+
 export function monthName(month: Month): string {
     return monthNames[month];
 }