Create French to JDN conversion function

src/Calendar.test.ts

@@ -1,4 +1,4 @@
-import {gregorianJDN} from "./Calendar";
+import {frJDN, gregorianJDN} from './Calendar';
 
 describe('gregorianJDN', () => {
     it('works', () => {
@@ -16,3 +16,16 @@ describe('gregorianJDN', () => {
         expect(gregorianJDN(7504, 7, 22)).toBe(4462042);
     });
 });
+
+describe('frJDN', () => {
+    it('works for sample dates', () => {
+        expect(frJDN(1, 1, 1)).toBe(2375840);
+        expect(frJDN(8, 2, 18)).toBe(2378444);
+    });
+
+    it('works in years starting/ending near midnight', () => {
+        expect(frJDN( 111, 1, 1)).toBe(2416017); // equinox 1902-09-23T23:55:19 UT1
+        expect(frJDN( 206, 1, 1)).toBe(2450715); // equinox 1997-09-22T23:55:46 UT1
+        expect(frJDN(2490, 1, 1)).toBe(3284926); // equinox 4281-09-20T23:50:38 UT1
+    });
+});

src/Calendar.ts

@@ -1,3 +1,15 @@
+import data from '../cal.json';
+
+const startJD = data.start_jd;
+const startYear = data.start_year;
+const leaps: Array<number> = [0];
+
+let leapFromStart = 0;
+data.leap.forEach(leap => {
+    leapFromStart += leap;
+    leaps.push(leapFromStart);
+});
+
 export function gregorianJDN(year: number, month: number, day: number) {
     const g = year + 4716 - (month <= 2 ? 1 : 0);
     const f = (month + 9) % 12;
@@ -6,3 +18,9 @@ export function gregorianJDN(year: number, month: number, day: number) {
     const dg = 38 - Math.floor(Math.floor((g + 184) / 100) * 3 / 4);
     return J + dg;
 }
+
+export function frJDN(year: number, month: number, day: number) {
+    const dy = year - startYear;
+    const dd = month * 30 + day - 31;
+    return startJD + 365 * dy + leaps[dy] + dd;
+}