Add JDN to French Republican calculation

src/App.tsx

@@ -1,7 +1,7 @@
 import React from 'react';
 import './App.css';
 import {Calendar} from './Calendar';
-import {gregorianJDN, Month} from "./dates";
+import {gregorianJDN, jdnFrench, Month} from "./dates";
 
 type YearMonth = {
     year: number;
@@ -28,12 +28,13 @@ class App extends React.Component<{}, AppState> {
 
     constructor(props: {}) {
         super(props);
-        const current = {year: 230, month: 5};
         const today = new Date();
+        const todayJDN = gregorianJDN(today.getFullYear(), today.getMonth() + 1, today.getDay());
+        const {year, month} = jdnFrench(todayJDN);
 
         this.state = {
-            ...(parseURL() || current as YearMonth),
-            todayJDN: gregorianJDN(today.getFullYear(), today.getMonth() + 1, today.getDay()),
+            ...(parseURL() || {year, month}),
+            todayJDN,
         };
         this.updateURL();
         this.updateStateFromURL = this.updateStateFromURL.bind(this);

src/dates.test.ts

@@ -1,4 +1,4 @@
-import {dateName, frJDN, gregorianJDN, jdnGregorian, jdnLongCount, monthName} from './dates';
+import {dateName, frJDN, gregorianJDN, jdnFrench, jdnGregorian, jdnLongCount, monthName} from './dates';
 
 describe('gregorianJDN', () => {
     it('works', () => {
@@ -30,6 +30,22 @@ describe('frJDN', () => {
     });
 });
 
+describe('jdnFrench', () => {
+    it('works for sample dates', () => {
+        expect(jdnFrench(2375840)).toEqual({year: 1, month: 1, day: 1});
+        expect(jdnFrench(2378444)).toEqual({year: 8, month: 2, day: 18});
+    });
+
+    it('works in years starting/ending near midnight', () => {
+        expect(jdnFrench(2416017)).toEqual({year:  111, month: 1, day: 1});
+        expect(jdnFrench(2450715)).toEqual({year:  206, month: 1, day: 1});
+        expect(jdnFrench(3284926)).toEqual({year: 2490, month: 1, day: 1});
+        expect(jdnFrench(2416016)).toEqual({year:  110, month: 13, day: 6});
+        expect(jdnFrench(2450714)).toEqual({year:  205, month: 13, day: 6});
+        expect(jdnFrench(3284925)).toEqual({year: 2489, month: 13, day: 5});
+    });
+});
+
 describe('jdnGregorian', () => {
     it('works', () => {
         expect(jdnGregorian(0)).toEqual(new Date(-4713, 10, 24));

src/dates.ts

@@ -34,10 +34,10 @@ export type Day =
     | 29
     | 30;
 
-export type GregorianDate = {
+export type FrenchDate = {
     year: number,
-    month: number,
-    day: number,
+    month: Month,
+    day: Day,
 };
 
 const monthNames: {
@@ -96,6 +96,26 @@ export function jdnGregorian(jdn: number): Date {
     return new Date(year, month - 1, day);
 }
 
+export function jdnFrench(jdn: number): FrenchDate {
+    let lo = 0;
+    let hi = leaps.length;
+
+    while (lo + 1 < hi) {
+        const mid = Math.floor((lo + hi) / 2);
+        if (startJD + 365 * mid + leaps[mid] <= jdn)
+            lo = mid;
+        else
+            hi = mid;
+    }
+
+    const dd = jdn - (startJD + 365 * lo + leaps[lo]);
+    return {
+        year: startYear + lo,
+        month: Math.floor(dd / 30) + 1 as Month,
+        day: dd % 30 + 1 as Day,
+    }
+}
+
 export function jdnLongCount(jdn: number): string | null {
     let z = jdn - 584283;
     if (z < 0)