add holiday calculation cache

lib/holidays.js

@@ -29,6 +29,15 @@ const variableOffsets = {
   603: 60
 };
 
+const holidayCache = {};
+function getDateHolidays(countryCode) {
+  if (!(countryCode in holidayCache)) {
+    holidayCache[countryCode] = new DateHolidays(countryCode).getHolidays(2022);
+  }
+
+  return holidayCache[countryCode];
+}
+
 function Holidays(year) {
   function getEasterSunday() {
     if (!easterSundays[year]) {
@@ -143,9 +152,7 @@ function Holidays(year) {
 
   this.getAllHolidays = function (info, epochSpan, cb) {
     if (info.countryCode && ['DE', 'GB', 'US'].includes(info.countryCode)) {
-      const dateHolidays = new DateHolidays(info.countryCode);
-      const holidayDays = dateHolidays
-        .getHolidays(2021)
+      const holidayDays = getDateHolidays(info.countryCode)
         .filter(x => x.type === 'public')
         .map(x => {
           const dateDay = x.date.slice(0, 10); // strip hours