From bf2af799d506546b7f085f29aab4dae4a617ee27 Mon Sep 17 00:00:00 2001
From: Alex van Andel <me@alexvanandel.com>
Date: Thu, 7 Dec 2023 09:46:45 +0000
Subject: [PATCH] perf: Improve performance of working hours processing
 (#12687)

---
 packages/lib/date-ranges.ts | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/packages/lib/date-ranges.ts b/packages/lib/date-ranges.ts
index 1032bba1bd..fabd7cb5c7 100644
--- a/packages/lib/date-ranges.ts
+++ b/packages/lib/date-ranges.ts
@@ -21,9 +21,10 @@ export function processWorkingHours({
   dateFrom: Dayjs;
   dateTo: Dayjs;
 }) {
+  const utcDateTo = dateTo.utc();
   const results = [];
-  for (let date = dateFrom.tz(timeZone).startOf("day"); dateTo.isAfter(date); date = date.add(1, "day")) {
-    const fromOffset = dateFrom.tz(timeZone).startOf("day").utcOffset();
+  for (let date = dateFrom.startOf("day"); utcDateTo.isAfter(date); date = date.add(1, "day")) {
+    const fromOffset = dateFrom.startOf("day").utcOffset();
     const offset = date.tz(timeZone).utcOffset();
 
     // it always has to be start of the day (midnight) even when DST changes
@@ -44,7 +45,7 @@ export function processWorkingHours({
     start = start.add(offsetDiff, "minute");
     end = end.add(offsetDiff, "minute");
 
-    const startResult = dayjs.max(start, dateFrom.tz(timeZone));
+    const startResult = dayjs.max(start, dateFrom);
     const endResult = dayjs.min(end, dateTo.tz(timeZone));
 
     if (endResult.isBefore(startResult)) {
@@ -92,10 +93,13 @@ export function buildDateRanges({
   dateFrom: Dayjs;
   dateTo: Dayjs;
 }): DateRange[] {
+  const dateFromOrganizerTZ = dateFrom.tz(timeZone);
   const groupedWorkingHours = groupByDate(
     availability.reduce((processed: DateRange[], item) => {
       if ("days" in item) {
-        processed = processed.concat(processWorkingHours({ item, timeZone, dateFrom, dateTo }));
+        processed = processed.concat(
+          processWorkingHours({ item, timeZone, dateFrom: dateFromOrganizerTZ, dateTo })
+        );
       }
       return processed;
     }, [])