📝 Commit Details:

backend/node_modules/date-fns/getOverlappingDaysInIntervals.js

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+import { getTimezoneOffsetInMilliseconds } from "./_lib/getTimezoneOffsetInMilliseconds.js";
+import { millisecondsInDay } from "./constants.js";
+import { toDate } from "./toDate.js";
+
+/**
+ * @name getOverlappingDaysInIntervals
+ * @category Interval Helpers
+ * @summary Get the number of days that overlap in two time intervals
+ *
+ * @description
+ * Get the number of days that overlap in two time intervals. It uses the time
+ * between dates to calculate the number of days, rounding it up to include
+ * partial days.
+ *
+ * Two equal 0-length intervals will result in 0. Two equal 1ms intervals will
+ * result in 1.
+ *
+ * @param intervalLeft - The first interval to compare.
+ * @param intervalRight - The second interval to compare.
+ * @param options - An object with options
+ *
+ * @returns The number of days that overlap in two time intervals
+ *
+ * @example
+ * // For overlapping time intervals adds 1 for each started overlapping day:
+ * getOverlappingDaysInIntervals(
+ *   { start: new Date(2014, 0, 10), end: new Date(2014, 0, 20) },
+ *   { start: new Date(2014, 0, 17), end: new Date(2014, 0, 21) }
+ * )
+ * //=> 3
+ *
+ * @example
+ * // For non-overlapping time intervals returns 0:
+ * getOverlappingDaysInIntervals(
+ *   { start: new Date(2014, 0, 10), end: new Date(2014, 0, 20) },
+ *   { start: new Date(2014, 0, 21), end: new Date(2014, 0, 22) }
+ * )
+ * //=> 0
+ */
+
+export function getOverlappingDaysInIntervals(intervalLeft, intervalRight) {
+  const [leftStart, leftEnd] = [
+    +toDate(intervalLeft.start),
+    +toDate(intervalLeft.end),
+  ].sort((a, b) => a - b);
+  const [rightStart, rightEnd] = [
+    +toDate(intervalRight.start),
+    +toDate(intervalRight.end),
+  ].sort((a, b) => a - b);
+
+  // Prevent NaN result if intervals don't overlap at all.
+  const isOverlapping = leftStart < rightEnd && rightStart < leftEnd;
+  if (!isOverlapping) return 0;
+
+  // Remove the timezone offset to negate the DST effect on calculations.
+  const overlapLeft = rightStart < leftStart ? leftStart : rightStart;
+  const left = overlapLeft - getTimezoneOffsetInMilliseconds(overlapLeft);
+  const overlapRight = rightEnd > leftEnd ? leftEnd : rightEnd;
+  const right = overlapRight - getTimezoneOffsetInMilliseconds(overlapRight);
+
+  // Ceil the number to include partial days too.
+  return Math.ceil((right - left) / millisecondsInDay);
+}
+
+// Fallback for modularized imports:
+export default getOverlappingDaysInIntervals;