skill issue

src/api/attendanceSeed.js

@@ -39,8 +39,8 @@ const WINDOW_DAYS = 56;
  * Local rather than UTC because a shift belongs to the day it was worked in the
  * place it was worked, and `periodRange` windows on local day boundaries too.
  */
-function daysAgo(n, hour = 9, minute = 0) {
-  const d = new Date();
+function daysAgo(n, hour = 9, minute = 0, anchor = new Date()) {
+  const d = new Date(anchor.getTime());
   d.setDate(d.getDate() - n);
   d.setHours(hour, minute, 0, 0);
   return d;
@@ -154,10 +154,10 @@ const BEHAVIOUR = {
 };
 
 /** Every shift date for one worker, most recent first. */
-function shiftOffsets(weekdays) {
+function shiftOffsets(weekdays, anchor) {
   const offsets = [];
   for (let offset = 0; offset <= WINDOW_DAYS; offset += 1) {
-    const day = daysAgo(offset).getDay();
+    const day = daysAgo(offset, 9, 0, anchor).getDay();
     if (weekdays.includes(day)) offsets.push(offset);
   }
   return offsets;
@@ -166,14 +166,19 @@ function shiftOffsets(weekdays) {
 const pad = (n) => String(n).padStart(2, '0');
 const localDate = (d) => `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}`;
 
-function buildShifts() {
+/**
+ * @param {Date} [anchor] the day to count back from. Defaults to now, which is
+ *   the point of this collection; a caller passes one only to hold the window
+ *   still — see buildShiftsAt.
+ */
+function buildShifts(anchor = new Date()) {
   const records = [];
 
   for (const worker of ROSTER) {
-    const offsets = shiftOffsets(worker.weekdays);
+    const offsets = shiftOffsets(worker.weekdays, anchor);
 
     offsets.forEach((offset, i) => {
-      const scheduledStart = daysAgo(offset, worker.startHour);
+      const scheduledStart = daysAgo(offset, worker.startHour, 0, anchor);
       const weekday = scheduledStart.getDay();
       const scheduledEnd = new Date(scheduledStart.getTime() + worker.scheduledHours * HOUR);
 
@@ -231,3 +236,21 @@ function buildShifts() {
 }
 
 export const SHIFT_RECORDS = buildShifts();
+
+/**
+ * The same shifts, counted back from a day you choose.
+ *
+ * The distribution is deterministic given its anchor but not across anchors:
+ * which shifts fall in a complete week depends on where the anchor sits in one,
+ * so an assertion about the seeded overtime climb held on a Thursday and failed
+ * on a Friday. That is a test that reports the calendar, not the code.
+ *
+ * The app keeps `SHIFT_RECORDS` above, anchored to now, because attendance
+ * "last week" has to mean last week on the day somebody looks. Pin the anchor
+ * here instead, and pass the SAME date as `now` to whatever reads the result —
+ * shifts built around one day and bucketed around another describe two
+ * different windows.
+ */
+export function buildShiftsAt(anchor) {
+  return buildShifts(anchor);
+}