skill issue

src/lib/attendance.js

@@ -241,6 +241,14 @@ const RECENT_WEEKS = 2;
  * Each finding carries the figures it was derived from, so a caller can state
  * *why* rather than asserting that something is wrong.
  */
+/** The middle value, for asking what a typical week looks like. */
+function median(values) {
+  const sorted = [...values].filter((n) => Number.isFinite(n)).sort((a, b) => a - b);
+  if (!sorted.length) return 0;
+  const mid = Math.floor(sorted.length / 2);
+  return sorted.length % 2 ? sorted[mid] : (sorted[mid - 1] + sorted[mid]) / 2;
+}
+
 export function attendanceAnomalies(shifts = [], { now = new Date() } = {}) {
   const findings = [];
   const trend = weeklyTrend(shifts, { weeks: 12, now });
@@ -311,7 +319,34 @@ export function attendanceAnomalies(shifts = [], { now = new Date() } = {}) {
    * against whole ones reports a fall in overtime every Monday morning.
    */
   const thisWeekStart = startOfWeek(now).getTime();
-  const complete = trend.filter((w) => new Date(w.weekStart).getTime() < thisWeekStart);
+  const withinWindow = trend.filter((w) => new Date(w.weekStart).getTime() < thisWeekStart);
+
+  /* And the same again at the other end.
+   *
+   * The rule above drops the week in progress because a part-week is not
+   * comparable to a whole one. The oldest week has exactly that problem and was
+   * kept: records begin somewhere, and wherever that is, it lands mid-week — so
+   * the first bucket holds whatever part of a week the data reaches back to.
+   *
+   * Per-shift normalises for a week's size but not for its sample: two shifts
+   * carrying one long night read as a very high rate, and that inflated figure
+   * became the baseline every later week was measured against. The effect was
+   * a blind spot with a calendar in it — the seeded overtime climb was reported
+   * Sunday through Thursday and vanished on Friday and Saturday, as the oldest
+   * week thinned from 8 scheduled shifts to 5 to 2.
+   *
+   * A week rostered well below the usual is dropped from the front rather than
+   * trusted. Only from the front: a genuine collapse in scheduled work is a
+   * finding, not noise, and this must not swallow one in the middle. */
+  const scheduledPerWeek = withinWindow.map((w) => w.scheduled);
+  const typicalWeek = median(scheduledPerWeek);
+  let firstFull = 0;
+  while (
+    firstFull < withinWindow.length - 1
+    && typicalWeek > 0
+    && withinWindow[firstFull].scheduled < typicalWeek * 0.6
+  ) firstFull += 1;
+  const complete = withinWindow.slice(firstFull);
 
   if (complete.length >= 4) {
     const perShift = complete.map((w) => (w.scheduled ? w.overtimeHours / w.scheduled : 0));