feat: algorithm module with unit tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

js/algorithm.js

@@ -0,0 +1,97 @@
+function toISO(year, month, day) {
+  return year + '-' + String(month).padStart(2, '0') + '-' + String(day).padStart(2, '0');
+}
+
+// Returns 1=Mo, 2=Di, 3=Mi, 4=Do, 5=Fr, 6=Sa, 7=So
+function getWeekday(isoDate) {
+  const d = new Date(isoDate + 'T12:00:00Z');
+  const day = d.getUTCDay(); // 0=Sun
+  return day === 0 ? 7 : day;
+}
+
+function isOnVacation(employee, isoDate) {
+  return employee.constraints.vacations.some(v => v.from <= isoDate && isoDate <= v.to);
+}
+
+function daysBetween(isoA, isoB) {
+  return Math.abs(
+    (new Date(isoA + 'T12:00:00Z') - new Date(isoB + 'T12:00:00Z')) / 86400000
+  );
+}
+
+// Returns sorted array of ISO date strings that are workdays in the given month
+export function getWorkdays(year, month, holidays, companyClosures) {
+  const days = [];
+  const daysInMonth = new Date(year, month, 0).getDate();
+  for (let d = 1; d <= daysInMonth; d++) {
+    const iso = toISO(year, month, d);
+    const wd = getWeekday(iso);
+    if (wd >= 6) continue; // weekend
+    if (holidays.includes(iso) || companyClosures.includes(iso)) continue;
+    days.push(iso);
+  }
+  return days;
+}
+
+function isEligible(employee, isoDate, monthAssignments, allHistory) {
+  const c = employee.constraints;
+  const wd = getWeekday(isoDate);
+
+  if (c.neverDays.includes(wd)) return false;
+  if (c.onlyDays.length > 0 && !c.onlyDays.includes(wd)) return false;
+  if (isOnVacation(employee, isoDate)) return false;
+
+  if (c.maxPerMonth !== null) {
+    const count = monthAssignments.filter(a => a.employeeId === employee.id).length;
+    if (count >= c.maxPerMonth) return false;
+  }
+
+  if (c.minGapDays > 0) {
+    const allEntries = [...allHistory, ...monthAssignments];
+    const lastDate = allEntries
+      .filter(a => a.employeeId === employee.id && a.date < isoDate)
+      .map(a => a.date)
+      .sort()
+      .at(-1);
+    if (lastDate && daysBetween(lastDate, isoDate) < c.minGapDays) return false;
+  }
+
+  return true;
+}
+
+// randomFn injectable so tests can pass () => 0.5 for deterministic output
+function scoreCandidate(employee, isoDate, allHistory, randomFn) {
+  const wd = getWeekday(isoDate);
+  const myTotal = allHistory.filter(h => h.employeeId === employee.id).length;
+
+  // Fewer past assignments → higher (less negative) score
+  let score = -myTotal;
+
+  // Low priority: increase negative score (less likely to be picked)
+  if (employee.constraints.lowPriorityDays.includes(wd)) score -= 5;
+
+  // Jitter ±0.49
+  score += (randomFn() - 0.5) * 0.98;
+
+  return score;
+}
+
+// Returns [{ date, employeeId, manual: false }] for each assigned workday
+export function generatePlan(workdays, employees, history, randomFn = Math.random) {
+  const monthAssignments = [];
+
+  for (const date of workdays) {
+    const candidates = employees.filter(e =>
+      isEligible(e, date, monthAssignments, history)
+    );
+    if (candidates.length === 0) continue;
+
+    const scored = candidates
+      .map(e => ({ employee: e, score: scoreCandidate(e, date, history, randomFn) }))
+      .sort((a, b) => b.score - a.score);
+
+    monthAssignments.push({ date, employeeId: scored[0].employee.id, manual: false });
+  }
+
+  return monthAssignments;
+}