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;
}