// Reads employee names from first column of xlsx (skips header row 0)
export function readEmployeeNames(arrayBuffer) {
  const wb = XLSX.read(arrayBuffer, { type: 'array' });
  const sheet = wb.Sheets[wb.SheetNames[0]];
  const rows = XLSX.utils.sheet_to_json(sheet, { header: 1 });
  return rows.slice(1)
    .map(row => String(row[0] ?? '').trim())
    .filter(name => name.length > 0);
}

// Pure helper: returns { names: [...], dates: [...] } columns for a schedule.
// Multiple employees per date are joined alphabetically with " & ".
export function buildScheduleColumns(assignments, employees) {
  const nameById = Object.fromEntries(employees.map(e => [e.id, e.name]));
  const byDate = new Map();
  for (const a of assignments) {
    if (!byDate.has(a.date)) byDate.set(a.date, []);
    byDate.get(a.date).push(nameById[a.employeeId] ?? '');
  }
  const sortedDates = [...byDate.keys()].sort();
  const names = sortedDates.map(d =>
    byDate.get(d).slice().sort((x, y) => x.localeCompare(y, 'de')).join(' & ')
  );
  const dates = sortedDates.map(d => {
    const [y, m, day] = d.split('-');
    return day + '.' + m + '.' + y;
  });
  return { names, dates };
}

// Exports schedule as xlsx blob
// assignments: [{ date: "2026-04-01", employeeId }] — multiple entries per date allowed
// employees: [{ id, name }]
export function exportSchedule(assignments, employees) {
  const { names, dates } = buildScheduleColumns(assignments, employees);
  const ws = XLSX.utils.aoa_to_sheet([names, dates]);
  const wb = XLSX.utils.book_new();
  XLSX.utils.book_append_sheet(wb, ws, 'Moderationsplan');
  const buf = XLSX.write(wb, { type: 'array', bookType: 'xlsx' });
  return new Blob([buf], { type: 'application/octet-stream' });
}

export function downloadBlob(blob, filename) {
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = filename;
  a.click();
  URL.revokeObjectURL(url);
}