function formatDate(date) {
  const year = date.getFullYear();
  const month = String(date.getMonth() + 1).padStart(2, '0');
  const day = String(date.getDate()).padStart(2, '0');
  return `${year}-${month}-${day}`;
}

function parseDateInput(dateString) {
  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(String(dateString ?? ''));
  if (!match) {
    return null;
  }

  const [, yearText, monthText, dayText] = match;
  const year = Number(yearText);
  const month = Number(monthText);
  const day = Number(dayText);
  const date = new Date(year, month - 1, day);

  if (
    date.getFullYear() !== year ||
    date.getMonth() !== month - 1 ||
    date.getDate() !== day
  ) {
    return null;
  }

  return date;
}

function getWeekRange(dateString) {
  const baseDate = parseDateInput(dateString);
  if (!baseDate) {
    return null;
  }

  const day = baseDate.getDay();
  const diffToMonday = day === 0 ? -6 : 1 - day;

  const start = new Date(baseDate);
  start.setDate(baseDate.getDate() + diffToMonday);

  const end = new Date(start);
  end.setDate(start.getDate() + 6);

  return {
    startDate: formatDate(start),
    endDate: formatDate(end),
  };
}

module.exports = {
  formatDate,
  getWeekRange,
  parseDateInput,
};