export function formatDate(date, format) {
  const padZero = (num) => { return num.toString().padStart(2, '0'); }
  const replacements = {
    '%Y': date.getFullYear(),
    '%m': padZero(date.getMonth() + 1),
    '%d': padZero(date.getDate()),
    '%H': padZero(date.getHours()),
    '%M': padZero(date.getMinutes()),
    '%S': padZero(date.getSeconds()),
    '%A': date.toLocaleString('en-US', { weekday: 'long' }),
    '%a': date.toLocaleString('en-US', { weekday: 'short' }),
    '%b': date.toLocaleString('en-US', { month: 'short' }),
    '%u': date.getDay() === 0 ? 7 : date.getDay(),
    '%U': padZero(getSunWeekNum(date)),
    '%V': padZero(getISOWeekNum(date)),
  };
  return format.replace(/%[a-zA-Z]/g, match => replacements[match]);
}

function getISOWeekNum(date) {
  const tempDate = new Date(date.getTime());
  tempDate.setHours(0, 0, 0, 0);
  tempDate.setDate(tempDate.getDate() + 3 - (tempDate.getDay() + 6) % 7);
  const week1 = new Date(tempDate.getFullYear(), 0, 4);
  return 1 + Math.round(((tempDate.getTime() - week1.getTime()) / 86400000 - 3 + (week1.getDay() + 6) % 7) / 7);
}

function getSunWeekNum(date) {
  const sunday = new Date(date);
  sunday.setDate(date.getDate() - date.getDay());
  const firstSunday = new Date(sunday.getFullYear(), 0, 1);
  if (firstSunday.getDay() !== 0) firstSunday.setMonth(0, 1 + (7 - firstSunday.getDay()) % 7);
  return 1 + Math.floor((sunday - firstSunday) / 604800000);
}