/util/date/weekdata.mjs

weekdata.mjs

import range from '/util/iterator/range.mjs';

// Week start data pulled from similarly golfed answer on SO:
// https://stackoverflow.com/a/57102881/1631952
//
// I’m not thrilled with this; we probably ought to be generating it since the
// data is pretty obscure. OTOH I’d be far less thrilled with loading hefty
// chunks of ICU data and this is pretty solid and compact!
//
// Note that the RegExp given for lang tags in that answer is a bit off. The one
// below is more accurate but deliberately doesn’t bother matching any segments
// after region and doesn’t include grandfathered tags etc, since browsers
// should not be returning those values. It also does not bother matching region
// tag segments which use the three-digit format since we don’t have data for
// those below (and again I doubt browsers ever surface language tags that way).
//
// The exports here are live. Queue a render on languagechange in elements that
// consume this data if you want it to respond to system language changes. In
// practice, such events are very rare and on Windows they don’t reliably
// occur, so it’s your call whether it’s worthwhile in a given case.

const LANG_TAG_REGION =
  /^(?:[a-z]{2,3}(?:-[a-z]{3}){0,3}|[a-z]{4,8})(?:-[a-z]{4})?-([a-z]{2})/i;
  //   ^ lang (1)    ^ extlangs      ^ lang (2)    ^ script    ^ region

const [ SATURDAYS, SUNDAYS ] = [
  'AEAFBHDJDZEGIQIRJOKWLYOMQASDSY',
  'AGARASAUBDBRBSBTBWBZCACNCODMDOETGTGUHKHNIDILINJMJPKEKHKRLAMHMMMOMTMXMZNINP' +
  'PAPEPHPKPRPTPYSASGSVTHTTTWUMUSVEVIWSYEZAZW'
].map(src => new Set(src.split(/(?=(..)+$)/)));

////////////////////////////////////////////////////////////////////////////////

export let indices, longNames, shortNames, start;

update();
addEventListener('languagechange', update);

////////////////////////////////////////////////////////////////////////////////

function getRegion() {
  for (const tag of navigator.languages) {
    const match = LANG_TAG_REGION.exec(tag);

    if (match) {
      return match[1].toUpperCase();
    }
  }
}

function getWeekNames(format) {
  return indices.map(i => {
    const date = new Date(0, 0, i);

    return date.toLocaleDateString(navigator.languages, {
      calendar: 'gregory',
      weekday: format
    });
  });
}

function update() {
  const region = getRegion();

  start = SATURDAYS.has(region) ? 6 : SUNDAYS.has(region) ? 0 : 1;
  indices = Array.from(range(0, 7), i => (start + i) % 7);
  shortNames = getWeekNames('short');
  longNames = getWeekNames('long');
}