/**
 * Every entry carries the timezone it was written in, and every stamp is
 * formatted in that timezone rather than in the reader's current one - a
 * journal written on a trip should still read as the local time of the moment.
 * Timestamps cross the wire as UNIX seconds, so nothing here parses a string.
 */

const asFormatterCache = new Map();

function getFormatter(sLocale, sTimezone, asOptions) {
	const sKey = sLocale + '|' + sTimezone + '|' + JSON.stringify(asOptions);

	if(!asFormatterCache.has(sKey)) {
		let oFormatter;
		try {
			oFormatter = new Intl.DateTimeFormat(sLocale, {...asOptions, timeZone: sTimezone});
		}
		catch{
			//An unknown IANA name (renamed zone, hand-edited row) must not take
			//the whole book down - fall back to the browser's own zone.
			oFormatter = new Intl.DateTimeFormat(sLocale, asOptions);
		}
		asFormatterCache.set(sKey, oFormatter);
	}

	return asFormatterCache.get(sKey);
}

export function getBrowserTimezone() {
	try {
		return Intl.DateTimeFormat().resolvedOptions().timeZone || '';
	}
	catch{
		return '';
	}
}

/** 'YYYY-MM-DD' for the given instant, as seen in the given timezone. */
export function getDayKey(iUnix, sTimezone) {
	return getFormatter('en-CA', sTimezone, {year: 'numeric', month: '2-digit', day: '2-digit'}).format(iUnix * 1000);
}

/** 'YYYY-MM-DD' for a local Date, without the UTC shift toISOString() adds. */
export function getLocalDayKey(oDate) {
	const sMonth = String(oDate.getMonth() + 1).padStart(2, '0');
	const sDay = String(oDate.getDate()).padStart(2, '0');
	return oDate.getFullYear() + '-' + sMonth + '-' + sDay;
}

export function formatDate(iUnix, sTimezone, sLocale) {
	return getFormatter(sLocale, sTimezone, {day: 'numeric', month: 'short', year: 'numeric'}).format(iUnix * 1000);
}

export function formatShortDate(iUnix, sTimezone, sLocale) {
	return getFormatter(sLocale, sTimezone, {day: 'numeric', month: 'short'}).format(iUnix * 1000);
}

export function formatTime(iUnix, sTimezone, sLocale) {
	return getFormatter(sLocale, sTimezone, {hour: '2-digit', minute: '2-digit'}).format(iUnix * 1000);
}

export function formatWeekday(iUnix, sTimezone, sLocale) {
	return getFormatter(sLocale, sTimezone, {weekday: 'long'}).format(iUnix * 1000);
}

export function formatFull(iUnix, sTimezone, sLocale) {
	return getFormatter(sLocale, sTimezone, {
		weekday: 'long', day: 'numeric', month: 'long', year: 'numeric', hour: '2-digit', minute: '2-digit'
	}).format(iUnix * 1000);
}

export function formatMonthTitle(oDate, sLocale) {
	return new Intl.DateTimeFormat(sLocale, {month: 'long', year: 'numeric'}).format(oDate);
}

/**
 * Weekday initials starting on Monday, in the reader's locale.
 */
export function getWeekdayInitials(sLocale) {
	const oFormatter = new Intl.DateTimeFormat(sLocale, {weekday: 'short'});
	const asLabels = [];

	//2024-01-01 was a Monday, which anchors the week without hard-coding names.
	for(let iDay = 0; iDay < 7; iDay++) {
		asLabels.push(oFormatter.format(new Date(Date.UTC(2024, 0, 1 + iDay))));
	}

	return asLabels;
}

/** Short "how long ago", used by the autosave indicator only. */
export function formatSince(iUnix, sLocale, sJustNow) {
	const iSeconds = Math.round(Date.now() / 1000) - iUnix;
	if(iSeconds < 45) return sJustNow;

	const oFormatter = new Intl.RelativeTimeFormat(sLocale, {numeric: 'auto'});
	if(iSeconds < 3600) return oFormatter.format(-Math.round(iSeconds / 60), 'minute');
	if(iSeconds < 86400) return oFormatter.format(-Math.round(iSeconds / 3600), 'hour');
	return oFormatter.format(-Math.round(iSeconds / 86400), 'day');
}