when-am-i / files / src / utils / dateRange.ts

dateRange.ts

1	// Derives the year-slider's lower bound from the loaded calendar data, so the
2	// timeline doesn't waste most of its width on millennia with no eras plotted.
3	//
4	// Only eras with an explicit numeric `start` count — `start: null` means "since
5	// the earliest time" in the schema (e.g. the Hebrew or Ethiopian calendars'
6	// traditional epochs run far earlier, but we never invent a date for them), so
7	// those eras stay open-ended rather than pushing the floor back further.
8
9	import type { CalendarRegion } from '../types';
10	import { parseGregorianDate } from './julianDay';
11
12	const FALLBACK_MIN_YEAR = -1999; // 2000 BCE, used if no era has a dated start.
13	const MARGIN_FRACTION = 0.1; // headroom before the oldest era, as a fraction of its span to "now".
14
15	/**
16	* The earliest astronomical year any era explicitly starts in `regions`, minus
17	* a margin (MARGIN_FRACTION of the span from that year to `nowYear`), so the
18	* oldest era isn't pinned at the very edge of the slider. Falls back to
19	* FALLBACK_MIN_YEAR if no era has a dated (non-null) start.
20	*/
21	export function deriveMinYear(regions: CalendarRegion[], nowYear: number): number {
22	let oldest: number \| undefined;
23	for (const { eras } of regions) {
24	for (const era of eras) {
25	if (!era.start) continue;
26	const year = parseGregorianDate(era.start).year;
27	if (oldest === undefined \|\| year < oldest) oldest = year;
28	}
29	}
30	if (oldest === undefined) return FALLBACK_MIN_YEAR;
31
32	const margin = (nowYear - oldest) * MARGIN_FRACTION;
33	return Math.floor(oldest - margin);
34	}