when-am-i / files / src / utils / dateRange.ts
dateRange.ts
8fa8f3cb 2026-06-21 Pavel Safronov | 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 | } |