when-am-i / files / src / utils / historicalDate.ts
historicalDate.ts
| 1 | // Bridge between the date-picker UI and the engine's HistoricalDate. |
| 2 | // |
| 3 | // The picker works in the way people actually say dates — a positive year plus |
| 4 | // a BCE/CE choice — while the engine uses astronomical year numbering |
| 5 | // (year 0 = 1 BCE). These helpers convert between the two and validate ranges. |
| 6 | |
| 7 | import type { HistoricalDate } from '../types'; |
| 8 | |
| 9 | export type Era = 'CE' | 'BCE'; |
| 10 | |
| 11 | /** A date as entered in the picker: a positive year plus an era. */ |
| 12 | export interface PickerDate { |
| 13 | year: number; // >= 1 |
| 14 | month: number; // 1–12 |
| 15 | day: number; // 1–31 |
| 16 | era: Era; |
| 17 | } |
| 18 | |
| 19 | /** Whether a proleptic-Gregorian year is a leap year. */ |
| 20 | export function isLeapYear(year: number): boolean { |
| 21 | return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0; |
| 22 | } |
| 23 | |
| 24 | /** Days in a proleptic-Gregorian month (astronomical year numbering). */ |
| 25 | export function daysInMonth(year: number, month: number): number { |
| 26 | if (month === 2) return isLeapYear(year) ? 29 : 28; |
| 27 | return [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month - 1] ?? 31; |
| 28 | } |
| 29 | |
| 30 | /** Days in a proleptic-Gregorian year (365 or 366). */ |
| 31 | export function daysInYear(year: number): number { |
| 32 | return isLeapYear(year) ? 366 : 365; |
| 33 | } |
| 34 | |
| 35 | /** 1-based ordinal day within the year for a (month, day). */ |
| 36 | export function dayOfYear(year: number, month: number, day: number): number { |
| 37 | let doy = day; |
| 38 | for (let m = 1; m < month; m++) doy += daysInMonth(year, m); |
| 39 | return doy; |
| 40 | } |
| 41 | |
| 42 | /** Convert a 1-based ordinal day-of-year back to a (month, day), clamped. */ |
| 43 | export function fromDayOfYear(year: number, doy: number): { month: number; day: number } { |
| 44 | let remaining = Math.max(1, Math.min(Math.round(doy), daysInYear(year))); |
| 45 | let month = 1; |
| 46 | while (month < 12 && remaining > daysInMonth(year, month)) { |
| 47 | remaining -= daysInMonth(year, month); |
| 48 | month++; |
| 49 | } |
| 50 | return { month, day: remaining }; |
| 51 | } |
| 52 | |
| 53 | /** Convert a picker date to the engine's astronomical HistoricalDate. */ |
| 54 | export function toHistoricalDate({ year, month, day, era }: PickerDate): HistoricalDate { |
| 55 | // BCE uses astronomical numbering: 1 BCE = 0, 2 BCE = -1, … |
| 56 | const astronomicalYear = era === 'BCE' ? 1 - year : year; |
| 57 | return { year: astronomicalYear, month, day }; |
| 58 | } |
| 59 | |
| 60 | /** Convert an engine HistoricalDate back to picker form. */ |
| 61 | export function fromHistoricalDate({ year, month, day }: HistoricalDate): PickerDate { |
| 62 | return year > 0 |
| 63 | ? { year, month, day, era: 'CE' } |
| 64 | : { year: 1 - year, month, day, era: 'BCE' }; |
| 65 | } |
| 66 | |
| 67 | /** Clamp a picker date to a valid day-of-month (e.g. after changing month). */ |
| 68 | export function clampDay(date: PickerDate): PickerDate { |
| 69 | const astronomicalYear = date.era === 'BCE' ? 1 - date.year : date.year; |
| 70 | const max = daysInMonth(astronomicalYear, date.month); |
| 71 | return date.day > max ? { ...date, day: max } : date; |
| 72 | } |