src/hebrew/utils/dateConversion.ts

import {
  daysInHebrewMonth,
  monthsInHebrewYear,
  roshHashanah,
} from "./calendarMath.js";
import { GREGORIAN_EPOCH, type HebrewDate, MS_PER_DAY } from "./constants.js";

/** Convert a Gregorian date to an absolute day number from the epoch. */
function dateToAbsolute(date: Date): number {
  // Years < 100 must use UTC components to avoid JS's 1900 offset; for normal
  // years keep local components so we don't reintroduce UTC/local skew in the
  // rendered month grid.
  const useUTC = date.getFullYear() < 100;
  const year = useUTC ? date.getUTCFullYear() : date.getFullYear();
  const month = useUTC ? date.getUTCMonth() : date.getMonth();
  const day = useUTC ? date.getUTCDate() : date.getDate();
  const normalized = new Date(0);
  normalized.setUTCFullYear(year, month, day);
  normalized.setUTCHours(0, 0, 0, 0);
  return Math.floor((normalized.getTime() - GREGORIAN_EPOCH) / MS_PER_DAY) + 1;
}

/** Convert an absolute day number back to a Gregorian date. */
function absoluteToDate(absolute: number): Date {
  const utc = new Date(GREGORIAN_EPOCH + (absolute - 1) * MS_PER_DAY);
  const result = new Date(0);
  result.setFullYear(utc.getUTCFullYear(), utc.getUTCMonth(), utc.getUTCDate());
  result.setHours(0, 0, 0, 0);
  return result;
}

/** Convert a Hebrew date to an absolute day number so it can be compared. */
function absoluteFromHebrew({ year, monthIndex, day }: HebrewDate): number {
  let days = day - 1;
  for (let index = 0; index < monthIndex; index += 1) {
    days += daysInHebrewMonth(year, index);
  }
  return roshHashanah(year) + days;
}

/** Convert an absolute day number to the equivalent Hebrew date. */
function hebrewFromAbsolute(absolute: number): HebrewDate {
  const date = new Date(GREGORIAN_EPOCH + (absolute - 1) * MS_PER_DAY);
  let year = date.getUTCFullYear() + 3760;
  if (date.getUTCMonth() >= 8) {
    year += 1;
  }

  while (absolute >= roshHashanah(year + 1)) {
    year += 1;
  }
  while (absolute < roshHashanah(year)) {
    year -= 1;
  }

  let dayOfYear = absolute - roshHashanah(year);
  const monthCount = monthsInHebrewYear(year);
  let monthIndex = 0;
  while (monthIndex < monthCount) {
    const monthDays = daysInHebrewMonth(year, monthIndex);
    if (dayOfYear < monthDays) {
      break;
    }
    dayOfYear -= monthDays;
    monthIndex += 1;
  }

  return {
    year,
    monthIndex,
    day: dayOfYear + 1,
  };
}

/** Converts a Gregorian date to the corresponding Hebrew date. */
export function toHebrewDate(date: Date): HebrewDate {
  return hebrewFromAbsolute(dateToAbsolute(date));
}

/** Converts a Hebrew date back to the Gregorian calendar. */
export function toGregorianDate(hebrew: HebrewDate): Date {
  return absoluteToDate(absoluteFromHebrew(hebrew));
}