tibetan-date-calculator/src/conversions/julian-from-tibetan.ts

A library to calculate tibetan calendar details. It is based on Svante Janson's paper www2.math.uu.se/~svante/papers/calendars/tibet.pdf

import monthCountFromTibetan from './month-count-from-tibetan';
import getTrueDate from './true-date-from-month-count-day';

/**
 * Gives the Julian date for a Tibetan year, month number (leap or not) and Tibetan day.
 *
 * Does not check that the tibetan day actually exists:
 *  - If given the date of a skipped day, will return the same Julian date as the day before.
 *  - If given the date of a duplicate day, returns the Julian date of the second of the two.
 *
 * @param {number} year - Tibetan year
 * @param {number} month - Tibetan month
 * @param {boolean} isLeapMonth - true if leap month
 * @param {number} day - Tibetan day
 * @returns {number} - Julian date
 */
const julianFromTibetan = (year: number, month: number, isLeapMonth: boolean, day: number): number => {
  const monthCount = monthCountFromTibetan({ year, month, isLeapMonth });

  return Math.floor(getTrueDate(day, monthCount));
};

export default julianFromTibetan;