jalaliday/src/calendar.ts

Persian (Jalali, Khorshidi) Plugin for Day.js

/*
 JavaScript functions for the Fourmilab Calendar Converter
 by John Walker  --  September, MIM
 http://www.fourmilab.ch/documents/calendar/
 This program is in the public domain.
 */

/*  MOD  --  Modulus function which works for non-integers.  */
const $floor = Math.floor
function mod(a, b) {
  return a - (b * $floor(a / b))
}

//  LEAP_GREGORIAN  --  Is a given year in the Gregorian calendar a leap year ?

function lg(year) {
  return ((year % 4) === 0)
    && (!(((year % 100) === 0) && ((year % 400) !== 0)))
}

//  GREGORIAN_TO_JD  --  Determine Julian day number from Gregorian calendar date

// GREGORIAN_EPOCH
const GE = 1721425.5

function g2j(year, month, day) {
  return (GE - 1)
    + (365 * (year - 1))
    + $floor((year - 1) / 4)
    + (-$floor((year - 1) / 100))
    + $floor((year - 1) / 400)
    + $floor((((367 * month) - 362) / 12)
      + ((month <= 2)
        ? 0
        : (lg(year) ? -1 : -2)
      )
      + day)
}

//  JD_TO_GREGORIAN  --  Calculate Gregorian calendar date from Julian day

function j2g(jd) {
  const wjd = $floor(jd - 0.5) + 0.5
  const depoch = wjd - GE
  const quadricent = $floor(depoch / 146097)
  const dqc = mod(depoch, 146097)

  const cent = $floor(dqc / 36524)
  const dcent = mod(dqc, 36524)
  const quad = $floor(dcent / 1461)
  const dquad = mod(dcent, 1461)
  const yindex = $floor(dquad / 365)
  let year = (quadricent * 400) + (cent * 100) + (quad * 4) + yindex
  if (!((cent === 4) || (yindex === 4))) {
    year++
  }
  const yearday = wjd - g2j(year, 1, 1)
  const leapadj = ((wjd < g2j(year, 3, 1))
    ? 0
    : (lg(year) ? 1 : 2)
  )
  const month = $floor((((yearday + leapadj) * 12) + 373) / 367)
  const day = (wjd - g2j(year, month, 1)) + 1

  return [year, month, day]
}

// PERSIAN_EPOCH
const PE = 1948320.5

//  PERSIAN_TO_JD  --  Determine Julian day from Persian date

function p2j(year, month, day) {
  const epbase = year - ((year >= 0) ? 474 : 473)
  const epyear = 474 + mod(epbase, 2820)

  return day
    + ((month <= 7)
      ? ((month - 1) * 31)
      : (((month - 1) * 30) + 6)
    )
    + $floor(((epyear * 682) - 110) / 2816)
    + (epyear - 1) * 365
    + $floor(epbase / 2820) * 1029983
    + (PE - 1)
}

//  JD_TO_PERSIAN  --  Calculate Persian date from Julian day

function j2p(jd) {
  let year, ycycle, aux1, aux2

  jd = $floor(jd) + 0.5

  const depoch = jd - p2j(475, 1, 1)
  const cycle = $floor(depoch / 1029983)
  const cyear = mod(depoch, 1029983)
  if (cyear === 1029982) {
    ycycle = 2820
  }
  else {
    aux1 = $floor(cyear / 366)
    aux2 = mod(cyear, 366)
    ycycle = $floor(((2134 * aux1) + (2816 * aux2) + 2815) / 1028522)
      + aux1 + 1
  }
  year = ycycle + (2820 * cycle) + 474
  if (year <= 0) {
    year--
  }
  const yday = (jd - p2j(year, 1, 1)) + 1
  const month = (yday <= 186) ? Math.ceil(yday / 31) : Math.ceil((yday - 6) / 30)
  const day = (jd - p2j(year, month, 1)) + 1
  return [year, month, day]
}

export default {
  J: (y, m, d) => j2p(g2j(y, m, d)),
  G: (y, m, d) => j2g(p2j(y, m, d)),
}