path: root/content/cal/lunar.js

// Part 1: Converter functions

// Sin function with degree instead of radian
function sind(degree) {
	return Math.sin(degree * Math.PI / 180)
}

// Convert Gregorian date to Julian day
function getJulianDay(date) {
	const year = date.getFullYear()
	const month = date.getMonth() + 1
	const day = date.getDate()

	let a = parseInt((14 - month) / 12)
	let y = year + 4800 - a
	y = y * 365 + parseInt(y / 4) - parseInt(y / 100) + parseInt(y / 400)
	let m = month + 12 * a - 3
	return parseInt(day + (153 * m + 2) / 5) + y - 32045
}

// Convert Julian day to Gregorian date
function getDateFromJulianDay(jd) {
	const a = jd + 32044
	const b = parseInt((4 * a + 3) / 146097)
	const c = a - parseInt((b * 146097) / 4)
	const d = parseInt((4 * c + 3) / 1461)
	const e = c - parseInt((1461 * d) / 4)
	const m = parseInt((5 * e + 2) / 153)
	const dd = e - parseInt((153 * m + 2) / 5) + 1
	const mm = m + 2 - 12 * parseInt(m / 10)
	const yy = b * 100 + d - 4800 + parseInt(m / 10)
	return new Date(yy, mm, dd)
}

// Get kth new moon day since 1900-01-01 in Julian day
function getNewMoonDay(k, tzOffset) {
	const t = k / 1236.85
	const t2 = t * t
	const t3 = t2 * t

	let jd1 = 2415020.75933 + 29.53058868 * k + 0.0001178 * t2 - 0.000000155 * t3
	jd1 += 0.00033 * sind(166.56 + 132.87 * t - 0.009173 * t2)

	const m = 359.2242 + 29.10535608 * k - 0.0000333 * t2 - 0.00000347 * t3
	const mpr = 306.0253 + 385.81691806 * k + 0.0107306 * t2 + 0.00001236 * t3
	const f = 21.2964 + 390.67050646 * k - 0.0016528 * t2 - 0.00000239 * t3

	let c1 = (0.1734 - 0.000393 * t) * sind(m) + 0.0021 * sind(2 * m)
	c1 -= 0.4068 * sind(mpr) + 0.0161 * sind(2 * mpr)
	c1 -= 0.0004 * sind(3 * mpr)
	c1 += 0.0104 * sind(2 * f) - 0.0051 * sind(m + mpr)
	c1 -= 0.0074 * sind(m - mpr) + 0.0004 * sind(2 * f + m)
	c1 -= 0.0004 * sind(2 * f - m) - 0.0006 * sind(2 * f + mpr)
	c1 += 0.0010 * sind(2 * f - mpr) + 0.0005 * sind(2 * mpr + m)

	let dt
	if (t < -11) {
		dt = 0.001 + 0.000839 * t + 0.0002261 * t2 - 0.00000845 * t3 - 0.000000081 * t * t3
	} else {
		dt = -0.000278 + 0.000265 * t + 0.000262 * t2
	}
	let jdNew = jd1 + c1 - dt
	return parseInt(jdNew + 0.5 + tzOffset / 24)
}

// Get solar term from Julian day, from 0 to 23, corresponding to Chunfen to Jingzhe.
function getSolarTerm(jd, tzOffset) {
	t = (jd - 2451545.5 - tzOffset / 24) / 36525
	t2 = t * t
	m = 357.52910 + 35999.05030 * t - 0.0001559 * t2 - 0.00000048 * t * t2
	l0 = 280.46645 + 36000.76983 * t + 0.0003032 * t2
	dl = (1.914600 - 0.004817 * t - 0.000014 * t2) * sind(m)
	dl = dl + (0.019993 - 0.000101 * t) * sind(2 * m) + 0.000290 * sind(3 * m)
	l = l0 + dl
	l %= 360  // normalize to (0, 360)
	return parseInt(l * 24 / 360)
}

// Get the solar longitude of the day, from 0 to 11.
// 0 is corresponding to 0 degree to 30 degree and solar term Chunfen (spring equinox) - Guyu
// 1 is corresponding to 30 degree to 60 degree and solar term Guyu - Xiaoman
// 2 is corresponding to 60 degree to 90 degree and solar term Xiaoman - Xiazhi (summer solstince)
// etc
function getSunLongitude(jd, tzOffset) {
	t = (jd - 2451545.5 - tzOffset / 24) / 36525
	t2 = t * t
	m = 357.52910 + 35999.05030 * t - 0.0001559 * t2 - 0.00000048 * t * t2
	l0 = 280.46645 + 36000.76983 * t + 0.0003032 * t2
	dl = (1.914600 - 0.004817 * t - 0.000014 * t2) * sind(m)
	dl = dl + (0.019993 - 0.000101 * t) * sind(2 * m) + 0.000290 * sind(3 * m)
	l = l0 + dl
	l %= 360  // normalize to (0, 360 deg)
	return parseInt(l * 12 / 360)
}

// Get Julian day for the first day of 11th lunar month of the year.
function getMonth11(year, tzOffset) {
	off = getJulianDay(new Date(year, 11, 31)) - 2415021
	k = parseInt(off / 29.530588853)
	nm = getNewMoonDay(k, tzOffset)
	sun_long = getSunLongitude(nm, tzOffset)
	if (sun_long >= 9) {
		nm = getNewMoonDay(k-1, tzOffset)
	}
	return nm
}

// Get the leap month offset from the 11th month before it.
//
// a11: Julian day of the first day of the 11th month
function getLeapMonthOffset(a11, tzOffset) {
	k = parseInt((a11 - 2415021.076998695) / 29.530588853 + 0.5)
	i = 1
	arc = getSunLongitude(getNewMoonDay(k + i, tzOffset), tzOffset)
	last = 0
	while (arc != last && i < 14) {
		last = arc
		i++
		arc = getSunLongitude(getNewMoonDay(k + i, tzOffset), tzOffset)
	}
	return i - 1
}

function convertSolarToLunar(date, tzOffset) {
	const jd = getJulianDay(date)

	let k = parseInt((jd - 2415021.076998695) / 29.530588853)
	let monthStart = getNewMoonDay(k + 1, tzOffset)
	if (monthStart > jd) {
		monthStart = getNewMoonDay(k, tzOffset)
	}
	let a11 = getMonth11(date.getFullYear(), tzOffset)
	let b11 = a11
	let lunarYear
	if (a11 >= monthStart) {
		lunarYear = date.getFullYear()
		a11 = getMonth11(lunarYear - 1, tzOffset)
	} else {
		lunarYear = date.getFullYear() + 1
		b11 = getMonth11(lunarYear, tzOffset)
	}
	let lunarDay = jd - monthStart + 1
	let diff = parseInt((monthStart - a11) / 29)
	let isLeapMonth = false
	let lunarMonth = diff + 11
	if (b11 - a11 > 365) {
		leapMonthDiff = getLeapMonthOffset(a11, tzOffset)
		if (diff >= leapMonthDiff) {
			lunarMonth = diff + 10
			if (diff == leapMonthDiff) {
				isLeapMonth = true
			}
		}
	}
	if (lunarMonth > 12) {
		lunarMonth -= 12
	}
	if (lunarMonth >= 11 && diff < 4) {
		lunarYear -= 1
	}
	return {
		year: lunarYear,
		month: lunarMonth,
		day: lunarDay,
		isLeap: isLeapMonth,
		jd: jd
	}
}

CAN_VI = ['Giáp', 'Ất', 'Bính', 'Đinh', 'Mậu', 'Kỷ', 'Canh', 'Tân', 'Nhâm', 'Quý']
CAN_ZH = []
CHI_VI = ['Tý', 'Sửu', 'Dần', 'Mão', 'Thìn', 'Tỵ', 'Ngọ', 'Mùi', 'Thân', 'Dậu', 'Tuất', 'Hợi']
CHI_ZH = []

// Get can/chi of a lunar date
function getZodiac({year, month, day, jd}) {
	let yearZodiac = {
		can: (year + 6) % 10,
		chi: (year + 8) % 12
	}
	let monthZodiac = {
		can: (year * 12 + month + 3) % 10,
		chi: (month + 1) % 12
	}
	let dayZodiac = {
		can: (jd + 9) % 10,
		chi: (jd + 1) % 12
	}
	return {yearZodiac, monthZodiac, dayZodiac}
}

function getZodiacText(zodiac, lang) {
	switch (lang) {
		case "vi":
			return CAN_VI[zodiac.can] + " " + CHI_VI[zodiac.chi]
		case "zh":
			return CAN_ZH[zodiac.can] + CHI_ZH[zodiac.chi]
		default:
			throw("Unsupported")
	}
}

// Part 2: Update DOM

// DOM
const dateInput = document.querySelector('#solar-date')
const solarYearOutput = document.querySelector('#solar-year')
const solarMonthOutputEN = document.querySelector('#solar-month-en')
const solarMonthOutputVI = document.querySelector('#solar-month-vi')
const solarDayOutput = document.querySelector('#solar-day')
const solarWeekdayOutputEN = document.querySelector('#solar-weekday-en')
const solarWeekdayOutputVI = document.querySelector('#solar-weekday-vi')
const solarWeekdayOutputZH = document.querySelector('#solar-weekday-zh')
const lunarYearOutputVI = document.querySelector('#lunar-year-vi')
const lunarMonthOutputVI = document.querySelector('#lunar-month-vi')
const lunarDayOutputVI = document.querySelector('#lunar-day-vi')
const lunarYearOutputZH = document.querySelector('#lunar-year-zh')
const lunarMonthOutputZH = document.querySelector('#lunar-month-zh')
const lunarDayOutputZH = document.querySelector('#lunar-day-zh')

const monthFormatEN = new Intl.DateTimeFormat("en-US", {month: "long"})
const monthFormatVI = new Intl.DateTimeFormat("vi-VN", {month: "long"})
const weekdayFormatEN = new Intl.DateTimeFormat("en-US", {weekday: "long"})
const weekdayFormatVI = new Intl.DateTimeFormat("vi-VN", {weekday: "long"})
const weekdayFormatZH = new Intl.DateTimeFormat("zh-CN", {weekday: "long"})

const LUNAR_MONTH_VI = ["một", "hai", "ba", "tư", "năm", "sáu", "bảy", "tám", "chín", "mười", "mười một", "chạp"]

function updateOutputs() {
	if (dateInput.valueAsDate === null) {
		dateInput.valueAsDate = new Date()
	}
	const date = dateInput.valueAsDate
	const tzOffset = 7
	solarYearOutput.innerText = date.getFullYear()
	solarMonthOutputEN.innerText = monthFormatEN.format(date)
	solarMonthOutputVI.innerText = monthFormatVI.format(date)
	solarDayOutput.innerText = date.getDate()
	solarWeekdayOutputEN.innerText = weekdayFormatEN.format(date)
	solarWeekdayOutputVI.innerText = weekdayFormatVI.format(date)
	solarWeekdayOutputZH.innerText = weekdayFormatZH.format(date)

	const lunarDate = convertSolarToLunar(date, tzOffset)
	const zodiac = getZodiac(lunarDate)
	lunarYearOutputVI.innerText = "Năm " + getZodiacText(zodiac.yearZodiac, "vi")
	lunarMonthOutputVI.innerText = "Tháng " + LUNAR_MONTH_VI[lunarDate.month - 1]
	if (lunarDate.isLeap) {
		lunarMonthOutputVI.innerText += " (nhuận)"
	}
	lunarDayOutputVI.innerText = "Ngày " + lunarDate.day
}

document.addEventListener("DOMContentLoaded", updateOutputs)

dateInput.onchange = updateOutputs