ck/julian/julian.go

package julian

import "time"

// TODO: describe how time doesn't go past BCE, and therefore we don't either
// Keep in mind, Go's time.Time gregorian dates are `proleptic`, ie. there is
// no need to give special care to dates before 15. October 1582.
type Date struct {
	year  int
	month time.Month
	day   int
}

func greaterOrEqual(a, b Date) bool {
	if a.year != b.year {
		return a.year > b.year
	} else if a.month != b.month {
		return a.month > b.month
	} else {
		return a.day >= b.day
	}
}

func monthLength(year int, month time.Month) int {
	lengths := map[time.Month]int{
		time.January:   31,
		time.March:     31,
		time.April:     30,
		time.May:       31,
		time.June:      30,
		time.July:      31,
		time.August:    31,
		time.September: 30,
		time.October:   31,
		time.November:  30,
		time.December:  31,
	}
	if _, ok := lengths[month]; ok {
		return lengths[month]
	}
	// February
	if year%4 == 0 {
		return 29
	}
	return 28
}

func dateDiff(d Date, days int) Date {
	// keep in mind throughout this code that the original "days" value can
	// be negative
	var fourYearPeriods int
	var periodLen int = (3 * 365) + 366
	// a difference which is an exact multiple of 4 years can be dealt with
	// easily, so we only have to take greater care with the remainder
	fourYearPeriods, days = days/periodLen, days%periodLen
	// this `if' will happen if days was originally <0, AND is not an exact
	// multiple of periodLen
	if days < 0 {
		fourYearPeriods -= 1
		days += periodLen
	}
	// now days must be 0 <= days < periodLen, so our job is easier
	// first we shift in the big periods
	d.year += fourYearPeriods * 4

	// We shift the date to the first of the month to simplify our
	// calculation
	days += d.day - 1
	d.day = 1
	// since days < periodLen+31-1, we will go through this loop at most
	// 4*12+1 = 49 times
	for days >= monthLength(d.year, d.month) {
		days -= monthLength(d.year, d.month)
		d.year, d.month = nextMonth(d.year, d.month)
	}
	d.day += days

	return d
}

func nextMonth(year int, month time.Month) (int, time.Month) {
	if month == time.December {
		return year + 1, time.January
	} else {
		return year, month + 1
	}
}

func New(year int, month time.Month, day int) Date {
	// TODO: deal with users giving negative dates
	for day > monthLength(year, month) {
		day -= monthLength(year, month)
		year, month = nextMonth(year, month)
	}
	return Date{year, month, day}
}

// The algorithm, in large part: follows this article:
// https://en.wikipedia.org/wiki/Conversion_between_Julian_and_Gregorian_calendars
func ToTime(d Date) time.Time {
	offset := -2
	borderDate := Date{100, time.March, 2}
	// We don't increase the offset for years divisible by 400, ie. every
	// fourth loop, we keep track of that with skipYear, we can't simply
	// check borderDate.year % 400, because when we reach 31. December
	// 8299, the year will not be divisible by 100 anymore
	skipYear := 1
	for greaterOrEqual(d, borderDate) {
		// We can simply add 100 to the year because in the julian
		// calendar, if a date exists in year y, it always exists in
		// year y+100
		borderDate.year += 100
		if skipYear%4 != 0 {
			borderDate = dateDiff(borderDate, -1)
			offset++
		}
		skipYear++
	}

	d = dateDiff(d, offset)
	return time.Date(d.year, d.month, d.day, 0, 0, 0, 0, time.UTC)
}