// JDToCalendar returns the calendar date for the given jd.
//
// Note that this function returns a date in either the Julian or Gregorian
// Calendar, as appropriate.
func JDToCalendar(jd float64) (year, month int, day float64) {
zf, f := math.Modf(jd + .5)
z := int64(zf)
a := z
if z >= 2299151 {
α := base.FloorDiv64(z*100-186721625, 3652425)
a = z + 1 + α - base.FloorDiv64(α, 4)
}
b := a + 1524
c := base.FloorDiv64(b*100-12210, 36525)
d := base.FloorDiv64(36525*c, 100)
e := int(base.FloorDiv64((b-d)*1e4, 306001))
// compute return values
day = float64(int(b-d)-base.FloorDiv(306001*e, 1e4)) + f
switch e {
default:
month = e - 1
case 14, 15:
month = e - 13
}
switch month {
default:
year = int(c) - 4716
case 1, 2:
year = int(c) - 4715
}
return
}
// CalendarJulianToJD converts a Julian year, month, and day of month to Julian day.
//
// Negative years are valid, back to JD 0. The result is not valid for
// dates before JD 0.
func CalendarJulianToJD(y, m int, d float64) float64 {
switch m {
case 1, 2:
y--
m += 12
}
return float64(base.FloorDiv64(36525*(int64(y+4716)), 100)) +
float64(base.FloorDiv(306*(m+1), 10)) + d - 1524.5
}
// CalendarGregorianToJD converts a Gregorian year, month, and day of month
// to Julian day.
//
// Negative years are valid, back to JD 0. The result is not valid for
// dates before JD 0.
func CalendarGregorianToJD(y, m int, d float64) float64 {
switch m {
case 1, 2:
y--
m += 12
}
a := base.FloorDiv(y, 100)
b := 2 - a + base.FloorDiv(a, 4)
// (7.1) p. 61
return float64(base.FloorDiv64(36525*(int64(y+4716)), 100)) +
float64(base.FloorDiv(306*(m+1), 10)+b) + d - 1524.5
}
func ExampleFloorDiv64() {
// compare to / operator examples in Go spec at
// https://golang.org/ref/spec#Arithmetic_operators
fmt.Println(base.FloorDiv64(+5, +3))
fmt.Println(base.FloorDiv64(-5, +3))
fmt.Println(base.FloorDiv64(+5, -3))
fmt.Println(base.FloorDiv64(-5, -3))
fmt.Println()
// exact divisors, no remainders
fmt.Println(base.FloorDiv64(+6, +3))
fmt.Println(base.FloorDiv64(-6, +3))
fmt.Println(base.FloorDiv64(+6, -3))
fmt.Println(base.FloorDiv64(-6, -3))
// Output:
// 1
// -2
// -2
// 1
//
// 2
// -2
// -2
// 2
}