Merge pull request #9 from goark/add-function-in-jdn

Add function in jdn package

README.md

@@ -263,7 +263,6 @@ func main() {
 
 ```
 $ go run sample/sample4.go 2022-01-01
-$ go run sample/sample4.go 2022-01-01
 Julian Day Number of 2022-01-01 is 2459580
 ```
 

jdn/jdn.go

@@ -5,27 +5,27 @@ import (
 	"time"
 )
 
-// GetJD returns Julian Day from time.Time.
+// GetJD returns Julian Date from time.Time.
 func GetJD(dt time.Time) *big.Rat {
 	dt = dt.In(time.UTC)
 	y := intRat(int64(dt.Year()))
 	m := int64(dt.Month())
 	d := int64(dt.Day())
 	k := floorRat(quoInt(intRat(14-m), 12))
-	jdn := floorRat(mulRat(addInt(subRat(y, k), 4800), fracInt(1461, 4)))
-	jdn = addRat(jdn, floorRat(mulRat(addInt(mulInt(k, 12), m-2), fracInt(367, 12))))
-	jdn = subRat(jdn, floorRat(mulRat(floorRat(quoInt(addInt(subRat(y, k), 4900), 100)), fracInt(3, 4))))
-	jdn = addInt(jdn, d-32075)
-	jdn = addRat(jdn, subRat(quoRat(addRat(intRat(int64(dt.Second()+dt.Minute()*60+dt.Hour()*3600)), fracInt(int64(dt.Nanosecond()), 999999999)), floatRat((24*time.Hour).Seconds())), floatRat(0.5)))
-	return jdn
+	j := floorRat(mulRat(addInt(subRat(y, k), 4800), fracInt(1461, 4)))
+	j = addRat(j, floorRat(mulRat(addInt(mulInt(k, 12), m-2), fracInt(367, 12))))
+	j = subRat(j, floorRat(mulRat(floorRat(quoInt(addInt(subRat(y, k), 4900), 100)), fracInt(3, 4))))
+	j = addInt(j, d-32075)
+	j = addRat(j, subRat(quoRat(addRat(intRat(int64(dt.Second()+dt.Minute()*60+dt.Hour()*3600)), fracInt(int64(dt.Nanosecond()), 999999999)), floatRat((24*time.Hour).Seconds())), floatRat(0.5)))
+	return j
 }
 
 // GetJDN returns Julian Day Number from time.Time.
 func GetJDN(dt time.Time) int64 {
 	return floorRat(GetJD(dt)).Num().Int64()
 }
 
-// GetMJD returns Modified Julian Day from time.Time.
+// GetMJD returns Modified Julian Date from time.Time.
 func GetMJD(dt time.Time) *big.Rat {
 	return subRat(GetJD(dt), floatRat(2400000.5))
 }
@@ -35,6 +35,21 @@ func GetMJDN(dt time.Time) int64 {
 	return floorRat(GetMJD(dt)).Num().Int64()
 }
 
+// FromJDN returns time.Time instance form Julian Day Number.
+func FromJD(jn int64) time.Time {
+	l := intRat(jn + 68569)
+	n := floorRat(mulInt(quoInt(l, 146097), 4))
+	l = subRat(l, floorRat(quoInt(addInt(mulInt(n, 146097), 3), 4)))
+	i := floorRat(quoInt(mulInt(addInt(l, 1), 4000), 1461001))
+	l = addInt(subRat(l, floorRat(quoInt(mulInt(i, 1461), 4))), 31)
+	j := floorRat(quoInt(mulInt(l, 80), 2447))
+	day := subRat(l, floorRat(quoInt(mulInt(j, 2447), 80)))
+	l = floorRat(quoInt(j, 11))
+	month := subRat(addInt(j, 2), mulInt(l, 12))
+	year := addRat(mulInt(addInt(n, -49), 100), addRat(i, l))
+	return time.Date(int(year.Num().Int64()), time.Month(int(month.Num().Int64())), int(day.Num().Int64()), 12, 0, 0, 0, time.UTC)
+}
+
 func intRat(x int64) *big.Rat {
 	return fracInt(x, 1)
 }

jdn/jdn_test.go

@@ -9,23 +9,28 @@ import (
 func TestGetJDN(t *testing.T) {
 	jst := time.FixedZone("JST", int((9 * time.Hour).Seconds())) // Japan standard Time
 	testCases := []struct {
-		inp   time.Time
-		outp1 *big.Rat
-		outp2 int64
-		outp3 *big.Rat
-		outp4 int64
+		inp    time.Time
+		outp1  *big.Rat
+		outp2  int64
+		outpDt time.Time
+		outp3  *big.Rat
+		outp4  int64
 	}{
-		{inp: time.Date(2015, time.January, 1, 0, 0, 0, 0, time.UTC), outp1: floatRat(2457023.5), outp2: 2457023, outp3: floatRat(57023.0), outp4: 57023},
-		{inp: time.Date(2022, time.January, 1, 0, 0, 0, 0, jst), outp1: floatRat(2459580.125), outp2: 2459580, outp3: floatRat(59579.625), outp4: 59579},
+		{inp: time.Date(2015, time.January, 1, 0, 0, 0, 0, time.UTC), outp1: floatRat(2457023.5), outp2: 2457023, outpDt: time.Date(2015, time.January, 0, 12, 0, 0, 0, time.UTC), outp3: floatRat(57023.0), outp4: 57023},
+		{inp: time.Date(2022, time.January, 1, 0, 0, 0, 0, jst), outp1: floatRat(2459580.125), outp2: 2459580, outpDt: time.Date(2022, time.January, 0, 12, 0, 0, 0, time.UTC), outp3: floatRat(59579.625), outp4: 59579},
 	}
 	for _, tc := range testCases {
 		jd := GetJD(tc.inp)
 		if jd.Cmp(tc.outp1) != 0 {
 			t.Errorf("GetJD(%v) is %v, want %v.", tc.inp, jd.FloatString(5), tc.outp1.FloatString(5))
 		}
-		jdn := GetJDN(tc.inp)
-		if jdn != tc.outp2 {
-			t.Errorf("GetJDN(%v) is %v, want %v.", tc.inp, jdn, tc.outp2)
+		jn := GetJDN(tc.inp)
+		if jn != tc.outp2 {
+			t.Errorf("GetJDN(%v) is %v, want %v.", tc.inp, jn, tc.outp2)
+		}
+		dt := FromJD(jn)
+		if !dt.Equal(tc.outpDt) {
+			t.Errorf("FromJD(%v) is %v, want %v.", jn, dt, tc.outpDt)
 		}
 		mjd := GetMJD(tc.inp)
 		if mjd.Cmp(tc.outp3) != 0 {