125 lines
3.2 KiB
Go
125 lines
3.2 KiB
Go
// Package julianday provides Time to Julian day conversions.
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package julianday
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import (
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"bytes"
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"errors"
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"math"
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"strconv"
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"time"
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)
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const secs_per_day = 86_400
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const nsec_per_sec = 1_000_000_000
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const nsec_per_day = nsec_per_sec * secs_per_day
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const epoch_days = 2_440_587
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const epoch_secs = secs_per_day / 2
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func jd(t time.Time) (day, nsec int64) {
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sec := t.Unix()
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// guaranteed not to overflow
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day, sec = sec/secs_per_day+epoch_days, sec%secs_per_day+epoch_secs
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return day, sec*nsec_per_sec + int64(t.Nanosecond())
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}
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// Date returns the Julian day number for t,
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// and the nanosecond offset within that day,
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// in the range [0, 86399999999999].
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func Date(t time.Time) (day, nsec int64) {
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day, nsec = jd(t)
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switch {
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case nsec < 0:
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day -= 1
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nsec += nsec_per_day
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case nsec >= nsec_per_day:
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day += 1
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nsec -= nsec_per_day
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}
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return day, nsec
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}
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// Float returns the Julian date for t as a float64.
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//
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// In the XXI century, this has submillisecond precision.
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func Float(t time.Time) float64 {
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day, nsec := jd(t)
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// converting day and nsec to float64 is exact
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return float64(day) + float64(nsec)/nsec_per_day
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}
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// Format returns the Julian date for t as a string.
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//
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// This has nanosecond precision.
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func Format(t time.Time) string {
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var buf [32]byte
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return string(AppendFormat(buf[:0], t))
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}
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// AppendFormat is like Format but appends the textual representation to dst
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// and returns the extended buffer.
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func AppendFormat(dst []byte, t time.Time) []byte {
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day, nsec := Date(t)
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if day < 0 && nsec != 0 {
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dst = append(dst, '-')
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day = ^day
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nsec = nsec_per_day - nsec
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}
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var buf [20]byte
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dst = strconv.AppendInt(dst, day, 10)
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frac := strconv.AppendFloat(buf[:0], float64(nsec)/nsec_per_day, 'f', 15, 64)
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return append(dst, bytes.TrimRight(frac[1:], ".0")...)
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}
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// Time returns the UTC Time corresponding to the Julian day number
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// and nanosecond offset within that day.
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// Not all day values have a corresponding time value.
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func Time(day, nsec int64) time.Time {
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return time.Unix((day-epoch_days)*secs_per_day-epoch_secs, nsec).UTC()
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}
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// FloatTime returns the UTC Time corresponding to a Julian date.
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// Not all date values have a corresponding time value.
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//
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// In the XXI century, this has submillisecond precision.
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func FloatTime(date float64) time.Time {
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day, frac := math.Modf(date)
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nsec := math.Floor(frac * nsec_per_day)
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return Time(int64(day), int64(nsec))
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}
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// Parse parses a formatted Julian date and returns the UTC Time it represents.
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//
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// This has nanosecond precision.
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func Parse(s string) (time.Time, error) {
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digits := 0
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dot := len(s)
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for i, b := range []byte(s) {
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if '0' <= b && b <= '9' {
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digits++
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continue
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}
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if b == '.' && i < dot {
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dot = i
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continue
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}
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if (b == '+' || b == '-') && i == 0 {
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continue
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}
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return time.Time{}, errors.New("julianday: invalid syntax")
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}
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if digits == 0 {
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return time.Time{}, errors.New("julianday: invalid syntax")
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}
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day, err := strconv.ParseInt(s[:dot], 10, 64)
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if err != nil && dot > 0 {
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return time.Time{}, errors.New("julianday: value out of range")
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}
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frac, _ := strconv.ParseFloat(s[dot:], 64)
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nsec := int64(math.Round(frac * nsec_per_day))
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if s[0] == '-' {
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nsec = -nsec
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}
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return Time(day, nsec), nil
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}
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