99 lines
3.1 KiB
Go
99 lines
3.1 KiB
Go
package xls
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import (
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"math"
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"time"
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)
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const MJD_0 float64 = 2400000.5
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const MJD_JD2000 float64 = 51544.5
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func shiftJulianToNoon(julianDays, julianFraction float64) (float64, float64) {
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switch {
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case -0.5 < julianFraction && julianFraction < 0.5:
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julianFraction += 0.5
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case julianFraction >= 0.5:
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julianDays += 1
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julianFraction -= 0.5
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case julianFraction <= -0.5:
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julianDays -= 1
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julianFraction += 1.5
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}
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return julianDays, julianFraction
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}
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// Return the integer values for hour, minutes, seconds and
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// nanoseconds that comprised a given fraction of a day.
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func fractionOfADay(fraction float64) (hours, minutes, seconds, nanoseconds int) {
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f := 5184000000000000 * fraction
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nanoseconds = int(math.Mod(f, 1000000000))
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f = f / 1000000000
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seconds = int(math.Mod(f, 60))
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f = f / 3600
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minutes = int(math.Mod(f, 60))
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f = f / 60
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hours = int(f)
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return hours, minutes, seconds, nanoseconds
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}
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func julianDateToGregorianTime(part1, part2 float64) time.Time {
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part1I, part1F := math.Modf(part1)
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part2I, part2F := math.Modf(part2)
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julianDays := part1I + part2I
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julianFraction := part1F + part2F
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julianDays, julianFraction = shiftJulianToNoon(julianDays, julianFraction)
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day, month, year := doTheFliegelAndVanFlandernAlgorithm(int(julianDays))
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hours, minutes, seconds, nanoseconds := fractionOfADay(julianFraction)
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return time.Date(year, time.Month(month), day, hours, minutes, seconds, nanoseconds, time.UTC)
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}
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// By this point generations of programmers have repeated the
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// algorithm sent to the editor of "Communications of the ACM" in 1968
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// (published in CACM, volume 11, number 10, October 1968, p.657).
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// None of those programmers seems to have found it necessary to
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// explain the constants or variable names set out by Henry F. Fliegel
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// and Thomas C. Van Flandern. Maybe one day I'll buy that jounal and
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// expand an explanation here - that day is not today.
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func doTheFliegelAndVanFlandernAlgorithm(jd int) (day, month, year int) {
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l := jd + 68569
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n := (4 * l) / 146097
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l = l - (146097*n+3)/4
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i := (4000 * (l + 1)) / 1461001
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l = l - (1461*i)/4 + 31
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j := (80 * l) / 2447
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d := l - (2447*j)/80
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l = j / 11
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m := j + 2 - (12 * l)
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y := 100*(n-49) + i + l
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return d, m, y
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}
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// Convert an excelTime representation (stored as a floating point number) to a time.Time.
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func timeFromExcelTime(excelTime float64, date1904 bool) time.Time {
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var date time.Time
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var intPart int64 = int64(excelTime)
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// Excel uses Julian dates prior to March 1st 1900, and
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// Gregorian thereafter.
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if intPart <= 61 {
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const OFFSET1900 = 15018.0
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const OFFSET1904 = 16480.0
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var date time.Time
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if date1904 {
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date = julianDateToGregorianTime(MJD_0+OFFSET1904, excelTime)
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} else {
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date = julianDateToGregorianTime(MJD_0+OFFSET1900, excelTime)
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}
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return date
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}
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var floatPart float64 = excelTime - float64(intPart)
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var dayNanoSeconds float64 = 24 * 60 * 60 * 1000 * 1000 * 1000
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if date1904 {
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date = time.Date(1904, 1, 1, 0, 0, 0, 0, time.UTC)
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} else {
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date = time.Date(1899, 12, 30, 0, 0, 0, 0, time.UTC)
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}
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durationDays := time.Duration(intPart) * time.Hour * 24
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durationPart := time.Duration(dayNanoSeconds * floatPart)
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return date.Add(durationDays).Add(durationPart)
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}
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