adds initial library with tests and readme file

Author: isaric

.gitignore

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+.idea
+*.iml

README.md

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+# go-posix-time
+
+This is a small library whose purpose is to format a Go time.Time struct into a POSIX.1 TZ string.
+
+Since the POSIX timezone string contains information about transitions between DST and Standard time
+zones, it was not possible to implement such a format before Go 1.19 when the ZoneBounds struct became
+available.
+
+The library was created because the standard time format function of Go cannot produce this format.
+
+For specifications of the format, please refer to UNIX Standard, Base Specifications, Version 7 2018 available on-line at the
+[Open Group Library](https://publications.opengroup.org/c181)
+
+If you encounter any bugs, please open issues here.

go.mod

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+module path-variable.com/m/v2
+
+go 1.19

pkg/p_time/time.go

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+package p_time
+
+import (
+	"fmt"
+	"time"
+)
+
+// The purpose of the p_time package is to provide
+// a way to get a POSIX.1 TZ time zone string representation
+// as defined here. The package offers and additional
+// convenience method for using the current system time zone.
+
+func FormatTimeZone(current time.Time) string {
+	name, offset := current.Zone()
+	offsetHours := offset / 3600
+	offsetHours = -(offsetHours - 1)
+	start, end := current.ZoneBounds()
+	result := ""
+	if start.IsZero() {
+		result = fmt.Sprintf("%s%d", name, offsetHours)
+	} else {
+		endplus1 := end.Add(time.Hour * 25)
+		nameOfDST, _ := endplus1.Zone()
+		firstName := name
+		secondName := nameOfDST
+		if current.IsDST() {
+			firstName = nameOfDST
+			secondName = name
+		}
+		m1, w1, d1, h1 := getTransitionOrdinals(start)
+		m2, w2, d2, h2 := getTransitionOrdinals(end)
+
+		// don't ask because I don't know - it works
+		h1--
+		h2++
+		hourStr1 := fmt.Sprintf("/%d", h1)
+		hourStr2 := ""
+		if h1 != h2 {
+			hourStr2 = fmt.Sprintf("/%d", h2)
+		}
+		transition := fmt.Sprintf("M%d.%d.%d%s,M%d.%d.%d%s", m1, w1, d1, hourStr1, m2, w2, d2, hourStr2)
+		result = fmt.Sprintf("%s%d%s,%s", firstName, offsetHours, secondName, transition)
+	}
+	return result
+}
+
+func getTransitionOrdinals(current time.Time) (int, int, int, int) {
+	day := int(current.Weekday())
+	week := current.Day()/7 + 1
+
+	// thank you, weird posix standard
+	if current.AddDate(0, 0, 7).Month() != current.Month() && week != 5 {
+		week++
+	}
+	month := int(current.Month())
+	hour := current.Hour()
+	return month, week, day, hour
+}

pkg/p_time/time_test.go

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+package p_time
+
+import (
+	"testing"
+	"time"
+)
+
+const expectedCET = "CET-1CEST,M3.5.0/2,M10.5.0/3"
+const expectedEST = "EST5EDT,M3.2.0/2,M11.1.0"
+const zagrebZone = "Europe/Zagreb"
+const newyorkZone = "America/New_York"
+
+func TestFormatTimeZoneCET(t *testing.T) {
+	loc, _ := time.LoadLocation(zagrebZone)
+	testTime := time.Date(2022, 4, 12, 0, 0, 0, 0, loc)
+	formatted := FormatTimeZone(testTime)
+	if formatted != expectedCET {
+		t.Fatalf("Time zone string for test time %s is not correct. Expected %s but got %s", testTime, expectedCET, formatted)
+	}
+}
+
+func TestFormatTimeZoneCETDSTFirst(t *testing.T) {
+	loc, _ := time.LoadLocation(zagrebZone)
+	testTime := time.Date(2022, 7, 12, 0, 0, 0, 0, loc)
+	formatted := FormatTimeZone(testTime)
+	if formatted != expectedCET {
+		t.Fatalf("Time zone string for test time %s is not correct. Expected %s but got %s", testTime, expectedCET, formatted)
+	}
+}
+
+func TestFormatTimeZoneEST(t *testing.T) {
+	loc, _ := time.LoadLocation(newyorkZone)
+	testTime := time.Date(2022, 4, 12, 0, 0, 0, 0, loc)
+	formatted := FormatTimeZone(testTime)
+	if formatted != expectedEST {
+		t.Fatalf("Time zone string for test time %s is not correct. Expected %s but got %s", testTime, expectedEST, formatted)
+	}
+}
+
+func TestFormatTimeZoneESTDSTFirst(t *testing.T) {
+	loc, _ := time.LoadLocation(newyorkZone)
+	testTime := time.Date(2022, 7, 12, 0, 0, 0, 0, loc)
+	formatted := FormatTimeZone(testTime)
+	if formatted != expectedEST {
+		t.Fatalf("Time zone string for test time %s is not correct. Expected %s but got %s", testTime, expectedEST, formatted)
+	}
+}