add changes to changeover code based on unit tests

go.mod

@@ -1,3 +1,3 @@
 module github.com/taigrr/most-specific-period
 
-go 1.17
+go 1.18

msp/changeover.go

@@ -11,11 +11,20 @@ func GetChangeOvers(periods ...Period) (changeovers []time.Time) {
 		timeStamps = append(timeStamps, x.GetEndTime())
 		timeStamps = append(timeStamps, x.GetStartTime())
 	}
+	if len(timeStamps) == 0 {
+		return
+	}
 	sort.Slice(timeStamps, func(i, j int) bool {
 		return timeStamps[i].Before(timeStamps[j])
 	})
-
+	// timeStamps is sorted, so this will always result in an unused time
+	// struct, as it's before the first
+	previousTs := timeStamps[0].Add(-10 * time.Nanosecond)
 	for _, ts := range timeStamps {
+		if ts.Equal(previousTs) {
+			continue
+		}
+		previousTs = ts
 		before := ts.Add(-1 * time.Nanosecond)
 		after := ts.Add(1 * time.Nanosecond)
 		from, _ := MostSpecificPeriod(before, periods...)