Added check for valid interval in Add function. Added overlap testcases.

add.go

@@ -1,8 +1,17 @@
 package interval
 
-func (intvls *intervals) Add(itvl *Interval) {
+func (intvls *intervals) Add(itvl *Interval) error {
+	low := intvls.getInclusiveLow(itvl.Low)
+	high := intvls.getInclusiveHigh(itvl.High)
+
+	err := intvls.checkValidInterval(low, high)
+	if err != nil {
+		return err
+	}
+
 	intvls.Intervals = append(intvls.Intervals, itvl)
 	intvls.reset()
+	return nil
 }
 
 func (intvls *intervals) reset() {