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

inclusives.go

@@ -1,22 +1,28 @@
 package interval
 
-import (
-	"fmt"
-)
+import "fmt"
 
 func (intvls *intervals) getInclusives(intervalLow, intervalHigh int) (int, int, error) {
 	low := intvls.getInclusiveLow(intervalLow)
 	high := intvls.getInclusiveHigh(intervalHigh)
 
+	err := intvls.checkValidInterval(low, high)
+	if err != nil {
+		return 0, 0, err
+	}
+	return low, high, nil
+}
+
+func (intvls *intervals) checkValidInterval(low, high int) error {
 	// if we get an incoherent result it's because it's invalid, for example in this case:
 	//		(low,high)=(0,1) --> if we add 1 to the Low and substract 1 to the High ends with (1,0) which is not valid
 	if low > high {
-		return 0, 0, fmt.Errorf("low (%v) can not be bigger than high (%v)", low, high)
+		return fmt.Errorf("low (%v) can not be bigger than high (%v)", low, high)
 	}
 	if high < low {
-		return 0, 0, fmt.Errorf("high (%v) can not be smaller than low (%v)", high, low)
+		return fmt.Errorf("high (%v) can not be smaller than low (%v)", high, low)
 	}
-	return low, high, nil
+	return nil
 }
 
 func (intvls *intervals) getInclusiveLow(value int) int {