add more test

storage/util/dynamic_delay.go

@@ -65,7 +65,9 @@ func NewDelay(targetPercentile float64, increaseRate float64, initialDelay, minD
 	if initialDelay > maxDelay {
 		initialDelay = maxDelay
 	}
-	// See http://google3/net/rpc/contrib/hedged_call/dynamic_delay.h?l=35&rcl=194811090
+
+	// Compute increaseFactor and decreaseFactor such that:
+	// (increaseFactor ^ (1 - targetPercentile)) * (decreaseFactor ^ targetPercentile) = 1
 	increaseFactor := math.Exp(math.Log(2) / increaseRate)
 	if increaseFactor < 1.001 {
 		increaseFactor = 1.001

storage/util/dynamic_delay_test.go

@@ -162,3 +162,15 @@ func TestOverflow(t *testing.T) {
 		t.Fatalf("unexpected d.Value: got %v, want %v", got, want)
 	}
 }
+
+func TestInvalidArgument(t *testing.T) {
+	_, err := NewDelay(1-0.1, 15, 1*time.Millisecond, 2*time.Hour, 1*time.Hour)
+	if err == nil {
+		t.Fatal("unexpected, should throw error as minDelay is greater than maxDelay")
+	}
+
+	_, err = NewDelay(1-0.1, 0, 1*time.Millisecond, 2*time.Hour, 1*time.Hour)
+	if err == nil {
+		t.Fatal("unexpected, should throw error as increaseRate can't be zero")
+	}
+}