Change method Set to panic when called on an invalid Interval

intervalset.go

@@ -27,14 +27,16 @@ type Interval[E Elem[E]] struct {
 
 // One returns an Interval that only contains a single element v.
 //
-// If v is the maximum value of E, One returns an invalid Interval.
+// If there is no element next to v, One returns an invalid Interval.
+// Note that calling Set method on an invalid Interval panics.
 func One[E Enum[E]](v E) Interval[E] {
 	return Range(v, v.Next())
 }
 
 // Range returns an Interval of range [lo, hi).
 //
 // If lo.Compare(hi) >= 0, Range returns an invalid Interval.
+// Note that calling Set method on an invalid Interval panics.
 func Range[E Elem[E]](lo, hi E) Interval[E] {
 	return Interval[E]{lo, hi}
 }
@@ -45,14 +47,17 @@ func (r Interval[E]) Equal(r2 Interval[E]) bool {
 }
 
 // Set returns the set of elements that are in r.
-//
-// If r is an invalid Interval, Set returns an empty set.
+// If r is the zero value, Set returns an empty set.
+// If r is an invalid Interval, Set panics.
 func (r Interval[E]) Set() Set[E] {
-	if r.Low.Compare(r.High) >= 0 {
+	switch c := r.Low.Compare(r.High); {
+	case c < 0:
+		return Set[E]{r}
+	case c == 0 && r.Equal(Interval[E]{}):
 		return nil
 	}
 
-	return Set[E]{r}
+	panic("invalid Interval")
 }
 
 // A Set is a slice of separate intervals sorted in ascending order.

intervalset_test.go

@@ -22,7 +22,7 @@ func TestCreation(t *testing.T) {
 			Set[E]{{1, 5}},
 		},
 		{
-			Range[E](5, 1).Set(),
+			Interval[E]{}.Set(),
 			nil,
 		},
 		{
@@ -53,6 +53,9 @@ func TestCreation(t *testing.T) {
 			t.Logf("Case %v: want %v, but got %v", i, c.Expected, c.Actual)
 		}
 	}
+
+	shouldPanic(t, func() { _ = Range[E](5, 1).Set() }, "Range[E](5, 1).Set() didn't panic.")
+	shouldPanic(t, func() { _ = Range[E](5, 5).Set() }, "Range[E](5, 5).Set() didn't panic.")
 }
 
 func TestAdd(t *testing.T) {
@@ -204,3 +207,16 @@ func TestExtent(t *testing.T) {
 		}
 	}
 }
+
+func shouldPanic(t *testing.T, f func(), name string) {
+	t.Helper()
+
+	defer func() {
+		if recover() == nil {
+			t.Log(name, "did not panic.")
+			t.Fail()
+		}
+	}()
+
+	f()
+}