Set should have a difference function

pkg/util/set.go

@@ -62,6 +62,22 @@ func (s StringSet) HasAll(items ...string) bool {
 	return true
 }
 
+// Difference returns a set of objects that are not in s2
+// For example:
+// s1 = {1, 2, 3}
+// s2 = {1, 2, 4, 5}
+// s1.Difference(s2) = {3}
+// s2.Difference(s1) = {4, 5}
+func (s StringSet) Difference(s2 StringSet) StringSet {
+	result := NewStringSet()
+	for key := range s {
+		if !s2.Has(key) {
+			result.Insert(key)
+		}
+	}
+	return result
+}
+
 // IsSuperset returns true iff s1 is a superset of s2.
 func (s1 StringSet) IsSuperset(s2 StringSet) bool {
 	for item := range s2 {

pkg/util/set_test.go

@@ -98,3 +98,22 @@ func TestStringSetList(t *testing.T) {
 		t.Errorf("List gave unexpected result: %#v", s.List())
 	}
 }
+
+func TestStringSetDifference(t *testing.T) {
+	a := NewStringSet("1", "2", "3")
+	b := NewStringSet("1", "2", "4", "5")
+	c := a.Difference(b)
+	d := b.Difference(a)
+	if len(c) != 1 {
+		t.Errorf("Expected len=1: %d", len(c))
+	}
+	if !c.Has("3") {
+		t.Errorf("Unexpected contents: %#v", c.List())
+	}
+	if len(d) != 2 {
+		t.Errorf("Expected len=2: %d", len(d))
+	}
+	if !d.Has("4") || !d.Has("5") {
+		t.Errorf("Unexpected contents: %#v", d.List())
+	}
+}