dag: Set difference

dag/set.go

@@ -48,6 +48,9 @@ func (s *Set) Include(v interface{}) bool {
 // Intersection computes the set intersection with other.
 func (s *Set) Intersection(other *Set) *Set {
 	result := new(Set)
+	if s == nil {
+		return result
+	}
 	if other != nil {
 		for _, v := range s.m {
 			if other.Include(v) {
@@ -59,6 +62,25 @@ func (s *Set) Intersection(other *Set) *Set {
 	return result
 }
 
+// Difference returns a set with the elements that s has but
+// other doesn't.
+func (s *Set) Difference(other *Set) *Set {
+	result := new(Set)
+	if s != nil {
+		for k, v := range s.m {
+			var ok bool
+			if other != nil {
+				_, ok = other.m[k]
+			}
+			if !ok {
+				result.Add(v)
+			}
+		}
+	}
+
+	return result
+}
+
 // Len is the number of items in the set.
 func (s *Set) Len() int {
 	if s == nil {

dag/set_test.go

@@ -0,0 +1,56 @@
+package dag
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestSetDifference(t *testing.T) {
+	cases := []struct {
+		Name     string
+		A, B     []interface{}
+		Expected []interface{}
+	}{
+		{
+			"same",
+			[]interface{}{1, 2, 3},
+			[]interface{}{3, 1, 2},
+			[]interface{}{},
+		},
+
+		{
+			"A has extra elements",
+			[]interface{}{1, 2, 3},
+			[]interface{}{3, 2},
+			[]interface{}{1},
+		},
+
+		{
+			"B has extra elements",
+			[]interface{}{1, 2, 3},
+			[]interface{}{3, 2, 1, 4},
+			[]interface{}{},
+		},
+	}
+
+	for i, tc := range cases {
+		t.Run(fmt.Sprintf("%d-%s", i, tc.Name), func(t *testing.T) {
+			var one, two, expected Set
+			for _, v := range tc.A {
+				one.Add(v)
+			}
+			for _, v := range tc.B {
+				two.Add(v)
+			}
+			for _, v := range tc.Expected {
+				expected.Add(v)
+			}
+
+			actual := one.Difference(&two)
+			match := actual.Intersection(&expected)
+			if match.Len() != expected.Len() {
+				t.Fatalf("bad: %#v", actual.List())
+			}
+		})
+	}
+}