sets: add sets intersection

sets/sets.go

@@ -72,9 +72,26 @@ func (s *Set[T]) Remove(item T) bool {
 // Difference returns a set containing the elements of s that are not in x.
 func (s *Set[T]) Difference(x *Set[T]) *Set[T] {
 	result := New[T](max(0, s.Size()-x.Size()))
-	for i := range s.items {
-		if !x.Contains(i) {
-			result.items[i] = struct{}{}
+	for item := range s.items {
+		if !x.Contains(item) {
+			result.items[item] = struct{}{}
+		}
+	}
+	return result
+}
+
+// Intersection returns a set containing the elements that are both in s and x.
+func (s *Set[T]) Intersection(x *Set[T]) *Set[T] {
+	result := New[T](0)
+	// loop over the smaller set (thanks to https://github.com/deckarep/golang-set)
+	smaller := s
+	bigger := x
+	if smaller.Size() > bigger.Size() {
+		smaller, bigger = bigger, smaller
+	}
+	for item := range smaller.items {
+		if bigger.Contains(item) {
+			result.items[item] = struct{}{}
 		}
 	}
 	return result

sets/sets_test.go

@@ -142,6 +142,65 @@ func TestDifference(t *testing.T) {
 	}
 }
 
+func TestIntersection(t *testing.T) {
+	type testCase struct {
+		name     string
+		s        *sets.Set[int]
+		x        *sets.Set[int]
+		wantList []int
+	}
+
+	test := func(t *testing.T, tc testCase) {
+		result := tc.s.Intersection(tc.x)
+		sorted := result.OrderedList()
+
+		assert.DeepEqual(t, sorted, tc.wantList)
+	}
+
+	testCases := []testCase{
+		{
+			name:     "both empty",
+			s:        sets.From[int](),
+			x:        sets.From[int](),
+			wantList: []int{},
+		},
+		{
+			name:     "empty x returns empty",
+			s:        sets.From(1, 2, 3),
+			x:        sets.From[int](),
+			wantList: []int{},
+		},
+		{
+			name:     "nothing in common returns empty",
+			s:        sets.From(1, 2, 3),
+			x:        sets.From(4, 5),
+			wantList: []int{},
+		},
+		{
+			name:     "one in common",
+			s:        sets.From(1, 2, 3),
+			x:        sets.From(4, 2),
+			wantList: []int{2},
+		},
+		{
+			name:     "s subset of x returns s",
+			s:        sets.From(1, 2, 3),
+			x:        sets.From(1, 2, 3, 12),
+			wantList: []int{1, 2, 3},
+		},
+		{
+			name:     "x subset of s returns x",
+			s:        sets.From(1, 2, 3, 12),
+			x:        sets.From(1, 2, 3),
+			wantList: []int{1, 2, 3},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) { test(t, tc) })
+	}
+}
+
 func TestRemoveFound(t *testing.T) {
 	type testCase struct {
 		name     string