Add a NewStringSet() function

Also beef up tests to cover len(ss).

pkg/util/set.go

@@ -21,6 +21,13 @@ type empty struct{}
 // A set of strings, implemented via map[string]struct{} for minimal memory consumption.
 type StringSet map[string]empty
 
+// NewStringSet creates a StringSet from a list of values.
+func NewStringSet(items ...string) StringSet {
+	ss := StringSet{}
+	ss.Insert(items...)
+	return ss
+}
+
 // Insert adds items to the set.
 func (s StringSet) Insert(items ...string) {
 	for _, item := range items {

pkg/util/set_test.go

@@ -22,7 +22,13 @@ import (
 
 func TestStringSet(t *testing.T) {
 	s := StringSet{}
+	if len(s) != 0 {
+		t.Errorf("Expected len=0: %d", len(s))
+	}
 	s.Insert("a", "b")
+	if len(s) != 2 {
+		t.Errorf("Expected len=2: %d", len(s))
+	}
 	s.Insert("c")
 	if s.Has("d") {
 		t.Errorf("Unexpected contents: %#v", s)
@@ -35,3 +41,13 @@ func TestStringSet(t *testing.T) {
 		t.Errorf("Unexpected contents: %#v", s)
 	}
 }
+
+func TestNewStringSet(t *testing.T) {
+	s := NewStringSet("a", "b", "c")
+	if len(s) != 3 {
+		t.Errorf("Expected len=3: %d", len(s))
+	}
+	if !s.Has("a") || !s.Has("b") || !s.Has("c") {
+		t.Errorf("Unexpected contents: %#v", s)
+	}
+}