Refactor auth stringSet into common.StringSet

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

common/stringset.go

@@ -0,0 +1,35 @@
+package common
+
+// StringSet is a useful type for looking up strings.
+type StringSet map[string]struct{}
+
+// NewStringSet creates a new StringSet with the given strings.
+func NewStringSet(keys ...string) StringSet {
+	ss := make(StringSet, len(keys))
+	ss.Add(keys...)
+	return ss
+}
+
+// Add inserts the given keys into this StringSet.
+func (ss StringSet) Add(keys ...string) {
+	for _, key := range keys {
+		ss[key] = struct{}{}
+	}
+}
+
+// Contains returns whether the given key is in this StringSet.
+func (ss StringSet) Contains(key string) bool {
+	_, ok := ss[key]
+	return ok
+}
+
+// Keys returns a slice of all keys in this StringSet.
+func (ss StringSet) Keys() []string {
+	keys := make([]string, 0, len(ss))
+
+	for key := range ss {
+		keys = append(keys, key)
+	}
+
+	return keys
+}