constant-time lookups for tag inserts

message.go

@@ -11,19 +11,23 @@ type Tag struct {
 	Value string
 }
 
-// Tags is an key-order-preserving, last-insert-wins slice
+// Tags is an key-order-preserving, last-insert-wins "set" of Tag
 type Tags struct {
+	kv   map[string]int // quick lookups for inserts
 	tags []Tag
 }
 
 func (t *Tags) Insert(new Tag) {
-	for i := range t.tags {
+	if t.kv == nil {
-		if t.tags[i].Key == new.Key {
+		t.kv = map[string]int{}
-			t.tags[i] = new
+	}
-			return
+	i, ok := t.kv[new.Key]
-		}
+	if ok {
+		t.tags[i] = new
+	} else {
+		t.kv[new.Key] = len(t.tags)
+		t.tags = append(t.tags, new)
 	}
-	t.tags = append(t.tags, new)
 }
 
 func (t *Tags) GetSlice() []Tag {