improve References header collapse

main.go

@@ -202,29 +202,37 @@ func (b backend) AllowPost() bool {
 	return true
 }
 
-func (b backend) findParent(child *nntp.Article) *nntp.Article {
-	ref := child.Header.Get("References")
+func (b backend) findOP(ref string) *nntp.Article {
 	if ref == "" {
 		return nil
 	}
+	// all references should have the same OP so just take the first
+	msgID := strings.SplitN(ref, " ", 2)[0]
 
-	var parent *nntp.Article
+	// traverse backwards expecting most reply activity concentrated late in the total history
 	for i := len(b.messages) - 1; i >= 0; i-- {
-		a := b.messages[i]
-		if a.MessageID() == ref {
-			parent = a
-			ref = a.Header.Get("References")
-			if ref == "" {
-				break
-			}
+		article := b.messages[i]
+		if article.MessageID() != msgID {
+			continue
 		}
+
+		gpID := article.Header.Get("References")
+		if gpID != "" {
+			return b.findOP(gpID)
+		}
+		return article
 	}
-	return parent
+
+	return nil
 }
 
 func (b backend) Post(article *nntp.Article) error {
 	b.logger("Post()")
-	parent := b.findParent(article)
+
+	// iris replies are all made to a top-level post, there is no grandchild nesting.
+	//
+	// but NNTP supports this, so collapse the provided "References" header up to the OP.
+	parent := b.findOP(article.Header.Get("References"))
 	if parent != nil {
 		article.Header.Set("References", parent.MessageID())
 	}