Avoid intermediate conversation history copies

internal/domain/exchange.go

@@ -51,10 +51,42 @@ func (e *Exchange) Messages() []RenderedMessage {
 	if e == nil {
 		return nil
 	}
-	messages := make([]RenderedMessage, e.count)
-	for current := e; current != nil; current = current.parent {
-		start := current.count - len(current.messages)
-		copyRenderedMessages(messages[start:current.count], current.messages)
+	return e.MessagesWithSuffix()
+}
+
+// MessagesWithSuffix returns independently owned history followed by each part.
+// A nil exchange supplies no history.
+func (e *Exchange) MessagesWithSuffix(parts ...[]RenderedMessage) []RenderedMessage {
+	count := e.MessageCount()
+	for _, part := range parts {
+		count += len(part)
+	}
+	messages := make([]RenderedMessage, count)
+	e.visitChunks(func(start int, chunk []RenderedMessage) {
+		copyRenderedMessages(messages[start:], chunk)
+	})
+	offset := e.MessageCount()
+	for _, part := range parts {
+		copyRenderedMessages(messages[offset:], part)
+		offset += len(part)
 	}
 	return messages
 }
+
+// VisitMessages visits indexed history entries without materializing a slice.
+// Entries and their cache metadata are borrowed read-only values. Visits need
+// not be in index order; callers can fill a destination using the supplied index.
+func (e *Exchange) VisitMessages(visit func(int, RenderedMessage)) {
+	e.visitChunks(func(start int, chunk []RenderedMessage) {
+		for index, message := range chunk {
+			visit(start+index, message)
+		}
+	})
+}
+
+func (e *Exchange) visitChunks(visit func(int, []RenderedMessage)) {
+	for current := e; current != nil; current = current.parent {
+		start := current.count - len(current.messages)
+		visit(start, current.messages)
+	}
+}