Keep observer composition storage proportional to selected callbacks

observer_composition_contract_test.go

@@ -31,6 +31,31 @@ func TestComposeObserversCopiesAndOrders(t *testing.T) {
 	promptkit.ComposeObservers(func(promptkit.ExecutionEvent) { panic("panic") }, second)(promptkit.ExecutionEvent{})
 }
 
+func TestComposeObserversSparseSelection(t *testing.T) {
+	for _, selected := range []int{0, 1, 2} {
+		calls := 0
+		observers := make([]promptkit.ExecutionObserver, 1024)
+		callback := func(promptkit.ExecutionEvent) { calls++ }
+		for index := 0; index < selected; index++ {
+			observers[len(observers)-1-index] = callback
+		}
+		composed := promptkit.ComposeObservers(observers...)
+		for index := range observers {
+			observers[index] = func(promptkit.ExecutionEvent) { calls += 100 }
+		}
+		if selected == 0 {
+			if composed != nil {
+				t.Fatal("all-nil composition must be nil")
+			}
+			continue
+		}
+		composed(promptkit.ExecutionEvent{})
+		if calls != selected {
+			t.Fatalf("selected %d callbacks, got %d calls", selected, calls)
+		}
+	}
+}
+
 func TestCoordinateObserverCompositionAndNestedSelection(t *testing.T) {
 	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}))
 	ctx := context.Background()