package observation

// Compose copies and synchronously invokes non-nil observers in order.
func Compose[T any, F ~func(T)](observers ...F) F {
	count := 0
	var single F
	for _, observer := range observers {
		if observer != nil {
			count++
			single = observer
		}
	}
	if count == 0 {
		return nil
	}
	if count == 1 {
		return single
	}
	selected := make([]F, 0, count)
	for _, observer := range observers {
		if observer != nil {
			selected = append(selected, observer)
		}
	}
	return func(event T) {
		for _, observer := range selected {
			observer(event)
		}
	}
}