package patterns

// Observer pattern implementation in Go
type Observer interface {
	Update()
}

// Subject interface defines methods for registering, unregistering, and notifying observers
type Subject interface {
	Register(o Observer)
	Unregister(o Observer)
	Notify()
}

// Concrete implementation of the Subject interface. Lowercase 'subject' to make it unexported, as it will be created through the NewSubject function.
type subject struct {
	observers map[Observer]bool // Using a map to store observers for efficient add/remove operations
}

// NewSubject creates and returns a new instance of the subject
func NewSubject() Subject {
	// Needs to return a pointer to the subject struct to ensure that the same instance is modified when registering/unregistering observers
	return &subject{
		observers: make(map[Observer]bool),
	}
}

// Register adds an observer to the subject's set of observers
// Pointer receiver is used to modify the subject's state (the observers map) when registering an observer
func (s *subject) Register(o Observer) {
	s.observers[o] = true
}

// Unregister removes an observer from the subject's set of observers
// Pointer receiver is used to modify the subject's state (the observers map) when unregistering an observer
func (s *subject) Unregister(o Observer) {
	delete(s.observers, o)
}

// Notify calls the Update method on all registered observers to notify them of a change
func (s *subject) Notify() {
	for o := range s.observers {
		o.Update()
	}
}