package server import ( "context" "errors" "log" "math/rand" "sync" "time" ) // topic represents a channel to which subscribers can subscribe, and publishers // can publish a message type topic struct { id string subscribers map[int]subscriber messages int last time.Time ctx context.Context cancel context.CancelFunc mu sync.Mutex } // subscriber is a function that is called for every new message on a topic type subscriber func(msg *message) error // newTopic creates a new topic func newTopic(id string) *topic { ctx, cancel := context.WithCancel(context.Background()) return &topic{ id: id, subscribers: make(map[int]subscriber), last: time.Now(), ctx: ctx, cancel: cancel, } } func (t *topic) Subscribe(s subscriber) int { t.mu.Lock() defer t.mu.Unlock() subscriberID := rand.Int() t.subscribers[subscriberID] = s t.last = time.Now() return subscriberID } func (t *topic) Unsubscribe(id int) int { t.mu.Lock() defer t.mu.Unlock() delete(t.subscribers, id) return len(t.subscribers) } func (t *topic) Publish(m *message) error { t.mu.Lock() defer t.mu.Unlock() if len(t.subscribers) == 0 { return errors.New("no subscribers") } t.last = time.Now() t.messages++ for _, s := range t.subscribers { if err := s(m); err != nil { log.Printf("error publishing message to subscriber") } } return nil } func (t *topic) Stats() (subscribers int, messages int) { t.mu.Lock() defer t.mu.Unlock() return len(t.subscribers), t.messages } func (t *topic) Close() { t.cancel() }