refactor notify func, add slice to english formatter

cmd/themis-server/main.go

@@ -524,33 +524,30 @@ func main() {
 		cancel()
 	}()
 
-	go func() {
-		for {
-			select {
-			case <-ctx.Done():
-				return
-			case <-notifChan:
+	go notifier.NotifyFunc(ctx, func() {
 		absentees, err := store.GetAbsentees(ctx, themis.NextMonday())
 		if err != nil {
 			log.Error().Err(err).Msg("failed to get absentees for next session")
 			return
 		}
 
+		for i, a := range absentees {
+			a = fmt.Sprintf("<@%s>", a)
+			absentees[i] = a
+		}
+
 		var msg string
 		if len(absentees) == 0 {
 			msg = "Everybody can make it next Monday, see you then!"
 		} else {
-					msg = fmt.Sprintf("%s can't make it next Monday.", strings.Join(absentees, ", "))
+			msg = fmt.Sprintf("%s can't make it next Monday.", themis.FormatStringSlice(absentees))
 		}
 
 		_, err = discord.ChannelMessageSend(channelId, msg)
 		if err != nil {
 			log.Error().Err(err).Msg("failed to send scheduled notification")
 		}
-			}
-			time.Sleep(5 * time.Minute)
-		}
-	}()
+	})
 
 	<-ctx.Done()
 	log.Info().Msg("context cancelled, exiting")

fmt.go

@@ -70,3 +70,27 @@ func FormatRows(rows *sql.Rows) (string, error) {
 
 	return sb.String(), nil
 }
+
+func FormatStringSlice(s []string) string {
+	if len(s) == 0 {
+		return ""
+	}
+
+	sb := strings.Builder{}
+	for len(s) > 0 {
+		curr, rest := s[0], s[1:]
+		sb.WriteString(curr)
+
+		if len(rest) == 0 {
+			break
+		}
+
+		if len(rest) > 1 {
+			sb.WriteString(", ")
+		} else {
+			sb.WriteString(" and ")
+		}
+		s = rest
+	}
+	return sb.String()
+}

notify.go

@@ -29,7 +29,7 @@ func (n *Notifier) Start(ctx context.Context) {
 		sat = sat.AddDate(0, 0, 7)
 	}
 
-	t, err := time.ParseInLocation(time.DateTime, fmt.Sprintf("%s 16:35:00", sat.Format(time.DateOnly)), loc)
+	t, err := time.ParseInLocation(time.DateTime, fmt.Sprintf("%s 17:00:00", sat.Format(time.DateOnly)), loc)
 	if err != nil {
 		panic("failed to parse next monday notif time. this is likely a bug.")
 	}
@@ -54,3 +54,15 @@ func (n *Notifier) Start(ctx context.Context) {
 		time.Sleep(time.Hour)
 	}
 }
+
+func (n *Notifier) NotifyFunc(ctx context.Context, f func()) {
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-n.c:
+			f()
+		}
+		time.Sleep(5 * time.Minute)
+	}
+}