feat: persist booked dates in JSON file and exclude booked dates from selection

main.go

@@ -46,6 +46,7 @@ func main() {
 	}
 
 	http.HandleFunc("/api/submit", handleSubmit)
+	http.HandleFunc("/api/booked-dates", handleBookedDates)
 
 	// Serve static files from webDir directory
 	fs := http.FileServer(http.Dir(webDir))
@@ -81,6 +82,10 @@ func handleSubmit(w http.ResponseWriter, r *http.Request) {
 
 	log.Printf("Received survey submission from %s for date %s (%s)", payload.Name, payload.Date, payload.Activity)
 
+	if err := recordBookedDate(payload.Date); err != nil {
+		log.Printf("Error recording booked date: %v", err)
+	}
+
 	err = sendEmail(payload)
 	if err != nil {
 		log.Printf("Error sending email: %v", err)
@@ -101,6 +106,80 @@ func handleSubmit(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+func handleBookedDates(w http.ResponseWriter, r *http.Request) {
+	if enableCORS(w, r) {
+		return
+	}
+
+	if r.Method != http.MethodGet {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	dates, err := getBookedDates()
+	if err != nil {
+		log.Printf("Error reading booked dates: %v", err)
+		dates = []string{}
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	json.NewEncoder(w).Encode(dates)
+}
+
+func getBookedDatesFilePath() string {
+	path := os.Getenv("BOOKED_DATES_FILE")
+	if path == "" {
+		path = "booked_dates.json"
+	}
+	return path
+}
+
+func getBookedDates() ([]string, error) {
+	filePath := getBookedDatesFilePath()
+	file, err := os.Open(filePath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return []string{}, nil
+		}
+		return nil, err
+	}
+	defer file.Close()
+
+	var dates []string
+	if err := json.NewDecoder(file).Decode(&dates); err != nil {
+		return []string{}, nil
+	}
+	return dates, nil
+}
+
+func recordBookedDate(dateStr string) error {
+	if strings.TrimSpace(dateStr) == "" {
+		return nil
+	}
+
+	dates, err := getBookedDates()
+	if err != nil {
+		dates = []string{}
+	}
+
+	for _, d := range dates {
+		if d == dateStr {
+			return nil
+		}
+	}
+
+	dates = append(dates, dateStr)
+
+	filePath := getBookedDatesFilePath()
+	data, err := json.MarshalIndent(dates, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(filePath, data, 0644)
+}
+
 func enableCORS(w http.ResponseWriter, r *http.Request) bool {
 	w.Header().Set("Access-Control-Allow-Origin", "*")
 	w.Header().Set("Access-Control-Allow-Methods", "POST, GET, OPTIONS")