import datetime
from django.apps import apps
from django.db import transaction

def generate_invoice_number():
    """
    Generira novi broj fakture u formatu INV-YYYY-XXXX.
    Koristi transakciju kako bi se spriječilo dupliciranje brojeva.
    """
    with transaction.atomic():
        Invoice = apps.get_model('invoicing', 'Invoice')

        current_year = datetime.date.today().year
        
        # Zaključavamo tablicu za čitanje kako bismo izbjegli Race Condition
        last_invoice = Invoice.objects.filter(
            invoice_number__startswith=f"INV-{current_year}-"
        ).select_for_update().order_by('-invoice_number').first()

        if last_invoice:
            last_num = int(last_invoice.invoice_number.split('-')[-1])
            new_num = last_num + 1
        else:
            new_num = 1
            
        return f"INV-{current_year}-{new_num:04d}"