from __future__ import annotations
import re

def slugify(value: str, max_len: int = 40) -> str:
    v = value.strip()
    v = re.sub(r"\s+", "_", v)
    v = re.sub(r"[^A-Za-z0-9_\-]+", "", v)
    v = v.strip("_-")
    if len(v) > max_len:
        v = v[:max_len].rstrip("_-")
    return v or "Kunde"

def default_invoice_folder_name(invoice_number: str, customer: str | None = None) -> str:
    cust = slugify(customer or "Kunde")
    inv = slugify(invoice_number, max_len=32)
    return f"Rechnung_{inv}_{cust}"