"""Fast monthly service billing from existing customer presets."""
import calendar
import tempfile
from .invoice_number import invoice_root_lock
import re
from datetime import date, timedelta
from decimal import Decimal
from pathlib import Path
from .init import init_invoice_folder
from .presets import load_yaml
from .models import Invoice
from .init import _dump_yaml
from .folder_naming import default_invoice_folder_name

MONTHS = ('Januar Februar März April Mai Juni Juli August September Oktober November Dezember').split()

def _monthly_invoice(root: Path, repo_root: Path, logger, customer: str, month: str,
                    hours: Decimal, number: str, issued: date | None = None,
                    rate: Decimal | None = None, due_days: int = 30) -> Path:
    if not re.fullmatch(r'\d{4}-\d{2}', month):
        raise ValueError('Month must be YYYY-MM')
    year, m = map(int, month.split('-'))
    start, end = date(year, m, 1), date(year, m, calendar.monthrange(year, m)[1])
    if not hours.is_finite() or hours <= 0 or due_days < 0:
        raise ValueError('Hours must be positive and due_days nonnegative')
    target = root.expanduser().resolve() / default_invoice_folder_name(number, customer)
    # Explicit number required: existing PDF invoices may live outside this root.
    if target.exists():
        raise FileExistsError(f'Invoice folder exists: {target}')
    for existing in root.expanduser().glob('*/invoice.yaml'):
        if str(load_yaml(existing).get('meta', {}).get('invoice_number', '')) == number:
            raise ValueError(f'Invoice number already used: {number}')
    with tempfile.TemporaryDirectory(prefix='.draft-', dir=root.expanduser().resolve()) as staging:
        work = Path(staging) / 'invoice'
        init_invoice_folder(work, repo_root, logger, customer=customer, invoice_number=number, due_days=due_days)
        path = work / 'invoice.yaml'
        data = load_yaml(path)
        if len(data['items']) != 1 or data['items'][0]['unit'] != 'HUR':
            raise ValueError('Monthly billing requires a preset with one HUR line')
        issued = issued or date.today()
        data['meta'].update(invoice_date=str(issued), service_date=str(end),
                            service_period_start=str(start), service_period_end=str(end))
        data['payment']['due_date'] = str(issued + timedelta(days=due_days))
        item = data['items'][0]
        item['quantity'] = str(hours)
        item['name'] = f'Geleistete Stunden {MONTHS[m-1]} {year}'
        if rate is not None:
            item['unit_price_net'] = str(rate)
        Invoice.model_validate(data)
        _dump_yaml(data, path)
        work.rename(target)
    return target


def monthly_invoice(root: Path, *args, **kwargs) -> Path:
    with invoice_root_lock(root):
        return _monthly_invoice(root, *args, **kwargs)