from datetime import date, datetime
from decimal import Decimal, InvalidOperation
from .extensions import db
from .models import AppSetting

KEYS = {
    "enabled": "global_vat_override_enabled",
    "name": "global_vat_override_name",
    "rate": "global_vat_override_rate",
    "start": "global_vat_override_start",
    "end": "global_vat_override_end",
}


def _value(key, default=""):
    setting = db.session.get(AppSetting, KEYS[key])
    return setting.value if setting else default


def global_vat_override():
    rate_raw = _value("rate")
    try:
        rate = Decimal(rate_raw) if rate_raw != "" else None
    except InvalidOperation:
        rate = None
    return {
        "enabled": _value("enabled", "0") == "1",
        "name": _value("name", "Projekt rządowy"),
        "rate": rate,
        "start": _parse_date(_value("start")),
        "end": _parse_date(_value("end")),
    }


def _parse_date(value):
    if not value:
        return None
    try:
        return date.fromisoformat(value)
    except ValueError:
        return None


def effective_vat_rate(nominal_rate, on_date=None):
    override = global_vat_override()
    if not override["enabled"] or override["rate"] is None:
        return Decimal(str(nominal_rate))
    if isinstance(on_date, datetime):
        on_date = on_date.date()
    on_date = on_date or date.today()
    if override["start"] and on_date < override["start"]:
        return Decimal(str(nominal_rate))
    if override["end"] and on_date > override["end"]:
        return Decimal(str(nominal_rate))
    return override["rate"]



def vat_override_for_date(nominal_rate, on_date=None):
    """Return a complete, immutable VAT-action snapshot for a date."""
    override = global_vat_override()
    if isinstance(on_date, datetime):
        on_date = on_date.date()
    on_date = on_date or date.today()
    applies = bool(
        override["enabled"]
        and override["rate"] is not None
        and (not override["start"] or on_date >= override["start"])
        and (not override["end"] or on_date <= override["end"])
    )
    return {
        "applied": applies,
        "name": override["name"] if applies else None,
        "rate": override["rate"] if applies else Decimal(str(nominal_rate)),
        "start": override["start"] if applies else None,
        "end": override["end"] if applies else None,
    }


def freeze_vat_action(entry, nominal_rate, previous_fueled_at=None):
    """Attach VAT action conditions to an entry without rewriting valid history."""
    fueled_date = entry.fueled_at.date() if isinstance(entry.fueled_at, datetime) else entry.fueled_at
    previous_date = previous_fueled_at.date() if isinstance(previous_fueled_at, datetime) else previous_fueled_at
    if entry.vat_action_frozen and (previous_date is None or previous_date == fueled_date):
        return

    snapshot = vat_override_for_date(nominal_rate, fueled_date)
    entry.vat_action_frozen = True
    entry.vat_action_applied = snapshot["applied"]
    entry.vat_action_name = snapshot["name"]
    entry.vat_action_rate = snapshot["rate"] if snapshot["applied"] else None
    entry.vat_action_start = snapshot["start"]
    entry.vat_action_end = snapshot["end"]


def entry_vat_rate(entry, nominal_rate):
    if entry.vat_action_frozen:
        if entry.vat_action_applied and entry.vat_action_rate is not None:
            return Decimal(str(entry.vat_action_rate))
        return Decimal(str(nominal_rate))
    return effective_vat_rate(nominal_rate, entry.fueled_at)

def save_global_vat_override(form):
    enabled = "enabled" in form
    name = (form.get("name") or "Projekt rządowy").strip()
    rate_raw = (form.get("rate") or "").strip()
    start_raw = (form.get("start") or "").strip()
    end_raw = (form.get("end") or "").strip()

    if enabled:
        try:
            rate = Decimal(rate_raw)
        except InvalidOperation as exc:
            raise ValueError("Podaj prawidłową globalną stawkę VAT.") from exc
        if rate < 0 or rate > 100:
            raise ValueError("Globalna stawka VAT musi mieścić się w zakresie 0–100%.")
        if start_raw and not _parse_date(start_raw):
            raise ValueError("Nieprawidłowa data rozpoczęcia.")
        if end_raw and not _parse_date(end_raw):
            raise ValueError("Nieprawidłowa data zakończenia.")
        if start_raw and end_raw and start_raw > end_raw:
            raise ValueError("Data zakończenia nie może być wcześniejsza niż rozpoczęcia.")

    values = {
        KEYS["enabled"]: "1" if enabled else "0",
        KEYS["name"]: name,
        KEYS["rate"]: rate_raw,
        KEYS["start"]: start_raw,
        KEYS["end"]: end_raw,
    }
    for key, value in values.items():
        db.session.merge(AppSetting(key=key, value=value))