from decimal import Decimal

from django.db import transaction

from apps.billing.models import CreditAccount, CreditLedger, CreditReservation


@transaction.atomic
def reserve_credit(*, team, user, task, amount: Decimal) -> CreditReservation:
    account, _ = CreditAccount.objects.select_for_update().get_or_create(team=team)
    available = account.balance - account.reserved_balance
    if available < amount:
        raise ValueError("insufficient credit")

    account.reserved_balance += amount
    account.save(update_fields=["reserved_balance", "updated_at"])
    reservation = CreditReservation.objects.create(
        team=team,
        user=user,
        project=task.project,
        task=task,
        amount=amount,
    )
    CreditLedger.objects.create(
        team=team,
        user=user,
        project=task.project,
        task=task,
        ledger_type=CreditLedger.Type.RESERVE,
        amount=amount,
        balance_after=account.balance,
        reason="reserve ai task credit",
    )
    return reservation


@transaction.atomic
def release_credit(*, reservation: CreditReservation, reason: str = "") -> None:
    account = CreditAccount.objects.select_for_update().get(team=reservation.team)
    if reservation.status != CreditReservation.Status.ACTIVE:
        return

    account.reserved_balance -= reservation.amount
    account.save(update_fields=["reserved_balance", "updated_at"])
    reservation.status = CreditReservation.Status.RELEASED
    reservation.save(update_fields=["status", "updated_at"])
    CreditLedger.objects.create(
        team=reservation.team,
        user=reservation.user,
        project=reservation.project,
        task=reservation.task,
        ledger_type=CreditLedger.Type.RELEASE,
        amount=reservation.amount,
        balance_after=account.balance,
        reason=reason or "release reserved credit",
    )


@transaction.atomic
def charge_reserved_credit(*, reservation: CreditReservation, actual_amount: Decimal) -> None:
    account = CreditAccount.objects.select_for_update().get(team=reservation.team)
    if reservation.status != CreditReservation.Status.ACTIVE:
        raise ValueError("reservation is not active")
    if actual_amount > reservation.amount:
        raise ValueError("actual amount exceeds reserved amount")

    account.balance -= actual_amount
    account.reserved_balance -= reservation.amount
    account.save(update_fields=["balance", "reserved_balance", "updated_at"])
    reservation.status = CreditReservation.Status.CHARGED
    reservation.save(update_fields=["status", "updated_at"])
    CreditLedger.objects.create(
        team=reservation.team,
        user=reservation.user,
        project=reservation.project,
        task=reservation.task,
        ledger_type=CreditLedger.Type.CHARGE,
        amount=actual_amount,
        balance_after=account.balance,
        reason="charge ai task credit",
    )
    if reservation.amount > actual_amount:
        CreditLedger.objects.create(
            team=reservation.team,
            user=reservation.user,
            project=reservation.project,
            task=reservation.task,
            ledger_type=CreditLedger.Type.RELEASE,
            amount=reservation.amount - actual_amount,
            balance_after=account.balance,
            reason="release unused reserved credit",
        )