from .charity_payroll_giving_types import PayrollGiving
from .money_amount import Money, money
from .payroll_income_tax import income_tax
from .payroll_tax_period_types import PayFrequency


def payroll_giving(
    tax_code: str,
    frequency: PayFrequency,
    period: int,
    pay: Money,
    donation: Money,
    previous_pay_to_date: Money,
    previous_tax_to_date: Money,
    pay_date: str,
) -> PayrollGiving:
    """Payroll Giving (ITEPA 2003 s713): the donation is deducted from pay
    before PAYE is worked out, so the relief is whatever PAYE the lower pay
    saves, by the same tax table routines the payroll uses. PAYE rounds
    taxable pay down to whole pounds, so a 50p donation can save nothing.
    """
    if donation.currency != "GBP":
        raise ValueError("payroll amounts must be in GBP, received %s for donation" % (donation.currency,))
    if donation.minor < 0:
        raise ValueError("donation must not be negative, received %d" % (donation.minor,))
    if donation.minor > pay.minor:
        raise ValueError("donation of %d is more than pay of %d" % (donation.minor, pay.minor))
    taxable_pay = money(pay.minor - donation.minor, "GBP")
    without = income_tax(tax_code, frequency, period, pay, previous_pay_to_date, previous_tax_to_date, pay_date).tax.minor
    with_giving = income_tax(
        tax_code, frequency, period, taxable_pay, previous_pay_to_date, previous_tax_to_date, pay_date
    ).tax.minor
    return PayrollGiving(
        taxable_pay=taxable_pay,
        tax_without_giving=money(without, "GBP"),
        tax_with_giving=money(with_giving, "GBP"),
        tax_saved=money(without - with_giving, "GBP"),
        net_cost=money(donation.minor - (without - with_giving), "GBP"),
    )