from typing import List, Sequence

from .dates_days_between import days_between
from .finance_tax_vat_return_types import VatReturn, VatTransaction
from .math_round_div import round_div
from .money_add import add_money
from .money_amount import Money, money

_ZERO_VAT_SALES = ("exempt", "outside-scope", "domestic-reverse-charge", "services-reverse-charge", "ni-eu-goods")
_ZERO_VAT_PURCHASES = ("exempt", "outside-scope")
_TREATMENTS = (
    "taxable",
    "exempt",
    "outside-scope",
    "domestic-reverse-charge",
    "services-reverse-charge",
    "postponed-import-vat",
    "ni-eu-goods",
)


def _whole_pounds(amount: Money) -> Money:
    """Drop the pence, towards zero: boxes 6 to 9 are whole pounds."""
    return money(round_div(amount.minor, 100, "down") * 100, amount.currency)


def vat_return(transactions: Sequence[VatTransaction], period_start: str, period_end: str) -> VatReturn:
    """The nine boxes of a UK VAT return for a period.

    Each transaction feeds the boxes its direction and treatment call for
    (VAT Notice 700/12 sections 3 and 4.6); boxes 6 to 9 drop the pence after
    totalling.
    """
    if days_between(period_start, period_end) < 0:
        raise ValueError("VAT return period ends %s, before it starts %s" % (period_end, period_start))
    box: List[Money] = [money(0, "GBP") for _ in range(10)]
    count = 0
    for index, t in enumerate(transactions):
        n = index + 1
        if t.direction not in ("sale", "purchase"):
            raise ValueError('transaction %d: unknown direction "%s"' % (n, t.direction))
        if t.treatment not in _TREATMENTS:
            raise ValueError('transaction %d: unknown VAT treatment "%s"' % (n, t.treatment))
        if t.net.currency != "GBP" or t.vat.currency != "GBP":
            raise ValueError("transaction %d: amounts must be in GBP" % (n,))
        sale = t.direction == "sale"
        if sale and t.treatment == "postponed-import-vat":
            raise ValueError("transaction %d: postponed-import-vat applies to purchases only" % (n,))
        if t.vat.minor != 0 and t.treatment in (_ZERO_VAT_SALES if sale else _ZERO_VAT_PURCHASES):
            raise ValueError("transaction %d: %s %ss must have zero VAT" % (n, t.treatment, t.direction))
        if days_between(period_start, t.date) < 0 or days_between(t.date, period_end) < 0:
            continue
        count += 1

        def add(b: int, amount: Money) -> None:
            box[b] = add_money(box[b], amount)

        if sale:
            if t.treatment == "taxable":
                add(1, t.vat)
            if t.treatment != "outside-scope":
                add(6, t.net)
            if t.treatment == "ni-eu-goods":
                add(8, t.net)
        elif t.treatment == "taxable":
            add(4, t.vat)
            add(7, t.net)
        elif t.treatment == "exempt":
            add(7, t.net)
        elif t.treatment in ("domestic-reverse-charge", "services-reverse-charge", "postponed-import-vat"):
            if t.treatment == "services-reverse-charge":
                add(6, t.net)
            add(1, t.vat)
            add(4, t.vat)
            add(7, t.net)
        elif t.treatment == "ni-eu-goods":
            add(2, t.vat)
            add(4, t.vat)
            add(7, t.net)
            add(9, t.net)

    box3 = add_money(box[1], box[2])
    difference = box3.minor - box[4].minor
    return VatReturn(
        box1=box[1],
        box2=box[2],
        box3=box3,
        box4=box[4],
        box5=money(abs(difference), "GBP"),
        box6=_whole_pounds(box[6]),
        box7=_whole_pounds(box[7]),
        box8=_whole_pounds(box[8]),
        box9=_whole_pounds(box[9]),
        repayment=difference < 0,
        transactions=count,
    )