from dataclasses import dataclass
from decimal import Decimal, ROUND_HALF_UP
from .models import Invoice

CENT = Decimal('0.01')

def rounded(value: Decimal) -> Decimal:
    return value.quantize(CENT, rounding=ROUND_HALF_UP)

def line_net(item) -> Decimal:
    return rounded(item.quantity * item.unit_price_net)

@dataclass(frozen=True)
class Totals:
    net_total: Decimal
    tax_total: Decimal
    gross_total: Decimal
    tax_by_rate: dict[Decimal, Decimal]
    basis_by_rate: dict[Decimal, Decimal]

def compute_totals(inv: Invoice) -> Totals:
    bases = {}
    for item in inv.items:
        bases[item.tax_rate] = bases.get(item.tax_rate, Decimal(0)) + line_net(item)
    taxes = {rate: rounded(base * rate / 100) for rate, base in bases.items()}
    net = sum(bases.values(), Decimal(0))
    tax = sum(taxes.values(), Decimal(0))
    return Totals(net, tax, net + tax, taxes, bases)