from __future__ import annotations
from dataclasses import dataclass
from typing import Dict
from .models import Invoice

@dataclass(frozen=True)
class Totals:
    net_total: float
    tax_total: float
    gross_total: float
    tax_by_rate: Dict[float, float]

def compute_totals(inv: Invoice) -> Totals:
    net = 0.0
    tax_total = 0.0
    tax_by_rate: Dict[float, float] = {}
    for it in inv.items:
        line_net = float(it.quantity) * float(it.unit_price_net)
        net += line_net
        tax = line_net * float(it.tax_rate) / 100.0
        tax_total += tax
        tax_by_rate[it.tax_rate] = tax_by_rate.get(it.tax_rate, 0.0) + tax
    gross = net + tax_total
    return Totals(net_total=net, tax_total=tax_total, gross_total=gross, tax_by_rate=tax_by_rate)