File

from dataclasses import dataclass

from typing import Iterable

@dataclass(frozen=True)

class InvoiceLine:

    unit_price: float

    quantity: int = 1

    def total(self) -> float:

        if self.unit_price < 0:

            raise ValueError("unit_price must be >= 0")

        if self.quantity <= 0:

            raise ValueError("quantity must be > 0")

        return round(self.unit_price * self.quantity, 2)

def calculate_invoice(lines: Iterable[InvoiceLine], discount_pct: float = 0.0, tax_pct: float = 0.0) -> dict:

    if not 0 <= discount_pct <= 100:

        raise ValueError("discount_pct must be between 0 and 100")

    if tax_pct < 0:

        raise ValueError("tax_pct must be >= 0")

    items = list(lines)

    subtotal = round(sum(line.total() for line in items), 2)

    discount = round(subtotal * discount_pct / 100, 2)

    taxable = round(subtotal - discount, 2)

    tax = round(taxable * tax_pct / 100, 2)

    total = round(taxable + tax, 2)

    return {"subtotal": subtotal, "discount": discount, "taxable": taxable, "tax": tax, "total": total, "line_count": len(items)}