"""Modul prosedural validasi checkout (FR-ORD-02, FR-ORD-03, FR-ORD-08).

Harga diambil dari katalog, bukan dari input klien. Satu order hanya boleh
berisi produk dari satu toko. Keranjang kosong mengembalikan total 0; penolakan
keranjang kosong menjadi tanggung jawab pemanggil.
"""

from dataclasses import dataclass


class CheckoutError(Exception):
    """Kelas dasar semua kegagalan validasi checkout."""


class ProductNotFoundError(CheckoutError):
    pass


class InvalidQuantityError(CheckoutError):
    pass


class MultiShopError(CheckoutError):
    pass


class InsufficientStockError(CheckoutError):
    pass


@dataclass(frozen=True)
class CartItem:
    product_id: str
    quantity: int


@dataclass(frozen=True)
class CheckoutSummary:
    shop_id: str | None
    total_amount: int


def validate_checkout(items, products):
    """Validasi keranjang terhadap katalog lalu hitung total dalam rupiah."""
    shop_id = None
    total = 0
    for item in items:
        product = products.get(item.product_id)
        if product is None:
            raise ProductNotFoundError(item.product_id)
        if item.quantity <= 0:
            raise InvalidQuantityError(item.product_id)
        if shop_id is None:
            shop_id = product.shop_id
        elif product.shop_id != shop_id:
            raise MultiShopError(item.product_id)
        if product.available < item.quantity:
            raise InsufficientStockError(item.product_id)
        total += product.price * item.quantity
    return CheckoutSummary(shop_id, total)