from __future__ import annotations

import csv
from dataclasses import dataclass
from decimal import Decimal, InvalidOperation
from pathlib import Path


@dataclass(frozen=True, slots=True)
class RawCatalogItem:
    sku: str
    name: str
    unit: str
    price: Decimal


def load_raw_catalog(path: Path) -> tuple[RawCatalogItem, ...]:
    if not path.is_file():
        raise FileNotFoundError(f"Catalog file does not exist: {path}")

    items: list[RawCatalogItem] = []
    with path.open("r", encoding="utf-8-sig", newline="") as file:
        reader = csv.DictReader(file)
        expected = {"sku", "name", "unit", "price"}
        if set(reader.fieldnames or ()) != expected:
            raise ValueError(
                f"Unexpected catalog columns: {reader.fieldnames!r}; expected {sorted(expected)!r}"
            )

        seen_skus: set[str] = set()
        for line_number, row in enumerate(reader, start=2):
            sku = (row.get("sku") or "").strip()
            name = (row.get("name") or "").strip()
            unit = (row.get("unit") or "").strip()
            price_text = (row.get("price") or "").strip()

            if not sku or not name or not unit or not price_text:
                raise ValueError(f"Catalog line {line_number} contains an empty required field")
            if sku in seen_skus:
                raise ValueError(f"Duplicate SKU at catalog line {line_number}: {sku}")

            try:
                price = Decimal(price_text)
            except InvalidOperation as exc:
                raise ValueError(
                    f"Invalid price at catalog line {line_number}: {price_text!r}"
                ) from exc

            seen_skus.add(sku)
            items.append(RawCatalogItem(sku=sku, name=name, unit=unit, price=price))

    if not items:
        raise ValueError("Catalog is empty")
    return tuple(items)