"""Ranking rules for a product line.

Kept out of ``app.py`` so it can be imported and tested without executing the
Streamlit page. Deciding which 10 of 99 products to spend an hour analyzing is
real logic, not presentation.
"""
from __future__ import annotations

# Label shown in the picker → (field to sort on, descending?).
# Ordered by how often each is the right question to ask of a line.
LINE_SORTS: dict[str, tuple[str, bool]] = {
    "Sales velocity (30d)": ("avg_30d", True),
    "Inventory on hand": ("inventory", True),
    "Cover days — lowest first": ("cover_days", False),
    "Reorder size (min PO)": ("min_po_quantity", True),
    "A–Z": ("sku", False),
}


def line_sort_key(product: dict, field: str):
    """Sort key tolerating both text and numeric fields, and missing values.

    Two things it has to get right:

    * **Mixed types.** A single numeric key raises ``TypeError`` the moment the
      user picks A–Z, because it compares ``str`` against ``0``.
    * **Missing values sort last, in either direction.** A SKU with no cover-days
      reading is not the most urgent item in the line — but a plain ``or 0``
      makes it exactly that when sorting ascending.
    """
    value = product.get(field)
    if isinstance(value, str):
        return (False, value.upper())
    return (value is None, value if value is not None else 0)


def rank(products: list[dict], sort_label: str) -> list[dict]:
    """Order a line by one of `LINE_SORTS`, falling back to the first rule."""
    field, reverse = LINE_SORTS.get(sort_label, next(iter(LINE_SORTS.values())))
    return sorted(products, key=lambda p: line_sort_key(p, field), reverse=reverse)