"""Nutrition math — the ONLY place nutrition is computed (spec §8.1 rule 1).

Routers never compute nutrition; the frontend never re-derives it.
`quantity` is interpreted by context (spec §2.1):
  - unit_type "weight": quantity is grams; nutrition = (quantity / 100) × per_unit
  - unit_type "count":  quantity is item count; nutrition = quantity × per_unit
  - meal:               quantity is a scaling factor (1.0 = one full meal)
"""

from models import Food


def scale_to_quantity(per_unit: float | None, quantity: float, unit_type: str) -> float:
    """Scale a per-unit nutrition value to a logged quantity. Missing values count as 0."""
    if per_unit is None:
        return 0.0
    if unit_type == "weight":
        return per_unit * quantity / 100.0
    if unit_type == "count":
        return per_unit * quantity
    raise ValueError(f"unknown unit_type: {unit_type!r}")


def entry_calories(food: Food, quantity: float) -> float:
    """Calories for a single (non-meal) food at a logged quantity.

    TODO: handle is_meal foods by summing scaled component nutrition
    (recursively, with cycle detection — spec §2.2).
    """
    return scale_to_quantity(food.calories_per_unit, quantity, food.unit_type)