"""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

# All nutrition fields tracked on foods. Each has a corresponding *_per_unit
# column on the Food model. Used by entry_nutrition() to iterate over fields.
NUTRITION_FIELDS = [
    "calories", "protein_g", "carbs_g", "fat_g",
    "fiber_g", "saturated_fat_g", "sugars_g", "sodium_g",
]

# Maps each NUTRITION_FIELDS key to the Food model's per_unit column name.
# Naming is slightly irregular (e.g. "protein_g" → "protein_per_unit",
# not "protein_g_per_unit").
_FIELD_TO_PER_UNIT_COL = {
    "calories": "calories_per_unit",
    "protein_g": "protein_per_unit",
    "carbs_g": "carbs_per_unit",
    "fat_g": "fat_per_unit",
    "fiber_g": "fiber_per_unit",
    "saturated_fat_g": "saturated_fat_per_unit",
    "sugars_g": "sugars_per_unit",
    "sodium_g": "sodium_per_unit",
}


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)


def entry_nutrition(food: Food, quantity: float) -> dict[str, float]:
    """Return all nutrition fields scaled to a logged quantity.

    Each field is resolved via scale_to_quantity using the food's unit_type.
    NULL per-unit values contribute 0.0, not an error.

    Meal foods (is_meal=True) have null per_unit fields per the CHECK
    constraint, so they naturally contribute 0 for all fields.
    TODO: TICKET-007 — real meal nutrition will sum scaled component
    foods recursively. Until then, meal entries contribute 0.
    """
    return {
        field: scale_to_quantity(
            getattr(food, _FIELD_TO_PER_UNIT_COL[field], None),
            quantity,
            food.unit_type,
        )
        for field in NUTRITION_FIELDS
    }