"""Cost calculation for meter rate components."""

import logging
from dataclasses import dataclass

from hameter.config import RateComponent

logger = logging.getLogger(__name__)


@dataclass
class CostResult:
    """Result of a cost calculation for a single reading delta."""

    delta: float
    per_unit_cost: float
    component_costs: list[dict]
    total_incremental_cost: float


def calculate_incremental_cost(
    delta: float,
    cost_factors: list[RateComponent],
) -> CostResult:
    """Calculate the incremental cost for a usage delta.

    Only per_unit rate components contribute to incremental cost.
    Fixed charges are NOT included — they are handled separately
    via the billing period reset / manual add workflow.

    Args:
        delta: Usage delta in calibrated units (e.g., kWh).
        cost_factors: List of rate components from meter config.

    Returns:
        CostResult with per-component breakdown and total.
    """
    component_costs = []
    per_unit_total = 0.0

    for cf in cost_factors:
        if cf.type == "per_unit":
            cost = round(delta * cf.rate, 4)
            per_unit_total += cost
        else:
            cost = 0.0

        component_costs.append({
            "name": cf.name,
            "rate": cf.rate,
            "type": cf.type,
            "cost": cost,
        })

    return CostResult(
        delta=delta,
        per_unit_cost=round(per_unit_total, 4),
        component_costs=component_costs,
        total_incremental_cost=round(per_unit_total, 4),
    )