"""Thermal hydraulics approximations."""

from __future__ import annotations

from dataclasses import dataclass
import logging

import math

from . import constants
from .state import CoolantLoopState, CoreState

LOGGER = logging.getLogger(__name__)


def heat_transfer(primary: CoolantLoopState, secondary: CoolantLoopState, core_power_mw: float) -> float:
    """Return MW transferred to the secondary loop."""
    delta_t = max(0.0, primary.temperature_out - secondary.temperature_in)
    conductance = 0.15  # steam generator effectiveness
    efficiency = 1.0 - math.exp(-conductance * delta_t)
    transferred = min(core_power_mw, core_power_mw * efficiency)
    LOGGER.debug("Heat transfer %.2f MW with ΔT=%.1fK", transferred, delta_t)
    return transferred


def temperature_rise(power_mw: float, mass_flow: float) -> float:
    if mass_flow <= 0:
        return 0.0
    return (power_mw * constants.MEGAWATT) / (mass_flow * constants.COOLANT_HEAT_CAPACITY)


@dataclass
class ThermalSolver:
    primary_volume_m3: float = 300.0

    def step_core(self, core: CoreState, primary: CoolantLoopState, power_mw: float, dt: float) -> None:
        temp_rise = temperature_rise(power_mw, primary.mass_flow_rate)
        primary.temperature_out = primary.temperature_in + temp_rise
        # Fuel heats from any power not immediately convected away, and cools toward the primary outlet.
        heating = 0.005 * max(0.0, power_mw - temp_rise) * dt
        cooling = 0.05 * max(0.0, core.fuel_temperature - primary.temperature_out) * dt
        core.fuel_temperature += heating - cooling
        # Keep fuel temperature bounded and never below the coolant outlet temperature.
        core.fuel_temperature = min(max(primary.temperature_out, core.fuel_temperature), constants.MAX_CORE_TEMPERATURE)
        LOGGER.debug(
            "Primary loop: flow=%.0f kg/s temp_out=%.1fK core_temp=%.1fK",
            primary.mass_flow_rate,
            primary.temperature_out,
            core.fuel_temperature,
        )

    def step_secondary(self, secondary: CoolantLoopState, transferred_mw: float) -> None:
        delta_t = temperature_rise(transferred_mw, secondary.mass_flow_rate)
        secondary.temperature_out = secondary.temperature_in + delta_t
        secondary.steam_quality = min(1.0, max(0.0, delta_t / 100.0))
        secondary.pressure = min(constants.MAX_PRESSURE, 6.0 + delta_t * 0.01)
        LOGGER.debug(
            "Secondary loop: transferred=%.1fMW temp_out=%.1fK quality=%.2f",
            transferred_mw,
            secondary.temperature_out,
            secondary.steam_quality,
        )