# tests/cryo_tank/test_properties.py
"""Tests for CoolProp property wrappers."""
import pytest
import sys
sys.path.insert(0, "src")

from cryo_tank.properties import (
    ln2_rho, ln2_h, ln2_u, ln2_T_from_u,
    ln2_drho_dT_const_p, ln2_du_dT_const_p,
    he_u, he_h, he_cp, he_cv,
)
from cryo_tank.config import P_WORKING


class TestLN2Properties:
    """Liquid nitrogen properties at P = 0.17 MPa."""

    def test_ln2_density_at_78K(self):
        rho = ln2_rho(78.0, P_WORKING)
        assert 800 < rho < 810  # ~803 kg/m3

    def test_ln2_enthalpy_at_77K(self):
        h = ln2_h(77.0, P_WORKING)
        assert -130000 < h < -110000  # ~-122695 J/kg

    def test_ln2_internal_energy_at_78K(self):
        u = ln2_u(78.0, P_WORKING)
        assert -130000 < u < -110000  # ~-120865 J/kg

    def test_ln2_T_from_u_roundtrip(self):
        T_orig = 78.0
        u = ln2_u(T_orig, P_WORKING)
        T_recovered = ln2_T_from_u(u, P_WORKING)
        assert abs(T_recovered - T_orig) < 0.01

    def test_ln2_density_derivative_matches_finite_difference(self):
        T = 78.0
        step = 1e-3
        actual = ln2_drho_dT_const_p(T, P_WORKING)
        expected = (
            ln2_rho(T + step, P_WORKING)
            - ln2_rho(T - step, P_WORKING)
        ) / (2.0 * step)
        assert actual < 0.0
        assert abs(actual - expected) / abs(expected) < 1e-5

    def test_ln2_internal_energy_derivative_is_positive(self):
        du_dT = ln2_du_dT_const_p(78.0, P_WORKING)
        assert du_dT > 0.0


class TestHeliumProperties:
    """Helium properties."""

    def test_he_cp_near_5196(self):
        cp = he_cp()
        assert abs(cp - 5196.2) < 10

    def test_he_cv_near_3117(self):
        cv = he_cv()
        assert abs(cv - 3117.1) < 10

    def test_he_enthalpy_at_100K(self):
        h = he_h(100.0)
        # CoolProp gives ~524762 J/kg at 100K
        assert 500000 < h < 550000

    def test_he_internal_energy_at_78K(self):
        u = he_u(78.0)
        assert 200000 < u < 280000  # ~247932 J/kg