"""Exact-math verification tests for the DCF engine.

Each scenario independently recomputes the expected projection, discounting,
terminal value, enterprise value, equity value, and per-share intrinsic value,
then asserts the implementation matches. This makes the test itself auditable.
"""

import pandas as pd
import pytest

from app.services.data_service import _run_dcf


def _fcf_series(prior: float, base: float) -> pd.Series:
    """Return a two-point historical FCF series: prior -> base (most recent)."""
    return pd.Series([prior, base], index=pd.to_datetime(["2023-09-30", "2024-09-30"]))


def _expected_dcf(
    base_fcf: float,
    growth_rate: float,
    wacc: float,
    terminal_growth: float,
    projection_years: int,
    shares_outstanding: float,
    total_debt: float = 0.0,
    cash_and_equivalents: float = 0.0,
    preferred_equity: float = 0.0,
    minority_interest: float = 0.0,
) -> dict:
    """Independent reference implementation of the DCF math used by `_run_dcf`."""
    projected = [base_fcf * ((1 + growth_rate) ** yr) for yr in range(1, projection_years + 1)]
    discounted = [fcf / ((1 + wacc) ** (year - 0.5)) for year, fcf in enumerate(projected, start=1)]
    fcf_pv_sum = sum(discounted)

    terminal_fcf = projected[-1] * (1 + terminal_growth)
    terminal_value = terminal_fcf / (wacc - terminal_growth)
    terminal_value_pv = terminal_value / ((1 + wacc) ** (projection_years - 0.5))

    enterprise_value = fcf_pv_sum + terminal_value_pv
    net_debt = total_debt - cash_and_equivalents
    equity_value = enterprise_value - net_debt - preferred_equity - minority_interest
    intrinsic_value_per_share = equity_value / shares_outstanding

    return {
        "fcf_pv_sum": fcf_pv_sum,
        "terminal_value_pv": terminal_value_pv,
        "enterprise_value": enterprise_value,
        "net_debt": net_debt,
        "equity_value": equity_value,
        "intrinsic_value_per_share": intrinsic_value_per_share,
    }


_APPROX = pytest.approx


def test_run_dcf_one_year_with_equity_bridge() -> None:
    """1-year projection with debt, cash, preferred, and minority claims."""
    fcf = _fcf_series(100.0, 105.0)  # exact 5% growth
    expected = _expected_dcf(
        base_fcf=105.0,
        growth_rate=0.05,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=1,
        shares_outstanding=10.0,
        total_debt=20.0,
        cash_and_equivalents=5.0,
        preferred_equity=2.0,
        minority_interest=3.0,
    )

    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=1,
        total_debt=20.0,
        cash_and_equivalents=5.0,
        preferred_equity=2.0,
        minority_interest=3.0,
    )

    assert result["base_fcf"] == 105.0
    assert result["growth_rate_used"] == _APPROX(0.05)
    assert result["fcf_pv_sum"] == _APPROX(expected["fcf_pv_sum"])
    assert result["terminal_value_pv"] == _APPROX(expected["terminal_value_pv"])
    assert result["enterprise_value"] == _APPROX(expected["enterprise_value"])
    assert result["net_debt"] == expected["net_debt"]
    assert result["equity_value"] == _APPROX(expected["equity_value"])
    assert result["intrinsic_value_per_share"] == _APPROX(expected["intrinsic_value_per_share"])


def test_run_dcf_three_years_no_claims() -> None:
    """3-year projection with a clean capital structure."""
    fcf = _fcf_series(100.0, 105.0)
    expected = _expected_dcf(
        base_fcf=105.0,
        growth_rate=0.05,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=3,
        shares_outstanding=10.0,
    )

    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=3,
    )

    assert result["base_fcf"] == 105.0
    assert result["fcf_pv_sum"] == _APPROX(expected["fcf_pv_sum"])
    assert result["terminal_value_pv"] == _APPROX(expected["terminal_value_pv"])
    assert result["enterprise_value"] == _APPROX(expected["enterprise_value"])
    assert result["equity_value"] == _APPROX(expected["equity_value"])
    assert result["intrinsic_value_per_share"] == _APPROX(expected["intrinsic_value_per_share"])
    assert result["net_debt"] == 0.0


def test_run_dcf_five_years_default_horizon() -> None:
    """5-year projection matching the API default horizon."""
    fcf = _fcf_series(100.0, 105.0)
    expected = _expected_dcf(
        base_fcf=105.0,
        growth_rate=0.05,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
        shares_outstanding=10.0,
    )

    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
    )

    assert result["base_fcf"] == 105.0
    assert result["fcf_pv_sum"] == _APPROX(expected["fcf_pv_sum"])
    assert result["terminal_value_pv"] == _APPROX(expected["terminal_value_pv"])
    assert result["enterprise_value"] == _APPROX(expected["enterprise_value"])
    assert result["equity_value"] == _APPROX(expected["equity_value"])
    assert result["intrinsic_value_per_share"] == _APPROX(expected["intrinsic_value_per_share"])


def test_run_dcf_zero_growth_flat_fcf() -> None:
    """Edge of the model: zero growth projects a flat FCF stream."""
    fcf = _fcf_series(100.0, 100.0)  # exact 0% growth
    expected = _expected_dcf(
        base_fcf=100.0,
        growth_rate=0.0,
        wacc=0.10,
        terminal_growth=0.02,
        projection_years=5,
        shares_outstanding=10.0,
    )

    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.02,
        projection_years=5,
    )

    assert result["growth_rate_used"] == _APPROX(0.0)
    assert result["fcf_pv_sum"] == _APPROX(expected["fcf_pv_sum"])
    assert result["terminal_value_pv"] == _APPROX(expected["terminal_value_pv"])
    assert result["enterprise_value"] == _APPROX(expected["enterprise_value"])
    assert result["intrinsic_value_per_share"] == _APPROX(expected["intrinsic_value_per_share"])


def test_run_dcf_uses_historical_growth_rate() -> None:
    """End-to-end check that the growth-rate estimator feeds the projection.

    A series rising exactly 5% per year should produce the same 5-year result as
    the hand-computed 5-year scenario (within float tolerance).
    """
    fcf = _fcf_series(100.0, 105.0)
    expected = _expected_dcf(
        base_fcf=105.0,
        growth_rate=0.05,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
        shares_outstanding=10.0,
    )

    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
    )

    assert result["growth_rate_used"] == _APPROX(0.05)
    assert result["intrinsic_value_per_share"] == _APPROX(expected["intrinsic_value_per_share"])