"""Boundary and invalid-input tests for the DCF engine."""

import pandas as pd
import pytest

from app.services.data_service import _run_dcf


def _fcf_series(base: float, growth: float) -> pd.Series:
    """Return a two-point historical FCF series whose median YoY growth equals `growth`."""
    prior = base / (1 + growth)
    return pd.Series([prior, base], index=pd.to_datetime(["2023-09-30", "2024-09-30"]))


def _expected_mid_year_enterprise_value(
    base_fcf: float,
    growth: float,
    wacc: float,
    terminal_growth: float,
    projection_years: int,
) -> float:
    projected = [base_fcf * ((1 + growth) ** year) for year in range(1, projection_years + 1)]
    discounted = [fcf / ((1 + wacc) ** (year - 0.5)) for year, fcf in enumerate(projected, start=1)]
    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))
    return sum(discounted) + terminal_value_pv


def test_run_dcf_two_year_horizon_uses_mid_year_discounting() -> None:
    fcf = _fcf_series(100.0, 0.05)
    expected_enterprise_value = _expected_mid_year_enterprise_value(
        base_fcf=100.0,
        growth=0.05,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=2,
    )

    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=2,
    )

    assert result["enterprise_value"] == pytest.approx(expected_enterprise_value)


def test_run_dcf_terminal_growth_equals_wacc_is_error() -> None:
    fcf = _fcf_series(100.0, 0.05)
    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.10,
        projection_years=5,
    )
    assert "error" in result
    assert "Terminal growth" in result["error"]
    assert "intrinsic_value_per_share" not in result


def test_run_dcf_terminal_growth_above_wacc_is_error() -> None:
    fcf = _fcf_series(100.0, 0.05)
    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.11,
        projection_years=5,
    )
    assert "error" in result
    assert "Terminal growth" in result["error"]
    assert "intrinsic_value_per_share" not in result


def test_run_dcf_zero_wacc_is_error() -> None:
    fcf = _fcf_series(100.0, 0.05)
    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.0,
        terminal_growth=0.03,
        projection_years=5,
    )
    assert "error" in result
    assert "WACC" in result["error"]
    assert "intrinsic_value_per_share" not in result


def test_run_dcf_negative_base_fcf_is_error() -> None:
    """The most recent FCF is negative, making the DCF not meaningful."""
    fcf = pd.Series([100.0, -50.0], index=pd.to_datetime(["2023-09-30", "2024-09-30"]))
    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
    )
    assert "error" in result
    assert "negative" in result["error"].lower() or "zero" in result["error"].lower()
    assert "intrinsic_value_per_share" not in result


def test_run_dcf_zero_base_fcf_is_error() -> None:
    fcf = pd.Series([100.0, 0.0], index=pd.to_datetime(["2023-09-30", "2024-09-30"]))
    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
    )
    assert "error" in result
    assert "zero" in result["error"].lower()
    assert "intrinsic_value_per_share" not in result


def test_run_dcf_zero_shares_returns_empty() -> None:
    fcf = _fcf_series(100.0, 0.05)
    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=0.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
    )
    assert result == {}


def test_run_dcf_negative_shares_returns_empty() -> None:
    fcf = _fcf_series(100.0, 0.05)
    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=-1.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
    )
    assert result == {}


def test_run_dcf_insufficient_history_returns_empty() -> None:
    fcf = pd.Series([100.0], index=pd.to_datetime(["2024-09-30"]))
    result = _run_dcf(
        fcf_series=fcf,
        shares_outstanding=10.0,
        wacc=0.10,
        terminal_growth=0.03,
        projection_years=5,
    )
    assert result == {}