"""
Unit tests for WalkForwardEngine window generation logic.
"""
from datetime import date

import pytest

from app.services.backtest.walkforward_engine import WalkForwardEngine, Window


class TestGenerateWindows:
    """Test _generate_windows static method."""

    def test_basic_windows(self):
        """2-year period with 12m train, 3m test, 3m step -> 4 windows."""
        windows = WalkForwardEngine._generate_windows(
            start=date(2020, 1, 1),
            end=date(2021, 12, 31),
            train_months=12,
            test_months=3,
            step_months=3,
        )
        assert len(windows) == 4
        assert windows[0].index == 0
        assert windows[0].train_start == date(2020, 1, 1)
        assert windows[0].train_end == date(2020, 12, 31)
        assert windows[0].test_start == date(2021, 1, 1)
        assert windows[0].test_end == date(2021, 3, 31)

    def test_single_window(self):
        """Exactly 15 months -> 1 window with 12m train + 3m test."""
        windows = WalkForwardEngine._generate_windows(
            start=date(2020, 1, 1),
            end=date(2021, 3, 31),
            train_months=12,
            test_months=3,
            step_months=3,
        )
        assert len(windows) == 1
        assert windows[0].train_start == date(2020, 1, 1)
        assert windows[0].test_end == date(2021, 3, 31)

    def test_no_windows_period_too_short(self):
        """Period shorter than train + test -> 0 windows."""
        windows = WalkForwardEngine._generate_windows(
            start=date(2020, 1, 1),
            end=date(2020, 12, 31),
            train_months=12,
            test_months=3,
            step_months=3,
        )
        assert len(windows) == 0

    def test_partial_last_window(self):
        """Last window with partial test period is included."""
        windows = WalkForwardEngine._generate_windows(
            start=date(2020, 1, 1),
            end=date(2021, 2, 15),
            train_months=12,
            test_months=3,
            step_months=3,
        )
        assert len(windows) == 1
        assert windows[0].test_end == date(2021, 2, 15)

    def test_step_larger_than_test(self):
        """step_months > test_months creates non-overlapping test windows."""
        windows = WalkForwardEngine._generate_windows(
            start=date(2019, 1, 1),
            end=date(2022, 12, 31),
            train_months=12,
            test_months=3,
            step_months=6,
        )
        assert len(windows) >= 2
        # test windows should not overlap
        for i in range(1, len(windows)):
            assert windows[i].test_start > windows[i - 1].test_end

    def test_monthly_step(self):
        """step_months=1 creates many overlapping windows."""
        windows = WalkForwardEngine._generate_windows(
            start=date(2020, 1, 1),
            end=date(2021, 6, 30),
            train_months=6,
            test_months=3,
            step_months=1,
        )
        assert len(windows) >= 9

    def test_window_indices_sequential(self):
        """Window indices should be sequential starting from 0."""
        windows = WalkForwardEngine._generate_windows(
            start=date(2019, 1, 1),
            end=date(2022, 12, 31),
            train_months=12,
            test_months=3,
            step_months=3,
        )
        for i, w in enumerate(windows):
            assert w.index == i

    def test_window_dates_consistent(self):
        """train_end < test_start and test_start <= test_end for all windows."""
        windows = WalkForwardEngine._generate_windows(
            start=date(2019, 1, 1),
            end=date(2023, 12, 31),
            train_months=12,
            test_months=6,
            step_months=3,
        )
        for w in windows:
            assert w.train_start < w.train_end
            assert w.train_end < w.test_start
            assert w.test_start <= w.test_end