"""Tests for the ELO calculation module."""

import pytest

from app.elo import calculate_elo


class TestCalculateElo:
    """Tests for calculate_elo."""

    def test_equal_ratings(self):
        """Equal ratings should produce symmetric changes."""
        new_w, new_l = calculate_elo(1500.0, 1500.0, k_factor=32.0)
        assert new_w > 1500.0
        assert new_l < 1500.0
        assert new_w - 1500.0 == pytest.approx(1500.0 - new_l, abs=0.01)

    def test_higher_rated_wins(self):
        """Higher-rated winner should get a small gain."""
        new_w, new_l = calculate_elo(1800.0, 1200.0, k_factor=32.0)
        change = new_w - 1800.0
        assert 0 < change < 16.0  # expected win → small gain

    def test_lower_rated_wins(self):
        """Lower-rated winner (upset) should get a large gain."""
        new_w, new_l = calculate_elo(1200.0, 1800.0, k_factor=32.0)
        change = new_w - 1200.0
        assert change > 16.0  # upset → large gain

    def test_k_factor_scales_change(self):
        """Higher K-factor should produce larger rating changes."""
        _, _ = calculate_elo(1500.0, 1500.0, k_factor=16.0)
        new_w_16, _ = calculate_elo(1500.0, 1500.0, k_factor=16.0)
        new_w_64, _ = calculate_elo(1500.0, 1500.0, k_factor=64.0)
        assert (new_w_64 - 1500.0) > (new_w_16 - 1500.0)

    def test_total_elo_preserved(self):
        """Total ELO should be preserved (zero-sum)."""
        new_w, new_l = calculate_elo(1600.0, 1400.0, k_factor=32.0)
        assert new_w + new_l == pytest.approx(1600.0 + 1400.0, abs=0.01)

    def test_result_is_rounded(self):
        """Results should be rounded to 2 decimal places."""
        new_w, new_l = calculate_elo(1500.0, 1500.0, k_factor=32.0)
        assert new_w == round(new_w, 2)
        assert new_l == round(new_l, 2)