trueskill-tt/tests/degenerate_inputs.rs

//! Degenerate, boundary, and error-path coverage.
//!
//! These run in both debug and release: the defects they pin were all
//! guarded only by `debug_assert!`, so a debug-only suite never saw them.

use trueskill_tt::{
    ConstantDrift, ConvergenceOptions, Game, GameOptions, Gaussian, History, InferenceError,
    Outcome, Rating,
};

type R = Rating<i64, ConstantDrift>;

fn rating() -> R {
    R::new(
        Gaussian::from_ms(25.0, 25.0 / 3.0),
        25.0 / 6.0,
        ConstantDrift(25.0 / 300.0),
    )
}

fn assert_finite(g: Gaussian, what: &str) {
    assert!(
        g.mu().is_finite() && g.sigma().is_finite(),
        "{what} must be finite, got mu={} sigma={}",
        g.mu(),
        g.sigma()
    );
}

#[test]
fn record_draw_without_draw_probability_is_rejected() {
    let mut h = History::default();
    let err = h.record_draw(&"a", &"b", 1).unwrap_err();
    assert!(matches!(
        err,
        InferenceError::TieWithoutDrawProbability { .. }
    ));
}

#[test]
fn builder_draw_without_draw_probability_is_rejected() {
    let mut h = History::default();
    let err = h
        .event(1)
        .team(["a"])
        .team(["b"])
        .draw()
        .commit()
        .unwrap_err();
    assert!(matches!(
        err,
        InferenceError::TieWithoutDrawProbability { .. }
    ));
}

#[test]
fn draw_with_positive_draw_probability_is_finite() {
    let mut h = History::builder().p_draw(0.25).build();
    h.record_draw(&"a", &"b", 1).unwrap();
    let report = h.converge().unwrap();

    assert_finite(h.current_skill("a").unwrap(), "drawn competitor skill");
    assert_finite(h.current_skill("b").unwrap(), "drawn competitor skill");
    assert!(report.log_evidence.is_finite());
    assert!(report.converged);
}

#[test]
fn game_ranked_rejects_tie_without_draw_probability() {
    let a = [rating()];
    let b = [rating()];
    let teams: Vec<&[R]> = vec![&a, &b];
    let err = Game::ranked(&teams, Outcome::draw(2), &GameOptions::default()).unwrap_err();
    assert!(matches!(
        err,
        InferenceError::TieWithoutDrawProbability { .. }
    ));
}

/// `Outcome::winner(w, n)` ties every loser, so any n >= 3 free-for-all hits
/// the tie path even though the caller never asked for a draw.
#[test]
fn winner_of_three_or_more_requires_draw_probability() {
    let a = [rating()];
    let b = [rating()];
    let c = [rating()];
    let teams: Vec<&[R]> = vec![&a, &b, &c];

    let err = Game::ranked(&teams, Outcome::winner(0, 3), &GameOptions::default()).unwrap_err();
    assert!(matches!(
        err,
        InferenceError::TieWithoutDrawProbability { .. }
    ));

    let opts = GameOptions {
        p_draw: 0.1,
        ..GameOptions::default()
    };
    let game = Game::ranked(&teams, Outcome::winner(0, 3), &opts).unwrap();
    for team in game.posteriors() {
        for skill in team {
            assert_finite(skill, "3-team winner posterior");
        }
    }
}

#[test]
fn full_ranking_without_ties_needs_no_draw_probability() {
    let a = [rating()];
    let b = [rating()];
    let c = [rating()];
    let teams: Vec<&[R]> = vec![&a, &b, &c];
    let game = Game::ranked(&teams, Outcome::ranking([0, 1, 2]), &GameOptions::default()).unwrap();

    for team in game.posteriors() {
        for skill in team {
            assert_finite(skill, "strict ranking posterior");
        }
    }
}

#[test]
fn empty_history_converges_trivially() {
    let mut h = History::default();
    let report = h.converge().unwrap();
    assert_eq!(report.iterations, 0);
    assert!(report.converged);
}

#[test]
fn empty_event_stream_then_converge() {
    let mut h = History::default();
    h.add_events(std::iter::empty()).unwrap();
    let report = h.converge().unwrap();
    assert_eq!(report.iterations, 0);
}

#[test]
fn empty_history_queries_do_not_panic() {
    let h = History::default();
    assert!(h.learning_curves().is_empty());
    assert!(h.learning_curve("nobody").is_empty());
    assert!(h.current_skill("nobody").is_none());
}

#[test]
fn single_event_history_converges() {
    let mut h = History::default();
    h.record_winner(&"a", &"b", 1).unwrap();
    let report = h.converge().unwrap();
    assert!(report.converged);
    assert_finite(h.current_skill("a").unwrap(), "single-event skill");
}

#[test]
fn scored_event_rejects_non_positive_sigma() {
    let mut h = History::builder().score_sigma(2.0).build();
    let err = h
        .event(1)
        .team(["a"])
        .team(["b"])
        .scores_with_sigma([3.0, 1.0], f64::NAN)
        .commit()
        .unwrap_err();
    assert!(matches!(
        err,
        InferenceError::InvalidParameter {
            name: "score_sigma",
            ..
        }
    ));
}

#[test]
fn convergence_reports_are_finite_across_many_teams() {
    let opts = GameOptions {
        p_draw: 0.1,
        convergence: ConvergenceOptions::default(),
        ..GameOptions::default()
    };
    let holders: Vec<[R; 1]> = (0..12).map(|_| [rating()]).collect();
    let teams: Vec<&[R]> = holders.iter().map(|t| t.as_slice()).collect();
    let game = Game::ranked(&teams, Outcome::ranking(0..12), &opts).unwrap();

    assert!(
        game.log_evidence().is_finite(),
        "12-team log-evidence must be finite, got {}",
        game.log_evidence()
    );
    for team in game.posteriors() {
        for skill in team {
            assert_finite(skill, "12-team posterior");
        }
    }
}

/// A long diff chain underflows a linear evidence product: each link
/// contributes a probability in (0, 1], so ~1000 links flush the product to
/// exactly 0.0 and `ln(0.0)` is `-inf`. Accumulating in log space keeps it
/// finite.
#[test]
fn log_evidence_survives_a_long_diff_chain() {
    let holders: Vec<[R; 1]> = (0..1200).map(|_| [rating()]).collect();
    let teams: Vec<&[R]> = holders.iter().map(|t| t.as_slice()).collect();
    let game = Game::ranked(
        &teams,
        Outcome::ranking(0..holders.len() as u32),
        &GameOptions::default(),
    )
    .unwrap();

    let log_evidence = game.log_evidence();
    assert!(
        log_evidence.is_finite(),
        "1200-team log-evidence must be finite, got {log_evidence}"
    );
    assert!(
        log_evidence < 0.0,
        "log-evidence of a probability must be negative, got {log_evidence}"
    );
}

/// A near-certain outcome rounds the losing tail to exactly zero in the
/// `erfc` approximation; the evidence floor keeps `ln` finite.
#[test]
fn log_evidence_finite_for_near_certain_outcome() {
    let overwhelming = R::new(Gaussian::from_ms(5_000.0, 0.5), 1.0, ConstantDrift(0.0));
    let hopeless = R::new(Gaussian::from_ms(-5_000.0, 0.5), 1.0, ConstantDrift(0.0));
    let a = [overwhelming];
    let b = [hopeless];
    let teams: Vec<&[R]> = vec![&a, &b];

    let game = Game::ranked(&teams, Outcome::winner(0, 2), &GameOptions::default()).unwrap();
    assert!(
        game.log_evidence().is_finite(),
        "got {}",
        game.log_evidence()
    );

    // And the reverse — a colossal upset — must also stay finite.
    let upset = Game::ranked(&teams, Outcome::winner(1, 2), &GameOptions::default()).unwrap();
    assert!(
        upset.log_evidence().is_finite(),
        "upset log-evidence must be finite, got {}",
        upset.log_evidence()
    );
}