trueskill-tt/tests/convergence_strictness.rs

//! Stopping short of convergence is an error, not a flag on a success.
//!
//! A fit that hits `max_iter` is wrong by a little: every rating is finite,
//! the ordering looks sensible, and nothing in the numbers says they were
//! still moving. When that was `Ok` with `converged: false`, detecting it was
//! opt-in and `let _ = h.converge()` was the natural way to opt out — which is
//! how a real defect once hid in this crate's own suite.

use smallvec::smallvec;
use trueskill_tt::{
    ConstantDrift, ConvergenceOptions, Event, History, InferenceError, Member, Outcome, Team,
};

type H = History;

fn duel(a: &'static str, b: &'static str, t: i64) -> Event<i64, &'static str> {
    Event {
        time: t,
        teams: smallvec![
            Team::with_members([Member::new(a)]),
            Team::with_members([Member::new(b)]),
        ],
        outcome: Outcome::scores([3.0, 1.0]),
    }
}

fn capped(max_iter: usize) -> H {
    History::builder()
        .mu(0.0)
        .sigma(6.0)
        .beta(1.0)
        .score_sigma(2.0)
        .drift(ConstantDrift::new(0.5))
        .convergence(ConvergenceOptions {
            max_iter,
            epsilon: 1e-13,
            alpha: 1.0,
        })
        .build()
}

fn fill(h: &mut H) {
    h.add_events((1..=6).map(|t| duel("a", "b", t)).collect::<Vec<_>>())
        .unwrap();
}

#[test]
fn hitting_the_cap_is_an_error() {
    let mut h = capped(1);
    fill(&mut h);
    let err = h.converge().unwrap_err();
    match err {
        InferenceError::NotConverged {
            iterations,
            final_step,
            epsilon,
            ..
        } => {
            assert_eq!(iterations, 1);
            assert!(
                final_step.0 > epsilon || final_step.1 > epsilon,
                "{final_step:?}"
            );
        }
        other => panic!("expected NotConverged, got {other:?}"),
    }
}

/// The message has to name what to do about it, since the fit looks fine.
#[test]
fn the_error_says_how_to_fix_it() {
    let mut h = capped(1);
    fill(&mut h);
    let text = h.converge().unwrap_err().to_string();
    assert!(text.contains("did not converge in 1 iterations"), "{text}");
    assert!(text.contains("max_iter"), "{text}");
    assert!(text.contains("alpha"), "{text}");
}

/// The escape hatch: a deliberately capped fit is still reachable.
#[test]
fn converge_partial_returns_the_short_fit() {
    let mut h = capped(1);
    fill(&mut h);
    let report = h.converge_partial().unwrap();
    assert_eq!(report.iterations, 1);
    assert!(!report.converged);
    assert!(h.current_skill(&"a").is_some());
}

/// Both agree when the fit does converge, so the strict path costs nothing.
#[test]
fn the_two_agree_on_a_converged_fit() {
    let mut strict = capped(20_000);
    fill(&mut strict);
    let a = strict.converge().unwrap();

    let mut partial = capped(20_000);
    fill(&mut partial);
    let b = partial.converge_partial().unwrap();

    assert!(a.converged && b.converged);
    assert_eq!(a.iterations, b.iterations);
    assert_eq!(a.final_step, b.final_step);
}

/// The default cap must be high enough that an ordinary history clears it.
/// At the old value of 30 this history stopped short and said nothing.
#[test]
fn the_default_cap_clears_an_ordinary_history() {
    let mut h: History<String> = History::builder()
        .key_type::<String>()
        .mu(0.0)
        .sigma(6.0)
        .beta(1.0)
        .score_sigma(2.0)
        .drift(ConstantDrift::new(0.05))
        .build();

    let mut events = Vec::new();
    for t in 0..20i64 {
        for j in 0..8usize {
            let k = (t as usize) * 8 + j;
            events.push(Event {
                time: t,
                teams: smallvec![
                    Team::with_members([Member::new(format!("p{}", k % 100))]),
                    Team::with_members([Member::new(format!("p{}", (k + 37) % 100))]),
                ],
                outcome: Outcome::scores([3.0, 1.0]),
            });
        }
    }
    h.add_events(events).unwrap();

    let report = h
        .converge()
        .expect("an ordinary history must converge by default");
    assert!(
        report.iterations > 30,
        "needed {} sweeps",
        report.iterations
    );
    assert!(report.iterations < trueskill_tt::ITERATIONS);
}

/// An empty history converges trivially rather than erroring.
#[test]
fn an_empty_history_converges() {
    let mut h = capped(1);
    let report = h.converge().unwrap();
    assert!(report.converged);
    assert_eq!(report.iterations, 0);
}