trueskill-tt/tests/time_axis.rs

//! The `Time` generic, exercised end to end.
//!
//! `History<T: Time, ..>` has always been generic over the time axis, `Untimed`
//! has always been exported, and `Drift<T>` is generic specifically so that
//! "seasonal or calendar-aware drift is expressible without going through
//! `i64`". None of it was reachable: every construction route pinned `T = i64`,
//! `HistoryBuilder`'s fields are private, and its `Default` existed only for the
//! `i64` instantiation.
//!
//! Nothing in the repository constructed a non-`i64` history, which is why that
//! went unnoticed. This file is the guard against it recurring — it is as much
//! about the generic being *exercised* as about any single assertion.

use trueskill_tt::{ConstantDrift, Drift, History, HistoryBuilder, Time, Untimed};

/// A domain time type: a season number. Exactly what the `Time` trait exists
/// to support, and what a consumer with `chrono` dates would write.
#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
struct Season(u16);

impl Time for Season {
    fn elapsed_to(&self, later: &Self) -> i64 {
        i64::from(later.0.saturating_sub(self.0))
    }
}

/// Drift that only accumulates between seasons, not within one — the
/// calendar-aware case the trait's own docs cite.
#[derive(Copy, Clone, Debug)]
struct SeasonalDrift {
    per_season: f64,
}

impl Drift<Season> for SeasonalDrift {
    fn variance_delta(&self, from: &Season, to: &Season) -> f64 {
        self.variance_for_elapsed(from.elapsed_to(to))
    }

    fn variance_for_elapsed(&self, elapsed: i64) -> f64 {
        elapsed.max(0) as f64 * self.per_season * self.per_season
    }
}

#[test]
fn an_untimed_history_fits_through_the_builder() {
    let mut h = History::builder().time_type::<Untimed>().build();
    for _ in 0..5 {
        h.record_winner(&"alice", &"bob", Untimed).unwrap();
    }
    assert!(h.converge().unwrap().converged);

    let alice = h.current_skill(&"alice").unwrap();
    let bob = h.current_skill(&"bob").unwrap();
    assert!(alice.mu() > bob.mu(), "{alice:?} vs {bob:?}");
    assert!(alice.sigma().is_finite() && alice.sigma() > 0.0);
}

/// `Untimed::elapsed_to` is always 0, so no drift accumulates however many
/// events there are. That is the property the type exists for, and it had never
/// been checked.
#[test]
fn untimed_accumulates_no_drift() {
    fn final_sigma<T: Time + Copy>(time: T, drift: ConstantDrift) -> f64 {
        let mut h = History::builder().time_type::<T>().drift(drift).build();
        for _ in 0..8 {
            h.record_winner(&"a", &"b", time).unwrap();
        }
        let _ = h.converge().unwrap();
        h.current_skill(&"a").unwrap().sigma()
    }

    // Under Untimed the drift setting cannot matter, because elapsed is always 0.
    let none = final_sigma(Untimed, ConstantDrift::new(0.0));
    let large = final_sigma(Untimed, ConstantDrift::new(5.0));
    assert_eq!(
        none.to_bits(),
        large.to_bits(),
        "Untimed must ignore drift entirely: {none} vs {large}"
    );
}

#[test]
fn a_custom_time_type_and_a_custom_drift_work_together() {
    let mut h = History::builder()
        .time_type::<Season>()
        .drift(SeasonalDrift { per_season: 0.5 })
        .build();

    for season in 1..=4u16 {
        for _ in 0..3 {
            h.record_winner(&"veteran", &"rookie", Season(season))
                .unwrap();
        }
    }
    assert!(h.converge().unwrap().converged);

    let curve = h.learning_curve(&"veteran").unwrap();
    assert_eq!(curve.len(), 4, "one point per season: {curve:?}");
    for (season, g) in &curve {
        assert!(
            g.mu().is_finite() && g.sigma() > 0.0,
            "season {season:?}: {g:?}"
        );
    }
    // Times come back as the domain type, not as an integer.
    assert_eq!(curve[0].0, Season(1));
    assert_eq!(curve[3].0, Season(4));
}

/// Seasonal drift must actually widen a gap across seasons — otherwise the
/// custom `Drift` is being ignored and the test above would pass regardless.
#[test]
fn a_custom_drift_is_actually_consulted() {
    fn sigma_with(per_season: f64) -> f64 {
        let mut h = History::builder()
            .time_type::<Season>()
            .drift(SeasonalDrift { per_season })
            .build();
        for season in 1..=6u16 {
            h.record_winner(&"a", &"b", Season(season)).unwrap();
        }
        let _ = h.converge().unwrap();
        h.current_skill(&"a").unwrap().sigma()
    }

    let still = sigma_with(0.0);
    let drifting = sigma_with(2.0);
    assert!(
        drifting > still * 1.05,
        "a drifting fit must be less certain: {drifting} vs {still}"
    );
}

/// The other axis: a custom key type, through the same mechanism.
#[test]
fn key_type_replaces_builder_with_key() {
    let mut h = History::builder().key_type::<String>().build();
    h.record_winner(&"alice".to_string(), &"bob".to_string(), 1)
        .unwrap();
    assert!(h.converge().unwrap().converged);
    assert!(h.current_skill("alice").is_some());
}

/// Both axes at once, via the explicit constructor rather than the setters.
#[test]
fn new_constructs_on_any_axis_directly() {
    let mut h = HistoryBuilder::<String, Season>::new().build();
    h.record_winner(&"a".to_string(), &"b".to_string(), Season(7))
        .unwrap();
    assert!(h.converge().unwrap().converged);
    assert_eq!(h.learning_curve("a").unwrap()[0].0, Season(7));
}