trueskill-tt/tests/rating_rule.rs

//! `HistoryBuilder::default_rating_for`: configuring a *class* of competitors
//! rather than one at a time (#53).
//!
//! Every test carries a control — a key the rule does not match — so none can
//! pass by the rule firing for everybody, which would be indistinguishable
//! from changing the history defaults.

use trueskill_tt::{
    ConstantDrift, Gaussian, History, HistoryBuilder, InferenceError, Member, NullObserver,
    RatingRule, StartingPoint,
};

/// Pinned: no drift, and a tight prior at a known strength.
fn pinned() -> StartingPoint {
    StartingPoint::new()
        .prior(Gaussian::from_ms(5.0, 0.5))
        .drift_scale(0.0)
}

fn play<R: RatingRule<&'static str>>(
    h: &mut History<&'static str, i64, ConstantDrift, NullObserver, R>,
) {
    for t in 1..=6 {
        h.event(t)
            .team(["layout_a"])
            .team(["alice"])
            .scores([3.0, 1.0])
            .commit()
            .expect("ingests");
    }
    h.converge().expect("converges");
}

#[test]
fn a_rule_configures_every_matching_key_without_naming_them() {
    let mut ruled = History::builder()
        .gamma(0.5)
        .default_rating_for(|key: &&'static str| key.starts_with("layout_").then(pinned))
        .build();
    play(&mut ruled);

    let mut plain = History::builder().gamma(0.5).build();
    play(&mut plain);

    let layout = ruled.current_skill("layout_a").expect("played");

    // The rule pinned the layout: tight prior, no drift.
    assert!(
        layout.sigma() < 0.5,
        "the layout should stay near its pinned prior, got sigma {}",
        layout.sigma()
    );
    assert_ne!(
        layout.sigma(),
        plain.current_skill("layout_a").unwrap().sigma(),
        "the rule must actually change the fit"
    );

    // The control is the *configuration*, not the posterior. Alice's posterior
    // legitimately moves — she is playing a differently-configured opponent,
    // and what she learns from beating it depends on how sure the model is
    // about it. What must not move is what the rule was asked about.
    let alice = ruled.rating("alice").expect("played");
    assert_eq!(
        alice.drift_scale(),
        1.0,
        "a non-matching key keeps the default drift"
    );
    assert_eq!(
        (alice.prior().mu(), alice.prior().sigma()),
        {
            let p = plain.rating("alice").expect("played").prior();
            (p.mu(), p.sigma())
        },
        "a non-matching key keeps the history's prior"
    );
}

#[test]
fn a_rule_fires_for_a_competitor_first_seen_through_record_winner() {
    // `record_winner` cannot carry configuration, which is the case a rule
    // exists for.
    let mut h = History::builder()
        .default_rating_for(|key: &&'static str| key.starts_with("bot_").then(pinned))
        .build();
    h.record_winner(&"bot_1", &"human", 1).expect("ingests");
    h.converge().expect("converges");

    assert_eq!(h.rating("bot_1").expect("known").drift_scale(), 0.0);
    assert_eq!(h.rating("human").expect("known").drift_scale(), 1.0);
}

#[test]
fn explicit_configuration_overrides_a_rule_field_by_field() {
    let mut h = History::builder()
        .default_rating_for(|_: &&'static str| Some(pinned()))
        .build();

    // Sets only the prior, so the rule's `drift_scale` must survive.
    h.register(Member::new("a").with_prior(Gaussian::from_ms(-9.0, 2.0)))
        .expect("new");
    // Sets neither: the rule supplies both.
    h.register(Member::new("b")).expect("new");

    let a = h.rating("a").expect("registered");
    assert_eq!(a.prior().mu(), -9.0, "explicit prior wins");
    assert_eq!(a.drift_scale(), 0.0, "the rule's drift_scale survives");

    let b = h.rating("b").expect("registered");
    assert_eq!(b.prior().mu(), 5.0);
    assert_eq!(b.drift_scale(), 0.0);
}

#[test]
fn two_explicit_declarations_that_disagree_are_still_an_error() {
    // Precedence resolves rule-vs-explicit. It does not weaken the check
    // between two explicit declarations, neither of which is more specific.
    let mut h = History::builder()
        .default_rating_for(|_: &&'static str| Some(pinned()))
        .build();

    let err = h
        .add_events(vec![
            event(1, "x", Gaussian::from_ms(1.0, 1.0)),
            event(2, "x", Gaussian::from_ms(2.0, 1.0)),
        ])
        .expect_err("two different priors for one competitor");
    assert!(
        matches!(err, InferenceError::ConflictingCompetitorConfig { .. }),
        "{err:?}"
    );
}

fn event(time: i64, key: &'static str, prior: Gaussian) -> trueskill_tt::Event<i64, &'static str> {
    trueskill_tt::Event {
        time,
        teams: [
            trueskill_tt::Team::with_members([Member::new(key).with_prior(prior)]),
            trueskill_tt::Team::with_members([Member::new("opponent")]),
        ]
        .into_iter()
        .collect(),
        outcome: trueskill_tt::Outcome::scores([2.0, 1.0]),
    }
}

/// A named rule type, so the `History<..>` can be written down in a field.
struct StaticLayouts;

impl RatingRule<&'static str> for StaticLayouts {
    fn starting_point(&self, key: &&'static str) -> Option<StartingPoint> {
        key.starts_with("layout_").then(pinned)
    }
}

/// The reason this is a trait rather than a bare `Fn` bound: a consumer holds
/// its history in application state and has to name the type.
struct Ladder {
    history: History<&'static str, i64, ConstantDrift, NullObserver, StaticLayouts>,
}

#[test]
fn a_named_rule_type_can_be_stored_in_a_struct_field() {
    let mut ladder = Ladder {
        history: HistoryBuilder::default().rating_rule(StaticLayouts).build(),
    };
    play(&mut ladder.history);

    assert!(
        ladder
            .history
            .current_skill("layout_a")
            .expect("played")
            .sigma()
            < 0.5
    );
    assert_eq!(
        ladder
            .history
            .rating("alice")
            .expect("played")
            .drift_scale(),
        1.0
    );
}

#[test]
fn no_rule_is_the_default_and_costs_nothing_to_spell() {
    // The whole point of defaulting the parameter: `History<K>` still works.
    let h: History<String> = History::builder().key_type::<String>().build();
    assert_eq!(h.competitor_count(), 0);
}