Skill drift models how a player's true skill can change between appearances. Each time a player reappears after a gap, their skill uncertainty is widened by the drift model before the new evidence is incorporated.

Drift is represented by the Drift trait:

pub trait Drift: Copy + Debug {
    fn variance_delta(&self, elapsed: i64) -> f64;
}

variance_delta returns the amount to add to σ² given the elapsed time since the player last played. Internally, Gaussian::forget uses this to compute the new sigma: σ_new = sqrt(σ² + variance_delta).

ConstantDrift

The built-in ConstantDrift implements a linear random walk — skill uncertainty grows proportionally to time:

variance_delta = elapsed * γ²

This is the standard TrueSkill Through Time model. Use it by passing a ConstantDrift(gamma) when constructing a Player:

use trueskill_tt::{Player, Gaussian, drift::ConstantDrift};

// gamma = 0.1 means skill can shift ~0.1 per time unit
let player = Player::new(Gaussian::from_ms(0.0, 6.0), 1.0, ConstantDrift(0.1));

Custom drift

Implement Drift to express any other model. For example, a drift that saturates after a long absence (uncertainty grows with the square root of elapsed time instead of linearly):

use trueskill_tt::drift::Drift;

#[derive(Clone, Copy, Debug)]
struct SqrtDrift {
    gamma: f64,
}

impl Drift for SqrtDrift {
    fn variance_delta(&self, elapsed: i64) -> f64 {
        (elapsed as f64).sqrt() * self.gamma * self.gamma
    }
}

let player = Player::new(Gaussian::from_ms(0.0, 6.0), 1.0, SqrtDrift { gamma: 0.5 });

To use a custom drift type with History, use the .drift() builder method instead of .gamma():

let h = History::builder()
    .drift(SqrtDrift { gamma: 0.5 })
    .build();

Scored outcomes

Use Outcome::scores([...]) when you have continuous per-team scores rather than just ranks. Adjacent score margins flow into a MarginFactor that adds soft Gaussian evidence about the latent performance diff. Configure HistoryBuilder::score_sigma(σ) to control how much you trust the margins (smaller σ = more trust).

use trueskill_tt::{History, Outcome};

let mut h = History::builder().score_sigma(2.0).build();
h.event(1)
    .team(["alice"])
    .team(["bob"])
    .scores([21.0, 9.0])
    .commit()
    .unwrap();
h.converge().unwrap();

Todo

Implement approx for Gaussian
Add more tests from TrueSkillThroughTime.jl
Generalise a time axis — Time is now a trait (Untimed, i64), not an enum
Add examples (examples/atp.rs, examples/scored.rs)
Add Observer (Observer / NullObserver)
Benchmark the inference loop (benches/batch.rs, benches/history_converge.rs, benches/ingest.rs)
Cross-check quality() against sublee/trueskill — N-group support works and is covered by invariants, but no reference values are asserted

License

Licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

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TrueSkill - Through Time

Other implementations

Drift