TrueSkill - Through Time

Rust port of TrueSkillThroughTime.py.

Other implementations

• ttt-scala
• ChessAnalysis #F
• TrueSkillThroughTime.jl
• TrueSkillThroughTime.R
• TrueSkill Through Time: Revisiting the History of Chess
• TrueSkill Through Time. The full scientific documentation

Drift

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();

Todo

• Implement approx for Gaussian
• Add more tests from TrueSkillThroughTime.jl
• Add tests for quality() (Use sublee/trueskill as reference)
• Benchmark Batch::iteration()
• Time needs to be an enum so we can have multiple states (see batch::compute_elapsed())
• Add examples (use same TrueSkillThroughTime.(py|jl))
• Add Observer (see argmin for inspiration)