Make quality a free standing function instead

src/lib.rs

@@ -18,6 +18,7 @@ pub mod player;
 pub use game::Game;
 pub use gaussian::Gaussian;
 pub use history::History;
+use matrix::Matrix;
 use message::DiffMessage;
 pub use player::Player;
 
@@ -255,6 +256,67 @@ pub(crate) fn evidence(d: &[DiffMessage], margin: &[f64], tie: &[bool], e: usize
     }
 }
 
+pub fn quality(rating_groups: &[&[Gaussian]], beta: f64) -> f64 {
+    let flatten_ratings = rating_groups
+        .iter()
+        .flat_map(|group| group.iter())
+        .collect::<Vec<_>>();
+
+    let flatten_weights = vec![1.0; flatten_ratings.len()].into_boxed_slice();
+
+    let length = flatten_ratings.len();
+
+    let mut mean_matrix = Matrix::new(length, 1);
+
+    for (i, rating) in flatten_ratings.iter().enumerate() {
+        mean_matrix[(i, 0)] = rating.mu;
+    }
+
+    let mut variance_matrix = Matrix::new(length, length);
+
+    for (i, rating) in flatten_ratings.iter().enumerate() {
+        variance_matrix[(i, i)] = rating.sigma.powi(2);
+    }
+
+    let mut rotated_a_matrix = Matrix::new(rating_groups.len() - 1, length);
+
+    let mut t = 0;
+    let mut x = 0;
+
+    for (row, group) in rating_groups.windows(2).enumerate() {
+        let current = group[0];
+        let next = group[1];
+
+        for n in t..t + current.len() {
+            rotated_a_matrix[(row, n)] = flatten_weights[n];
+
+            x += 1;
+        }
+
+        t += current.len();
+
+        for n in x..x + next.len() {
+            rotated_a_matrix[(row, n)] = -flatten_weights[n];
+        }
+
+        x += next.len();
+    }
+
+    let a_matrix = rotated_a_matrix.transpose();
+
+    let ata = beta.powi(2) * &rotated_a_matrix * &a_matrix;
+    let atsa = &rotated_a_matrix * &variance_matrix * &a_matrix;
+
+    let start = mean_matrix.transpose() * &a_matrix;
+    let middle = &ata + &atsa;
+    let end = &rotated_a_matrix * &mean_matrix;
+
+    let e_arg = (-0.5 * &start * &middle.inverse() * &end).determinant();
+    let s_arg = ata.determinant() / middle.determinant();
+
+    e_arg.exp() * s_arg.sqrt()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;