Struct Partition 

pub struct Partition {
    pub classes: Vec<Vec<usize>>,
    pub class_of: Vec<usize>,
    pub pure: Vec<bool>,
}

The mechanical partition: every hunk assigned to a shape class. Classes are ordered by descending member count (ties broken by first appearance, so the ordering is deterministic); class_of[i] is the class index of canonical hunk i. Coverage is total by construction.

Fields

classes: Vec<Vec<usize>>

Hunk indices per class, in canonical order within each class.

class_of: Vec<usize>

Canonical hunk index -> class index.

pure: Vec<bool>

Per class: is the exemplar a pure substitution?