Function reorder_sccs

pub fn reorder_sccs<Id: Debug + Copy + Hash + Eq>(
    get_id_dependencies: &dyn Fn(Id) -> Vec<Id>,
    ids: &Vec<Id>,
    sccs: &[Vec<Id>],
) -> SCCs<Id>

Provided we computed the SCCs (Strongly Connected Components) of a set of identifier, and those identifiers are ordered, compute the set of SCCs where the order of the SCCs and the order of the identifiers inside the SCCs attempt to respect as much as possible the original order between the identifiers. The ids vector gives the ordered set of identifiers.

This is used in several places, for instance to generate the translated definitions in a consistent and stable manner. For instance, let’s say we extract 4 definitions f, g1, g2 and h, where:

• g1 and g2 are mutually recursive
• h calls g1

When translating those functions, we group together the mutually recursive functions, because they have to be extracted in one single group, and thus apply Tarjan’s algorithm on the call graph to find out which functions are mutually recursive. The implementation of Tarjan’s algorithm we use gives us the Strongly Connected SCCs of the call graph in an arbitrary order, so we can have: [[f], [g1, g2], [h]], but also [[h], [f], [g2, g1]], etc.

If the user defined those functions in the order: f, g1, h, g2, we want to reorder them into: f, g1, g2, h, so that we can translate the mutually recursive functions together, while performing a minimal amount of reordering. And if reordering is not needed, because the user defined those functions in the order f, g1, g2, h, or g1, g2, f, h or … then we want to translate them in this exact order.

This function performs just that: provided the order in which the definitions were defined, and the SCCs of the call graph, return an order suitable for translation and where the amount of reorderings is minimal with regards to the initial order.

This function is also used to generate the backward functions in a stable manner: the order is provided by the order in which the user listed the region parameters in the function signature, and the graph is the hierarchy graph (or region subtyping graph) between those regions.