Subset constraints in general graphs

!!! info inline end "See also"

- [Subpath constraints in DAGs](subpath-constraints.md)

To any of the models on directed graphs (possibly with cycles) that are based on (or inherit from) the AbstractWalkModelDiGraph class you can add subset constraints. As opposed to the subpath constraints that you can add for directed acyclic graphs, here they mean the following.

Say that you have prior knowledge of some set of edges that must appear together in at least one solution walk of you model. These constrain the space of possible solution walks.

Let's consider the Minimum Flow Decomposition problem in graphs with cycles, and let's take the example graph from there. Let's assume that you want the set [(a,b),(c,a)] (which we draw in brown) to appear in at last one solution walk.

flowchart LR
    s((s))
    a((a))
    t((t))
    b((b))
    c((c))

    s -->|7| a
    a -->|7| t
    a -->|5| b
    b -->|5| a
    a -->|2| c
    c -->|2| a
    linkStyle 2,5 stroke:brown,stroke-width:3;

For example, the following flow decomposition doesn't contain the full set [(a,b),(c,a)], in the sense that neither red nor blue walks contain it.

flowchart LR
    s((s))
    a((a))
    t((t))
    b((b))
    c((c))

    s -->|5| a
    a -->|5| t
    a -->|5| b
    b -->|5| a


    s -->|2| a
    a -->|2| c
    c -->|2| a
    a -->|2| t
    linkStyle 0,1,2,3 stroke:red,stroke-width:3;
    linkStyle 4,5,6,7 stroke:blue,stroke-width:3;

A valid decomposition that contains it, and has the minimum number of walks among these, is the following. Note that [(a,b),(c,a)] now appears in the orange walk.

Note also that if the orange walk is $s$, $a$, $c$, $a$, $b$, $a$, $t$ then it does not contain the edges in the order (a,b),(c,a). In fact, the subpath constraints cannot guarantee any order in which the edges in the set appear in a walk containing them.

flowchart LR
    s((s))
    a((a))
    t((t))
    b((b))
    c((c))

    s -->|3| a
    a -->|3| t
    s -->|2| a
    a -->|2| t
    a -->|2| b
    b -->|2| a
    a -->|3| b
    b -->|3| a


    s -->|2| a
    a -->|2| c
    c -->|2| a
    a -->|2| t
    linkStyle 2,4,5,9,10,3 stroke:orange,stroke-width:3;
    linkStyle 0,6,7,1 stroke:red,stroke-width:3;
    linkStyle 8,11 stroke:blue,stroke-width:3;

!!! tip "Note 1" Any existing decomposition model based on AbstractWalkModelDiGraph supports subset constraints.

!!! tip "Note 2"

- The subset constraints can be any set of edges that don't necessarily share endpoints. 
- The model does not guarantee that the edges in a subset constraint appear in a solution walk in any given order. It is only guaranteed that at least one solution walk contain this sequence of edges.