Replace generic hash table by functorial in WeakTopological

[TDacik]

In WeakTopological module, a generic hash table is used to store vertices. This could result in the error when a custom compare function is used. A trivial example is:

open Graph

module V = struct
  type t = int * int
  let compare x y = Pervasives.compare (fst x) (fst y)
  let equal x y = Pervasives.(=) (fst x) (fst y)
  let hash x = Hashtbl.hash (fst x)
end

module G = Imperative.Digraph.ConcreteBidirectional(V)
module WTO = WeakTopological.Make(G)

let () =
  let g = G.create () in
  G.add_edge g (1,0) (2,0);
  G.add_edge g (1,0) (3,0);
  G.add_edge g (2,0) (4,0);
  G.add_edge g (3,0) (4,1);
  let _ = WTO.recursive_scc g (1,0) in
  ()

During iterating on successors of the vertex (3,0) at WeakTopological:L65, due to use of Pervasives.compare function by the generic hash table, search of the vertex (4,1) raises Not_found exception, because only the vertex (4,0) is present.

I believe this could be fixed by using a functorial hash table.

[thizanne]

The fix looks perfect. Do you think your example should be added to tests/test_wto.ml ?

[backtracking]

Good catch. Many thanks for the fix.

[TDacik]

The fix looks perfect. Do you think your example should be added to tests/test_wto.ml ?

I don't think this can be reintroduced again so adding the test is not necessary.