This attribute are lazy to compute the list use get_all_predecessors
for each needed vertexe.
Warning the cache must be invalidated after add_arc
# Cache of all predecessors for each vertex.
# This attribute are lazy to compute the list use `get_all_predecessors` for each needed vertexe.
# Warning the cache must be invalidated after `add_arc`
private var cache_all_predecessors = new HashMap[V, Set[V]]
lib/graph/digraph.nit:866,2--869,60