Function spfa 

pub fn spfa<G, F>(
    graph: &G,
    source: G::Node,
    edge_cost: F,
) -> Result<Option<BellmanFord<G::Node>>>
where
    G: IndexGraphView,
    F: Fn(G::Edge) -> i64,

Computes signed single-source paths with the queue-based SPFA algorithm.

Errors

Returns an error for arithmetic overflow or a reachable negative cycle.