Function regularity 

pub fn regularity(graph: &Graph) -> IgraphResult<Option<u32>>

Return the regularity degree of the graph, or None if the graph is not regular.

For undirected graphs, returns the common degree. For directed graphs, returns the common out-degree (which equals the common in-degree in a regular directed graph where out-degree == in-degree for all vertices; if out-degrees are all equal and in-degrees are all equal but differ from each other, returns None since such a graph is not considered regular in the standard sense for simple regularity queries).

An empty graph returns None (no vertices, no degree).

Examples

use rust_igraph::{Graph, regularity};

// K3 is 2-regular
let mut g = Graph::with_vertices(3);
g.add_edge(0, 1).unwrap();
g.add_edge(1, 2).unwrap();
g.add_edge(2, 0).unwrap();
assert_eq!(regularity(&g).unwrap(), Some(2));

// Star graph is not regular
let mut g = Graph::with_vertices(4);
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(0, 3).unwrap();
assert_eq!(regularity(&g).unwrap(), None);