Function shell_diversity 

pub fn shell_diversity(graph: &Graph) -> IgraphResult<f64>

Shannon entropy of the k-shell size distribution, normalised to [0, 1].

The k-shell of order k is the set of vertices with coreness exactly k. This function computes the entropy of the distribution of shell sizes and normalises by log(number_of_distinct_shells) so the result is in [0, 1]. A value of 1 means all shells have equal size; a value near 0 means vertices are concentrated in one shell.

Returns 0.0 for empty or edgeless graphs (single shell).

Examples

use rust_igraph::{Graph, shell_diversity};

// Path 0-1-2: coreness = [1, 1, 1], single shell → diversity = 0
let g = Graph::from_edges(&[(0, 1), (1, 2)], false, Some(3)).unwrap();
assert!(shell_diversity(&g).unwrap().abs() < 1e-10);