Function reach_decay 

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

Average fraction of vertices reachable within half the diameter.

For each vertex, computes the fraction of other vertices reachable within floor(diameter / 2) hops, then averages across all vertices. Measures how quickly connectivity “fills in” relative to the graph’s diameter.

Returns 0.0 for disconnected or trivial graphs.

Examples

use rust_igraph::{Graph, reach_decay};

// K4: diameter=1, half=0 → no vertex reachable in 0 hops → 0
// Actually half_diam = floor(1/2) = 0, so reach = 0
let g = Graph::from_edges(
    &[(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)],
    false,
    Some(4),
)
.unwrap();
let r = reach_decay(&g).unwrap();
assert!(r >= 0.0 && r <= 1.0);