sequential parent-local numbering of nodes in a petgraph - https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1951210f28c2ef1caa9b815155472712

gistfile1.txt

// petgraph v0.6.5
use petgraph::graph::{NodeIndex, DiGraph};

fn number_children(graph: &DiGraph<(), ()>, parent: NodeIndex, depth: usize) -> Vec<(NodeIndex, usize, usize)> {
    let mut tree = Vec::new();
    let mut index = 0;

    // Start numbering the children from index 0
    for child in graph.neighbors(parent) {
        tree.push((child, index, depth));
        index += 1;
    }

    // Recursively number children of children
    for child in graph.neighbors(parent) {
        tree.extend(number_children(graph, child, depth + 1));
    }

    tree
}

fn main() {
    let mut graph = DiGraph::new();
    let a = graph.add_node(());
    let b = graph.add_node(());
    let c = graph.add_node(());
    let d = graph.add_node(());
    let e = graph.add_node(());
    let f = graph.add_node(());
    let g = graph.add_node(());
    let h = graph.add_node(());
    let i = graph.add_node(());
    let j = graph.add_node(());

    graph.add_edge(a, b, ());
    graph.add_edge(a, c, ());
    graph.add_edge(b, d, ());
    graph.add_edge(b, e, ());
    graph.add_edge(c, f, ());
    graph.add_edge(c, g, ());
    graph.add_edge(d, h, ());
    graph.add_edge(d, i, ());
    graph.add_edge(f, j, ());

    let tree = number_children(&graph, a, 0);

    for (node, id, depth) in tree {
        println!("{:indent$}{:?} has ID {}", "", node, id, indent = depth * 2);
    }
}