Function topological_sorting 

pub fn topological_sorting(
    graph: &Graph,
    mode: DijkstraMode,
) -> IgraphResult<Vec<VertexId>>

Returns a topological ordering of graph’s vertices.

mode controls the direction of “precedes”:

• DijkstraMode::Out (most common): for every edge u → v, u appears before v in the result. Vertices with no incoming edges come first.
• DijkstraMode::In: reversed — for every edge u → v, v appears before u. Vertices with no outgoing edges come first.
• DijkstraMode::All: rejected with IgraphError::InvalidArgument — topological sorting only makes sense in a directional sense.

Self-loops are ignored when computing degrees (matches upstream): a single self-loop with no other in/out edges does not block sorting.

Errors:

• IgraphError::InvalidArgument if the graph is undirected.
• IgraphError::InvalidArgument if the graph contains a non-trivial cycle (a topological ordering does not exist).

Counterpart of igraph_topological_sorting(_, _, mode) from references/igraph/src/properties/dag.c:54.

Examples

use rust_igraph::{Graph, topological_sorting, DijkstraMode};

// 0 → 1, 0 → 2, 1 → 3, 2 → 3: a DAG with two valid orderings.
let mut g = Graph::new(4, true).unwrap();
g.add_edges(vec![(0u32, 1u32), (0, 2), (1, 3), (2, 3)]).unwrap();
let order = topological_sorting(&g, DijkstraMode::Out).unwrap();
// 0 must come first; 3 must come last.
assert_eq!(order[0], 0);
assert_eq!(order[3], 3);