Function breadth_first_search

pub fn breadth_first_search<G, I, F, C>(graph: G, starts: I, visitor: F) -> C
where
    G: IntoEdges + Visitable,
    I: IntoIterator<Item = G::NodeId>,
    F: FnMut(BfsEvent<G::NodeId, &G::EdgeWeight>) -> C,
    C: ControlFlow,

An iterative breadth first search.

Starting points are the nodes in the iterator starts (specify just one start vertex x by using Some(x)).

The traversal emits discovery and finish events for each reachable vertex, and edge classification of each reachable edge. visitor is called for each event, see BfsEvent for possible values.

The return value should implement the trait ControlFlow, and can be used to change the control flow of the search.

Control Implements ControlFlow such that Control::Continue resumes the search. Control::Break will stop the visit early, returning the contained value. Control::Prune will stop traversing any additional edges from the current node and proceed immediately to the Finish event.

There are implementations of ControlFlow for (), and Result<C, E> where C: ControlFlow. The implementation for () will continue until finished. For Result, upon encountering an E it will break, otherwise acting the same as C.

*Panics if you attempt to prune a node from its Finish event.

The pseudo-code for the BFS algorithm is listed below, with the annotated event points, for which the given visitor object will be called with the appropriate method.

BFS(G, s)
  for each vertex u in V
      color[u] := WHITE
  end for
  color[s] := GRAY
  EQUEUE(Q, s)                             discover vertex s
  while (Q != Ø)
      u := DEQUEUE(Q)
      for each vertex v in Adj[u]          (u,v) is a tree edge
          if (color[v] = WHITE)
              color[v] = GRAY
          else                             (u,v) is a non - tree edge
              if (color[v] = GRAY)         (u,v) has a gray target
                  ...
              else if (color[v] = BLACK)   (u,v) has a black target
                  ...
      end for
      color[u] := BLACK                    finish vertex u
  end while

Example returning Control.

Find a path from vertex 0 to 5, and exit the visit as soon as we reach the goal vertex.

use rustworkx_core::petgraph::prelude::*;
use rustworkx_core::petgraph::graph::node_index as n;
use rustworkx_core::petgraph::visit::Control;

use rustworkx_core::traversal::{BfsEvent, breadth_first_search};

let gr: Graph<(), ()> = Graph::from_edges(&[
    (0, 1), (0, 2), (0, 3),
    (1, 3),
    (2, 3), (2, 4),
    (4, 0), (4, 5),
]);

// record each predecessor, mapping node → node
let mut predecessor = vec![NodeIndex::end(); gr.node_count()];
let start = n(0);
let goal = n(5);
breadth_first_search(&gr, Some(start), |event| {
    if let BfsEvent::TreeEdge(u, v, _) = event {
        predecessor[v.index()] = u;
        if v == goal {
            return Control::Break(v);
        }
    }
    Control::Continue
});

let mut next = goal;
let mut path = vec![next];
while next != start {
    let pred = predecessor[next.index()];
    path.push(pred);
    next = pred;
}
path.reverse();
assert_eq!(&path, &[n(0), n(2), n(4), n(5)]);

Example returning a Result.

use rustworkx_core::petgraph::graph::node_index as n;
use rustworkx_core::petgraph::prelude::*;

use rustworkx_core::traversal::{BfsEvent, breadth_first_search};

let gr: Graph<(), ()> = Graph::from_edges(&[(0, 1), (1, 2), (1, 1), (2, 1)]);
let start = n(0);
let mut non_tree_edges = 0;

#[derive(Debug)]
struct NonTreeEdgeFound {
    source: NodeIndex,
    target: NodeIndex,
}

// Stop the search, the first time a BackEdge is encountered.
let result = breadth_first_search(&gr, Some(start), |event| {
    match event {
        BfsEvent::NonTreeEdge(u, v, _) => {
            non_tree_edges += 1;
            // the implementation of ControlFlow for Result,
            // treats this Err value as Continue::Break
            Err(NonTreeEdgeFound {source: u, target: v})
        }
        // In the cases where Ok(()) is returned,
        // Result falls back to the implementation of Control on the value ().
        // In the case of (), this is to always return Control::Continue.
        // continuing the search.
        _ => Ok(()),
    }
});

assert_eq!(non_tree_edges, 1);
println!("number of non-tree edges encountered: {}", non_tree_edges);
println!("non-tree edge: ({:?})", result.unwrap_err());