Note

This is the documentation for the current state of the development branch of rustworkx. The documentation or APIs here can change prior to being released.

rustworkx.digraph_dfs_search

digraph_dfs_search(graph, source=None, visitor=None)

Depth-first traversal of a directed graph with several source vertices.

Pseudo-code for the depth-first search algorithm with a single source vertex is listed below with annotated event points at which a method of the given DFSVisitor is called.

# G - graph, s - single source node
DFS(G, s)
  let color be a mapping                        # color[u] - vertex u color WHITE/GRAY/BLACK
  for each u in G                               # u - vertex in G
    color[u] := WHITE                           # color all as undiscovered
  end for
  time := 0
  let S be a stack
  PUSH(S, (s, iterator of OutEdges(G, s)))      # S - stack of vertices and edge iterators
  color[s] := GRAY                              # event: discover_vertex(s, time)
  while (S is not empty)
    let (u, iterator) := LAST(S)
    flag := False                               # whether edge to undiscovered vertex found
    for each v, w in iterator                   # v - target vertex, w - edge weight
      if (WHITE = color[v])                     # event: tree_edge((u, v, w))
        time := time + 1
        color[v] := GRAY                        # event: discover_vertex(v, time)
        flag := True
        break
      elif (GRAY = color[v])                    # event: back_edge((u, v, w))
        ...
      elif (BLACK = color[v])                   # event: forward_or_cross_edge((u, v, w))
        ...
    end for
    if (flag is True)
      PUSH(S, (v, iterator of OutEdges(G, v)))
    elif (flag is False)
      time := time + 1
      color[u] := BLACK                         # event: finish_vertex(u, time)
      POP(S)
  end while

For several source nodes, the DFS algorithm is applied on source nodes by the given order.

If an exception is raised inside the callback method of the DFSVisitor instance, the graph traversal will be stopped immediately. You can exploit this to exit early by raising a StopSearch exception. You can also prune part of the search tree by raising PruneSearch.

In the following example we keep track of the tree edges:

import rustworkx as rx
from rustworkx.visit import DFSVisitor

class TreeEdgesRecorder(DFSVisitor):

    def __init__(self):
        self.edges = []

    def tree_edge(self, edge):
        self.edges.append(edge)

graph = rx.PyDiGraph()
graph.extend_from_edge_list([(1, 3), (0, 1), (2, 1), (0, 2)])
vis = TreeEdgesRecorder()
rx.digraph_dfs_search(graph, [0], vis)
print('Tree edges:', vis.edges)

Tree edges: [(0, 2, None), (2, 1, None), (1, 3, None)]

Note

Graph can not be mutated while traversing. Trying to do so raises an exception.

Note

An exception is raised if the PruneSearch is raised in the finish_vertex event.

Parameters:

• graph (PyDiGraph) – The graph to be used.

• source – An optional list of node indices to use as the starting nodes for the depth-first search. If None or not specified then a source will be chosen arbitrarily and repeated until all components of the graph are searched. This can be a Sequence[int] or None.

• visitor – A visitor object that is invoked at the event points inside the algorithm. This should be a subclass of DFSVisitor. This has a default value of None as a backwards compatibility artifact (to preserve argument ordering from an earlier version) but it is a required argument and will raise a TypeError if not specified.