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authorS. Solomon Darnell2025-03-28 21:52:21 -0500
committerS. Solomon Darnell2025-03-28 21:52:21 -0500
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+"""Basic algorithms for depth-first searching the nodes of a graph."""
+
+from collections import defaultdict
+
+import networkx as nx
+
+__all__ = [
+ "dfs_edges",
+ "dfs_tree",
+ "dfs_predecessors",
+ "dfs_successors",
+ "dfs_preorder_nodes",
+ "dfs_postorder_nodes",
+ "dfs_labeled_edges",
+]
+
+
+@nx._dispatchable
+def dfs_edges(G, source=None, depth_limit=None, *, sort_neighbors=None):
+ """Iterate over edges in a depth-first-search (DFS).
+
+ Perform a depth-first-search over the nodes of `G` and yield
+ the edges in order. This may not generate all edges in `G`
+ (see `~networkx.algorithms.traversal.edgedfs.edge_dfs`).
+
+ Parameters
+ ----------
+ G : NetworkX graph
+
+ source : node, optional
+ Specify starting node for depth-first search and yield edges in
+ the component reachable from source.
+
+ depth_limit : int, optional (default=len(G))
+ Specify the maximum search depth.
+
+ sort_neighbors : function (default=None)
+ A function that takes an iterator over nodes as the input, and
+ returns an iterable of the same nodes with a custom ordering.
+ For example, `sorted` will sort the nodes in increasing order.
+
+ Yields
+ ------
+ edge: 2-tuple of nodes
+ Yields edges resulting from the depth-first-search.
+
+ Examples
+ --------
+ >>> G = nx.path_graph(5)
+ >>> list(nx.dfs_edges(G, source=0))
+ [(0, 1), (1, 2), (2, 3), (3, 4)]
+ >>> list(nx.dfs_edges(G, source=0, depth_limit=2))
+ [(0, 1), (1, 2)]
+
+ Notes
+ -----
+ If a source is not specified then a source is chosen arbitrarily and
+ repeatedly until all components in the graph are searched.
+
+ The implementation of this function is adapted from David Eppstein's
+ depth-first search function in PADS [1]_, with modifications
+ to allow depth limits based on the Wikipedia article
+ "Depth-limited search" [2]_.
+
+ See Also
+ --------
+ dfs_preorder_nodes
+ dfs_postorder_nodes
+ dfs_labeled_edges
+ :func:`~networkx.algorithms.traversal.edgedfs.edge_dfs`
+ :func:`~networkx.algorithms.traversal.breadth_first_search.bfs_edges`
+
+ References
+ ----------
+ .. [1] http://www.ics.uci.edu/~eppstein/PADS
+ .. [2] https://en.wikipedia.org/wiki/Depth-limited_search
+ """
+ if source is None:
+ # edges for all components
+ nodes = G
+ else:
+ # edges for components with source
+ nodes = [source]
+ if depth_limit is None:
+ depth_limit = len(G)
+
+ get_children = (
+ G.neighbors
+ if sort_neighbors is None
+ else lambda n: iter(sort_neighbors(G.neighbors(n)))
+ )
+
+ visited = set()
+ for start in nodes:
+ if start in visited:
+ continue
+ visited.add(start)
+ stack = [(start, get_children(start))]
+ depth_now = 1
+ while stack:
+ parent, children = stack[-1]
+ for child in children:
+ if child not in visited:
+ yield parent, child
+ visited.add(child)
+ if depth_now < depth_limit:
+ stack.append((child, get_children(child)))
+ depth_now += 1
+ break
+ else:
+ stack.pop()
+ depth_now -= 1
+
+
+@nx._dispatchable(returns_graph=True)
+def dfs_tree(G, source=None, depth_limit=None, *, sort_neighbors=None):
+ """Returns oriented tree constructed from a depth-first-search from source.
+
+ Parameters
+ ----------
+ G : NetworkX graph
+
+ source : node, optional
+ Specify starting node for depth-first search.
+
+ depth_limit : int, optional (default=len(G))
+ Specify the maximum search depth.
+
+ sort_neighbors : function (default=None)
+ A function that takes an iterator over nodes as the input, and
+ returns an iterable of the same nodes with a custom ordering.
+ For example, `sorted` will sort the nodes in increasing order.
+
+ Returns
+ -------
+ T : NetworkX DiGraph
+ An oriented tree
+
+ Examples
+ --------
+ >>> G = nx.path_graph(5)
+ >>> T = nx.dfs_tree(G, source=0, depth_limit=2)
+ >>> list(T.edges())
+ [(0, 1), (1, 2)]
+ >>> T = nx.dfs_tree(G, source=0)
+ >>> list(T.edges())
+ [(0, 1), (1, 2), (2, 3), (3, 4)]
+
+ See Also
+ --------
+ dfs_preorder_nodes
+ dfs_postorder_nodes
+ dfs_labeled_edges
+ :func:`~networkx.algorithms.traversal.edgedfs.edge_dfs`
+ :func:`~networkx.algorithms.traversal.breadth_first_search.bfs_tree`
+ """
+ T = nx.DiGraph()
+ if source is None:
+ T.add_nodes_from(G)
+ else:
+ T.add_node(source)
+ T.add_edges_from(dfs_edges(G, source, depth_limit, sort_neighbors=sort_neighbors))
+ return T
+
+
+@nx._dispatchable
+def dfs_predecessors(G, source=None, depth_limit=None, *, sort_neighbors=None):
+ """Returns dictionary of predecessors in depth-first-search from source.
+
+ Parameters
+ ----------
+ G : NetworkX graph
+
+ source : node, optional
+ Specify starting node for depth-first search.
+ Note that you will get predecessors for all nodes in the
+ component containing `source`. This input only specifies
+ where the DFS starts.
+
+ depth_limit : int, optional (default=len(G))
+ Specify the maximum search depth.
+
+ sort_neighbors : function (default=None)
+ A function that takes an iterator over nodes as the input, and
+ returns an iterable of the same nodes with a custom ordering.
+ For example, `sorted` will sort the nodes in increasing order.
+
+ Returns
+ -------
+ pred: dict
+ A dictionary with nodes as keys and predecessor nodes as values.
+
+ Examples
+ --------
+ >>> G = nx.path_graph(4)
+ >>> nx.dfs_predecessors(G, source=0)
+ {1: 0, 2: 1, 3: 2}
+ >>> nx.dfs_predecessors(G, source=0, depth_limit=2)
+ {1: 0, 2: 1}
+
+ Notes
+ -----
+ If a source is not specified then a source is chosen arbitrarily and
+ repeatedly until all components in the graph are searched.
+
+ The implementation of this function is adapted from David Eppstein's
+ depth-first search function in `PADS`_, with modifications
+ to allow depth limits based on the Wikipedia article
+ "`Depth-limited search`_".
+
+ .. _PADS: http://www.ics.uci.edu/~eppstein/PADS
+ .. _Depth-limited search: https://en.wikipedia.org/wiki/Depth-limited_search
+
+ See Also
+ --------
+ dfs_preorder_nodes
+ dfs_postorder_nodes
+ dfs_labeled_edges
+ :func:`~networkx.algorithms.traversal.edgedfs.edge_dfs`
+ :func:`~networkx.algorithms.traversal.breadth_first_search.bfs_tree`
+ """
+ return {
+ t: s
+ for s, t in dfs_edges(G, source, depth_limit, sort_neighbors=sort_neighbors)
+ }
+
+
+@nx._dispatchable
+def dfs_successors(G, source=None, depth_limit=None, *, sort_neighbors=None):
+ """Returns dictionary of successors in depth-first-search from source.
+
+ Parameters
+ ----------
+ G : NetworkX graph
+
+ source : node, optional
+ Specify starting node for depth-first search.
+ Note that you will get successors for all nodes in the
+ component containing `source`. This input only specifies
+ where the DFS starts.
+
+ depth_limit : int, optional (default=len(G))
+ Specify the maximum search depth.
+
+ sort_neighbors : function (default=None)
+ A function that takes an iterator over nodes as the input, and
+ returns an iterable of the same nodes with a custom ordering.
+ For example, `sorted` will sort the nodes in increasing order.
+
+ Returns
+ -------
+ succ: dict
+ A dictionary with nodes as keys and list of successor nodes as values.
+
+ Examples
+ --------
+ >>> G = nx.path_graph(5)
+ >>> nx.dfs_successors(G, source=0)
+ {0: [1], 1: [2], 2: [3], 3: [4]}
+ >>> nx.dfs_successors(G, source=0, depth_limit=2)
+ {0: [1], 1: [2]}
+
+ Notes
+ -----
+ If a source is not specified then a source is chosen arbitrarily and
+ repeatedly until all components in the graph are searched.
+
+ The implementation of this function is adapted from David Eppstein's
+ depth-first search function in `PADS`_, with modifications
+ to allow depth limits based on the Wikipedia article
+ "`Depth-limited search`_".
+
+ .. _PADS: http://www.ics.uci.edu/~eppstein/PADS
+ .. _Depth-limited search: https://en.wikipedia.org/wiki/Depth-limited_search
+
+ See Also
+ --------
+ dfs_preorder_nodes
+ dfs_postorder_nodes
+ dfs_labeled_edges
+ :func:`~networkx.algorithms.traversal.edgedfs.edge_dfs`
+ :func:`~networkx.algorithms.traversal.breadth_first_search.bfs_tree`
+ """
+ d = defaultdict(list)
+ for s, t in dfs_edges(
+ G,
+ source=source,
+ depth_limit=depth_limit,
+ sort_neighbors=sort_neighbors,
+ ):
+ d[s].append(t)
+ return dict(d)
+
+
+@nx._dispatchable
+def dfs_postorder_nodes(G, source=None, depth_limit=None, *, sort_neighbors=None):
+ """Generate nodes in a depth-first-search post-ordering starting at source.
+
+ Parameters
+ ----------
+ G : NetworkX graph
+
+ source : node, optional
+ Specify starting node for depth-first search.
+
+ depth_limit : int, optional (default=len(G))
+ Specify the maximum search depth.
+
+ sort_neighbors : function (default=None)
+ A function that takes an iterator over nodes as the input, and
+ returns an iterable of the same nodes with a custom ordering.
+ For example, `sorted` will sort the nodes in increasing order.
+
+ Returns
+ -------
+ nodes: generator
+ A generator of nodes in a depth-first-search post-ordering.
+
+ Examples
+ --------
+ >>> G = nx.path_graph(5)
+ >>> list(nx.dfs_postorder_nodes(G, source=0))
+ [4, 3, 2, 1, 0]
+ >>> list(nx.dfs_postorder_nodes(G, source=0, depth_limit=2))
+ [1, 0]
+
+ Notes
+ -----
+ If a source is not specified then a source is chosen arbitrarily and
+ repeatedly until all components in the graph are searched.
+
+ The implementation of this function is adapted from David Eppstein's
+ depth-first search function in `PADS`_, with modifications
+ to allow depth limits based on the Wikipedia article
+ "`Depth-limited search`_".
+
+ .. _PADS: http://www.ics.uci.edu/~eppstein/PADS
+ .. _Depth-limited search: https://en.wikipedia.org/wiki/Depth-limited_search
+
+ See Also
+ --------
+ dfs_edges
+ dfs_preorder_nodes
+ dfs_labeled_edges
+ :func:`~networkx.algorithms.traversal.edgedfs.edge_dfs`
+ :func:`~networkx.algorithms.traversal.breadth_first_search.bfs_tree`
+ """
+ edges = nx.dfs_labeled_edges(
+ G, source=source, depth_limit=depth_limit, sort_neighbors=sort_neighbors
+ )
+ return (v for u, v, d in edges if d == "reverse")
+
+
+@nx._dispatchable
+def dfs_preorder_nodes(G, source=None, depth_limit=None, *, sort_neighbors=None):
+ """Generate nodes in a depth-first-search pre-ordering starting at source.
+
+ Parameters
+ ----------
+ G : NetworkX graph
+
+ source : node, optional
+ Specify starting node for depth-first search and return nodes in
+ the component reachable from source.
+
+ depth_limit : int, optional (default=len(G))
+ Specify the maximum search depth.
+
+ sort_neighbors : function (default=None)
+ A function that takes an iterator over nodes as the input, and
+ returns an iterable of the same nodes with a custom ordering.
+ For example, `sorted` will sort the nodes in increasing order.
+
+ Returns
+ -------
+ nodes: generator
+ A generator of nodes in a depth-first-search pre-ordering.
+
+ Examples
+ --------
+ >>> G = nx.path_graph(5)
+ >>> list(nx.dfs_preorder_nodes(G, source=0))
+ [0, 1, 2, 3, 4]
+ >>> list(nx.dfs_preorder_nodes(G, source=0, depth_limit=2))
+ [0, 1, 2]
+
+ Notes
+ -----
+ If a source is not specified then a source is chosen arbitrarily and
+ repeatedly until all components in the graph are searched.
+
+ The implementation of this function is adapted from David Eppstein's
+ depth-first search function in `PADS`_, with modifications
+ to allow depth limits based on the Wikipedia article
+ "`Depth-limited search`_".
+
+ .. _PADS: http://www.ics.uci.edu/~eppstein/PADS
+ .. _Depth-limited search: https://en.wikipedia.org/wiki/Depth-limited_search
+
+ See Also
+ --------
+ dfs_edges
+ dfs_postorder_nodes
+ dfs_labeled_edges
+ :func:`~networkx.algorithms.traversal.breadth_first_search.bfs_edges`
+ """
+ edges = nx.dfs_labeled_edges(
+ G, source=source, depth_limit=depth_limit, sort_neighbors=sort_neighbors
+ )
+ return (v for u, v, d in edges if d == "forward")
+
+
+@nx._dispatchable
+def dfs_labeled_edges(G, source=None, depth_limit=None, *, sort_neighbors=None):
+ """Iterate over edges in a depth-first-search (DFS) labeled by type.
+
+ Parameters
+ ----------
+ G : NetworkX graph
+
+ source : node, optional
+ Specify starting node for depth-first search and return edges in
+ the component reachable from source.
+
+ depth_limit : int, optional (default=len(G))
+ Specify the maximum search depth.
+
+ sort_neighbors : function (default=None)
+ A function that takes an iterator over nodes as the input, and
+ returns an iterable of the same nodes with a custom ordering.
+ For example, `sorted` will sort the nodes in increasing order.
+
+ Returns
+ -------
+ edges: generator
+ A generator of triples of the form (*u*, *v*, *d*), where (*u*,
+ *v*) is the edge being explored in the depth-first search and *d*
+ is one of the strings 'forward', 'nontree', 'reverse', or 'reverse-depth_limit'.
+ A 'forward' edge is one in which *u* has been visited but *v* has
+ not. A 'nontree' edge is one in which both *u* and *v* have been
+ visited but the edge is not in the DFS tree. A 'reverse' edge is
+ one in which both *u* and *v* have been visited and the edge is in
+ the DFS tree. When the `depth_limit` is reached via a 'forward' edge,
+ a 'reverse' edge is immediately generated rather than the subtree
+ being explored. To indicate this flavor of 'reverse' edge, the string
+ yielded is 'reverse-depth_limit'.
+
+ Examples
+ --------
+
+ The labels reveal the complete transcript of the depth-first search
+ algorithm in more detail than, for example, :func:`dfs_edges`::
+
+ >>> from pprint import pprint
+ >>>
+ >>> G = nx.DiGraph([(0, 1), (1, 2), (2, 1)])
+ >>> pprint(list(nx.dfs_labeled_edges(G, source=0)))
+ [(0, 0, 'forward'),
+ (0, 1, 'forward'),
+ (1, 2, 'forward'),
+ (2, 1, 'nontree'),
+ (1, 2, 'reverse'),
+ (0, 1, 'reverse'),
+ (0, 0, 'reverse')]
+
+ Notes
+ -----
+ If a source is not specified then a source is chosen arbitrarily and
+ repeatedly until all components in the graph are searched.
+
+ The implementation of this function is adapted from David Eppstein's
+ depth-first search function in `PADS`_, with modifications
+ to allow depth limits based on the Wikipedia article
+ "`Depth-limited search`_".
+
+ .. _PADS: http://www.ics.uci.edu/~eppstein/PADS
+ .. _Depth-limited search: https://en.wikipedia.org/wiki/Depth-limited_search
+
+ See Also
+ --------
+ dfs_edges
+ dfs_preorder_nodes
+ dfs_postorder_nodes
+ """
+ # Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
+ # by D. Eppstein, July 2004.
+ if source is None:
+ # edges for all components
+ nodes = G
+ else:
+ # edges for components with source
+ nodes = [source]
+ if depth_limit is None:
+ depth_limit = len(G)
+
+ get_children = (
+ G.neighbors
+ if sort_neighbors is None
+ else lambda n: iter(sort_neighbors(G.neighbors(n)))
+ )
+
+ visited = set()
+ for start in nodes:
+ if start in visited:
+ continue
+ yield start, start, "forward"
+ visited.add(start)
+ stack = [(start, get_children(start))]
+ depth_now = 1
+ while stack:
+ parent, children = stack[-1]
+ for child in children:
+ if child in visited:
+ yield parent, child, "nontree"
+ else:
+ yield parent, child, "forward"
+ visited.add(child)
+ if depth_now < depth_limit:
+ stack.append((child, iter(get_children(child))))
+ depth_now += 1
+ break
+ else:
+ yield parent, child, "reverse-depth_limit"
+ else:
+ stack.pop()
+ depth_now -= 1
+ if stack:
+ yield stack[-1][0], parent, "reverse"
+ yield start, start, "reverse"