Add dfs-search

docs/source/api.rst

@@ -56,6 +56,7 @@ Traversal
    :toctree: stubs
 
    retworkx.dfs_edges
+   retworkx.dfs_search
    retworkx.bfs_successors
    retworkx.bfs_search
    retworkx.topological_sort
@@ -64,6 +65,7 @@ Traversal
    retworkx.ancestors
    retworkx.collect_runs
    retworkx.collect_bicolor_runs
+   retworkx.visit.DFSVisitor
    retworkx.visit.BFSVisitor
 
 .. _dag-algorithms:
@@ -250,6 +252,7 @@ the functions from the explicitly typed based on the data type.
    retworkx.digraph_all_pairs_dijkstra_path_lengths
    retworkx.digraph_k_shortest_path_lengths
    retworkx.digraph_dfs_edges
+   retworkx.digraph_dfs_search
    retworkx.digraph_find_cycle
    retworkx.digraph_transitivity
    retworkx.digraph_core_number
@@ -295,6 +298,7 @@ typed API based on the data type.
    retworkx.graph_k_shortest_path_lengths
    retworkx.graph_all_pairs_dijkstra_path_lengths
    retworkx.graph_dfs_edges
+   retworkx.graph_dfs_search
    retworkx.graph_transitivity
    retworkx.graph_core_number
    retworkx.graph_complement

releasenotes/notes/dfs-search-6083680bf62356b0.yaml

@@ -0,0 +1,28 @@
+---
+features:
+  - |
+    Added a new :func:`~retworkx.dfs_search` (and it's per type variants
+    :func:`~retworkx.graph_dfs_search` and :func:`~retworkx.digraph_dfs_search`)
+    that traverses the graph in a depth-first manner and emits events at specified
+    points. The events are handled by a visitor object that subclasses
+    :class:`~retworkx.visit.DFSVisitor` through the appropriate callback functions.
+    For example:
+
+    .. jupyter-execute::
+
+      import retworkx
+      from retworkx.visit import DFSVisitor
+
+      class TreeEdgesRecorder(DFSVisitor):
+
+          def __init__(self):
+              self.edges = []
+
+          def tree_edge(self, edge):
+              self.edges.append(edge)
+
+      graph = retworkx.PyGraph()
+      graph.extend_from_edge_list([(1, 3), (0, 1), (2, 1), (0, 2)])
+      vis = TreeEdgesRecorder()
+      retworkx.dfs_search(graph, [0], vis)
+      print('Tree edges:', vis.edges)

retworkx-core/src/traversal/dfs_visit.rs

@@ -0,0 +1,276 @@
+// Licensed under the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License. You may obtain
+// a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations
+// under the License.
+
+// This module is an iterative implementation of the upstream petgraph
+// ``depth_first_search`` function.
+// https://github.com/petgraph/petgraph/blob/0.6.0/src/visit/dfsvisit.rs
+
+use petgraph::visit::{
+    ControlFlow, EdgeRef, IntoEdges, Time, VisitMap, Visitable,
+};
+
+/// Return if the expression is a break value, execute the provided statement
+/// if it is a prune value.
+/// https://github.com/petgraph/petgraph/blob/0.6.0/src/visit/dfsvisit.rs#L27
+macro_rules! try_control {
+    ($e:expr, $p:stmt) => {
+        try_control!($e, $p, ());
+    };
+    ($e:expr, $p:stmt, $q:stmt) => {
+        match $e {
+            x => {
+                if x.should_break() {
+                    return x;
+                } else if x.should_prune() {
+                    $p
+                } else {
+                    $q
+                }
+            }
+        }
+    };
+}
+
+/// A depth first search (DFS) visitor event.
+#[derive(Copy, Clone, Debug)]
+pub enum DfsEvent<N, E> {
+    Discover(N, Time),
+    /// An edge of the tree formed by the traversal.
+    TreeEdge(N, N, E),
+    /// An edge to an already visited node.
+    BackEdge(N, N, E),
+    /// A cross or forward edge.
+    ///
+    /// For an edge *(u, v)*, if the discover time of *v* is greater than *u*,
+    /// then it is a forward edge, else a cross edge.
+    CrossForwardEdge(N, N, E),
+    /// All edges from a node have been reported.
+    Finish(N, Time),
+}
+
+/// An iterative depth first search.
+///
+/// It is an iterative implementation of the upstream petgraph
+/// [`depth_first_search`](https://docs.rs/petgraph/0.6.0/petgraph/visit/fn.depth_first_search.html) function.
+///
+/// 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 `petgraph::DfsEvent` 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.
+///
+/// # Example returning `Control`.
+///
+/// Find a path from vertex 0 to 5, and exit the visit as soon as we reach
+/// the goal vertex.
+///
+/// ```
+/// use retworkx_core::petgraph::prelude::*;
+/// use retworkx_core::petgraph::graph::node_index as n;
+/// use retworkx_core::petgraph::visit::Control;
+///
+/// use retworkx_core::traversal::{DfsEvent, depth_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);
+/// depth_first_search(&gr, Some(start), |event| {
+///     if let DfsEvent::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 retworkx_core::petgraph::graph::node_index as n;
+/// use retworkx_core::petgraph::prelude::*;
+/// use retworkx_core::petgraph::visit::Time;
+///
+/// use retworkx_core::traversal::{DfsEvent, depth_first_search};
+///
+/// let gr: Graph<(), ()> = Graph::from_edges(&[(0, 1), (1, 2), (1, 1), (2, 1)]);
+/// let start = n(0);
+/// let mut back_edges = 0;
+/// let mut discover_time = 0;
+///
+/// #[derive(Debug)]
+/// struct BackEdgeFound {
+///     source: NodeIndex,
+///     target: NodeIndex,
+/// }
+///
+/// // Stop the search, the first time a BackEdge is encountered.
+/// let result = depth_first_search(&gr, Some(start), |event| {
+///     match event {
+///         // 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.
+///         DfsEvent::Discover(_, Time(t)) => {
+///             discover_time = t;
+///             Ok(())
+///         }
+///         DfsEvent::BackEdge(u, v, _) => {
+///             back_edges += 1;
+///             // the implementation of ControlFlow for Result,
+///             // treats this Err value as Continue::Break
+///             Err(BackEdgeFound {source: u, target: v})
+///         }
+///         _ => Ok(()),
+///     }
+/// });
+///
+/// // Even though the graph has more than one cycle,
+/// // The number of back_edges visited by the search should always be 1.
+/// assert_eq!(back_edges, 1);
+/// println!("discover time:{:?}", discover_time);
+/// println!("number of backedges encountered: {}", back_edges);
+/// println!("back edge: ({:?})", result.unwrap_err());
+/// ```
+pub fn depth_first_search<G, I, F, C>(graph: G, starts: I, mut visitor: F) -> C
+where
+    G: IntoEdges + Visitable,
+    I: IntoIterator<Item = G::NodeId>,
+    F: FnMut(DfsEvent<G::NodeId, &G::EdgeWeight>) -> C,
+    C: ControlFlow,
+{
+    let time = &mut Time(0);
+    let discovered = &mut graph.visit_map();
+    let finished = &mut graph.visit_map();
+
+    for start in starts {
+        try_control!(
+            dfs_visitor(graph, start, &mut visitor, discovered, finished, time),
+            unreachable!()
+        );
+    }
+    C::continuing()
+}
+
+fn dfs_visitor<G, F, C>(
+    graph: G,
+    u: G::NodeId,
+    visitor: &mut F,
+    discovered: &mut G::Map,
+    finished: &mut G::Map,
+    time: &mut Time,
+) -> C
+where
+    G: IntoEdges + Visitable,
+    F: FnMut(DfsEvent<G::NodeId, &G::EdgeWeight>) -> C,
+    C: ControlFlow,
+{
+    if !discovered.visit(u) {
+        return C::continuing();
+    }
+
+    try_control!(visitor(DfsEvent::Discover(u, time_post_inc(time))), {}, {
+        let mut stack: Vec<(G::NodeId, <G as IntoEdges>::Edges)> = Vec::new();
+        stack.push((u, graph.edges(u)));
+
+        while let Some(elem) = stack.last_mut() {
+            let u = elem.0;
+            let adjacent_edges = &mut elem.1;
+            let mut next = None;
+
+            for edge in adjacent_edges {
+                let v = edge.target();
+                if !discovered.is_visited(&v) {
+                    try_control!(
+                        visitor(DfsEvent::TreeEdge(u, v, edge.weight())),
+                        continue
+                    );
+                    discovered.visit(v);
+                    try_control!(
+                        visitor(DfsEvent::Discover(v, time_post_inc(time))),
+                        continue
+                    );
+                    next = Some(v);
+                    break;
+                } else if !finished.is_visited(&v) {
+                    try_control!(
+                        visitor(DfsEvent::BackEdge(u, v, edge.weight())),
+                        continue
+                    );
+                } else {
+                    try_control!(
+                        visitor(DfsEvent::CrossForwardEdge(
+                            u,
+                            v,
+                            edge.weight()
+                        )),
+                        continue
+                    );
+                }
+            }
+
+            match next {
+                Some(v) => stack.push((v, graph.edges(v))),
+                None => {
+                    let first_finish = finished.visit(u);
+                    debug_assert!(first_finish);
+                    try_control!(
+                        visitor(DfsEvent::Finish(u, time_post_inc(time))),
+                        panic!("Pruning on the `DfsEvent::Finish` is not supported!")
+                    );
+                    stack.pop();
+                }
+            };
+        }
+    });
+
+    C::continuing()
+}
+
+fn time_post_inc(x: &mut Time) -> Time {
+    let v = *x;
+    x.0 += 1;
+    v
+}

retworkx-core/src/traversal/mod.rs

@@ -14,6 +14,8 @@
 
 mod bfs_visit;
 mod dfs_edges;
+mod dfs_visit;
 
 pub use bfs_visit::{breadth_first_search, BfsEvent};
 pub use dfs_edges::dfs_edges;
+pub use dfs_visit::{depth_first_search, DfsEvent};