Move dag longest path functions to rustworkx-core.

releasenotes/notes/migrate-longest_path-7c11cf2c8ac9781f.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    Added `longest_path` function to rustworkx-core. Before `longest_path` 
+    was only exposed via the Python interface. Now rust users can take 
+    advantage of this functionality in rustworkx-core. 
+    Refer to `#1168<https://github.com/Qiskit/rustworkx/issues/1168>`__ 
+    for more details. 
+

rustworkx-core/src/lib.rs

@@ -78,7 +78,7 @@ pub mod coloring;
 pub mod connectivity;
 pub mod generators;
 pub mod line_graph;
-
+pub mod longest_path;
 /// Module for maximum weight matching algorithms.
 pub mod max_weight_matching;
 pub mod planar;

rustworkx-core/src/longest_path.rs

@@ -0,0 +1,232 @@
+// 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.
+use hashbrown::HashMap;
+
+use petgraph::algo;
+use petgraph::graph::NodeIndex;
+use petgraph::visit::{
+    EdgeRef, GraphBase, GraphProp, IntoEdgesDirected, IntoNeighborsDirected, IntoNodeIdentifiers,
+    Visitable,
+};
+use petgraph::Directed;
+
+use num_traits::{Num, Zero};
+
+/// Calculates the longest path in a directed acyclic graph (DAG).
+///
+/// This function computes the longest path by weight in a given DAG. It will return the longest path
+/// along with its total weight, or `None` if the graph contains cycles which make the longest path
+/// computation undefined.
+///
+/// # Arguments
+/// * `graph`: Reference to a directed graph.
+/// * `weight_fn` - An input callable that will be passed the `EdgeRef` for each edge in the graph.
+///  The callable should return the weight of the edge as `Result<T, E>`. The weight must be a type that implements
+/// `Num`, `Zero`, `PartialOrd`, and `Copy`.
+///
+/// # Type Parameters
+/// * `G`: Type of the graph. Must be a directed graph.
+/// * `F`: Type of the weight function.
+/// * `T`: The type of the edge weight. Must implement `Num`, `Zero`, `PartialOrd`, and `Copy`.
+/// * `E`: The type of the error that the weight function can return.
+///
+/// # Returns
+/// * `None` if the graph contains a cycle.
+/// * `Some((Vec<usize>, T))` representing the longest path as a sequence of nodes and its total weight.
+/// * `Err(E)` if there is an error computing the weight of any edge.
+///
+/// # Example
+/// ```
+/// use petgraph::graph::DiGraph;
+/// use petgraph::graph::NodeIndex;
+/// use petgraph::Directed;
+/// use rustworkx_core::longest_path::longest_path;
+///
+/// let mut graph: DiGraph<(), i32> = DiGraph::new();
+/// let n0 = graph.add_node(());
+/// let n1 = graph.add_node(());
+/// let n2 = graph.add_node(());
+/// graph.add_edge(n0, n1, 1);
+/// graph.add_edge(n0, n2, 3);
+/// graph.add_edge(n1, n2, 1);
+///
+/// let weight_fn = |edge: petgraph::graph::EdgeReference<i32>| Ok::<i32, &str>(*edge.weight());
+/// let result = longest_path(&graph, weight_fn).unwrap();
+/// assert_eq!(result, Some((vec![n0.index(), n2.index()], 3)));
+/// ```
+pub fn longest_path<G, F, T, E>(graph: G, mut weight_fn: F) -> Result<Option<(Vec<usize>, T)>, E>
+where
+    G: GraphProp<EdgeType = Directed>
+        + IntoNodeIdentifiers
+        + IntoNeighborsDirected
+        + IntoEdgesDirected
+        + Visitable
+        + GraphBase<NodeId = NodeIndex>,
+    F: FnMut(G::EdgeRef) -> Result<T, E>,
+    T: Num + Zero + PartialOrd + Copy,
+{
+    let mut path: Vec<usize> = Vec::new();
+    let nodes = match algo::toposort(graph, None) {
+        Ok(nodes) => nodes,
+        Err(_) => return Ok(None), // Return None if the graph contains a cycle
+    };
+
+    if nodes.is_empty() {
+        return Ok(Some((path, T::zero())));
+    }
+
+    let mut dist: HashMap<NodeIndex, (T, NodeIndex)> = HashMap::with_capacity(nodes.len()); // Stores the distance and the previous node
+
+    // Iterate over nodes in topological order
+    for node in nodes {
+        let parents = graph.edges_directed(node, petgraph::Direction::Incoming);
+        let mut incoming_path: Vec<(T, NodeIndex)> = Vec::new(); // Stores the distance and the previous node for each parent
+        for p_edge in parents {
+            let p_node = p_edge.source();
+            let weight: T = weight_fn(p_edge)?;
+            let length = dist[&p_node].0 + weight;
+            incoming_path.push((length, p_node));
+        }
+        // Determine the maximum distance and corresponding parent node
+        let max_path: (T, NodeIndex) = incoming_path
+            .into_iter()
+            .max_by(|a, b| a.0.partial_cmp(&b.0).unwrap())
+            .unwrap_or((T::zero(), node)); // If there are no incoming edges, the distance is zero
+
+        // Store the maximum distance and the corresponding parent node for the current node
+        dist.insert(node, max_path);
+    }
+    let (first, _) = dist
+        .iter()
+        .max_by(|a, b| a.1.partial_cmp(b.1).unwrap())
+        .unwrap();
+    let mut v = *first;
+    let mut u: Option<NodeIndex> = None;
+    // Backtrack from this node to find the path
+    while u.map_or(true, |u| u != v) {
+        path.push(v.index());
+        u = Some(v);
+        v = dist[&v].1;
+    }
+    path.reverse(); // Reverse the path to get the correct order
+    let path_weight = dist[first].0; // The total weight of the longest path
+
+    Ok(Some((path, path_weight)))
+}
+
+#[cfg(test)]
+mod test_longest_path {
+    use super::*;
+    use petgraph::graph::DiGraph;
+    use petgraph::stable_graph::StableDiGraph;
+
+    #[test]
+    fn test_empty_graph() {
+        let graph: DiGraph<(), ()> = DiGraph::new();
+        let weight_fn = |_: petgraph::graph::EdgeReference<()>| Ok::<i32, &str>(0);
+        let result = longest_path(&graph, weight_fn);
+        assert_eq!(result, Ok(Some((vec![], 0))));
+    }
+
+    #[test]
+    fn test_single_node_graph() {
+        let mut graph: DiGraph<(), ()> = DiGraph::new();
+        graph.add_node(());
+        let weight_fn = |_: petgraph::graph::EdgeReference<()>| Ok::<i32, &str>(0);
+        let result = longest_path(&graph, weight_fn);
+        assert_eq!(result, Ok(Some((vec![0], 0))));
+    }
+
+    #[test]
+    fn test_dag_with_multiple_paths() {
+        let mut graph: DiGraph<(), i32> = DiGraph::new();
+        let n0 = graph.add_node(());
+        let n1 = graph.add_node(());
+        let n2 = graph.add_node(());
+        let n3 = graph.add_node(());
+        let n4 = graph.add_node(());
+        let n5 = graph.add_node(());
+        graph.add_edge(n0, n1, 3);
+        graph.add_edge(n0, n2, 2);
+        graph.add_edge(n1, n2, 1);
+        graph.add_edge(n1, n3, 4);
+        graph.add_edge(n2, n3, 2);
+        graph.add_edge(n3, n4, 2);
+        graph.add_edge(n2, n5, 1);
+        graph.add_edge(n4, n5, 3);
+        let weight_fn = |edge: petgraph::graph::EdgeReference<i32>| Ok::<i32, &str>(*edge.weight());
+        let result = longest_path(&graph, weight_fn);
+        assert_eq!(result, Ok(Some((vec![0, 1, 3, 4, 5], 12))));
+    }
+
+    #[test]
+    fn test_graph_with_cycle() {
+        let mut graph: DiGraph<(), i32> = DiGraph::new();
+        let n0 = graph.add_node(());
+        let n1 = graph.add_node(());
+        graph.add_edge(n0, n1, 1);
+        graph.add_edge(n1, n0, 1); // Creates a cycle
+
+        let weight_fn = |edge: petgraph::graph::EdgeReference<i32>| Ok::<i32, &str>(*edge.weight());
+        let result = longest_path(&graph, weight_fn);
+        assert_eq!(result, Ok(None));
+    }
+
+    #[test]
+    fn test_negative_weights() {
+        let mut graph: DiGraph<(), i32> = DiGraph::new();
+        let n0 = graph.add_node(());
+        let n1 = graph.add_node(());
+        let n2 = graph.add_node(());
+        graph.add_edge(n0, n1, -1);
+        graph.add_edge(n0, n2, 2);
+        graph.add_edge(n1, n2, -2);
+        let weight_fn = |edge: petgraph::graph::EdgeReference<i32>| Ok::<i32, &str>(*edge.weight());
+        let result = longest_path(&graph, weight_fn);
+        assert_eq!(result, Ok(Some((vec![0, 2], 2))));
+    }
+
+    #[test]
+    fn test_longest_path_in_stable_digraph() {
+        let mut graph: StableDiGraph<(), i32> = StableDiGraph::new();
+        let n0 = graph.add_node(());
+        let n1 = graph.add_node(());
+        let n2 = graph.add_node(());
+        graph.add_edge(n0, n1, 1);
+        graph.add_edge(n0, n2, 3);
+        graph.add_edge(n1, n2, 1);
+        let weight_fn =
+            |edge: petgraph::stable_graph::EdgeReference<'_, i32>| Ok::<i32, &str>(*edge.weight());
+        let result = longest_path(&graph, weight_fn);
+        assert_eq!(result, Ok(Some((vec![0, 2], 3))));
+    }
+
+    #[test]
+    fn test_error_handling() {
+        let mut graph: DiGraph<(), i32> = DiGraph::new();
+        let n0 = graph.add_node(());
+        let n1 = graph.add_node(());
+        let n2 = graph.add_node(());
+        graph.add_edge(n0, n1, 1);
+        graph.add_edge(n0, n2, 2);
+        graph.add_edge(n1, n2, 1);
+        let weight_fn = |edge: petgraph::graph::EdgeReference<i32>| {
+            if *edge.weight() == 2 {
+                Err("Error: edge weight is 2")
+            } else {
+                Ok::<i32, &str>(*edge.weight())
+            }
+        };
+        let result = longest_path(&graph, weight_fn);
+        assert_eq!(result, Err("Error: edge weight is 2"));
+    }
+}

src/dag_algo/longest_path.rs

@@ -11,69 +11,51 @@
 // under the License.
 
 use crate::{digraph, DAGHasCycle};
+use rustworkx_core::longest_path::longest_path as core_longest_path;
 
-use hashbrown::HashMap;
+use petgraph::stable_graph::EdgeReference;
+use petgraph::visit::EdgeRef;
 
 use pyo3::prelude::*;
 
-use petgraph::algo;
-use petgraph::prelude::*;
-use petgraph::stable_graph::NodeIndex;
-
 use num_traits::{Num, Zero};
 
+/// Calculate the longest path in a directed acyclic graph (DAG).
+///
+/// This function interfaces with the Python `PyDiGraph` object to compute the longest path
+/// using the provided weight function.
+///
+/// # Arguments
+/// * `graph`: Reference to a `PyDiGraph` object.
+/// * `weight_fn`: A callable that takes the source node index, target node index, and the weight
+///   object and returns the weight of the edge as a `PyResult<T>`.
+///
+/// # Type Parameters
+/// * `F`: Type of the weight function.
+/// * `T`: The type of the edge weight. Must implement `Num`, `Zero`, `PartialOrd`, and `Copy`.
+///
+/// # Returns
+/// * `PyResult<(Vec<usize>, T)>` representing the longest path as a sequence of node indices and its total weight.
 pub fn longest_path<F, T>(graph: &digraph::PyDiGraph, mut weight_fn: F) -> PyResult<(Vec<usize>, T)>
 where
     F: FnMut(usize, usize, &PyObject) -> PyResult<T>,
     T: Num + Zero + PartialOrd + Copy,
 {
     let dag = &graph.graph;
-    let mut path: Vec<usize> = Vec::new();
-    let nodes = match algo::toposort(&graph.graph, None) {
-        Ok(nodes) => nodes,
-        Err(_err) => return Err(DAGHasCycle::new_err("Sort encountered a cycle")),
+
+    // Create a new weight function that matches the required signature
+    let edge_cost = |edge_ref: EdgeReference<'_, PyObject>| -> Result<T, PyErr> {
+        let source = edge_ref.source().index();
+        let target = edge_ref.target().index();
+        let weight = edge_ref.weight();
+        weight_fn(source, target, weight)
     };
-    if nodes.is_empty() {
-        return Ok((path, T::zero()));
-    }
-    let mut dist: HashMap<NodeIndex, (T, NodeIndex)> = HashMap::new();
-    for node in nodes {
-        let parents = dag.edges_directed(node, petgraph::Direction::Incoming);
-        let mut us: Vec<(T, NodeIndex)> = Vec::new();
-        for p_edge in parents {
-            let p_node = p_edge.source();
-            let weight: T = weight_fn(p_node.index(), p_edge.target().index(), p_edge.weight())?;
-            let length = dist[&p_node].0 + weight;
-            us.push((length, p_node));
-        }
-        let maxu: (T, NodeIndex) = if !us.is_empty() {
-            *us.iter()
-                .max_by(|a, b| {
-                    let weight_a = a.0;
-                    let weight_b = b.0;
-                    weight_a.partial_cmp(&weight_b).unwrap()
-                })
-                .unwrap()
-        } else {
-            (T::zero(), node)
-        };
-        dist.insert(node, maxu);
-    }
-    let first = dist
-        .keys()
-        .max_by(|a, b| dist[*a].partial_cmp(&dist[*b]).unwrap())
-        .unwrap();
-    let mut v = *first;
-    let mut u: Option<NodeIndex> = None;
-    while match u {
-        Some(u) => u != v,
-        None => true,
-    } {
-        path.push(v.index());
-        u = Some(v);
-        v = dist[&v].1;
-    }
-    path.reverse();
-    let path_weight = dist[first].0;
+
+    let (path, path_weight) = match core_longest_path(dag, edge_cost) {
+        Ok(Some(result)) => result,
+        Ok(None) => return Err(DAGHasCycle::new_err("The graph contains a cycle")),
+        Err(e) => return Err(e),
+    };
+
     Ok((path, path_weight))
 }