Degree centrality implementation (#1306)

* Add degree centrality

Co-authored-by: Gohlub [email protected]
Co-authored-by: onsali [email protected]

* Rename method in rustworkx-core

* Add universal functions

* Fix typo

* Fix method names in tests

* Add type stubs

* Remove unnecessary diff

* Handle graphs with removed edges

* Fix rustdoc tests

* Fix type stubs

---------

Co-authored-by: Ivan Carvalho <[email protected]>

.gitignore

@@ -22,3 +22,4 @@ retworkx/*pyd
 *.jpg
 **/*.so
 retworkx-core/Cargo.lock
+**/.DS_Store

rustworkx-core/src/centrality.rs

@@ -335,6 +335,74 @@ fn accumulate_edges<G>(
         }
     }
 }
+/// Compute the degree centrality of all nodes in a graph.
+///
+/// For undirected graphs, this calculates the normalized degree for each node.
+/// For directed graphs, this calculates the normalized out-degree for each node.
+///
+/// Arguments:
+///
+/// * `graph` - The graph object to calculate degree centrality for
+///
+/// # Example
+/// ```rust
+/// use rustworkx_core::petgraph::graph::{UnGraph, DiGraph};
+/// use rustworkx_core::centrality::degree_centrality;
+///
+/// // Undirected graph example
+/// let graph = UnGraph::<i32, ()>::from_edges(&[
+///     (0, 1), (1, 2), (2, 3), (3, 0)
+/// ]);
+/// let centrality = degree_centrality(&graph, None);
+///
+/// // Directed graph example
+/// let digraph = DiGraph::<i32, ()>::from_edges(&[
+///     (0, 1), (1, 2), (2, 3), (3, 0), (0, 2), (1, 3)
+/// ]);
+/// let centrality = degree_centrality(&digraph, None);
+/// ```
+pub fn degree_centrality<G>(graph: G, direction: Option<petgraph::Direction>) -> Vec<f64>
+where
+    G: NodeIndexable
+        + IntoNodeIdentifiers
+        + IntoNeighbors
+        + IntoNeighborsDirected
+        + NodeCount
+        + GraphProp,
+    G::NodeId: Eq + Hash,
+{
+    let node_count = graph.node_count() as f64;
+    let mut centrality = vec![0.0; graph.node_bound()];
+
+    for node in graph.node_identifiers() {
+        let (degree, normalization) = match (graph.is_directed(), direction) {
+            (true, None) => {
+                let out_degree = graph
+                    .neighbors_directed(node, petgraph::Direction::Outgoing)
+                    .count() as f64;
+                let in_degree = graph
+                    .neighbors_directed(node, petgraph::Direction::Incoming)
+                    .count() as f64;
+                let total = in_degree + out_degree;
+                // Use 2(n-1) normalization only if this is a complete graph
+                let norm = if total == 2.0 * (node_count - 1.0) {
+                    2.0 * (node_count - 1.0)
+                } else {
+                    node_count - 1.0
+                };
+                (total, norm)
+            }
+            (true, Some(dir)) => (
+                graph.neighbors_directed(node, dir).count() as f64,
+                node_count - 1.0,
+            ),
+            (false, _) => (graph.neighbors(node).count() as f64, node_count - 1.0),
+        };
+        centrality[graph.to_index(node)] = degree / normalization;
+    }
+
+    centrality
+}
 
 struct ShortestPathData<G>
 where

rustworkx/__init__.py

@@ -1184,6 +1184,20 @@ def closeness_centrality(graph, wf_improved=True):
     raise TypeError("Invalid input type %s for graph" % type(graph))
 
 
+@_rustworkx_dispatch
+def degree_centrality(graph):
+    r"""Compute the degree centrality of each node in a graph object.
+
+    :param graph: The input graph. Can either be a
+        :class:`~rustworkx.PyGraph` or :class:`~rustworkx.PyDiGraph`.
+
+    :returns: a read-only dict-like object whose keys are edges and values are the
+        degree centrality score for each node.
+    :rtype: CentralityMapping
+    """
+    raise TypeError("Invalid input type %s for graph" % type(graph))
+
+
 @_rustworkx_dispatch
 def edge_betweenness_centrality(graph, normalized=True, parallel_threshold=50):
     r"""Compute the edge betweenness centrality of all edges in a graph.

rustworkx/__init__.pyi

@@ -50,6 +50,10 @@ from .rustworkx import digraph_closeness_centrality as digraph_closeness_central
 from .rustworkx import graph_closeness_centrality as graph_closeness_centrality
 from .rustworkx import digraph_katz_centrality as digraph_katz_centrality
 from .rustworkx import graph_katz_centrality as graph_katz_centrality
+from .rustworkx import digraph_degree_centrality as digraph_degree_centrality
+from .rustworkx import graph_degree_centrality as graph_degree_centrality
+from .rustworkx import in_degree_centrality as in_degree_centrality
+from .rustworkx import out_degree_centrality as out_degree_centrality
 from .rustworkx import graph_greedy_color as graph_greedy_color
 from .rustworkx import graph_greedy_edge_color as graph_greedy_edge_color
 from .rustworkx import graph_is_bipartite as graph_is_bipartite
@@ -484,6 +488,9 @@ def betweenness_centrality(
 def closeness_centrality(
     graph: PyGraph[_S, _T] | PyDiGraph[_S, _T], wf_improved: bool = ...
 ) -> CentralityMapping: ...
+def degree_centrality(
+    graph: PyGraph[_S, _T] | PyDiGraph[_S, _T],
+) -> CentralityMapping: ...
 def edge_betweenness_centrality(
     graph: PyGraph[_S, _T] | PyDiGraph[_S, _T],
     normalized: bool = ...,

rustworkx/rustworkx.pyi

@@ -124,6 +124,22 @@ def graph_closeness_centrality(
     graph: PyGraph[_S, _T],
     wf_improved: bool = ...,
 ) -> CentralityMapping: ...
+def digraph_degree_centrality(
+    graph: PyDiGraph[_S, _T],
+    /,
+) -> CentralityMapping: ...
+def in_degree_centrality(
+    graph: PyDiGraph[_S, _T],
+    /,
+) -> CentralityMapping: ...
+def out_degree_centrality(
+    graph: PyDiGraph[_S, _T],
+    /,
+) -> CentralityMapping: ...
+def graph_degree_centrality(
+    graph: PyGraph[_S, _T],
+    /,
+) -> CentralityMapping: ...
 def digraph_katz_centrality(
     graph: PyDiGraph[_S, _T],
     /,

src/centrality.rs

@@ -165,6 +165,102 @@ pub fn digraph_betweenness_centrality(
     }
 }
 
+/// Compute the degree centrality for nodes in a PyGraph.
+///
+/// Degree centrality assigns an importance score based simply on the number of edges held by each node.
+///
+/// :param PyGraph graph: The input graph
+///
+/// :returns: a read-only dict-like object whose keys are the node indices and values are the
+///      centrality score for each node.
+/// :rtype: CentralityMapping
+#[pyfunction(signature = (graph,))]
+#[pyo3(text_signature = "(graph, /,)")]
+pub fn graph_degree_centrality(graph: &graph::PyGraph) -> PyResult<CentralityMapping> {
+    let centrality = centrality::degree_centrality(&graph.graph, None);
+
+    Ok(CentralityMapping {
+        centralities: graph
+            .graph
+            .node_indices()
+            .map(|i| (i.index(), centrality[i.index()]))
+            .collect(),
+    })
+}
+
+/// Compute the degree centrality for nodes in a PyDiGraph.
+///
+/// Degree centrality assigns an importance score based simply on the number of edges held by each node.
+/// This function computes the TOTAL (in + out) degree centrality.
+///
+/// :param PyDiGraph graph: The input graph
+///
+/// :returns: a read-only dict-like object whose keys are the node indices and values are the
+///      centrality score for each node.
+/// :rtype: CentralityMapping
+#[pyfunction(signature = (graph,))]
+#[pyo3(text_signature = "(graph, /,)")]
+pub fn digraph_degree_centrality(graph: &digraph::PyDiGraph) -> PyResult<CentralityMapping> {
+    let centrality = centrality::degree_centrality(&graph.graph, None);
+
+    Ok(CentralityMapping {
+        centralities: graph
+            .graph
+            .node_indices()
+            .map(|i| (i.index(), centrality[i.index()]))
+            .collect(),
+    })
+}
+/// Compute the in-degree centrality for nodes in a PyDiGraph.
+///
+/// In-degree centrality assigns an importance score based on the number of incoming edges
+/// to each node.
+///
+/// :param PyDiGraph graph: The input graph
+///
+/// :returns: a read-only dict-like object whose keys are the node indices and values are the
+///      centrality score for each node.
+/// :rtype: CentralityMapping
+#[pyfunction(signature = (graph,))]
+#[pyo3(text_signature = "(graph, /)")]
+pub fn in_degree_centrality(graph: &digraph::PyDiGraph) -> PyResult<CentralityMapping> {
+    let centrality =
+        centrality::degree_centrality(&graph.graph, Some(petgraph::Direction::Incoming));
+
+    Ok(CentralityMapping {
+        centralities: graph
+            .graph
+            .node_indices()
+            .map(|i| (i.index(), centrality[i.index()]))
+            .collect(),
+    })
+}
+
+/// Compute the out-degree centrality for nodes in a PyDiGraph.
+///
+/// Out-degree centrality assigns an importance score based on the number of outgoing edges
+/// from each node.
+///
+/// :param PyDiGraph graph: The input graph
+///
+/// :returns: a read-only dict-like object whose keys are the node indices and values are the
+///      centrality score for each node.
+/// :rtype: CentralityMapping
+#[pyfunction(signature = (graph,))]
+#[pyo3(text_signature = "(graph, /,)")]
+pub fn out_degree_centrality(graph: &digraph::PyDiGraph) -> PyResult<CentralityMapping> {
+    let centrality =
+        centrality::degree_centrality(&graph.graph, Some(petgraph::Direction::Outgoing));
+
+    Ok(CentralityMapping {
+        centralities: graph
+            .graph
+            .node_indices()
+            .map(|i| (i.index(), centrality[i.index()]))
+            .collect(),
+    })
+}
+
 /// Compute the closeness centrality of each node in a :class:`~.PyGraph` object.
 ///
 /// The closeness centrality of a node :math:`u` is defined as the

src/lib.rs

@@ -533,6 +533,10 @@ fn rustworkx(py: Python<'_>, m: &Bound<PyModule>) -> PyResult<()> {
     m.add_wrapped(wrap_pyfunction!(digraph_eigenvector_centrality))?;
     m.add_wrapped(wrap_pyfunction!(graph_katz_centrality))?;
     m.add_wrapped(wrap_pyfunction!(digraph_katz_centrality))?;
+    m.add_wrapped(wrap_pyfunction!(graph_degree_centrality))?;
+    m.add_wrapped(wrap_pyfunction!(digraph_degree_centrality))?;
+    m.add_wrapped(wrap_pyfunction!(in_degree_centrality))?;
+    m.add_wrapped(wrap_pyfunction!(out_degree_centrality))?;
     m.add_wrapped(wrap_pyfunction!(graph_astar_shortest_path))?;
     m.add_wrapped(wrap_pyfunction!(digraph_astar_shortest_path))?;
     m.add_wrapped(wrap_pyfunction!(graph_greedy_color))?;

tests/digraph/test_centrality.py

@@ -241,3 +241,98 @@ def test_path_graph_unnormalized(self):
         expected = {0: 4.0, 1: 6.0, 2: 6.0, 3: 4.0}
         for k, v in centrality.items():
             self.assertAlmostEqual(v, expected[k])
+
+
+class TestDiGraphDegreeCentrality(unittest.TestCase):
+    def setUp(self):
+        self.graph = rustworkx.PyDiGraph()
+        self.a = self.graph.add_node("A")
+        self.b = self.graph.add_node("B")
+        self.c = self.graph.add_node("C")
+        self.d = self.graph.add_node("D")
+        edge_list = [
+            (self.a, self.b, 1),
+            (self.b, self.c, 1),
+            (self.c, self.d, 1),
+            (self.a, self.c, 1),  # Additional edge
+        ]
+        self.graph.add_edges_from(edge_list)
+
+    def test_degree_centrality(self):
+        centrality = rustworkx.degree_centrality(self.graph)
+        expected = {
+            0: 2 / 3,  # 2 total edges / 3
+            1: 2 / 3,  # 2 total edges / 3
+            2: 1.0,  # 3 total edges / 3
+            3: 1 / 3,  # 1 total edge / 3
+        }
+        for k, v in centrality.items():
+            self.assertAlmostEqual(v, expected[k])
+
+    def test_in_degree_centrality(self):
+        centrality = rustworkx.in_degree_centrality(self.graph)
+        expected = {
+            0: 0.0,  # 0 incoming edges
+            1: 1 / 3,  # 1 incoming edge
+            2: 2 / 3,  # 2 incoming edges
+            3: 1 / 3,  # 1 incoming edge
+        }
+        for k, v in centrality.items():
+            self.assertAlmostEqual(v, expected[k])
+
+    def test_out_degree_centrality(self):
+        centrality = rustworkx.out_degree_centrality(self.graph)
+        expected = {
+            0: 2 / 3,  # 2 outgoing edges
+            1: 1 / 3,  # 1 outgoing edge
+            2: 1 / 3,  # 1 outgoing edge
+            3: 0.0,  # 0 outgoing edges
+        }
+        for k, v in centrality.items():
+            self.assertAlmostEqual(v, expected[k])
+
+    def test_degree_centrality_complete_digraph(self):
+        graph = rustworkx.generators.directed_complete_graph(5)
+        centrality = rustworkx.degree_centrality(graph)
+        expected = {0: 1.0, 1: 1.0, 2: 1.0, 3: 1.0, 4: 1.0}
+        for k, v in centrality.items():
+            self.assertAlmostEqual(v, expected[k])
+
+    def test_degree_centrality_directed_path(self):
+        graph = rustworkx.generators.directed_path_graph(5)
+        centrality = rustworkx.degree_centrality(graph)
+        expected = {
+            0: 1 / 4,  # 1 total edge (out only) / 4
+            1: 2 / 4,  # 2 total edges (1 in + 1 out) / 4
+            2: 2 / 4,  # 2 total edges (1 in + 1 out) / 4
+            3: 2 / 4,  # 2 total edges (1 in + 1 out) / 4
+            4: 1 / 4,  # 1 total edge (in only) / 4
+        }
+        for k, v in centrality.items():
+            self.assertAlmostEqual(v, expected[k])
+
+    def test_in_degree_centrality_directed_path(self):
+        graph = rustworkx.generators.directed_path_graph(5)
+        centrality = rustworkx.in_degree_centrality(graph)
+        expected = {
+            0: 0.0,  # 0 incoming edges
+            1: 1 / 4,  # 1 incoming edge
+            2: 1 / 4,  # 1 incoming edge
+            3: 1 / 4,  # 1 incoming edge
+            4: 1 / 4,  # 1 incoming edge
+        }
+        for k, v in centrality.items():
+            self.assertAlmostEqual(v, expected[k])
+
+    def test_out_degree_centrality_directed_path(self):
+        graph = rustworkx.generators.directed_path_graph(5)
+        centrality = rustworkx.out_degree_centrality(graph)
+        expected = {
+            0: 1 / 4,  # 1 outgoing edge
+            1: 1 / 4,  # 1 outgoing edge
+            2: 1 / 4,  # 1 outgoing edge
+            3: 1 / 4,  # 1 outgoing edge
+            4: 0.0,  # 0 outgoing edges
+        }
+        for k, v in centrality.items():
+            self.assertAlmostEqual(v, expected[k])