Adding Transitive Reduction Function

docs/source/api/algorithm_functions/dag_algorithms.rst

@@ -12,3 +12,4 @@ DAG Algorithms
    rustworkx.dag_weighted_longest_path_length
    rustworkx.is_directed_acyclic_graph
    rustworkx.layers
+   rustworkx.transitive_reduction

releasenotes/notes/transitive-reduction-6db2b80351c15887.yaml

@@ -0,0 +1,36 @@
+---
+features:
+  - |
+    Added a new function, :func:`~.transitive_reduction` which returns the transtive reduction
+    of a given :class:`~rustworkx.PyDiGraph` and a dictionary with the mapping of indices from the given graph to the returned graph.
+    The given graph must be a Directed Acyclic Graph (DAG).
+    For example:
+
+    .. jupyter-execute::
+
+        from rustworkx import PyDiGraph
+        from rustworkx import transitive_reduction
+
+        graph = PyDiGraph()
+        a = graph.add_node("a")
+        b = graph.add_node("b")
+        c = graph.add_node("c")
+        d = graph.add_node("d")
+        e = graph.add_node("e")
+
+        graph.add_edges_from([
+            (a, b, 1),
+            (a, d, 1),
+            (a, c, 1),
+            (a, e, 1),
+            (b, d, 1),
+            (c, d, 1),
+            (c, e, 1),
+            (d, e, 1)
+        ])
+
+        tr, _ = transitive_reduction(graph)
+        list(tr.edge_list())
+
+    Ref: https://en.wikipedia.org/wiki/Transitive_reduction
+

src/dag_algo/mod.rs

@@ -12,12 +12,17 @@
 
 mod longest_path;
 
+use super::DictMap;
 use hashbrown::{HashMap, HashSet};
+use indexmap::IndexSet;
+use rustworkx_core::dictmap::InitWithHasher;
 use std::cmp::Ordering;
 use std::collections::BinaryHeap;
 
 use super::iterators::NodeIndices;
-use crate::{digraph, DAGHasCycle, InvalidNode};
+use crate::{digraph, DAGHasCycle, InvalidNode, StablePyGraph};
+
+use rustworkx_core::traversal::dfs_edges;
 
 use pyo3::exceptions::PyValueError;
 use pyo3::prelude::*;
@@ -637,3 +642,89 @@ pub fn collect_bicolor_runs(
 
     Ok(block_list)
 }
+
+/// Returns the transitive reduction of a directed acyclic graph
+///
+/// The transitive reduction of :math:`G = (V,E)` is a graph :math:`G\prime = (V,E\prime)`
+/// such that for all :math:`v` and :math:`w` in :math:`V` there is an edge :math:`(v, w)` in
+/// :math:`E\prime` if and only if :math:`(v, w)` is in :math:`E`
+/// and there is no path from :math:`v` to :math:`w` in :math:`G` with length greater than 1.
+///
+/// :param PyDiGraph graph: A directed acyclic graph
+///
+/// :returns: a directed acyclic graph representing the transitive reduction, and
+///     a map containing the index of a node in the original graph mapped to its
+///     equivalent in the resulting graph.
+/// :rtype: Tuple[PyGraph, dict]
+///
+/// :raises PyValueError: if ``graph`` is not a DAG
+
+#[pyfunction]
+#[pyo3(text_signature = "(graph, /)")]
+pub fn transitive_reduction(
+    graph: &digraph::PyDiGraph,
+    py: Python,
+) -> PyResult<(digraph::PyDiGraph, DictMap<usize, usize>)> {
+    let g = &graph.graph;
+    let mut index_map = DictMap::with_capacity(g.node_count());
+    if !is_directed_acyclic_graph(graph) {
+        return Err(PyValueError::new_err(
+            "Directed Acyclic Graph required for transitive_reduction",
+        ));
+    }
+    let mut tr = StablePyGraph::<Directed>::with_capacity(g.node_count(), 0);
+    let mut descendants = DictMap::new();
+    let mut check_count = HashMap::with_capacity(g.node_count());
+
+    for node in g.node_indices() {
+        let i = node.index();
+        index_map.insert(
+            node,
+            tr.add_node(graph.get_node_data(i).unwrap().clone_ref(py)),
+        );
+        check_count.insert(node, graph.in_degree(i));
+    }
+
+    for u in g.node_indices() {
+        let mut u_nbrs: IndexSet<NodeIndex> = g.neighbors(u).collect();
+        for v in g.neighbors(u) {
+            if u_nbrs.contains(&v) {
+                if !descendants.contains_key(&v) {
+                    let dfs = dfs_edges(&g, Some(v));
+                    descendants.insert(v, dfs);
+                }
+                for desc in &descendants[&v] {
+                    u_nbrs.remove(&NodeIndex::new(desc.1));
+                }
+            }
+            *check_count.get_mut(&v).unwrap() -= 1;
+            if check_count[&v] == 0 {
+                descendants.remove(&v);
+            }
+        }
+        for v in u_nbrs {
+            tr.add_edge(
+                *index_map.get(&u).unwrap(),
+                *index_map.get(&v).unwrap(),
+                graph
+                    .get_edge_data(u.index(), v.index())
+                    .unwrap()
+                    .clone_ref(py),
+            );
+        }
+    }
+    return Ok((
+        digraph::PyDiGraph {
+            graph: tr,
+            node_removed: false,
+            multigraph: graph.multigraph,
+            attrs: py.None(),
+            cycle_state: algo::DfsSpace::default(),
+            check_cycle: graph.check_cycle,
+        },
+        index_map
+            .iter()
+            .map(|(k, v)| (k.index(), v.index()))
+            .collect::<DictMap<usize, usize>>(),
+    ));
+}

src/lib.rs

@@ -359,6 +359,7 @@ fn rustworkx(py: Python<'_>, m: &PyModule) -> PyResult<()> {
     m.add_wrapped(wrap_pyfunction!(dag_longest_path_length))?;
     m.add_wrapped(wrap_pyfunction!(dag_weighted_longest_path))?;
     m.add_wrapped(wrap_pyfunction!(dag_weighted_longest_path_length))?;
+    m.add_wrapped(wrap_pyfunction!(transitive_reduction))?;
     m.add_wrapped(wrap_pyfunction!(number_connected_components))?;
     m.add_wrapped(wrap_pyfunction!(connected_components))?;
     m.add_wrapped(wrap_pyfunction!(is_connected))?;

tests/rustworkx_tests/digraph/test_transitive_reduction.py

@@ -0,0 +1,76 @@
+# 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.
+
+import unittest
+
+import rustworkx
+
+
+class TestTransitiveReduction(unittest.TestCase):
+    def test_tr1(self):
+        graph = rustworkx.PyDiGraph()
+        a = graph.add_node("a")
+        b = graph.add_node("b")
+        c = graph.add_node("c")
+        d = graph.add_node("d")
+        e = graph.add_node("e")
+        graph.add_edges_from(
+            [(a, b, 1), (a, d, 1), (a, c, 1), (a, e, 1), (b, d, 1), (c, d, 1), (c, e, 1), (d, e, 1)]
+        )
+        tr, _ = rustworkx.transitive_reduction(graph)
+        self.assertCountEqual(list(tr.edge_list()), [(0, 2), (0, 1), (1, 3), (2, 3), (3, 4)])
+
+    def test_tr2(self):
+        graph2 = rustworkx.PyDiGraph()
+        a = graph2.add_node("a")
+        b = graph2.add_node("b")
+        c = graph2.add_node("c")
+        graph2.add_edges_from(
+            [
+                (a, b, 1),
+                (b, c, 1),
+                (a, c, 1),
+            ]
+        )
+        tr2, _ = rustworkx.transitive_reduction(graph2)
+        self.assertCountEqual(list(tr2.edge_list()), [(0, 1), (1, 2)])
+
+    def test_tr3(self):
+        graph3 = rustworkx.PyDiGraph()
+        graph3.add_nodes_from([0, 1, 2, 3])
+        graph3.add_edges_from([(0, 1, 1), (0, 2, 1), (0, 3, 1), (1, 2, 1), (1, 3, 1)])
+        tr3, _ = rustworkx.transitive_reduction(graph3)
+        self.assertCountEqual(list(tr3.edge_list()), [(0, 1), (1, 2), (1, 3)])
+
+    def test_tr_with_deletion(self):
+        graph = rustworkx.PyDiGraph()
+        a = graph.add_node("a")
+        b = graph.add_node("b")
+        c = graph.add_node("c")
+        d = graph.add_node("d")
+        e = graph.add_node("e")
+
+        graph.add_edges_from(
+            [(a, b, 1), (a, d, 1), (a, c, 1), (a, e, 1), (b, d, 1), (c, d, 1), (c, e, 1), (d, e, 1)]
+        )
+
+        graph.remove_node(3)
+
+        tr, index_map = rustworkx.transitive_reduction(graph)
+
+        self.assertCountEqual(list(tr.edge_list()), [(0, 1), (0, 2), (2, 3)])
+        self.assertEqual(index_map[4], 3)
+
+    def test_tr_error(self):
+        digraph = rustworkx.generators.directed_cycle_graph(1000)
+        with self.assertRaises(ValueError):
+            rustworkx.transitive_reduction(digraph)