Add largest connected component method (#1562)

* Add largest connected component method

* Expose lcc to python

* Expose lcc to python

* optimise the function

* run cargo +nightly fmt

* Missed a bracket on the merge

* run cargo +nightly fmt

---------

Co-authored-by: Ben Steer <[email protected]>

python/tests/test_algorithms.py

@@ -31,6 +31,14 @@ def test_connected_components():
     assert actual.get("1") == 1
 
 
+def test_largest_connected_component():
+    g = gen_graph()
+    actual = g.largest_connected_component()
+    expected = ["1", "2", "3", "4", "5", "6", "7", "8"]
+    for node in expected:
+        assert actual.has_node(node)
+
+
 def test_in_components():
     g = gen_graph()
     actual = algorithms.in_components(g).get_all_with_names()

raphtory/src/algorithms/components/lcc.rs

@@ -0,0 +1,143 @@
+use crate::{
+    algorithms::components::connected_components::weakly_connected_components,
+    db::{
+        api::view::{GraphViewOps, StaticGraphViewOps},
+        graph::views::node_subgraph::NodeSubgraph,
+    },
+    prelude::Graph,
+};
+
+/// Gives the large connected component of a graph.
+/// The large connected component is the largest (i.e., with the highest number of nodes)
+/// connected sub-graph of the network.
+///
+/// # Example Usage:
+///
+/// g.largest_connected_component()
+///
+/// # Returns:
+///
+/// A raphtory graph, which essentially is a sub-graph of the graph `g`
+///
+pub trait LargestConnectedComponent {
+    fn largest_connected_component(&self) -> NodeSubgraph<Self>
+    where
+        Self: StaticGraphViewOps;
+}
+
+impl LargestConnectedComponent for Graph {
+    fn largest_connected_component(&self) -> NodeSubgraph<Self>
+    where
+        Self: StaticGraphViewOps,
+    {
+        let mut connected_components_map =
+            weakly_connected_components(self, usize::MAX, None).group_by();
+        let mut lcc_key = 0;
+        let mut key_length = 0;
+        let mut is_tie = false;
+
+        for (key, value) in &connected_components_map {
+            let length = value.len();
+            if length > key_length {
+                key_length = length;
+                lcc_key = *key;
+                is_tie = false;
+            } else if length == key_length {
+                is_tie = true
+            }
+        }
+        if is_tie {
+            println!("Warning: The graph has two or more connected components that are both the largest. \
+            The returned component has been picked arbitrarily.");
+        }
+        return match connected_components_map.remove(&lcc_key) {
+            Some(nodes) => self.subgraph(nodes),
+            None => self.subgraph(self.nodes()),
+        };
+    }
+}
+
+#[cfg(test)]
+mod largest_connected_component_test {
+    use super::*;
+    use crate::{
+        db::api::view::GraphViewOps,
+        prelude::{AdditionOps, Graph, NO_PROPS},
+    };
+
+    #[test]
+    fn test_empty_graph() {
+        let graph = Graph::new();
+        let subgraph = graph.largest_connected_component();
+        assert!(
+            subgraph.is_empty(),
+            "The subgraph of an empty graph should be empty"
+        );
+    }
+
+    #[test]
+    fn test_single_connected_component() {
+        let graph = Graph::new();
+        let edges = vec![(1, 1, 2), (2, 2, 1), (3, 3, 1)];
+        for (ts, src, dst) in edges {
+            graph.add_edge(ts, src, dst, NO_PROPS, None).unwrap();
+        }
+        let subgraph = graph.largest_connected_component();
+
+        let expected_nodes = vec![1, 2, 3];
+        for node in expected_nodes {
+            assert_eq!(
+                subgraph.has_node(node),
+                true,
+                "Node {} should be in the largest connected component.",
+                node
+            );
+        }
+        assert_eq!(subgraph.count_nodes(), 3);
+    }
+
+    #[test]
+    fn test_multiple_connected_components() {
+        let graph = Graph::new();
+        let edges = vec![
+            (1, 1, 2),
+            (2, 2, 1),
+            (3, 3, 1),
+            (1, 10, 11),
+            (2, 20, 21),
+            (3, 30, 31),
+        ];
+        for (ts, src, dst) in edges {
+            graph.add_edge(ts, src, dst, NO_PROPS, None).unwrap();
+        }
+        let subgraph = graph.largest_connected_component();
+        let expected_nodes = vec![1, 2, 3];
+        for node in expected_nodes {
+            assert_eq!(
+                subgraph.has_node(node),
+                true,
+                "Node {} should be in the largest connected component.",
+                node
+            );
+        }
+        assert_eq!(subgraph.count_nodes(), 3);
+    }
+
+    #[test]
+    fn test_same_size_connected_components() {
+        let graph = Graph::new();
+        let edges = vec![
+            (1, 1, 2),
+            (1, 2, 1),
+            (1, 3, 1),
+            (1, 5, 6),
+            (1, 11, 12),
+            (1, 12, 11),
+            (1, 13, 11),
+        ];
+        for (ts, src, dst) in edges {
+            graph.add_edge(ts, src, dst, NO_PROPS, None).unwrap();
+        }
+        let _subgraph = graph.largest_connected_component();
+    }
+}

raphtory/src/algorithms/components/mod.rs

@@ -1,9 +1,11 @@
 mod connected_components;
 mod in_components;
+mod lcc;
 mod out_components;
 mod scc;
 
 pub use connected_components::weakly_connected_components;
 pub use in_components::in_components;
+pub use lcc::LargestConnectedComponent;
 pub use out_components::out_components;
 pub use scc::strongly_connected_components;

raphtory/src/python/graph/graph.rs

@@ -5,10 +5,11 @@
 //! In Python, this class wraps around the rust graph.
 use super::utils;
 use crate::{
+    algorithms::components::LargestConnectedComponent,
     core::{entities::nodes::node_ref::NodeRef, utils::errors::GraphError, ArcStr},
     db::{
         api::view::internal::{CoreGraphOps, DynamicGraph, IntoDynamic, MaterializedGraph},
-        graph::{edge::EdgeView, node::NodeView},
+        graph::{edge::EdgeView, node::NodeView, views::node_subgraph::NodeSubgraph},
     },
     prelude::*,
     python::{
@@ -366,6 +367,18 @@ impl PyGraph {
         Ok(PyBytes::new(py, &bytes))
     }
 
+    /// Gives the large connected component of a graph.
+    ///
+    /// # Example Usage:
+    /// g.largest_connected_component()
+    ///
+    /// # Returns:
+    /// A raphtory graph, which essentially is a sub-graph of the graph `g`
+    ///
+    pub fn largest_connected_component(&self) -> NodeSubgraph<Graph> {
+        self.graph.largest_connected_component()
+    }
+
     /// Get persistent graph
     pub fn persistent_graph<'py>(&'py self) -> PyResult<Py<PyPersistentGraph>> {
         PyPersistentGraph::py_from_db_graph(self.graph.persistent_graph())