Implement InducedSubgraphView

include/networkit/graph/InducedSubgraphView.hpp

@@ -0,0 +1,130 @@
+/*
+ * InducedSubgraphView.hpp
+ *
+ *  Created on: 06.25.2026
+ *      Author: Ian Chen (ianchen3@illinois.edu)
+ */
+
+#ifndef NETWORKIT_GRAPH_INDUCED_SUBGRAPH_VIEW_HPP_
+#define NETWORKIT_GRAPH_INDUCED_SUBGRAPH_VIEW_HPP_
+
+#include <set>
+
+#include <networkit/graph/GraphR.hpp>
+
+namespace NetworKit {
+
+/**
+ * @ingroup graph
+ * InducedSubgraphView - Zero-copy induced subgraph.
+ *
+ * This class provides a memory-efficient graph view into a
+ * base graph, providing the same interface.
+ * Nodes can be added or removed as a batch.
+ *
+ * A mutable GraphW can be realized.
+ * Neighbors are computed on-the-fly.
+ */
+class InducedSubgraphView final : public GraphR {
+    const Graph &originalGraph;
+    std::set<node> nodeSubset;
+
+    count inducedNumberOfEdges = 0;
+    count inducedNumberOfSelfLoops = 0;
+    std::vector<count> inducedDegree;
+
+public:
+    /**
+     * Construct an induced subgraph view from the original graph and the node subset.
+     * @param originalGraph The original CSR graph
+     * @param subset The defining nodes of the induced subgraph.
+     */
+    InducedSubgraphView(const Graph &originalGraph, std::set<node> subset);
+
+    /**
+     * Add nodes to the subset.
+     */
+    void AddNodes(const std::set<node> &subset);
+
+    /**
+     * Remove nodes from the subset.
+     */
+    void RemoveNodes(const std::set<node> &subset);
+
+    /**
+     * Construct an explicit mutable subgraph equivalent to this view.
+     */
+    GraphW Realize() const;
+
+    /**
+     * Copy constructor
+     */
+    InducedSubgraphView(const InducedSubgraphView &other) = default;
+
+    /**
+     * Move constructor
+     */
+    InducedSubgraphView(InducedSubgraphView &&other) noexcept = default;
+
+    /**
+     * Copy assignment
+     */
+    InducedSubgraphView &operator=(const InducedSubgraphView &other) {
+        Graph::operator=(other);
+        return *this;
+    }
+
+    /**
+     * Move assignment
+     */
+    InducedSubgraphView &operator=(InducedSubgraphView &&other) noexcept {
+        Graph::operator=(std::move(other));
+        return *this;
+    }
+
+    /** Default destructor */
+    ~InducedSubgraphView() override = default;
+
+    // Override from Graph base
+    bool hasNode(node v) const noexcept;
+    count numberOfNodes() noexcept;
+    count numberOfEdges() noexcept;
+    count numberOfSelfLoops() noexcept;
+
+    // Implement pure virtual methods from Graph base class
+
+    count degree(node v) const override;
+    count degreeIn(node v) const override;
+
+    /**
+     * Return edge weight of edge {@a u,@a v}. Returns 0 if edge does not
+     * exist. If weights are provided during construction, returns the actual
+     * edge weight; otherwise returns defaultEdgeWeight (1.0).
+     *
+     * @param u Endpoint of edge.
+     * @param v Endpoint of edge.
+     * @return Edge weight of edge {@a u,@a v} or 0 if edge does not exist.
+     */
+    edgeweight weight(node u, node v) const override;
+
+    edgeid edgeId(node u, node v) const override;
+
+    node getIthNeighbor(Unsafe, node u, index i) const override;
+    edgeweight getIthNeighborWeight(node u, index i) const override;
+    node getIthNeighbor(node u, index i) const override;
+    node getIthInNeighbor(node u, index i) const override;
+    std::pair<node, edgeweight> getIthNeighborWithWeight(node u, index i) const override;
+    std::pair<node, edgeid> getIthNeighborWithId(node u, index i) const override;
+
+protected:
+    std::vector<node> getNeighborsVector(node u, bool inEdges = false) const override;
+    std::pair<std::vector<node>, std::vector<edgeweight>>
+    getNeighborsWithWeightsVector(node u, bool inEdges = false) const override;
+
+    index indexInInEdgeArray(node v, node u) const override;
+    index indexInOutEdgeArray(node u, node v) const override;
+};
+
+} // namespace NetworKit
+
+#endif // NETWORKIT_GRAPH_INDUCED_SUBGRAPH_VIEW_HPP_