Unify Graph Views

[IanChenUIUC]

Potentially, we can have CoarsenedGraphView and InducedSubgraphView share the same base class, e.g. this is the interface proposed by Arun. Any algorithm that takes a Graph instance should also work on these views, which do not copy the edges.

/** 
 * @ingroup graph
 * Base class for zero-copy graph views.
 */
class GraphView : public Graph {
protected:
    const Graph& original;

    // Common cached state (can be protected or accessed via virtuals)
    count n = 0;
    count m = 0;
    bool isDirected = false;
    bool isWeighted = false;
    // ... other flags you already have

    explicit GraphView(const Graph& g);

    // Helper for derived classes
    bool isValidNode(node u) const;           // to be implemented by subclass
    void invalidateCache();                   // when nodes change

public:
    // Common implementations that can be overridden
    count numberOfNodes() const override { return n; }
    count numberOfEdges() const override { return m; }
    bool isDirectedGraph() const override { return isDirected; }
    // ...
};