SpannerBaswanaSen.h

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#pragma once
 
#include <ogdf/basic/NodeSet.h>
#include <ogdf/graphalg/SpannerIteratedWrapper.h>
#include <ogdf/graphalg/SpannerModule.h>
 
namespace ogdf {
 
template<typename TWeight>
class SpannerBaswanaSen : public SpannerModule<TWeight> {
    GraphCopySimple m_G;
    NodeArray<node> m_cluster;
 
    EpsilonTest m_eps;
 
public:
    virtual bool preconditionsOk(const GraphAttributes& GA, double stretch,
            std::string& error) override {
        if (GA.directed()) {
            error = "The graph must be undirected";
            return false;
        }
        double integralPart;
        if (std::modf(stretch, &integralPart) != 0.0) {
            error = "The stretch is required to be an integer, not " + to_string(m_stretch);
            return false;
        }
        int intStretch = static_cast<int>(stretch);
        if (intStretch < 1) {
            error = "The stretch must be >= 1.0";
            return false;
        }
        if (intStretch % 2 == 0) {
            error = "The stretch must be odd";
            return false;
        }
        return true;
    }
 
private:
    virtual void init(const GraphAttributes& GA, double stretch, GraphCopySimple& spanner,
            EdgeArray<bool>& inSpanner) override {
        SpannerModule<TWeight>::init(GA, stretch, spanner, inSpanner);
        m_G.clear();
        m_G.init(GA.constGraph());
        m_cluster.init(m_G);
    }
 
    virtual typename SpannerModule<TWeight>::ReturnType execute() override {
        phase1();
        phase2();
        return SpannerModule<TWeight>::ReturnType::Feasible;
    }
 
    inline void addToSpanner(edge e) {
        m_spanner->newEdge(m_G.original(e));
        (*m_inSpanner)[m_G.original(e)] = true;
    }
 
    inline TWeight weight(edge e) { return getWeight(*m_GA, m_G.original(e)); }
 
    void phase1() {
        // init
        NodeSet<true> clusterCenters(m_G);
        for (node v : m_G.nodes) {
            m_cluster[v] = v; // At the beginning each node is it's own cluster
            clusterCenters.insert(v);
        }
 
        // A and B will be explained below. They are created here, so they can be reused.
        NodeArray<edge> A(m_G, nullptr);
        NodeArray<bool> B(m_G, false);
 
        // stretch = 2k-1
        // the stretch must be integer and uneven.
        // => k=(stretch+1)/2
        int k = (static_cast<int>(m_stretch) + 1) / 2;
        const double probability = pow(m_G.numberOfNodes(), -1.0 / k);
        for (int iteration = 1; iteration <= k - 1; iteration++) {
            assertTimeLeft();
 
            NodeArray<node> oldCluster = m_cluster; // oldCluster is the cluster of iteration i-1
 
            // 1: Sample new cluster centers
            NodeSet<true> sampledClusterCenters(m_G);
            for (node oldCenter : clusterCenters.nodes()) {
                if (randomDouble(0.0, 1.0) <= probability) {
                    sampledClusterCenters.insert(oldCenter);
                }
            }
 
            // 2 & 3: Nearest neighbors and growing clusters
            for (node v : m_G.nodes) {
                if (sampledClusterCenters.isMember(oldCluster[v])) {
                    continue;
                }
 
                // v is not a member of a sampled cluster, so we have to search for a nearest
                // neighbor
                // - v will be added to the cluster of the NN
                // - A[x] saves the least weight edge from v to the cluster member of the cluster x
                //   (or nullptr if there is no such edge.)
 
                // both "min" values are with respect to sampled clusters, not for all adjacent
                // clusters
                TWeight minWeight = std::numeric_limits<TWeight>::max();
                edge minEdge = nullptr;
 
                for (adjEntry a : v->adjEntries) {
                    node center = oldCluster[a->twinNode()];
                    edge e = a->theEdge();
 
                    // maybe add v to a new cluster
                    if (sampledClusterCenters.isMember(center)) {
                        // twin is an element of a sampled cluster, so it is a (possible nearest)
                        // neighbor
                        if (m_eps.less(weight(e), minWeight)) {
                            m_cluster[v] = center; // this line will automatically add v to the new
                                    // sampled cluster since the last assignment is
                                    // done on the min edge.
                            minWeight = weight(e);
                            minEdge = e;
                        }
                    }
 
                    // fill A[x]
                    if (A[center] == nullptr || m_eps.less(weight(e), weight(A[center]))) {
                        A[center] = e;
                    }
                }
 
                // B[<clustercenter>] stores whether all edges to a cluster should be deleted.
 
                // add sampled clusters (or all clusters) to v
                for (node center : m_G.nodes) {
                    // center is only a possible center. It is a center, if A[center] != nullptr.
                    // Some possibilities:
                    // - minEdge == nullptr (case (a) in paper): v is not adjacent to any sampled cluster.
                    //   So, add every least weight edge to the spanner.
                    // - minEdge == A[center] (case (b), first part, in paper): v is adjacent to a sampled
                    //   cluster and we have the min edge here: Add it!
                    // - not minEdge, but less weight (case (b), second part, in paper): v is adjacent to a
                    //   sampled cluster. The edge to the cluster has strictly less weight than the min edge.
                    if (A[center] != nullptr
                            && (minEdge == nullptr || minEdge == A[center]
                                    || m_eps.less(weight(A[center]), minWeight))) {
                        addToSpanner(A[center]);
                        B[center] = true;
                    }
                    A[center] = nullptr;
                }
 
#ifdef OGDF_DEBUG
                for (node _v : m_G.nodes) {
                    OGDF_ASSERT(A[_v] == nullptr);
                }
#endif
 
                // Finally, delete all edges from v indicated by B
                ListPure<adjEntry> adjs; // copy, so we can iterate and delete.
                v->allAdjEntries(adjs);
                for (adjEntry a : adjs) {
                    node center = oldCluster[a->twinNode()];
                    if (B[center]) {
                        m_G.delEdge(a->theEdge());
                    }
                    B[center] = false;
                }
 
#ifdef OGDF_DEBUG
                for (node _v : m_G.nodes) {
                    OGDF_ASSERT(!B[_v]);
                }
#endif
            }
 
            // 4: Removing intra-cluster edges
            for (edge _e : m_GA->constGraph().edges) {
                edge e = m_G.copy(_e);
                if (e == nullptr) {
                    continue;
                }
 
                if (m_cluster[e->source()] == m_cluster[e->target()]) {
                    // inter-cluster connection.
                    m_G.delEdge(e);
                }
            }
 
            clusterCenters = std::move(sampledClusterCenters);
        }
    }
 
    void phase2() {
        NodeArray<edge> A(m_G, nullptr);
        for (node v : m_G.nodes) {
            assertTimeLeft();
 
            for (adjEntry a : v->adjEntries) {
                node center = m_cluster[a->twinNode()];
                edge e = a->theEdge();
 
                // fill A[x]
                if (A[center] == nullptr || weight(e) < weight(A[center])) {
                    A[center] = e;
                }
            }
 
            for (node center : m_G.nodes) {
                if (A[center] != nullptr) {
                    addToSpanner(A[center]);
                    A[center] = nullptr;
                }
            }
 
            // Delete all edges from v in the original graph.
            adjEntry a;
            while ((a = v->firstAdj()) != nullptr) {
                m_G.delEdge(a->theEdge());
            }
 
#ifdef OGDF_DEBUG
            for (node _v : m_G.nodes) {
                OGDF_ASSERT(A[_v] == nullptr);
            }
#endif
        }
    }
 
    using SpannerModule<TWeight>::getWeight;
    using SpannerModule<TWeight>::assertTimeLeft;
    using SpannerModule<TWeight>::m_GA;
    using SpannerModule<TWeight>::m_stretch;
    using SpannerModule<TWeight>::m_spanner;
    using SpannerModule<TWeight>::m_inSpanner;
};
 
template<typename TWeight>
class SpannerBaswanaSenIterated : public SpannerIteratedWrapper<TWeight> {
public:
    SpannerBaswanaSenIterated()
        : SpannerIteratedWrapper<TWeight>(new SpannerBaswanaSen<TWeight>(), 1000) {};
};
 
}