SpannerBerman.h

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#pragma once
 
#include <ogdf/basic/extended_graph_alg.h>
#include <ogdf/basic/simple_graph_alg.h>
#include <ogdf/graphalg/SpannerModule.h>
 
#include <ogdf/external/coin.h>
 
#include <iomanip>
 
namespace ogdf {
 
template<typename TWeight>
class SpannerBerman : public SpannerModule<TWeight> {
public:
    static Logger logger;
 
    SpannerBerman() {
        m_OPT = 0;
        m_osi = nullptr;
    }
 
    virtual ~SpannerBerman() { resetLP(); }
 
    virtual bool preconditionsOk(const GraphAttributes& GA, double stretch,
            std::string& error) override {
        if (!isSimple(GA.constGraph())) {
            error = "The graph is not simple";
            return false;
        }
        if (!isConnected(GA.constGraph())) {
            error = "The graph is not connected";
            return false;
        }
        if (stretch < 1.0) {
            error = "The stretch must be >= 1.0";
            return false;
        }
        return true;
    }
 
private:
    enum class SeparationResult { NewConstraint, Fail, Solution };
 
    const Graph* m_G; 
    EdgeArray<TWeight> m_weight; 
    EdgeArray<TWeight> m_spannerWeight; 
    EdgeArray<bool> m_isThickEdge;
 
    // Some precalculated values that are needed all over the place.
    int m_nSquared; 
    double m_beta; 
    double m_sqrtlog; 
    int m_thickEdgeNodeAmountLimit; 
 
    EdgeArray<bool> m_E1; 
    EdgeArray<bool> m_E2; 
 
    NodeArray<NodeArray<TWeight>> m_inDistance; 
    NodeArray<NodeArray<TWeight>> m_outDistance; 
 
    // Solving the LP
    OsiSolverInterface* m_osi; 
    std::vector<CoinPackedVector*> m_constraints; 
    int m_OPT; 
 
    EpsilonTest m_eps;
 
    inline bool isThinEdge(edge e) { return !m_isThickEdge(e); }
 
    inline bool isThickEdge(edge e) { return m_isThickEdge(e); }
 
    void resetLP() {
        for (auto c : m_constraints) {
            delete c;
        }
        m_constraints.clear();
 
        delete m_osi;
        m_osi = nullptr;
    }
 
    virtual void init(const GraphAttributes& GA, double stretch, GraphCopySimple& spanner,
            EdgeArray<bool>& inSpanner) override {
        resetLP();
        SpannerModule<TWeight>::init(GA, stretch, spanner, inSpanner);
 
        m_G = &GA.constGraph();
        m_weight.init(*m_G);
        for (edge e : m_G->edges) {
            m_weight[e] = getWeight(*m_GA, e);
        }
        m_spannerWeight.init(*m_spanner);
        m_isThickEdge.init(*m_G, false);
 
        m_nSquared = m_G->numberOfNodes() * m_G->numberOfNodes();
        m_beta = sqrt(m_G->numberOfNodes()),
        m_thickEdgeNodeAmountLimit = ceil(m_G->numberOfNodes() / m_beta);
        m_sqrtlog = sqrt(m_G->numberOfNodes()) * log(m_G->numberOfNodes());
 
        m_E1.init(*m_G, false);
        m_E2.init(*m_G, false);
 
        m_inDistance.init(*m_G);
        m_outDistance.init(*m_G);
    }
 
    virtual typename SpannerModule<TWeight>::ReturnType execute() override {
        if (m_G->numberOfNodes() == 0 || m_G->numberOfEdges() == 0) {
            return SpannerModule<TWeight>::ReturnType::Feasible;
        }
        firstPart();
        printStats();
 
        logger.lout() << "\n2. Part\n" << std::endl;
        bool ok = secondPart();
 
        if (!ok) {
            return SpannerModule<TWeight>::ReturnType::Error;
        }
 
        printStats(true);
 
        // Fill m_inSpanner since only E1, E2 and m_spanner are correctly filled.
        for (edge e : m_G->edges) {
            (*m_inSpanner)[e] = m_E1[e] || m_E2[e];
        }
        return SpannerModule<TWeight>::ReturnType::Feasible;
    }
 
    void firstPart() {
        NodeArray<NodeArray<edge>> inPredecessor(*m_G);
        NodeArray<NodeArray<edge>> outPredecessor(*m_G);
        for (node n : m_G->nodes) {
            inArborescence(*m_GA, n, inPredecessor[n], m_inDistance[n]);
            outArborescence(*m_GA, n, outPredecessor[n], m_outDistance[n]);
            assertTimeLeft();
        }
 
        int max = ceil(m_beta * log(m_G->numberOfNodes())); // log is ln
        logger.lout() << "max=" << max << std::endl;
        for (int i = 0; i < max; i++) {
            node v = m_G->chooseNode();
            for (node n : m_G->nodes) {
                edge e1 = inPredecessor[v][n];
                if (e1) {
                    addEdgeToSpanner(e1);
                }
 
                edge e2 = outPredecessor[v][n];
                if (e2) {
                    addEdgeToSpanner(e2);
                }
            }
        }
        assertTimeLeft();
        logger.lout() << "thickEdgeNodeAmountLimit=" << m_thickEdgeNodeAmountLimit << std::endl;
        printStats();
 
        logger.lout() << "add unsettled thick edges" << std::endl;
        calculateThickEdges();
        addUnsettledThickEdges();
    }
 
    void inArborescence(const GraphAttributes& GA, node root, NodeArray<edge>& predecessor,
            NodeArray<TWeight>& distance) {
        Dijkstra<TWeight> dijkstra;
        dijkstra.callUnbound(*m_G, m_weight, root, predecessor, distance, true, true);
    }
 
    void outArborescence(const GraphAttributes& GA, node root, NodeArray<edge>& predecessor,
            NodeArray<TWeight>& distance) {
        Dijkstra<TWeight> dijkstra;
        dijkstra.callUnbound(*m_G, m_weight, root, predecessor, distance, true, false);
    }
 
    void addEdgeToSpanner(edge e) {
        if (!m_E1[e]) {
            m_E1[e] = true;
            m_spanner->newEdge(e);
            m_spannerWeight[m_spanner->copy(e)] = m_weight[e];
        }
    }
 
    void printStats(bool assert = false) {
        logger.lout() << "\nStats:" << std::endl;
        logger.lout() << m_spanner->numberOfEdges() << " covered of " << m_G->numberOfEdges()
                      << std::endl;
 
        int E1amount = 0;
        int E1amountThick = 0;
        int E1amountThinNotInE2 = 0;
        int E2amount = 0;
        int E2amountThin = 0;
        int E2amountThickNotInE1 = 0;
        int edgesCovered = 0;
        int duplicateCovered = 0;
        for (edge e : m_G->edges) {
            if (m_E1[e]) {
                E1amount++;
                if (isThickEdge(e)) {
                    E1amountThick++;
                } else if (!m_E2[e]) {
                    E1amountThinNotInE2++;
                }
            }
            if (m_E2[e]) {
                E2amount++;
                if (isThinEdge(e)) {
                    E2amountThin++;
                } else if (!m_E1[e]) {
                    E2amountThickNotInE1++;
                }
            }
            if (m_E1[e] || m_E2[e]) {
                edgesCovered++;
            }
            if (m_E1[e] && m_E2[e]) {
                duplicateCovered++;
            }
        }
 
        logger.lout() << "covered: " << edgesCovered << ", duplicate edges: " << duplicateCovered
                      << std::endl;
        logger.lout() << "E1: " << E1amount << " edges, " << E1amountThick << " thick, "
                      << (E1amount - E1amountThick) << " thin, " << E1amountThinNotInE2
                      << " thin edges not in E2" << std::endl;
        logger.lout() << "E2: " << E2amount << " edges, " << E2amountThin << " thin, "
                      << (E2amount - E2amountThin) << " thick, " << E2amountThickNotInE1
                      << " thick edges not in E1" << std::endl;
 
#if defined(OGDF_DEBUG)
        if (assert) {
            OGDF_ASSERT((E1amountThick + E2amountThin + E1amountThinNotInE2 + E2amountThickNotInE1)
                    == m_spanner->numberOfEdges());
        }
#endif
 
        OGDF_ASSERT(edgesCovered == m_spanner->numberOfEdges());
    }
 
    void calculateThickEdges() {
        for (edge e : m_G->edges) {
            m_isThickEdge[e] = _isThickEdge(e);
        }
    }
 
    bool _isThickEdge(edge e) {
        const TWeight maxDistance = m_stretch * m_weight[e];
 
        int amountNodesInShortestPaths = 0;
        for (node n : m_G->nodes) {
            TWeight sd = m_outDistance[e->source()][n];
            TWeight td = m_inDistance[e->target()][n];
            if (sd == std::numeric_limits<TWeight>::max()
                    || td == std::numeric_limits<TWeight>::max()) {
                continue;
            }
 
            OGDF_ASSERT(m_eps.geq(sd + td, static_cast<TWeight>(0)));
            OGDF_ASSERT(m_eps.less(sd + td, std::numeric_limits<TWeight>::max()));
 
            if (m_eps.leq((sd + td), maxDistance)) {
                amountNodesInShortestPaths++;
            }
 
            if (amountNodesInShortestPaths >= m_thickEdgeNodeAmountLimit) {
                return true;
            }
        }
        return false;
    }
 
    bool isSettledEdge(const edge e) { return isSettledEdge(e, *m_spanner, m_spannerWeight); }
 
    bool isSettledEdge(const edge e, const GraphCopySimple& _spanner,
            const EdgeArray<TWeight>& _spannerWeight) {
        const TWeight maxDistance = m_stretch * m_weight[e];
        node u = _spanner.copy(e->source());
        node v = _spanner.copy(e->target());
        TWeight currentSpannerDistance = distance(_spanner, _spannerWeight, u, v, ceil(maxDistance));
        return m_eps.leq(currentSpannerDistance, maxDistance);
    }
 
    TWeight distance(const GraphCopySimple& G, const EdgeArray<TWeight>& weights, const node s,
            const node t, int maxLookupDist) {
        NodeArray<TWeight> distances;
        NodeArray<edge> predecessor;
        Dijkstra<TWeight> dijkstra;
        dijkstra.callBound(G, weights, s, predecessor, distances, m_GA->directed(),
                false, // arcs are not reversed
                t, maxLookupDist);
        return distances[t];
    }
 
    void addUnsettledThickEdges() {
        for (edge e : m_G->edges) {
            if (m_E1[e]) {
                continue;
            }
 
            if (isThickEdge(e)) {
                if (!isSettledEdge(e)) {
                    addEdgeToSpanner(e);
                }
                assertTimeLeft();
            }
        }
    }
 
    bool secondPart() {
        logger.lout() << "Using LP-Solver: " << Configuration::whichLPSolver() << std::endl;
        m_osi = CoinManager::createCorrectOsiSolverInterface();
        m_osi->messageHandler()->setLogLevel(0); // 0=nothing .. 4=verbose
        m_osi->setObjSense(1); // minimize
 
        // One column per edge (x_e)
        CoinPackedVector zero;
        EdgeArray<int> indices(*m_G);
        int i = 0;
        for (edge e : m_G->edges) {
            m_osi->addCol(zero, 0, 1, 1); // vec, lower, upper, objective
            indices[e] = i++;
        }
 
 
        CoinPackedVector* optConstraint = new CoinPackedVector;
        for (edge e : m_G->edges) {
            optConstraint->insert(indices[e], 1);
        }
        // sense: 'E' ==   'G' >=   'L' <=
        m_osi->addRow(*optConstraint, 'L', 0, 0); // constraint, sense, rhs (will be set below), range
        m_constraints.push_back(optConstraint);
 
        // Set the initial value of the OPT constraint rhs to n-1
        setOpt(m_G->numberOfNodes() - 1);
 
        int amountSolverCalls = 0;
        while (true) {
            if (amountSolverCalls == 0) {
                m_osi->initialSolve();
            } else {
                m_osi->resolve();
            }
            amountSolverCalls++;
 
            assertTimeLeft();
 
            logger.lout() << amountSolverCalls << ". solve... ";
            if (m_osi->isProvenOptimal()) {
                logger.lout() << "-> optimal (" << m_osi->getObjValue() << ")" << std::endl;
                logger.lout() << "  separation... ";
                SeparationResult result = separation(m_osi->getColSolution(), indices);
 
                if (result == SeparationResult::Solution) {
                    for (edge e : m_G->edges) {
                        if (m_E2[e] && !m_E1[e]) {
                            m_spanner->newEdge(e);
                            m_spannerWeight[m_spanner->copy(e)] = m_weight[e];
                        }
                    }
                    logger.lout() << "-> Found solution\n" << std::endl;
                    logger.lout() << "solver calls: " << amountSolverCalls << std::endl;
                    logger.lout() << "cols: " << m_osi->getNumCols() << std::endl;
                    logger.lout() << "rows: " << m_osi->getNumRows() << std::endl;
                    return true;
                } else if (result == SeparationResult::NewConstraint) {
                    logger.lout() << "-> New constraint" << std::endl;
                } else { // Fail
                    logger.lout() << "-> Failed" << std::endl;
                    if (!setOpt(m_OPT + 1)) {
                        return false;
                    }
                }
            } else if (m_osi->isProvenPrimalInfeasible()) {
                logger.lout() << "-> Infeasible" << std::endl;
                if (!setOpt(m_OPT + 1)) {
                    return false;
                }
            } else if (m_osi->isProvenDualInfeasible()) {
                logger.lout() << "-> Unbounded" << std::endl;
                return false;
            } else {
                logger.lout() << "-> No solution found" << std::endl;
                return false;
            }
        };
    }
 
    bool setOpt(int opt) {
        m_OPT = opt;
        if (m_OPT > m_nSquared) {
            logger.lout() << "  OPT is too large. Abort." << std::endl;
            return false;
        }
        float percentage = static_cast<float>(m_OPT) / m_nSquared * 100.0;
        logger.lout() << "  Set OPT to " << m_OPT << " (" << std::fixed << std::setprecision(2)
                      << percentage << "% of " << m_nSquared << ")" << std::endl;
 
        m_osi->setRowBounds(0, 0.0,
                m_OPT); // this is the opt constraint; it is 0 <= sum(x_e) <= m_OPT
        return true;
    }
 
    SeparationResult separation(const double* solution, const EdgeArray<int>& indices) {
        EdgeArray<bool> out(*m_G, false);
        randomizedSelection(solution, out);
 
        GraphCopySimple copy;
        copy.setOriginalGraph(*m_G);
        for (node n : m_G->nodes) {
            copy.newNode(n);
        }
        EdgeArray<TWeight> copyWeight(copy);
        int amountCoveredEdges = 0;
        for (edge e : m_G->edges) {
            if (out[e]) {
                amountCoveredEdges++;
                copy.newEdge(e);
                copyWeight[copy.copy(e)] = m_weight[e];
            }
        }
 
        bool settlesAllThinEdges = true;
        edge unsettledThinEdge = nullptr;
        for (edge e : m_G->edges) {
            if (isThinEdge(e) && !isSettledEdge(e, copy, copyWeight)) {
                settlesAllThinEdges = false;
                unsettledThinEdge = e;
                break;
            }
        }
 
        if (settlesAllThinEdges) {
            if (amountCoveredEdges <= ceil(2.0 * m_OPT * m_sqrtlog)) {
                m_E2 = out;
                return SeparationResult::Solution;
            } else {
                return SeparationResult::Fail;
            }
        } else {
            EdgeArray<bool> antispanner(*m_G, false);
            createAntispanner(unsettledThinEdge, out, antispanner);
 
            double rowValue = 0.0;
            int i = 0;
            for (edge e : m_G->edges) {
                if (antispanner[e]) {
                    rowValue += solution[i];
                }
                i++;
            }
            if (m_eps.less(rowValue, 1.0)) {
                // create new constraint
                CoinPackedVector* c = new CoinPackedVector;
                for (edge e : m_G->edges) {
                    if (antispanner[e]) {
                        c->insert(indices[e], 1);
                    }
                }
                m_osi->addRow(*c, 'G', 1, 0);
                m_constraints.push_back(c);
                return SeparationResult::NewConstraint;
            } else {
                return SeparationResult::Fail;
            }
        }
    }
 
    void randomizedSelection(const double* fractions, EdgeArray<bool>& out) {
        int i = 0;
        for (edge e : m_G->edges) {
            double p = m_sqrtlog * fractions[i++]; // P may not be limited to 1: if it is >1 the
                    // result will always be true, which is ok.
            out[e] = randomDouble(0.0, 1.0) <= p;
        }
    }
 
    void createAntispanner(const edge unsettledThinEdge, const EdgeArray<bool>& out,
            EdgeArray<bool>& antispanner) {
        GraphCopySimple copy;
        copy.setOriginalGraph(*m_G);
        for (node n : m_G->nodes) {
            copy.newNode(n);
        }
        EdgeArray<TWeight> copyWeight(copy);
 
        const TWeight maxDistance = m_stretch * m_weight[unsettledThinEdge];
        for (edge e : m_G->edges) {
            // s->e->t
            TWeight s_e = m_outDistance[unsettledThinEdge->source()][e->source()];
            TWeight e_t = m_inDistance[unsettledThinEdge->target()][e->target()];
            bool isInLocalSubgraph = (s_e != std::numeric_limits<TWeight>::max()
                    && e_t != std::numeric_limits<TWeight>::max()
                    && m_eps.leq(s_e + m_weight[e] + e_t, maxDistance));
 
            // Graph to maximize is E_st\(E_st\E2) = E_st intersection E2
            // intersection[e] = isInLocalSubgraph && out[e];
            if (isInLocalSubgraph && out[e]) {
                copy.newEdge(e);
                copyWeight[copy.copy(e)] = m_weight[e];
            }
 
            // e in antispanner <=> e in E_st and not in E2 (=out)
            antispanner[e] = isInLocalSubgraph && !out[e];
        }
        assertTimeLeft();
 
        // minimize antispanner
        bool changed;
        do {
            changed = false;
            for (edge e : m_G->edges) {
                if (!antispanner[e]) {
                    continue;
                }
 
                // try to remove e and check, if still no path <=stretch*max exists
                // -> removing e from antispanner is equivalent to adding e to the graph
                copy.newEdge(e);
                copyWeight[copy.copy(e)] = m_weight[e];
                antispanner[e] = false;
 
                if (!isSettledEdge(unsettledThinEdge, copy, copyWeight)) {
                    // we've found an unnecessary edge.
                    changed = true;
                } else {
                    antispanner[e] = true;
                    copy.delEdge(copy.copy(e));
                }
            }
 
            assertTimeLeft();
        } while (changed);
    }
 
    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>
Logger SpannerBerman<TWeight>::logger(Logger::LogMode::Log,
        Logger::Level::High); // do not log by default
 
}