#include <iostream>
#include <numeric>
#include <random>
#include <vector>

class Edge {
  public:
    int u, v;

    bool operator==(const Edge &e) const { return u == e.u && v == e.v; }
};

// Its assumed that u,v are distinct.
// Check if all four vertices are distinct
bool edgeConflicts(const Edge &e1, const Edge &e2) {
    return (e1.u == e2.u || e1.u == e2.v || e1.v == e2.u || e1.v == e2.v);
}

std::ostream &operator<<(std::ostream &s, const Edge &e) {
    s << '{' << e.u << ',' << e.v << '}';
    return s;
}

class DiDegree {
  public:
    int in;
    int out;
};

typedef std::vector<int> DegreeSequence;
typedef std::vector<DiDegree> DiDegreeSequence;

class Graph {
  public:
    Graph() : edgeCount(0) {}

    Graph(int n) : edgeCount(0) { adj.resize(n); }

    ~Graph() {}

    bool createFromSequence(const DegreeSequence &d) {
        edgeCount = std::accumulate(d.begin(), d.end(), 0);

        //
        // TODO
        //

        return false;
    }

    bool hasEdge(const Edge &e) const {
        for (int v : adj[e.u]) {
            if (v == e.v)
                return true;
        }
        return false;
    }

    // There are two possible edge exchanges
    // switchType indicates which one is desired
    // Returns false if the switch is not possible
    bool exchangeEdges(const Edge &e1, const Edge &e2, bool switchType) {
        // The new edges configuration is one of these two
        // A) e1.u - e2.u and e1.v - e2.v
        // B) e1.u - e2.v and e1.v - e2.u
        // First check if the move is possible
        if (switchType) {
            if (hasEdge({e1.u, e2.u}) || hasEdge({e1.v, e2.v}))
                return false; // conflicting edges
        } else {
            if (hasEdge({e1.u, e2.v}) || hasEdge({e1.v, e2.u}))
                return false; // conflicting edges
        }

        // Find the edges in the adjacency lists
        int i1, j1, i2, j2;
        for (i1 = 0; i1 < (int)adj[e1.u].size(); ++i1) {
            if (adj[e1.u][i1] == e1.v)
                break;
        }
        for (j1 = 0; j1 < (int)adj[e1.v].size(); ++j1) {
            if (adj[e1.v][j1] == e1.u)
                break;
        }
        for (i2 = 0; i2 < (int)adj[e2.u].size(); ++i2) {
            if (adj[e2.u][i2] == e2.v)
                break;
        }
        for (j2 = 0; j2 < (int)adj[e2.v].size(); ++j2) {
            if (adj[e2.v][j2] == e2.u)
                break;
        }

        // Remove the old edges
        bool removedOne = false;
        for (auto iter = edges.begin(); iter != edges.end();) {
            if (*iter == e1) {
                iter = edges.erase(iter);
                if (removedOne)
                    break;
                removedOne = true;
            } else if (*iter == e2) {
                iter = edges.erase(iter);
                if (removedOne)
                    break;
                removedOne = true;
            } else {
                ++iter;
            }
        }

        // Add the new edges
        if (switchType) {
            adj[e1.u][i1] = e2.u;
            adj[e1.v][j1] = e2.v;
            adj[e2.u][i2] = e1.u;
            adj[e2.v][j2] = e1.v;
            edges.push_back({e1.u, e2.u});
            edges.push_back({e1.v, e2.v});
        } else {
            adj[e1.u][i1] = e2.v;
            adj[e1.v][j1] = e2.u;
            adj[e2.u][i2] = e1.v;
            adj[e2.v][j2] = e1.u;
            edges.push_back({e1.u, e2.v});
            edges.push_back({e1.v, e2.u});
        }
        return true;
    }

    // Graph is saved in two formats for speed
    // The two should be kept consistent at all times
    std::vector<std::vector<int>> adj;
    std::vector<Edge> edges;
    int edgeCount;
};

class SwitchChain {
  public:
    SwitchChain() : permutationDistribution(0, 2) {
        // random_device uses hardware entropy if available
        std::random_device rd;
        mt.seed(rd());
    }
    ~SwitchChain() {}

    bool initialize(const DegreeSequence &d) {
        if (!g.createFromSequence(d))
            return false;
        edgeDistribution.param(
            std::uniform_int_distribution<>::param_type(0, g.edgeCount - 1));
        return true;
    }

    bool doMove() {
        Edge e1 = g.edges[edgeDistribution(mt)];
        Edge e2 = g.edges[edgeDistribution(mt)];
        // Keep regenerating while conflicting edges
        int timeout = 0;
        while (edgeConflicts(e1, e2)) {
            e1 = g.edges[edgeDistribution(mt)];
            e2 = g.edges[edgeDistribution(mt)];
            ++timeout;
            if (timeout % 100 == 0) {
                std::cerr << "Warning: sampled " << timeout
                          << " random edges but they keep conflicting.\n";
            }
        }
        // Consider one of the three possible permutations
        // 1) e1.u - e1.v and e2.u - e2.v (original)
        // 2) e1.u - e2.u and e1.v - e2.v
        // 3) e1.u - e2.v and e1.v - e2.u

        // Note that it might be that these new edges already exist
        // in which case we also reject the move
        // This is checked in exchangeEdges

        int perm = permutationDistribution(mt);
        if (perm == 0) // Original permutation
            return false;
        return g.exchangeEdges(e1, e2, perm == 1);
    }

    Graph g;
    std::mt19937 mt;
    std::uniform_int_distribution<> edgeDistribution;
    std::uniform_int_distribution<> permutationDistribution;
};

int main() {
    SwitchChain chain;
    if (!chain.initialize({3, 2, 4, 2, 2, 1})) {
        std::cerr << "Could not initialize Markov chain.\n";
        return 1;
    }

    std::cout << "Starting switch Markov chain" << std::endl;
    int movesDone = 0;
    for (int i = 0; i < 100; ++i)
        if (chain.doMove())
            ++movesDone;

    std::cout << movesDone << "/100 moves succeeded." << std::endl;

    return 0;
}