/******************************************************************************
 *  Compilation:  javac GlobalMincut.java
 *  Execution:    java  GlobalMincut filename.txt
 *  Dependencies: EdgeWeightedGraph.java Edge.java UF.java
 *                IndexMaxPQ.java FlowNetwork.java FlowEdge.java
 *                FordFulkerson.java In.java StdOut.java
 *  Data files:   https://algs4.cs.princeton.edu/43mst/tinyEWG.txt
 *                https://algs4.cs.princeton.edu/43mst/mediumEWG.txt
 *
 *  Computes a minimum cut using Stoer-Wagner's algorithm.
 *
 *  % java GlobalMincut tinyEWG.txt
 *    Min cut: 5
 *    Min cut weight = 0.9500000000000001
 *
 *  % java GlobalMincut mediumEWG.txt
 *    Min cut: 25 60 63 96 199 237
 *    Min cut weight = 0.14021
 *
 ******************************************************************************/

/**
 *  The {@code GlobalMincut} class represents a data type for computing a
 *  <em>global minimum cut</em> in a graph with non-negative edge weights.
 *  A <em>cut</em> is a partition of the vertices into two nonempty subsets.
 *  An edge that has one
 *  endpoint in each subset of a cut is a <em>crossing edge</em>. The weight
 *  of a cut is the sum of the weights of its crossing edges.
 *  A <em>global minimum cut</em> whose weight is no larger than the weight
 *  of any other cut.
 *  <p>
 *  This is an implementation of <em>Stoer-Wagner's algorithm</em>.
 *  The constructor takes
 *  <em>O</em>(<em>V</em> (<em>V</em> + <em>E</em>) log <em>V</em>) time,
 *  where <em>V</em> is the number of vertices and <em>E</em> is the
 *  number of edges.
 *  The <em>weight</em> and <em>isCut</em> methods take &Theta;(1) time.
 *  It uses &Theta;(<em>V</em>) extra space (not including the graph).
 *  <p>
 *  For additional documentation, see
 *  <ul>
 *  <li>M. Stoer and F. Wagner (1997). A simple min-cut algorithm. <em>Journal of
 *  the ACM </em>, 44(4):585-591.
 *  </ul>
 *
 * @author Marcelo Silva
 */
public class GlobalMincut {
    private static final double FLOATING_POINT_EPSILON = 1.0E-11;

    // the weight of the minimum cut
    private double weight = Double.POSITIVE_INFINITY;

    // cut[v] = true if v is on the first subset of vertices of the minimum cut;
    // or false if v is on the second subset
    private boolean[] cut;

    // number of vertices
    private int V;

    /**
     * This helper class represents the <em>cut-of-the-phase</em>. The
     * cut-of-the-phase is a <em>minimum s-t-cut</em> in the current graph,
     * where {@code s} and {@code t} are the two vertices added last in the
     * phase.
     */
    private class CutPhase {
        private double weight; // the weight of the minimum s-t cut
        private int s;         // the vertex s
        private int t;         // the vertex t

        public CutPhase(double weight, int s, int t) {
            this.weight = weight;
            this.s = s;
            this.t = t;
        }
    }

    /**
     * Computes a minimum cut in an edge-weighted graph.
     *
     * @param G the edge-weighted graph
     * @throws IllegalArgumentException if the number of vertices of {@code G}
     *             is less than {@code 2}.
     * @throws IllegalArgumentException if any edge weight is negative
     */
    public GlobalMincut(EdgeWeightedGraph G) {
        V = G.V();
        validate(G);
        minCut(G, 0);
        assert check(G);
    }

    /**
     * Validates the edge-weighted graph.
     *
     * @param G the edge-weighted graph
     * @throws IllegalArgumentException if the number of vertices of {@code G}
     *             is less than {@code 2} or if any edge weight is negative
     */
    private void validate(EdgeWeightedGraph G) {
        if (G.V() < 2) throw new IllegalArgumentException("number of vertices of G is less than 2");
        for (Edge e : G.edges()) {
            if (e.weight() < 0) throw new IllegalArgumentException("edge " + e + " has negative weight");
        }
    }

    /**
     * Returns the weight of the minimum cut.
     *
     * @return the weight of the minimum cut
     */
    public double weight() {
        return weight;
    }

    /**
     * Returns {@code true} if the vertex {@code v} is one side of the
     * mincut and {@code false} otherwise. An edge <em>v</em>-<em>w</em>
     * crosses the mincut if and only if <em>v</em> and <em>w</em> have
     * opposite parity.
     *
     * @param v the vertex to check
     * @return {@code true} if the vertex {@code v} is on the first subset of
     *         vertices of the minimum cut; or {@code false} if the vertex
     *         {@code v} is on the second subset.
     * @throws IllegalArgumentException unless vertex {@code v} is between
     *             {@code 0 <= v < V}
     */
    public boolean cut(int v) {
        validateVertex(v);
        return cut[v];
    }

    /**
     * Makes a cut for the current edge-weighted graph by partitioning its
     * vertices into two nonempty subsets. The vertices connected to the
     * vertex {@code t} belong to the first subset. Other vertices not connected
     * to {@code t} belong to the second subset.
     *
     * @param t the vertex {@code t}
     * @param uf the union-find data type
     */
    private void makeCut(int t, UF uf) {
        for (int v = 0; v < cut.length; v++) {
            cut[v] = (uf.find(v) == uf.find(t));
        }
    }

    /**
     * Computes a minimum cut of the edge-weighted graph. Precisely, it computes
     * the lightest of the cuts-of-the-phase which yields the desired minimum
     * cut.
     *
     * @param G the edge-weighted graph
     * @param a the starting vertex
     */
    private void minCut(EdgeWeightedGraph G, int a) {
        UF uf = new UF(G.V());
        boolean[] marked = new boolean[G.V()];
        cut = new boolean[G.V()];
        CutPhase cp = new CutPhase(0.0, a, a);
        for (int v = G.V(); v > 1; v--) {
            minCutPhase(G, marked, cp);
            if (cp.weight < weight) {
                weight = cp.weight;
                makeCut(cp.t, uf);
            }
            G = contractEdge(G, cp.s, cp.t);
            marked[cp.t] = true;
            uf.union(cp.s, cp.t);
        }
    }

    /**
     * Returns the cut-of-the-phase. The cut-of-the-phase is a minimum s-t-cut
     * in the current graph, where {@code s} and {@code t} are the two vertices
     * added last in the phase. This algorithm is known in the literature as
     * <em>maximum adjacency search</em> or <em>maximum cardinality search</em>.
     *
     * @param G the edge-weighted graph
     * @param marked the array of contracted vertices, where {@code marked[v]}
     *            is {@code true} if the vertex {@code v} was already
     *            contracted; or {@code false} otherwise
     * @param cp the previous cut-of-the-phase
     * @return the cut-of-the-phase
     */
    private void minCutPhase(EdgeWeightedGraph G, boolean[] marked, CutPhase cp) {
        IndexMaxPQ<Double> pq = new IndexMaxPQ<Double>(G.V());
        for (int v = 0; v < G.V(); v++) {
            if (v != cp.s && !marked[v]) pq.insert(v, 0.0);
        }
        pq.insert(cp.s, Double.POSITIVE_INFINITY);
        while (!pq.isEmpty()) {
            int v = pq.delMax();
            cp.s = cp.t;
            cp.t = v;
            for (Edge e : G.adj(v)) {
                int w = e.other(v);
                if (pq.contains(w)) pq.increaseKey(w, pq.keyOf(w) + e.weight());
            }
        }
        cp.weight = 0.0;
        for (Edge e : G.adj(cp.t)) {
            cp.weight += e.weight();
        }
    }

    /**
     * Contracts the edges incidents on the vertices {@code s} and {@code t} of
     * the given edge-weighted graph.
     *
     * @param G the edge-weighted graph
     * @param s the vertex {@code s}
     * @param t the vertex {@code t}
     * @return a new edge-weighted graph for which the edges incidents on the
     *         vertices {@code s} and {@code t} were contracted
     */
    private EdgeWeightedGraph contractEdge(EdgeWeightedGraph G, int s, int t) {
        EdgeWeightedGraph H = new EdgeWeightedGraph(G.V());
        for (int v = 0; v < G.V(); v++) {
            for (Edge e : G.adj(v)) {
                int w = e.other(v);
                if (v == s && w == t || v == t && w == s) continue;
                if (v < w) {
                    if (w == t)      H.addEdge(new Edge(v, s, e.weight()));
                    else if (v == t) H.addEdge(new Edge(w, s, e.weight()));
                    else             H.addEdge(new Edge(v, w, e.weight()));
                }
            }
        }
        return H;
    }

    /**
     * Checks optimality conditions.
     *
     * @param G the edge-weighted graph
     * @return {@code true} if optimality conditions are fine
     */
    private boolean check(EdgeWeightedGraph G) {

        // compute min st-cut for all pairs s and t
        // shortcut: s must appear on one side of global mincut,
        // so it suffices to try all pairs s-v for some fixed s
        double value = Double.POSITIVE_INFINITY;
        for (int s = 0, t = 1; t < G.V(); t++) {
            FlowNetwork F = new FlowNetwork(G.V());
            for (Edge e : G.edges()) {
                int v = e.either(), w = e.other(v);
                F.addEdge(new FlowEdge(v, w, e.weight()));
                F.addEdge(new FlowEdge(w, v, e.weight()));
            }
            FordFulkerson maxflow = new FordFulkerson(F, s, t);
            value = Math.min(value, maxflow.value());
        }
        if (Math.abs(weight - value) > FLOATING_POINT_EPSILON) {
            System.err.println("Min cut weight = " + weight + " , max flow value = " + value);
            return false;
        }
        return true;
    }

    // throw an IllegalArgumentException unless {@code 0 <= v < V}
    private void validateVertex(int v) {
        if (v < 0 || v >= V)
            throw new IllegalArgumentException("vertex " + v + " is not between 0 and " + (V-1));
    }


    /**
     * Unit tests the {@code GlobalMincut} data type.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        In in = new In(args[0]);
        EdgeWeightedGraph G = new EdgeWeightedGraph(in);
        GlobalMincut mc = new GlobalMincut(G);
        StdOut.print("Min cut: ");
        for (int v = 0; v < G.V(); v++) {
            if (mc.cut(v)) StdOut.print(v + " ");
        }
        StdOut.println();
        StdOut.println("Min cut weight = " + mc.weight());
    }
}