FordFulkerson.java


Below is the syntax highlighted version of FordFulkerson.java.


/******************************************************************************
 *  Compilation:  javac FordFulkerson.java
 *  Execution:    java FordFulkerson V E
 *  Dependencies: FlowNetwork.java FlowEdge.java Queue.java
 *  Data files:   http://algs4.cs.princeton.edu/65maxflow/tinyFN.txt
 *
 *  Ford-Fulkerson algorithm for computing a max flow and 
 *  a min cut using shortest augmenting path rule.
 *
 ******************************************************************************/

package edu.princeton.cs.algs4;

/**
 *  The {@code FordFulkerson} class represents a data type for computing a
 *  <em>maximum st-flow</em> and <em>minimum st-cut</em> in a flow
 *  network.
 *  <p>
 *  This implementation uses the <em>Ford-Fulkerson</em> algorithm with
 *  the <em>shortest augmenting path</em> heuristic.
 *  The constructor takes time proportional to <em>E V</em> (<em>E</em> + <em>V</em>)
 *  in the worst case and extra space (not including the network)
 *  proportional to <em>V</em>, where <em>V</em> is the number of vertices
 *  and <em>E</em> is the number of edges. In practice, the algorithm will
 *  run much faster.
 *  Afterwards, the {@code inCut()} and {@code value()} methods take
 *  constant time.
 *  <p>
 *  If the capacities and initial flow values are all integers, then this
 *  implementation guarantees to compute an integer-valued maximum flow.
 *  If the capacities and floating-point numbers, then floating-point
 *  roundoff error can accumulate.
 *  <p>
 *  For additional documentation,
 *  see <a href="http://algs4.cs.princeton.edu/64maxflow">Section 6.4</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class FordFulkerson {
    private static final double FLOATING_POINT_EPSILON = 1E-11;

    private final int V;          // number of vertices
    private boolean[] marked;     // marked[v] = true iff s->v path in residual graph
    private FlowEdge[] edgeTo;    // edgeTo[v] = last edge on shortest residual s->v path
    private double value;         // current value of max flow
  
    /**
     * Compute a maximum flow and minimum cut in the network {@code G}
     * from vertex {@code s} to vertex {@code t}.
     *
     * @param  G the flow network
     * @param  s the source vertex
     * @param  t the sink vertex
     * @throws IllegalArgumentException unless {@code 0 <= s < V}
     * @throws IllegalArgumentException unless {@code 0 <= t < V}
     * @throws IllegalArgumentException if {@code s == t}
     * @throws IllegalArgumentException if initial flow is infeasible
     */
    public FordFulkerson(FlowNetwork G, int s, int t) {
        V = G.V();
        validate(s);
        validate(t);
        if (s == t)               throw new IllegalArgumentException("Source equals sink");
        if (!isFeasible(G, s, t)) throw new IllegalArgumentException("Initial flow is infeasible");

        // while there exists an augmenting path, use it
        value = excess(G, t);
        while (hasAugmentingPath(G, s, t)) {

            // compute bottleneck capacity
            double bottle = Double.POSITIVE_INFINITY;
            for (int v = t; v != s; v = edgeTo[v].other(v)) {
                bottle = Math.min(bottle, edgeTo[v].residualCapacityTo(v));
            }

            // augment flow
            for (int v = t; v != s; v = edgeTo[v].other(v)) {
                edgeTo[v].addResidualFlowTo(v, bottle); 
            }

            value += bottle;
        }

        // check optimality conditions
        assert check(G, s, t);
    }

    /**
     * Returns the value of the maximum flow.
     *
     * @return the value of the maximum flow
     */
    public double value()  {
        return value;
    }

    /**
     * Returns true if the specified vertex is on the {@code s} side of the mincut.
     *
     * @param  v vertex
     * @return {@code true} if vertex {@code v} is on the {@code s} side of the micut;
     *         {@code false} otherwise
     * @throws IllegalArgumentException unless {@code 0 <= v < V}
     */
    public boolean inCut(int v)  {
        validate(v);
        return marked[v];
    }

    // throw an IllegalArgumentException if v is outside prescibed range
    private void validate(int v)  {
        if (v < 0 || v >= V)
            throw new IllegalArgumentException("vertex " + v + " is not between 0 and " + (V-1));
    }


    // is there an augmenting path? 
    // if so, upon termination edgeTo[] will contain a parent-link representation of such a path
    // this implementation finds a shortest augmenting path (fewest number of edges),
    // which performs well both in theory and in practice
    private boolean hasAugmentingPath(FlowNetwork G, int s, int t) {
        edgeTo = new FlowEdge[G.V()];
        marked = new boolean[G.V()];

        // breadth-first search
        Queue<Integer> queue = new Queue<Integer>();
        queue.enqueue(s);
        marked[s] = true;
        while (!queue.isEmpty() && !marked[t]) {
            int v = queue.dequeue();

            for (FlowEdge e : G.adj(v)) {
                int w = e.other(v);

                // if residual capacity from v to w
                if (e.residualCapacityTo(w) > 0) {
                    if (!marked[w]) {
                        edgeTo[w] = e;
                        marked[w] = true;
                        queue.enqueue(w);
                    }
                }
            }
        }

        // is there an augmenting path?
        return marked[t];
    }



    // return excess flow at vertex v
    private double excess(FlowNetwork G, int v) {
        double excess = 0.0;
        for (FlowEdge e : G.adj(v)) {
            if (v == e.from()) excess -= e.flow();
            else               excess += e.flow();
        }
        return excess;
    }

    // return excess flow at vertex v
    private boolean isFeasible(FlowNetwork G, int s, int t) {

        // check that capacity constraints are satisfied
        for (int v = 0; v < G.V(); v++) {
            for (FlowEdge e : G.adj(v)) {
                if (e.flow() < -FLOATING_POINT_EPSILON || e.flow() > e.capacity() + FLOATING_POINT_EPSILON) {
                    System.err.println("Edge does not satisfy capacity constraints: " + e);
                    return false;
                }
            }
        }

        // check that net flow into a vertex equals zero, except at source and sink
        if (Math.abs(value + excess(G, s)) > FLOATING_POINT_EPSILON) {
            System.err.println("Excess at source = " + excess(G, s));
            System.err.println("Max flow         = " + value);
            return false;
        }
        if (Math.abs(value - excess(G, t)) > FLOATING_POINT_EPSILON) {
            System.err.println("Excess at sink   = " + excess(G, t));
            System.err.println("Max flow         = " + value);
            return false;
        }
        for (int v = 0; v < G.V(); v++) {
            if (v == s || v == t) continue;
            else if (Math.abs(excess(G, v)) > FLOATING_POINT_EPSILON) {
                System.err.println("Net flow out of " + v + " doesn't equal zero");
                return false;
            }
        }
        return true;
    }



    // check optimality conditions
    private boolean check(FlowNetwork G, int s, int t) {

        // check that flow is feasible
        if (!isFeasible(G, s, t)) {
            System.err.println("Flow is infeasible");
            return false;
        }

        // check that s is on the source side of min cut and that t is not on source side
        if (!inCut(s)) {
            System.err.println("source " + s + " is not on source side of min cut");
            return false;
        }
        if (inCut(t)) {
            System.err.println("sink " + t + " is on source side of min cut");
            return false;
        }

        // check that value of min cut = value of max flow
        double mincutValue = 0.0;
        for (int v = 0; v < G.V(); v++) {
            for (FlowEdge e : G.adj(v)) {
                if ((v == e.from()) && inCut(e.from()) && !inCut(e.to()))
                    mincutValue += e.capacity();
            }
        }

        if (Math.abs(mincutValue - value) > FLOATING_POINT_EPSILON) {
            System.err.println("Max flow value = " + value + ", min cut value = " + mincutValue);
            return false;
        }

        return true;
    }


    /**
     * Unit tests the {@code FordFulkerson} data type.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {

        // create flow network with V vertices and E edges
        int V = Integer.parseInt(args[0]);
        int E = Integer.parseInt(args[1]);
        int s = 0, t = V-1;
        FlowNetwork G = new FlowNetwork(V, E);
        StdOut.println(G);

        // compute maximum flow and minimum cut
        FordFulkerson maxflow = new FordFulkerson(G, s, t);
        StdOut.println("Max flow from " + s + " to " + t);
        for (int v = 0; v < G.V(); v++) {
            for (FlowEdge e : G.adj(v)) {
                if ((v == e.from()) && e.flow() > 0)
                    StdOut.println("   " + e);
            }
        }

        // print min-cut
        StdOut.print("Min cut: ");
        for (int v = 0; v < G.V(); v++) {
            if (maxflow.inCut(v)) StdOut.print(v + " ");
        }
        StdOut.println();

        StdOut.println("Max flow value = " +  maxflow.value());
    }

}

/******************************************************************************
 *  Copyright 2002-2016, Robert Sedgewick and Kevin Wayne.
 *
 *  This file is part of algs4.jar, which accompanies the textbook
 *
 *      Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
 *      Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
 *      http://algs4.cs.princeton.edu
 *
 *
 *  algs4.jar is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  algs4.jar is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with algs4.jar.  If not, see http://www.gnu.org/licenses.
 ******************************************************************************/


Last updated: Sun Apr 16 20:48:46 EDT 2017.