FlowNetwork.java

/******************************************************************************
 *  Compilation:  javac FlowNetwork.java
 *  Execution:    java FlowNetwork V E
 *  Dependencies: Bag.java FlowEdge.java
 *
 *  A capacitated flow network, implemented using adjacency lists.
 *
 ******************************************************************************/

package edu.princeton.cs.algs4;


/**
 *  The {@code FlowNetwork} class represents a capacitated network
 *  with vertices named 0 through <em>V</em> - 1, where each directed
 *  edge is of type {@link FlowEdge} and has a real-valued capacity
 *  and flow.
 *  It supports the following two primary operations: add an edge to the network,
 *  iterate over all of the edges incident to or from a vertex. It also provides
 *  methods for returning the number of vertices <em>V</em> and the number
 *  of edges <em>E</em>. Parallel edges and self-loops are permitted.
 *  <p>
 *  This implementation uses an adjacency-lists representation, which
 *  is a vertex-indexed array of {@link Bag} objects.
 *  All operations take constant time (in the worst case) except
 *  iterating over the edges incident to a given vertex, which takes
 *  time proportional to the number of such edges.
 *  <p>
 *  For additional documentation,
 *  see <a href="https://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 FlowNetwork {
    private static final String NEWLINE = System.getProperty("line.separator");

    private final int V;
    private int E;
    private Bag<FlowEdge>[] adj;

    /**
     * Initializes an empty flow network with {@code V} vertices and 0 edges.
     * @param V the number of vertices
     * @throws IllegalArgumentException if {@code V < 0}
     */
    public FlowNetwork(int V) {
        if (V < 0) throw new IllegalArgumentException("Number of vertices in a Graph must be non-negative");
        this.V = V;
        this.E = 0;
        adj = (Bag<FlowEdge>[]) new Bag[V];
        for (int v = 0; v < V; v++)
            adj[v] = new Bag<FlowEdge>();
    }

    /**
     * Initializes a random flow network with {@code V} vertices and <em>E</em> edges.
     * The capacities are integers between 0 and 99 and the flow values are zero.
     * @param V the number of vertices
     * @param E the number of edges
     * @throws IllegalArgumentException if {@code V < 0}
     * @throws IllegalArgumentException if {@code E < 0}
     */
    public FlowNetwork(int V, int E) {
        this(V);
        if (E < 0) throw new IllegalArgumentException("Number of edges must be non-negative");
        for (int i = 0; i < E; i++) {
            int v = StdRandom.uniformInt(V);
            int w = StdRandom.uniformInt(V);
            double capacity = StdRandom.uniformInt(100);
            addEdge(new FlowEdge(v, w, capacity));
        }
    }

    /**
     * Initializes a flow network from an input stream.
     * The format is the number of vertices <em>V</em>,
     * followed by the number of edges <em>E</em>,
     * followed by <em>E</em> pairs of vertices and edge capacities,
     * with each entry separated by whitespace.
     * @param in the input stream
     * @throws IllegalArgumentException if the endpoints of any edge are not in prescribed range
     * @throws IllegalArgumentException if the number of vertices or edges is negative
     */
    public FlowNetwork(In in) {
        this(in.readInt());
        int E = in.readInt();
        if (E < 0) throw new IllegalArgumentException("number of edges must be non-negative");
        for (int i = 0; i < E; i++) {
            int v = in.readInt();
            int w = in.readInt();
            validateVertex(v);
            validateVertex(w);
            double capacity = in.readDouble();
            addEdge(new FlowEdge(v, w, capacity));
        }
    }


    /**
     * Returns the number of vertices in the edge-weighted graph.
     * @return the number of vertices in the edge-weighted graph
     */
    public int V() {
        return V;
    }

    /**
     * Returns the number of edges in the edge-weighted graph.
     * @return the number of edges in the edge-weighted graph
     */
    public int E() {
        return E;
    }

    // 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));
    }

    /**
     * Adds the edge {@code e} to the network.
     * @param e the edge
     * @throws IllegalArgumentException unless endpoints of edge are between
     *         {@code 0} and {@code V-1}
     */
    public void addEdge(FlowEdge e) {
        int v = e.from();
        int w = e.to();
        validateVertex(v);
        validateVertex(w);
        adj[v].add(e);
        adj[w].add(e);
        E++;
    }

    /**
     * Returns the edges incident on vertex {@code v} (includes both edges pointing to
     * and from {@code v}).
     * @param v the vertex
     * @return the edges incident on vertex {@code v} as an Iterable
     * @throws IllegalArgumentException unless {@code 0 <= v < V}
     */
    public Iterable<FlowEdge> adj(int v) {
        validateVertex(v);
        return adj[v];
    }

    // return list of all edges - excludes self loops
    public Iterable<FlowEdge> edges() {
        Bag<FlowEdge> list = new Bag<FlowEdge>();
        for (int v = 0; v < V; v++)
            for (FlowEdge e : adj(v)) {
                if (e.to() != v)
                    list.add(e);
            }
        return list;
    }


    /**
     * Returns a string representation of the flow network.
     * This method takes time proportional to <em>E</em> + <em>V</em>.
     * @return the number of vertices <em>V</em>, followed by the number of edges <em>E</em>,
     *    followed by the <em>V</em> adjacency lists
     */
    public String toString() {
        StringBuilder s = new StringBuilder();
        s.append(V + " " + E + NEWLINE);
        for (int v = 0; v < V; v++) {
            s.append(v + ":  ");
            for (FlowEdge e : adj[v]) {
                if (e.to() != v) s.append(e + "  ");
            }
            s.append(NEWLINE);
        }
        return s.toString();
    }

    /**
     * Unit tests the {@code FlowNetwork} data type.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        In in = new In(args[0]);
        FlowNetwork G = new FlowNetwork(in);
        StdOut.println(G);
    }

}

/******************************************************************************
 *  Copyright 2002-2022, 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.
 ******************************************************************************/