code: /*
   * Copyright (c) 2017-2018 The Regents of the University of California
   *
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are
   * met:
   *
   * 1. Redistributions of source code must retain the above copyright
   * notice, this list of conditions and the following disclaimer.
   *
   * 2. Redistributions in binary form must reproduce the above copyright
   * notice, this list of conditions and the following disclaimer in the
   * documentation and/or other materials provided with the distribution.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
   * HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   */
  package edu.berkeley.cs.jqf.examples.jgrapht;
  
  import java.util.HashMap;
  import java.util.Map;
  import java.util.Random;
  
  import org.jgrapht.DirectedGraph;
  import org.jgrapht.EdgeFactory;
  import org.jgrapht.Graph;
  import org.jgrapht.VertexFactory;
  import org.jgrapht.generate.GraphGenerator;
  
  /**
   * @author Rohan Padhye
   */
  public class FastGnmRandomGraphGenerator<V, E, T> implements GraphGenerator<V, E, T> {
  
      private int n;
      private int m;
      private boolean loops;
      private Random rng;
  
      public FastGnmRandomGraphGenerator(int n, int m, Random rng, boolean loops) {
          this.n = n;
          this.m = m;
          this.loops = loops;
          this.rng = rng;
      }
  
      private int numEdges(boolean isDirected, boolean loops) {
          if (loops) {
              if (isDirected) {
                  return n * n;
              } else {
                  return (n * (n + 1)) / 2;
              }
          } else {
              if (isDirected) {
                  return n * (n - 1);
              } else {
                  return (n * (n - 1)) / 2;
              }
          }
      }
  
  
      @Override
      public void generateGraph(Graph<V, E> graph, VertexFactory<V> vertexFactory, Map<String, T> map) {
          // Create nodes
          V[] nodes = (V[]) new Object[n];
          for (int i = 0; i < n; i++) {
              V v = vertexFactory.createVertex();
              nodes[i] = v;
              graph.addVertex(v);
          }
  
          // Collect ALL edges into a list
          boolean isDirected = graph instanceof DirectedGraph;
          int maxEdges = numEdges(isDirected, loops);
          if (m > maxEdges) {
              throw new IllegalArgumentException("Cannot generate " + m + " edges; " +
                      "max allowed is " + maxEdges);
          }
          E[] edges = (E[]) new Object[maxEdges];
          EdgeFactory<V, E> edgeFactory = graph.getEdgeFactory();
          Map<E, V> sources = new HashMap<>();
          Map<E, V> targets = new HashMap<>();
  
          int k = 0;
          for (int i = 0; i < n; i++) {
              int start = isDirected ? 0 : i;
              for (int j = start; j < n; j++) {
                  if (i != j || loops) {
                      V source = nodes[i];
                      V target = nodes[j];
                      E e = edgeFactory.createEdge(source, target);
                      edges[k++] = e;
                      sources.put(e, source);
                      targets.put(e, target);
                  }
              }
          }
          assert(k == maxEdges);
  
          // Select `m` edges to add
          int available = maxEdges;
          for (int i = 0; i < m; i++) {
              // Select an edge in [0, available)
              int idx = rng.nextInt(available);
              E e = edges[idx];
              graph.addEdge(sources.get(e), targets.get(e), e);
              // Decrement upper bound and fill gap
              available--;
              edges[idx] = edges[available];
          }
  
  
      }
  }
  