public class RandomPermutation {

  // Permute the elements of array A. This affects
  // the original array A.

  public static void permute (int[] A) 
  {
    for (int i=0; i<A.length; i++) {
      // Pick a random index in i+1 ... A.length-1
      int k = (int) UniformRandom.uniform ( (int) i,  (int) A.length-1);
      // Swap.
      int temp = A[i];
      A[i] = A[k];
      A[k] = temp;
    }
  }


  // Generate a new array with elements permuted from A,
  // but without affecting A.

  public static int[] permutePreserve (int[] A) 
  {
    // Make new array.
    int[] B = new int [A.length];

    // Copy over.
    for (int i=0; i<A.length; i++) 
      B[i] = A[i];
    
    // Permute new array.
    for (int i=0; i<B.length; i++) {
      // Pick a random index in i+1 ... B.length-1
      int k = (int) UniformRandom.uniform ( (int) i,  (int) B.length-1);
      // Swap.
      int temp = B[i];
      B[i] = B[k];
      B[k] = temp;
    }

    return B;
  }
  

  /////////////////////////////////////////////////////////////////////////
  // TEST CODE

  public static void main (String[] argv)
  {
    try {
      if ( (argv == null) || (argv.length != 2) ) {
        System.out.println ("Usage: RandomPermutation <numTrials> <arraySize>");
        System.exit(1);
      }
      
      int numTrials = Integer.parseInt (argv[0]);
      int arraySize = Integer.parseInt (argv[1]);

      performTrials (numTrials, arraySize);
    }
    catch (Exception e) {
      e.printStackTrace();
    }
  }

  static void performTrials (int numTrials, int arraySize)
  {
    // First create array 1..n
    int[] A = new int [arraySize];
    for (int i=0; i<arraySize; i++)
      A[i] = i;

    double[] sum = new double [arraySize];
    for (int i=0; i<numTrials; i++) {
      int[] B = permutePreserve (A);
      for (int j=0; j<arraySize; j++)
        sum[j] += B[j];
      /*permute (A);
      for (int j=0; j<arraySize; j++)
      sum[j] += A[j];*/
    }

    System.out.println ("Histogram: ");
    System.out.println ("  Position  Average-value");
    for (int i=0; i<arraySize; i++)
      System.out.println ("        " + i + "        " + (sum[i]/numTrials));
  }

}