PermutationByBit.java

package ivory.lsh.data;

import static org.junit.Assert.assertTrue;
import static org.junit.Assert.fail;

import java.util.Random;

import tl.lin.data.array.ArrayListOfIntsWritable;

public class PermutationByBit extends Permutation {

  private Random rand;
  private ArrayListOfIntsWritable randPerm;
  private int length;

  /**
   * @param i
   * 		length of permutation
   */
  public PermutationByBit(int i){
    rand = new Random();
    randPerm = new ArrayListOfIntsWritable();
    for(int j = 0; j < i; j++){
      randPerm.add(j);
    }
    length = i;
  }

  public ArrayListOfIntsWritable nextPermutation() {
    for(int k = 0; k < length; k++)
    {
      int i = rand.nextInt(length);
      int j = randPerm.get(i);
      randPerm.set(i, randPerm.get(k));
      randPerm.set(k, j);
    }
    return new ArrayListOfIntsWritable(randPerm);
  }	

  public static void main(String[] args){
    int SIZE = 1000;
    PermutationByBit p = new PermutationByBit(SIZE);
    for(int k=0;k<100;k++){
      ArrayListOfIntsWritable a = p.nextPermutation();

      //make sure the permutation is not out of bounds
      for(int i=0;i<SIZE;i++){
        assertTrue(i+"-->"+a.get(i),a.get(i)<SIZE && a.get(i)>=0);
      }

      //make sure each position is included in the permutation exactly once
      int[] positions = new int[SIZE];
      for(int i=0;i<SIZE;i++){
        if(positions[a.get(i)]==1){
          fail("Same position included twice: "+a.get(i));
        }
        positions[a.get(i)]=1;
      }
      for(int i=0;i<SIZE;i++){
        if(positions[i]==0){
          //					System.out.println(java.util.Arrays.binarySearch(positions, i));
          fail("Position not included: "+i);
        }
      }
      assertTrue(sum(positions)+"",sum(positions)==SIZE);
    }
    System.out.println("done");
  }

  private static int sum(int[] positions){
    int sum=0;
    for(int i=0;i<positions.length;i++){
      sum+=positions[i];
    }
    return sum;
  }
}