PermutationUtils.java

/**
   Copyright (c) 2011, the SemanticVectors AUTHORS.

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 **/

package di.uniba.it.tri.vectors;

/**
 * Class that provides utilities for generating special permutations.
 *
 * Permutations themselves are just represented as int[] arrays saying.  It is presumed that
 * each such array is a permutation of the numbers from 1 to n, where n is the length of the
 * array.  This invariant could be enforced by making a Permutation class but this has not been
 * considered necessary to date.
 * 
 * @author Dominic Widdows
 */
public class PermutationUtils {

  private PermutationUtils() {}

  /**
   * Returns dimension for real and complex vectors, or dimension / 64 for binary vectors.
     * @param vectorType
     * @param dimension
     * @return 
   */
  public static int getPermutationLength(VectorType vectorType, int dimension) {
    return (vectorType != VectorType.BINARY) ? dimension : dimension / 64; 
  }
  
  /**
   * Creates a shift permutation that can be applied to vectors of this type and dimension.
   * 
   * @param vectorType necessary argument because binary vectors are only permuted in 64-bit chunks
   * @param dimension dimension of vectors to be permuted
   * @param shift number of places to shift the vector
   * @return array of length given by {@link #getPermutationLength}.
   */
  public static int[] getShiftPermutation(VectorType vectorType, int dimension, int shift) {
    int permutationLength = getPermutationLength(vectorType, dimension);
    int[] permutation = new int[permutationLength];

    for (int i = 0; i < permutationLength; ++i) {
      int entry = (i + shift) % permutationLength;
      if (entry < 0) entry += permutationLength;
      permutation[i] = entry;
    }
    return permutation;
  }

    /**
     *
     * @param permutation
     * @return
     */
    public static int[] getInversePermutation(int[] permutation) {
    int[] inversePermutation = new int[permutation.length];
    for (int x=0; x < permutation.length; x++) {
      inversePermutation[permutation[x]] = x;
    }
    return inversePermutation;  
  }
}