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Very simple java library to generate permutations, combinations and other combinatorial sequences.
There is a newer version: 2.3
/**
 * Combinatorics Library
 * Copyright 2012 Dmytro Paukov [email protected]
 */
package org.paukov.combinatorics;

import org.paukov.combinatorics.composition.IntegerCompositionGenerator;
import org.paukov.combinatorics.subsets.IntegerSubSetGenerator;

/**
 * This is a special factory to create integer vectors and generators.
 *
 * @author Dmytro Paukov
 * @version 2.0
 * @see IntegerVector
 * @see IntegerGenerator
 */
public class IntegerFactory {

  /**
   * Creates an empty integer vector instance
   *
   * @return The empty vector
   */
  public static IntegerVector createIntegerVector() {
    return new IntegerVector(0);
  }

  /**
   * Creates an empty integer vector instance
   *
   * @return The empty vector
   */
  public static IntegerVector createIntegerVector(int size) {
    return new IntegerVector(size);
  }

  /**
   * Creates an integer vector from the given integer array
   *
   * @param array The array
   * @return The integer vector
   */
  public static IntegerVector createIntegerVector(int[] array) {
    return new IntegerVector(array);
  }

  /**
   * Creates an integer vector from the given integer array by the length
   * len
   *
   * @param array The array
   * @param len the length
   * @return The integer vector
   */
  public static IntegerVector createIntegerVector(int[] array, int len) {
    return new IntegerVector(array, len);
  }

  /**
   * Creates a sub-set generator
   *
   * @param originalVector The initial vector
   */
  public static IntegerGenerator createIntegerSubSetGenerator(
      IntegerVector originalVector) {
    return new IntegerSubSetGenerator(originalVector);
  }

  /**
   * Creates a sub-set generator
   *
   * @param originalVector The initial vector
   * @param treatAsIdentical true if the sub sets have to be treated as identical
   * @return An instance of the generator
   * @see IntegerSubSetGenerator
   */
  public static IntegerGenerator createIntegerSubSetGenerator(
      IntegerVector originalVector, boolean treatAsIdentical) {
    return new IntegerSubSetGenerator(originalVector, treatAsIdentical);
  }

  /**
   * Creates a composition generator
   *
   * @param n The initial value
   */
  public static IntegerGenerator createIntegerCompositionGenerator(Integer n) {
    return new IntegerCompositionGenerator(n);
  }

  /**
   * This method creates a combinatorics vector of (1, 2,.., n)
   *
   * @param n The value of the number of the elements
   * @return The vector
   */
  public static IntegerVector range(int n) {
    if (n < 0) {
      throw new IllegalArgumentException(
          "Range value must be more or equal to 0");
    }

    // If range is zero return an empty vector
    if (n == 0) {
      return new IntegerVector();
    }

    int[] array = new int[n];
    for (int i = 0; i < n; i++) {
      array[i] = i + 1;
    }

    return new IntegerVector(array);
  }

  /**
   * This method creates a combinatorics vector of (from, from + 1,.., to-1,
   * to)
   *
   * @param from The first value
   * @param to The second value
   * @return The vector
   */
  public static IntegerVector range(int from, int to) {
    if (from > to) {
      throw new IllegalArgumentException(
          "From parameter must be less then To parameter");
    }

    int[] array = new int[to - from + 1];
    for (int i = 0; i < to - from + 1; i++) {
      array[i] = i + from;
    }

    return new IntegerVector(array);
  }
}