Support KPermutation

src/main/java/org/paukov/combinatorics3/KPermutationGenerator.java

@@ -0,0 +1,92 @@
+/*
+ * Combinatorics Library 3
+ * Copyright 2009-2016 Dmytro Paukov d.paukov@gmail.com
+ */
+package org.paukov.combinatorics3;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Spliterators;
+import java.util.stream.Stream;
+import java.util.stream.StreamSupport;
+
+/**
+ * This generator generates k-permutations of the specified initial vector
+ * <p>
+ * A k-permutation is an ordering of a subset in the subset of all possible
+ * orderings. For example, the set containing 2 digits of 123, has
+ * six permutations: 12, 21, 13, 31, 23 and 32.
+ * <p>
+ * This is an example of the k-permutations of 3 string items (apple, orange,
+ * cherry):
+ * <p>
+ * <blockquote>
+ *
+ * <pre>
+ *
+ *     List<List<String>> permutations =
+ *             Generator.permutation(Arrays.asList("apple", "orange", "cherry"))
+ *                     .k(2)
+ *                     .stream()
+ *                     .collect(toList());
+ *     permutations.stream().forEach(System.out::println);
+ *
+ * </pre>
+ *
+ * </blockquote>
+ * <p>
+ * And the result
+ * <p>
+ * <blockquote>
+ *
+ * <pre>
+ *   [apple, orange]
+ *   [orange, apple]
+ *   [apple, cherry]
+ *   [cherry, apple]
+ *   [orange, cherry]
+ *   [cherry, orange]
+ * </pre>
+ *
+ * </blockquote>
+ * <p>
+ *
+ * @param <T> Type of the elements in the permutations
+ * @author Charlie Feng
+ * @version 3.4
+ */
+class KPermutationGenerator<T> implements IGenerator<List<T>> {
+
+  final List<T> originalVector;
+  final int length;
+
+  /**
+   * Constructor
+   *
+   * @param vector Vector which is used for k-permutation generation
+   * @param length number of elements in k-permutation
+   */
+  KPermutationGenerator(Collection<T> vector,
+                        int length) {
+    this.originalVector = new ArrayList<>(vector);
+    this.length = length;
+  }
+
+  @Override
+  public Iterator<List<T>> iterator() {
+    return  Generator.combination(originalVector)
+            .simple(length)
+            .stream()
+            .flatMap(combination -> Generator.permutation(combination).simple().stream() )
+            .distinct()
+            .iterator();
+  }
+
+  @Override
+  public Stream<List<T>> stream() {
+    return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator(), 0), false);
+  }
+}
+

src/main/java/org/paukov/combinatorics3/PermutationGenerator.java

@@ -34,6 +34,15 @@ public IGenerator<List<T>> simple(TreatDuplicatesAs treatAsIdentical) {
         TreatDuplicatesAs.IDENTICAL.equals(treatAsIdentical));
   }
 
+  /**
+   * Generate KPermutationGenerator with treat duplicates as different.
+   * @param length the value of "k" in KPermutationGenerator
+   * @return KPermutationGenerator
+   */
+  public IGenerator<List<T>> k(int length) {
+    return new KPermutationGenerator<>(originalVector, length);
+  }
+
   public IGenerator<List<T>> withRepetitions(int permutationLength) {
     return new PermutationWithRepetitionGenerator<>(originalVector, permutationLength);
   }