Issue 665 feature

src/main/java/com/github/javafaker/Options.java

@@ -1,9 +1,14 @@
 package com.github.javafaker;
 
+import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
+import java.util.Random;
 
 public class Options {
     private final Faker faker;
+    private static final Random rand = new Random();
 
     protected Options(Faker faker) {
         this.faker = faker;
@@ -52,4 +57,36 @@ public <E> E nextElement(E[] array) {
     public <E> E nextElement(List<E> list) {
         return list.get(faker.random().nextInt(list.size()));
     }
+
+    /**
+     * Returns a random subset of the input list
+     * @param list input list
+     *
+     * @return A random subset.
+     */
+    public <E> List<E> getRandomSubset(final List<E> list) {
+        final List<E> toReturn = new ArrayList<E>();
+
+        final int inputSize = list.size();
+        if (inputSize == 0) {
+            return toReturn; // If we get an empty set, we have nothing to do but return the empty set back
+        }
+
+        final int subsetSize = rand.nextInt(inputSize); // Pick a subset size. Can also be the full list
+        final Set<Integer> subsetIndex = new HashSet<Integer>();
+
+        for (int i = 0; i < subsetSize; i++) {
+            int randomIndex = rand.nextInt(inputSize); // Pick a random index within the input list
+            while (subsetIndex.contains(randomIndex)) {
+                randomIndex = rand.nextInt(inputSize); // If the index is already picked, choose another one
+            }
+            subsetIndex.add(randomIndex);
+        }
+
+        for (final Integer index : subsetIndex) {
+            toReturn.add(list.get(index)); // Build the subset list
+        }
+
+        return toReturn;
+    }
 }

src/test/java/com/github/javafaker/OptionsTest.java

@@ -1,17 +1,25 @@
 package com.github.javafaker;
 
+import com.github.javafaker.repeating.Repeat;
 import org.junit.Before;
 import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Random;
 
 import static org.hamcrest.Matchers.isIn;
 import static org.hamcrest.Matchers.isOneOf;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertThat;
 
 public class OptionsTest extends AbstractFakerTest {
 
+    private static final Logger logger = LoggerFactory.getLogger(OptionsTest.class);
+
     private String[] options;
 
     @Before
@@ -60,4 +68,33 @@ public void testNextListElement() {
     public enum Day {
         MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
     }
+
+    @Test
+    @Repeat(times=10)
+    public void testRandomSubset1() {
+        List<Integer> list = new ArrayList<Integer>();
+        Random r = new Random();
+        // Keep the array size to a small size for runtime
+        int listSize = r.nextInt(100);
+        for (int i=0; i < listSize; i++) {
+            // Allow for any positive integer
+            list.add(r.nextInt());
+        }
+
+        List<Integer> subsetList = faker.options().getRandomSubset(list);
+        logger.info("Input list: {}", list);
+        logger.info("Random subset: {}", subsetList);
+
+        // Check that every element of the returned subset list is in fact in the original list
+        for (Integer integer : subsetList) {
+            assertThat(integer, isIn(list));
+        }
+    }
+
+    @Test
+    public void testRandomSubsetEmpty() {
+        List<Integer> list = new ArrayList<Integer>();
+        List<Integer> subsetList = faker.options().getRandomSubset(list);
+        assertEquals(list,subsetList);
+    }
 }