Made flatten() part of BloomFilterProducer

src/main/java/org/apache/commons/collections4/bloomfilter/BloomFilterProducer.java

@@ -68,6 +68,23 @@ default boolean forEachBloomFilterPair(final BloomFilterProducer other,
         return other.forEachBloomFilter(p) && p.forEachRemaining();
     }
 
+    /**
+     * Create a standard (non-layered) Bloom filter by merging all of the layers. If
+     * the filter is empty this method will return an empty Bloom filter.
+     *
+     * @return the merged bloom filter.
+     */
+    default BloomFilter flatten() {
+        BloomFilter bf[] = {null};
+        forEachBloomFilter( x -> {
+            if (bf[0]==null) {
+                bf[0] = new SimpleBloomFilter( x.getShape());
+            }
+            return bf[0].merge( x );
+        });
+        return bf[0];
+    }
+
     /**
      * Creates a BloomFilterProducer from an array of Bloom filters.
      *

src/main/java/org/apache/commons/collections4/bloomfilter/WrappedBloomFilter.java

@@ -27,6 +27,11 @@
 public abstract class WrappedBloomFilter implements BloomFilter {
     final BloomFilter wrapped;
 
+    /**
+     * Wraps a Bloom filter.  The wrapped filter is maintained as a reference
+     * not a copy.  Changes in one will be reflected in the other.
+     * @param bf The Bloom filter.
+     */
     public WrappedBloomFilter(BloomFilter bf) {
         this.wrapped = bf;
     }

src/test/java/org/apache/commons/collections4/bloomfilter/AbstractBloomFilterProducerTest.java

@@ -51,7 +51,7 @@ public void setup() {
         one.clear();
         one.merge(IndexProducer.fromIndexArray(1));
         two.clear();
-        two.merge(IndexProducer.fromIndexArray(1, 2));
+        two.merge(IndexProducer.fromIndexArray(2, 3));
         nullCount[0] = 0;
         nullCount[1] = 0;
         equalityCount[0] = 0;
@@ -123,4 +123,12 @@ public void testForEachPairReturnFalseEarly() {
         assertFalse(createUnderTest().forEachBloomFilterPair(BloomFilterProducer.fromBloomFilterArray(one, two, one),
                 (x, y) -> false));
     }
+
+    @Test
+    public void testFlatten() {
+        BloomFilter underTest = createUnderTest().flatten();
+        BloomFilter expected = new SimpleBloomFilter(shape);
+        expected.merge(IndexProducer.fromIndexArray(1, 2, 3));
+        assertArrayEquals(expected.asBitMapArray(), underTest.asBitMapArray());
+    }
 }