Add intersect method to bloom filter

src/main/java/com/clearspring/analytics/stream/membership/BloomFilter.java

@@ -144,6 +144,23 @@ public Filter merge(Filter... filters) {
         return merged;
     }
 
+    /**
+     * Intersect a Bloom Filter with a number of other bloom filters.
+     * @param filters
+     * @return a new Bloom Filter that has each filter bit set if and only if all the input bloom filters had that bit set
+     */
+    public Filter intersect(Filter... filters) {
+        BitSet intersection = (BitSet) this.filter().clone();
+        BloomFilter intersectionBloomFilter = new BloomFilter(this.getHashCount(), intersection);
+        for (Filter otherFilter : filters) {
+            if (!(otherFilter instanceof BloomFilter) || this.hashCount != otherFilter.hashCount) {
+                throw new IllegalArgumentException(("Cannot merge filters of different class or size"));
+            }
+            intersection.and(((BloomFilter) otherFilter).filter());
+        }
+        return intersectionBloomFilter;
+    }
+
     /**
      * @return a BloomFilter that always returns a positive match, for testing
      */

src/test/java/com/clearspring/analytics/stream/membership/BloomFilterTest.java

@@ -55,6 +55,7 @@ public BloomFilterTest() {
     @Before
     public void clear() {
         bf.clear();
+        bf2.clear();
     }
 
     @Test
@@ -76,6 +77,18 @@ public void testMerge() {
         assertTrue(mergeBf.isPresent("c"));
     }
 
+    @Test
+    public void testIntersect() {
+        bf.add("a");
+        bf.add("b");
+        bf2.add("a");
+        bf2.add("c");
+        BloomFilter intersectionBf = (BloomFilter) bf.intersect(bf2);
+        assertTrue(intersectionBf.isPresent("a"));
+        assertFalse(intersectionBf.isPresent("b"));
+        assertFalse(intersectionBf.isPresent("c"));
+    }
+
     @Test(expected=IllegalArgumentException.class)
     public void testMergeException() {
         BloomFilter bf3 = new BloomFilter(ELEMENTS*10, 1);