Fix SerDe Issue 165

Author: lrhodes

src/main/java/com/yahoo/sketches/quantiles/DoublesUnionImpl.java

@@ -77,9 +77,9 @@ static DoublesUnionImpl heapifyInstance(final DoublesSketch sketch) {
    * @return a DoublesUnion object
    */
   static DoublesUnionImpl heapifyInstance(final Memory srcMem) {
-    final long n = srcMem.getLong(PreambleUtil.N_LONG);
+    final int preLongs = srcMem.getByte(PreambleUtil.PREAMBLE_LONGS_BYTE) & 0xFF;
     final int k = srcMem.getShort(PreambleUtil.K_SHORT) & 0xFFFF;
-    final HeapUpdateDoublesSketch sketch = (n == 0)
+    final HeapUpdateDoublesSketch sketch = (preLongs == 1)
         ? HeapUpdateDoublesSketch.newInstance(k)
         : HeapUpdateDoublesSketch.heapifyInstance(srcMem);
     final DoublesUnionImpl union = new DoublesUnionImpl(k);

src/test/java/com/yahoo/sketches/quantiles/DoublesUnionImplTest.java

@@ -693,6 +693,15 @@ public void isSameResourceDirect() {
     Assert.assertFalse(union.isSameResource(mem2));
   }
 
+  @SuppressWarnings("unused")
+  @Test
+  public void checkSerDeIssue165() {
+    DoublesUnion union = DoublesUnion.builder().build();
+    byte[] byteArr = union.toByteArray();
+    Memory mem = Memory.wrap(byteArr);
+    DoublesUnion union2 = DoublesUnionBuilder.heapify(mem);
+  }
+
   @Test
   public void printlnTest() {
     println("PRINTING: " + this.getClass().getName());