Make various TDigest implementations Serializable (so that it can be …

…used with Spark).

pom.xml

@@ -76,6 +76,12 @@
             <version>0.0.5</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>3.4</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <dependencyManagement>

src/main/java/com/tdunning/math/stats/AVLGroupTree.java

@@ -17,6 +17,7 @@
 
 package com.tdunning.math.stats;
 
+import java.io.Serializable;
 import java.util.AbstractCollection;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -26,7 +27,7 @@
 /**
  * A tree of t-digest centroids.
  */
-final class AVLGroupTree extends AbstractCollection<Centroid> {
+final class AVLGroupTree extends AbstractCollection<Centroid> implements Serializable {
 
     /* For insertions into the tree */
     private double centroid;

src/main/java/com/tdunning/math/stats/ArrayDigest.java

@@ -17,6 +17,7 @@
 
 package com.tdunning.math.stats;
 
+import java.io.Serializable;
 import java.nio.ByteBuffer;
 import java.util.AbstractCollection;
 import java.util.ArrayList;
@@ -755,7 +756,7 @@ protected Index computeNext() {
     public final static int VERBOSE_ARRAY_DIGEST = 3;
     public final static int SMALL_ARRAY_DIGEST = 4;
 
-    class Index {
+    class Index implements Serializable {
         final int page, subPage;
 
         private Index(int page, int subPage) {
@@ -772,7 +773,7 @@ int count() {
         }
     }
 
-    private static class Page {
+    private static class Page implements Serializable {
         private final boolean recordAllData;
         private final int pageSize;
 

src/main/java/com/tdunning/math/stats/Centroid.java

@@ -17,14 +17,15 @@
 
 package com.tdunning.math.stats;
 
+import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicInteger;
 
 /**
  * A single centroid which represents a number of data points.
  */
-public class Centroid implements Comparable<Centroid> {
+public class Centroid implements Comparable<Centroid>, Serializable {
     private static final AtomicInteger uniqueCount = new AtomicInteger(1);
 
     private double centroid = 0;

src/main/java/com/tdunning/math/stats/GroupTree.java

@@ -17,6 +17,7 @@
 
 package com.tdunning.math.stats;
 
+import java.io.Serializable;
 import java.util.AbstractCollection;
 import java.util.ArrayDeque;
 import java.util.Deque;
@@ -27,7 +28,7 @@
  * A tree containing TDigest.Centroid.  This adds to the normal NavigableSet the
  * ability to sum up the size of elements to the left of a particular group.
  */
-public class GroupTree extends AbstractCollection<Centroid> {
+public class GroupTree extends AbstractCollection<Centroid> implements Serializable {
     private long count;
     private int size;
     private int depth;

src/main/java/com/tdunning/math/stats/IntAVLTree.java

@@ -1,5 +1,6 @@
 package com.tdunning.math.stats;
 
+import java.io.Serializable;
 import java.util.Arrays;
 
 
@@ -9,7 +10,7 @@
  * want to add data to the nodes, typically by using arrays and node
  * identifiers as indices.
  */
-abstract class IntAVLTree {
+abstract class IntAVLTree implements Serializable {
 
     /**
      * We use <tt>0</tt> instead of <tt>-1</tt> so that left(NIL) works without
@@ -507,7 +508,7 @@ void checkBalance(int node) {
     /**
      * A stack of int values.
      */
-    private static class IntStack {
+    private static class IntStack implements Serializable {
 
         private int[] stack;
         private int size;
@@ -535,7 +536,7 @@ void push(int v) {
 
     }
 
-    private static class NodeAllocator {
+    private static class NodeAllocator implements Serializable {
 
         private int nextNode;
         private final IntStack releasedNodes;

src/main/java/com/tdunning/math/stats/TDigest.java

@@ -17,6 +17,7 @@
 
 package com.tdunning.math.stats;
 
+import java.io.Serializable;
 import java.nio.ByteBuffer;
 import java.util.Collection;
 
@@ -39,7 +40,7 @@
  * <p/>
  * g) easy to adapt for use with map-reduce
  */
-public abstract class TDigest {
+public abstract class TDigest implements Serializable {
     /**
      * Creates an ArrayDigest with default page size.
      *

src/test/java/com/tdunning/math/stats/TDigestSerializationTest.java

@@ -0,0 +1,60 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.tdunning.math.stats;
+
+import org.apache.commons.lang3.SerializationUtils;
+import org.junit.*;
+
+import java.io.ByteArrayOutputStream;
+
+import static org.junit.Assert.assertNotNull;
+
+/**
+ * Verifies that the various TDigest implementations can be serialized.
+ *
+ * Serializability is important, for example, if we want to use t-digests with Spark.
+ */
+public class TDigestSerializationTest {
+
+    @Test
+    public void testTreeDigest() {
+        assertSerializesAndDeserializes(new TreeDigest(100));
+    }
+
+    @Test
+    public void testMergingDigest() {
+        assertSerializesAndDeserializes(new MergingDigest(100));
+    }
+
+    @Test
+    public void testAVLTreeDigest() {
+        assertSerializesAndDeserializes(new AVLTreeDigest(100));
+    }
+
+    @Test
+    public void testArrayDigest() {
+        assertSerializesAndDeserializes(new ArrayDigest(32, 100));
+    }
+
+    protected void assertSerializesAndDeserializes(TDigest tdigest) {
+        assertNotNull(SerializationUtils.deserialize(SerializationUtils.serialize(tdigest)));
+
+        tdigest.add(1);
+        assertNotNull(SerializationUtils.deserialize(SerializationUtils.serialize(tdigest)));
+    }
+}