Add t-digest

NOTICES

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+The code for the t-digest was originally authored by Ted Dunning
+
+Adrien Grand contributed the heart of the AVLTreeDigest (https://github.com/jpountz)

presto-main/src/main/java/com/facebook/presto/tdigest/Centroid.java

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+/*
+ * Licensed 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.
+ */
+
+/*
+ * Licensed to Ted Dunning 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.facebook.presto.tdigest;
+
+import javax.annotation.concurrent.NotThreadSafe;
+
+import java.io.Serializable;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static java.util.Objects.requireNonNull;
+
+/**
+ * A single centroid which represents a number of data points.
+ */
+@NotThreadSafe
+public class Centroid
+        implements Comparable<Centroid>, Serializable
+{
+    private static final AtomicInteger uniqueCount = new AtomicInteger(1);
+
+    private double centroid;
+    private int count;
+
+    // The ID is transient because it must be unique within a given JVM. A new
+    // ID should be generated from uniqueCount when a Centroid is deserialized.
+    private transient int id;
+
+    public Centroid(double x)
+    {
+        start(x, 1, uniqueCount.getAndIncrement());
+    }
+
+    public Centroid(double x, int w)
+    {
+        start(x, w, uniqueCount.getAndIncrement());
+    }
+
+    public Centroid(double x, int w, int id)
+    {
+        start(x, w, id);
+    }
+
+    private void start(double x, int w, int id)
+    {
+        this.id = id;
+        add(x, w);
+    }
+
+    public void add(double x, int w)
+    {
+        count += w;
+        centroid += w * (x - centroid) / count;
+    }
+
+    public double getMean()
+    {
+        return centroid;
+    }
+
+    public int getWeight()
+    {
+        return count;
+    }
+
+    public int getId()
+    {
+        return id;
+    }
+
+    @Override
+    public String toString()
+    {
+        return "Centroid{" +
+                "mean=" + centroid +
+                ", count=" + count +
+                '}';
+    }
+
+    @Override
+    public int hashCode()
+    {
+        return id;
+    }
+
+    @Override
+    public boolean equals(Object o)
+    {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        Centroid centroid = (Centroid) o;
+        return this.centroid == centroid.getMean() && this.count == centroid.getWeight();
+    }
+
+    @Override
+    public int compareTo(Centroid o)
+    {
+        requireNonNull(o);
+        int r = Double.compare(centroid, o.centroid);
+        if (r == 0) {
+            r = id - o.id;
+        }
+        return r;
+    }
+}