Github user sachingoel0101 commented on a diff in the pull request:
https://github.com/apache/flink/pull/861#discussion_r37556660
 Diff: flinkstaging/flinkml/src/main/scala/org/apache/flink/ml/statistics/ContinuousHistogram.scala

@@ 0,0 +1,315 @@
+/*
+ * 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/LICENSE2.0
+ *
+ * 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 org.apache.flink.ml.statistics
+
+import scala.Double.MaxValue
+import scala.collection.mutable
+
+/** Implementation of a continuous valued online histogram
+ * Adapted from BenHaim and YomTov's Streaming Decision Tree Algorithm
+ * Refer http://www.jmlr.org/papers/volume11/benhaim10a/benhaim10a.pdf
+ *
+ * =Parameters=
+ * [[capacity]]:
+ * Number of bins to be used in the histogram
+ *
+ * [[min]]:
+ * Lower limit on the elements
+ *
+ * [[max]]:
+ * Upper limit on the elements
+ */
+class ContinuousHistogram(capacity: Int, min: Double, max: Double) extends OnlineHistogram
{
+
+ private val lower = min
+ private val upper = max
+
+ require(capacity > 0, "Capacity should be a positive integer")
+ require(lower < upper, "Lower must be less than upper")
+
+ val data = new mutable.ArrayBuffer[(Double, Int)]()
+
+ /** Adds a new item to the histogram
+ *
+ * @param p value to be added
+ */
+ override def add(p: Double): Unit = {
+ require(p > lower && p < upper, p + " not in (" + lower + "," + upper
+ ")")
+ // search for the index where the value is just higher than p
+ val search = find(p)
+ // add the new value there, shifting everything to the right
+ data.insert(search, (p, 1))
+ // If we're over capacity or any two elements are within 1e9 of each other, merge.
+ // This will take care of the case if p was actually equal to some value in the histogram
and
+ // just increment the value there
+ mergeElements()
+ }
+
+ /** Merges the histogram with h and returns a histogram with capacity B
+ *
+ * @param h histogram to be merged
+ * @param B capacity of the resultant histogram
+ * @return Merged histogram with capacity B
+ */
+ override def merge(h: OnlineHistogram, B: Int): ContinuousHistogram = {
+ h match {
+ case temp: ContinuousHistogram => {
+ val m: Int = bins
+ val n: Int = temp.bins
+ var i, j: Int = 0
+ val mergeList = new mutable.ArrayBuffer[(Double, Int)]()
+ while (i < m  j < n) {
+ if (i >= m) {
+ mergeList += ((temp.getValue(j), temp.getCounter(j)))
+ j = j + 1
+ } else if (j >= n  getValue(i) <= temp.getValue(j)) {
+ mergeList += data.apply(i)
+ i = i + 1
+ } else {
+ mergeList += ((temp.getValue(j), temp.getCounter(j)))
+ j = j + 1
+ }
+ }
+ // the size will be brought to capacity while constructing the new histogram
itself
+ val finalLower = Math.min(lower, temp.lower)
+ val finalUpper = Math.max(upper, temp.upper)
+ val ret = new ContinuousHistogram(B, finalLower, finalUpper)
+ ret.loadData(mergeList.toArray)
+ ret
+ }
+ case default =>
+ throw new RuntimeException("Only a continuous histogram is allowed to be merged
with a " +
+ "continuous histogram")
+
+ }
+ }
+
+ /** Returns the qth quantile of the histogram
+ *
+ * @param q Quantile value in (0,1)
+ * @return Value at quantile q
+ */
+ def quantile(q: Double): Double = {
+ require(bins > 0, "Histogram is empty")
+ require(q > 0 && q < 1, "Quantile must be between 0 and 1")
+ val wantedSum = (q * total).round.toInt
+ var currSum = count(getValue(0))
+
+ if (wantedSum < currSum) {
+ require(lower > MaxValue, "Set a lower bound before proceeding")
 End diff 
Ah. Apologies. I was drawing the analogy from Java.

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