Github user chiwanpark commented on a diff in the pull request:
https://github.com/apache/flink/pull/861#discussion_r37137605
--- Diff:
flink-staging/flink-ml/src/main/scala/org/apache/flink/ml/statistics/CategoricalHistogram.scala
---
@@ -0,0 +1,135 @@
+/*
+ * 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 org.apache.flink.ml.statistics
+
+import scala.collection.mutable
+
+/** Implementation of a discrete valued online histogram
+ *
+ * =Parameters=
+ * -[[numCategories]]:
+ * Number of categories in the histogram
+ */
+case class CategoricalHistogram(numCategories: Int) extends
OnlineHistogram {
+
+ require(numCategories > 0, "Capacity must be greater than zero")
+ val data = new mutable.HashMap[Double, Int]()
+
+ /** Number of categories in the histogram
+ *
+ * @return number of categories
+ */
+ override def bins: Int = {
+ numCategories
+ }
+
+ /** Increment count of category c
+ *
+ * @param c category whose count needs to be incremented
+ */
+ override def add(c: Double): Unit = {
+ if (data.get(c).isEmpty) {
+ require(data.size < numCategories, "Insufficient capacity. Failed to
add.")
+ data.put(c, 1)
+ } else {
+ data.update(c, data.get(c).get + 1)
+ }
+ }
+
+ /** Merges the histogram with h and returns a new histogram
+ *
+ * @param h histogram to be merged
+ * @param B number of categories in the resultant histogram.
+ * (Default: ```0```, number of categories will be the size of
union of categories in
+ * both histograms)
+ * @return Merged histogram with capacity B
+ */
+ override def merge(h: OnlineHistogram, B: Int = 0): CategoricalHistogram
= {
+ h match {
+ case h1: CategoricalHistogram => {
+ val finalMap = new mutable.HashMap[Double, Int]()
+ data.iterator.foreach(x => finalMap.put(x._1, x._2))
+ h1.data.iterator.foreach(x => {
+ if (finalMap.get(x._1).isEmpty) {
+ finalMap.put(x._1, x._2)
+ } else {
+ finalMap.update(x._1, x._2 + finalMap.get(x._1).get)
+ }
+ })
--- End diff --
As I said above, we can reduce calling `get(x._1)` by using pattern
matching for `finalMap.get(x._1)`.
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