Github user ChengXiangLi commented on a diff in the pull request:
https://github.com/apache/flink/pull/949#discussion_r37373497
--- Diff:
flink-core/src/main/java/org/apache/flink/api/common/operators/util/PoissonSampler.java
---
@@ -0,0 +1,109 @@
+/*
+ * 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.api.common.operators.util;
+
+import com.google.common.base.Preconditions;
+import org.apache.commons.math3.distribution.PoissonDistribution;
+
+import java.util.Iterator;
+
+/**
+ * A sampler implementation based on Poisson Distribution. While sampling
elements with fraction and replacement,
+ * the selected number of each element follows a given poisson
distribution, so we could use poisson
+ * distribution to generate random variables for sample.
+ *
+ * @param <T> The type of sample.
+ * @see <a
href="https://en.wikipedia.org/wiki/Poisson_distribution";>https://en.wikipedia.org/wiki/Poisson_distribution</a>
+ */
+public class PoissonSampler<T> extends RandomSampler<T> {
+
+ private PoissonDistribution poissonDistribution;
+ private final double fraction;
+
+ /**
+ * Create a poisson sampler which would sample elements with
replacement.
+ *
+ * @param fraction The expected count of each element.
+ * @param seed Random number generator seed for internal
PoissonDistribution.
+ */
+ public PoissonSampler(double fraction, long seed) {
+ Preconditions.checkArgument(fraction >= 0, "fraction should be
positive.");
+ this.fraction = fraction;
+ if (this.fraction > 0) {
+ this.poissonDistribution = new
PoissonDistribution(fraction);
+ this.poissonDistribution.reseedRandomGenerator(seed);
+ }
+ }
+
+ /**
+ * Create a poisson sampler which would sample elements with
replacement.
+ *
+ * @param fraction The expected count of each element.
+ */
+ public PoissonSampler(double fraction) {
+ Preconditions.checkArgument(fraction >= 0, "fraction should be
non-negative.");
+ this.fraction = fraction;
+ if (this.fraction > 0) {
+ this.poissonDistribution = new
PoissonDistribution(fraction);
+ }
+ }
+
+ /**
+ * Sample the input elements, for each input element, generate its
count with poisson distribution random variables generation.
+ *
+ * @param input Elements to be sampled.
+ * @return The sampled result which is lazy computed upon input
elements.
+ */
+ @Override
+ public Iterator<T> sample(final Iterator<T> input) {
+ if (fraction == 0) {
+ return EMPTY_ITERABLE;
+ }
+
+ return new SampledIterator<T>() {
+ T currentElement;
+ int currentCount = 0;
+
+ @Override
+ public boolean hasNext() {
+ if (currentElement == null || currentCount ==
0) {
+ while (input.hasNext()) {
+ currentElement = input.next();
+ currentCount =
poissonDistribution.sample();
+ if (currentCount > 0) {
+ return true;
+ }
+ }
+ return false;
+ }
+ return true;
+ }
+
+ @Override
+ public T next() {
+ T result = currentElement;
+ if (currentCount == 0) {
+ currentElement = null;
+ return null;
+ }
--- End diff --
Thanks for the review, @tillrohrmann , I refactor this iterator, add else
condition branch as you mentioned. The previous one lazily computer next
element during hasNext(), which make this iterator only work while use invoke
hasNext() at first, and then next(), fix this issue in latest commit as well.
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at infrastruct...@apache.org or file a JIRA ticket
with INFRA.
---
