Github user dawidwys commented on a diff in the pull request:
https://github.com/apache/flink/pull/3624#discussion_r108232033
--- Diff:
flink-libraries/flink-cep/src/main/java/org/apache/flink/cep/pattern/conditions/IterativeCondition.java
---
@@ -0,0 +1,98 @@
+/*
+ * 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.cep.pattern.conditions;
+
+import org.apache.flink.api.common.functions.Function;
+
+import java.io.Serializable;
+
+/**
+ * A user-defined condition that decides if an element should be accepted
in the pattern or not.
+ * Accepting an element also signals a state transition for the
corresponding {@link org.apache.flink.cep.nfa.NFA}.
+ *
+ * <p>A condition can be a simple filter or a more complex condition that
iterates over the previously accepted
+ * elements in the pattern and decides to accept a new element or not
based on some statistic over these elements.
+ * In the former case, the condition should extend the {@link
SimpleCondition} class. In the later, the condition
+ * should extend this class, which gives you also access to the previously
accepted elements through a {@link Context}.
+ *
+ * <p>An iterative condition that accepts an element if i) its name is
middle, and ii) the sum of the prices of all
+ * accepted elements is less than {@code 5} would look like:
+ *
+ * <pre>
+ * {@code
+ * private class MyCondition extends IterativeCondition<Event> {
+ *
+ * @Override
+ * public boolean filter(Event value, Context<Event> ctx) throws
Exception {
+ * if (!value.getName().equals("middle")) {
+ * return false;
+ * }
+ *
+ * double sum = 0.0;
+ * for (Event e: ctx.getEventsForPattern("middle")) {
+ * sum += e.getPrice();
+ * }
+ * sum += value.getPrice();
+ * return Double.compare(sum, 5.0) <= 0;
+ * }
+ * }
+ * }
+ * </pre>
+ *
+ * <b>ATTENTION: </b> The call to {@link
Context#getEventsForPattern(String) getEventsForPattern(...)} has to find
+ * the elements that belong to the pattern among the elements stored by
the NFA. The cost of this operation can vary,
+ * so when implementing your condition, try to minimize the times the
method is called.
+ */
+public abstract class IterativeCondition<T> implements Function,
Serializable {
+
+ private static final long serialVersionUID = 7067817235759351255L;
+
+ /**
+ * The filter function that evaluates the predicate.
+ * <p>
+ * <strong>IMPORTANT:</strong> The system assumes that the function
does not
+ * modify the elements on which the predicate is applied. Violating
this assumption
+ * can lead to incorrect results.
+ *
+ * @param value The value to be tested.
+ * @param ctx The {@link Context} used for the evaluation of the
function and provides access to
+ * the already accepted events in the pattern (see {@link
Context#getEventsForPattern(String)}).
+ * @return {@code true} for values that should be retained, {@code
false}
+ * for values to be filtered out.
+ *
+ * @throws Exception This method may throw exceptions. Throwing an
exception will cause the operation
+ * to fail and may trigger recovery.
+ */
+ public abstract boolean filter(T value, Context<T> ctx) throws
Exception;
+
+ /**
+ * The context used when evaluating the {@link IterativeCondition
condition}.
+ */
+ public interface Context<T> extends Serializable {
+
+ /**
+ * @return An {@link Iterable} over the already accepted
elements
+ * for a given pattern. Elements are iterated in the order the
were
--- End diff --
misspell : the -> they
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