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https://issues.apache.org/jira/browse/FLINK-10470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16638074#comment-16638074
]
ASF GitHub Bot commented on FLINK-10470:
----------------------------------------
pnowojski commented on a change in pull request #6781: [FLINK-10470] Add method
to check if pattern can produce empty matches
URL: https://github.com/apache/flink/pull/6781#discussion_r222623342
##########
File path:
flink-libraries/flink-cep/src/main/java/org/apache/flink/cep/nfa/compiler/NFACompiler.java
##########
@@ -74,6 +77,36 @@
}
}
+ /**
+ * Verifies if the provided pattern can possibly generate empty match.
Example of patterns that can possibly
+ * generate empty matches are: A*, A?, A* B? etc.
+ *
+ * @param pattern pattern to check
+ * @return true if empty match could potentially match the pattern,
false otherwise
+ */
+ public static boolean canProduceEmptyMatches(final Pattern<?, ?>
pattern) {
+ NFAFactoryCompiler<?> compiler = new
NFAFactoryCompiler<>(checkNotNull(pattern));
+ compiler.compileFactory();
+ State<?> startState =
compiler.getStates().stream().filter(State::isStart).findFirst().get();
+
+ final Stack<State<?>> statesToCheck = new Stack<>();
+ statesToCheck.push(startState);
+ while (!statesToCheck.isEmpty()) {
+ final State<?> currentState = statesToCheck.pop();
+ for (StateTransition<?> transition :
currentState.getStateTransitions()) {
+ if (transition.getAction() ==
StateTransitionAction.PROCEED) {
+ if
(transition.getTargetState().isFinal()) {
+ return true;
+ } else {
+
statesToCheck.push(transition.getTargetState());
Review comment:
It would be better to at least throw `IllegalStateException` instead of
looping forever in case of some bug or if someone will refactor/add new feature
to the compiler.
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> Add method to check if pattern can produce empty matches
> --------------------------------------------------------
>
> Key: FLINK-10470
> URL: https://issues.apache.org/jira/browse/FLINK-10470
> Project: Flink
> Issue Type: Sub-task
> Components: CEP
> Reporter: Dawid Wysakowicz
> Assignee: Dawid Wysakowicz
> Priority: Major
> Labels: pull-request-available
>
> There is couple of inconsistencies how CEP library handles greedy and
> reluctant operators at the beginning at end of pattern. This results in
> subtle problems how empty matches should be generated for patterns like e.g.
> A? or A*?, where one is greedy and the other one is reluctant. In order to
> provide first version of MATCH_RECOGNIZE function we should have a
> possibility to disable patterns which can produce empty matches.
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