[GitHub] pnowojski commented on a change in pull request #6781: [FLINK-10470] Add method to check if pattern can produce empty matches

[GitBox <gi...@apache.org>]

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_r221577049
 
 

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 File path: flink-libraries/flink-cep/src/main/java/org/apache/flink/cep/nfa/compiler/NFACompiler.java
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 @@ -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:
   Can we loop here forever if `PROCEED` takes us to already visited node? Shouldn't we check for already visited states? 

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