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
########## 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: Can we loop here forever if `PROCEED` takes us to already visited node? Shouldn't we check for already visited states? ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org With regards, Apache Git Services