Github user dianfu commented on a diff in the pull request:

    https://github.com/apache/flink/pull/4331#discussion_r130266394
  
    --- Diff: 
flink-libraries/flink-cep/src/main/java/org/apache/flink/cep/nfa/NFA.java ---
    @@ -551,18 +579,47 @@ private boolean isSelfIgnore(final StateTransition<T> 
edge) {
                                                                nextVersion,
                                                                startTimestamp);
                                        }
    +
    +                                   switch (skipStrategy.getStrategy()) {
    +                                           case SKIP_PAST_LAST_EVENT:
    +                                                   if 
(nextState.isFinal()) {
    +                                                           
resultingComputationStates.add(createStartComputationState(computationState, 
event));
    +                                                   }
    +                                                   break;
    +                                           case SKIP_TO_FIRST:
    +                                                   if 
(nextState.getName().equals(skipStrategy.getPatternName()) &&
    +                                                           
!nextState.getName().equals(currentState.getName())) {
    +                                                           
ComputationState<T> startComputationState = 
createStartComputationState(computationState, event);
    +                                                           if (callLevel > 
0) {
    +                                                                   throw 
new RuntimeException("infinite loop! Will abort the match process, please 
rewrite your pattern query");
    +                                                           }
    +                                                           // feed current 
matched event to the state.
    +                                                           
Collection<ComputationState<T>> computationStates = 
computeNextStates(startComputationState, event, timestamp, callLevel++);
    +                                                           
resultingComputationStates.addAll(computationStates);
    +                                                   } else if 
(previousState == null && 
currentState.getName().equals(skipStrategy.getPatternName())) {
    +                                                           throw new 
RuntimeException("infinite loop! Will abort the match process, please rewrite 
your pattern query");
    +                                                   }
    +                                                   break;
    +                                           case SKIP_TO_LAST:
    +                                                   if 
(currentState.getName().equals(skipStrategy.getPatternName()) &&
    +                                                           
!nextState.getName().equals(currentState.getName())) {
    +                                                           
ComputationState<T> startComputationState = 
createStartComputationState(computationState, event);
    +                                                           if (callLevel > 
0) {
    +                                                                   throw 
new RuntimeException("infinite loop! Will abort the match process, please 
rewrite your pattern query");
    +                                                           }
    +                                                           // feed current 
matched event to the state.
    +                                                           
Collection<ComputationState<T>> computationStates = 
computeNextStates(startComputationState, event, timestamp, callLevel++);
    +                                                           
resultingComputationStates.addAll(computationStates);
    +                                                   }
    +                                                   break;
    +                                   }
                                        break;
                        }
                }
     
    -           if (computationState.isStartState()) {
    -                   int totalBranches = calculateIncreasingSelfState(
    -                                   outgoingEdges.getTotalIgnoreBranches(),
    -                                   outgoingEdges.getTotalTakeBranches());
    -
    -                   DeweyNumber startVersion = 
computationState.getVersion().increase(totalBranches);
    -                   ComputationState<T> startState = 
ComputationState.createStartState(this, computationState.getState(), 
startVersion);
    -                   resultingComputationStates.add(startState);
    +           if (computationState.isStartState() &&
    +                   skipStrategy.getStrategy() == 
AfterMatchSkipStrategy.SkipStrategy.SKIP_TO_NEXT_EVENT) {
    --- End diff --
    
    If the skip strategy is  @**SKIP_PAST_LAST_EVENT**, for pattern **a nextby 
b** and inputs **a1 c a2 b2**, the output should be **a2 b2**, but per the 
current implementation, I'm afraid there will be no output as after **a1** is 
processed, the start ComputationState will be consumed, but there is no chance 
to add the start ComputationState later and so **a2 b2** can not be matched. 
Right? For other skip strategy, I'm afraid there may be also such issues.


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