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https://issues.apache.org/jira/browse/LUCENE-7921?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16119548#comment-16119548
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Dawid Weiss commented on LUCENE-7921:
-------------------------------------

Automaton determinisation is a long topic... And has been for a long time too. 
:)

I quickly looked at the code - the difference is in:
{code}
  private Automaton toAutomatonInternal(Map<String,Automaton> automata,
      AutomatonProvider automaton_provider, int maxDeterminizedStates)
{code}

the two representations will undergo different paths. I wonder if we could 
minimize subautomatons before we apply repeat, for example here (but in 
multiple places, really):
{code}
      case REGEXP_REPEAT_MINMAX:
        a = Operations.repeat(
          exp1.toAutomatonInternal(automata, automaton_provider,
            maxDeterminizedStates),
          min,
          max);
        a = MinimizationOperations.minimize(a, maxDeterminizedStates);
        break;
{code}



> More efficient way to transform a RegExp to an Automaton
> --------------------------------------------------------
>
>                 Key: LUCENE-7921
>                 URL: https://issues.apache.org/jira/browse/LUCENE-7921
>             Project: Lucene - Core
>          Issue Type: Improvement
>    Affects Versions: 6.5.1
>            Reporter: Thomas Poppe
>            Priority: Minor
>
> Consider the following example:
> {code:title=ToAutomatonExample.java|borderStyle=solid}
>     public static void main(String[] args) {
>         org.apache.lucene.util.automaton.RegExp regExp =
>                 new 
> org.apache.lucene.util.automaton.RegExp("[a-z]{1,13}x[a-z][a-z]?[a-z]?[a-z]?[a-z]?[a-z]{0,8}");
>         org.apache.lucene.util.automaton.Automaton automaton = 
> regExp.toAutomaton();
>         System.out.println("states: " + automaton.getNumStates());
>         System.out.println("transitions: " + automaton.getNumTransitions());
>         System.out.println("-------------------------------");
>         try {
>             regExp = new 
> org.apache.lucene.util.automaton.RegExp("[a-z]{1,13}x[a-z]{1,13}");
>             automaton = regExp.toAutomaton();
>             System.out.println("Will not happen...");
>         } catch 
> (org.apache.lucene.util.automaton.TooComplexToDeterminizeException e) {
>             automaton = regExp.toAutomaton(1_000_000);
>             System.out.println("states: " + automaton.getNumStates());
>             System.out.println("transitions: " + 
> automaton.getNumTransitions());
>             System.out.println("-------------------------------");
>         }
>     }
> {code}
> Both regular expressions are equivalent, but it's much more efficient to 
> "unroll" the repetition.  It might be possible to optimize the 
> Regex#toAutomaton() method to handle this repetition without going over the 
> default number of determinized states, and using less memory and CPU?



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