[jira] [Created] (LUCENE-7921) More efficient way to transform a RegExp to an Automaton

[Thomas Poppe (JIRA)]

Thomas Poppe created LUCENE-7921:
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             Summary: 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:

    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("-------------------------------");
        }
    }

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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