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Date: Mon, 21 Feb 2011 08:51:38 +0000 (UTC)
From: "Dawid Weiss (JIRA)" <jira@apache.org>
To: dev@lucene.apache.org
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Subject: [jira] Created: (LUCENE-2933) Two-stage state expansion for the
 FST: distance-from-root and child-count criteria.
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Two-stage state expansion for the FST: distance-from-root and child-count criteria.
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                 Key: LUCENE-2933
                 URL: https://issues.apache.org/jira/browse/LUCENE-2933
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Index
            Reporter: Dawid Weiss
            Priority: Trivial
             Fix For: 4.0


In the current implementation FST states are expanded into a binary search on labels (from a vector of transitions) when the child count of a state exceeds a given predefined threshold (NUM_ARCS_FIXED_ARRAY). This threshold affects automaton size and traversal speed (as it turns out when benchmarked). For some degenerate (?) data sets, close-to-the-root nodes could have a small number of children (below the threshold) and yet be traversed on every single seek.

A fix of this is to introduce two control thresholds: 
  EXPAND state if (distance-from-root <= MIN_DISTANCE || children-count >= NUM_ARCS_FIXED_ARRAY)

My plan is to create a data set that will prove this first and then to implement the workaround above.

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