[jira] [Created] (LUCENE-5316) Taxonomy tree traversing improvement

["Gilad Barkai (JIRA)" <ji...@apache.org>]

Gilad Barkai created LUCENE-5316:
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             Summary: Taxonomy tree traversing improvement
                 Key: LUCENE-5316
                 URL: https://issues.apache.org/jira/browse/LUCENE-5316
             Project: Lucene - Core
          Issue Type: Improvement
          Components: modules/facet
            Reporter: Gilad Barkai
            Priority: Minor


The taxonomy traversing is done today utilizing the {{ParallelTaxonomyArrays}}. In particular, two taxonomy-size {{int}} arrays which hold for each ordinal it's (array #1) youngest child and (array #2) older sibling.

This is a compact way of holding the tree information in memory, but it's not perfect:
* Large (8 bytes per ordinal in memory)
* Exposes internal implementation
* Utilizing these arrays for tree traversing is not straight forward
* Lose reference locality while traversing (the array is accessed in increasing only entries, but they may be distant from one another)
* In NRT, a reopen is always (not worst case) done at O(Taxonomy-size)

This issue is about making the traversing more easy, the code more readable, and open it for future improvements (i.e memory footprint and NRT cost) - without changing any of the internals. 
A later issue(s?) could be opened to address the gaps once this one is done.



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