Michael McCandless created LUCENE-6595:
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Summary: CharFilter offsets correction is wonky
Key: LUCENE-6595
URL: https://issues.apache.org/jira/browse/LUCENE-6595
Project: Lucene - Core
Issue Type: Bug
Reporter: Michael McCandless
Spinoff from this original Elasticsearch issue:
https://github.com/elastic/elasticsearch/issues/11726
If I make a MappingCharFilter with these mappings:
{noformat}
( ->
) ->
{noformat}
i.e., just erase left and right paren, then tokenizing the string
"(F31)" with e.g. WhitespaceTokenizer, produces a single token F31,
with start offset 1 (good).
But for its end offset, I would expect/want 4, but it produces 5
today.
This can be easily explained given how the mapping works: each time a
mapping rule matches, we update the cumulative offset difference,
conceptually as an array like this (it's encoded more compactly):
{noformat}
Output offset: 0 1 2 3
Input offset: 1 2 3 5
{noformat}
When the tokenizer produces F31, it assigns it startOffset=0 and
endOffset=3 based on the characters it sees (F, 3, 1). It then asks
the CharFilter to correct those offsets, mapping them backwards
through the above arrays, which creates startOffset=1 (good) and
endOffset=5 (bad).
At first, to fix this, I thought this is an "off-by-1" and when
correcting the endOffset we really should return
1+correct(outputEndOffset-1), which would return the correct value (4)
here.
But that's too naive, e.g. here's another example:
{noformat}
cccc -> cc
{noformat}
If I then tokenize cccc, today we produce the correct offsets (0, 4)
but if we do this "off-by-1" fix for endOffset, we would get the wrong
endOffset (2).
I'm not sure what to do here...
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