Dawid Weiss created LUCENE-5977:
-----------------------------------
Summary: IW should safeguard against token streams returning
invalid offsets for multi-valued fields
Key: LUCENE-5977
URL: https://issues.apache.org/jira/browse/LUCENE-5977
Project: Lucene - Core
Issue Type: Improvement
Affects Versions: 4.10.1, 4.10, 4.9.1, 4.9
Reporter: Dawid Weiss
Priority: Minor
We have a custom token stream that emits information about offsets of each
token. My (wrong) assumption was that for multi-valued fields a token stream's
offset information is magically shifted, much like this is the case with
positions. It's not the case -- offsets should be increasing and monotonic
across all instances of a field, even if it has a custom token streams. So,
something like this:
{code}
doc.add(new Field("field-foo", new CannedTokenStream(token("bar", 1,
150, 160)), ftype));
doc.add(new Field("field-foo", new CannedTokenStream(token("bar", 1,
50, 60)), ftype));
{code}
where the token function is defined as:
{code}
token(String image, int positionIncrement, int startOffset, int endOffset)
{code}
will result in either a cryptic assertion thrown from IW:
{code}
Exception in thread "main" java.lang.AssertionError
at
org.apache.lucene.index.FreqProxTermsWriterPerField.writeOffsets(FreqProxTermsWriterPerField.java:99)
{code}
or nothing (or a codec error) if run without assertions.
Obviously returning non-shifted offsets from subsequent token streams makes
little sense but I wonder if it could be made more explicit (or asserted) that
offsets need to be increasing between multiple-values. The minimum is to add
some documentation to OffsetAttribute. I don't know if offsets should be
shifted automatically, as it's the case with positions -- this would change the
semantics of existing tokenizers and filters which implement such shifting
internally already.
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