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 custom token streams. So, something like this:
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));
where the token function is defined as:
token(String image, int positionIncrement, int startOffset, int endOffset)
will result in either a cryptic assertion thrown from IW:
Exception in thread "main" java.lang.AssertionError at org.apache.lucene.index.FreqProxTermsWriterPerField.writeOffsets(FreqProxTermsWriterPerField.java:99)
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.