Class FlattenGraphFilter

All Implemented Interfaces:
Closeable, AutoCloseable, Unwrappable<TokenStream>

public final class FlattenGraphFilter extends TokenFilter
Converts an incoming graph token stream, such as one from SynonymGraphFilter, into a flat form so that all nodes form a single linear chain with no side paths. Every path through the graph touches every node. This is necessary when indexing a graph token stream, because the index does not save PositionLengthAttribute and so it cannot preserve the graph structure. However, at search time, query parsers can correctly handle the graph and this token filter should not be used.

If the graph was not already flat to start, this is likely a lossy process, i.e. it will often cause the graph to accept token sequences it should not, and to reject token sequences it should not.

However, when applying synonyms during indexing, this is necessary because Lucene already does not index a graph and so the indexing process is already lossy (it ignores the PositionLengthAttribute).

WARNING: This API is experimental and might change in incompatible ways in the next release.