org.apache.lucene.codecs.memory

Class FSTTermsWriter

  • All Implemented Interfaces:
    Closeable, AutoCloseable


    public class FSTTermsWriter
    extends FieldsConsumer
    FST-based term dict, using metadata as FST output. The FST directly holds the mapping between <term, metadata>. Term metadata consists of three parts: 1. term statistics: docFreq, totalTermFreq; 2. monotonic long[], e.g. the pointer to the postings list for that term; 3. generic byte[], e.g. other information need by postings reader.

    File:

    Term Dictionary

    The .tst contains a list of FSTs, one for each field. The FST maps a term to its corresponding statistics (e.g. docfreq) and metadata (e.g. information for postings list reader like file pointer to postings list).

    Typically the metadata is separated into two parts:

    • Monotonical long array: Some metadata will always be ascending in order with the corresponding term. This part is used by FST to share outputs between arcs.
    • Generic byte array: Used to store non-monotonic metadata.
    File format:
    • TermsDict(.tst) --> Header, PostingsHeader, FieldSummary, DirOffset
    • FieldSummary --> NumFields, <FieldNumber, NumTerms, SumTotalTermFreq?, SumDocFreq, DocCount, LongsSize, TermFST >NumFields
    • TermFST --> FST<TermData>
    • TermData --> Flag, BytesSize?, LongDeltaLongsSize?, ByteBytesSize?, < DocFreq[Same?], (TotalTermFreq-DocFreq) > ?
    • Header --> IndexHeader
    • DirOffset --> Uint64
    • DocFreq, LongsSize, BytesSize, NumFields, FieldNumber, DocCount --> VInt
    • TotalTermFreq, NumTerms, SumTotalTermFreq, SumDocFreq, LongDelta --> VLong

    Notes:

    • The format of PostingsHeader and generic meta bytes are customized by the specific postings implementation: they contain arbitrary per-file data (such as parameters or versioning information), and per-term data (non-monotonic ones like pulsed postings data).
    • The format of TermData is determined by FST, typically monotonic metadata will be dense around shallow arcs, while in deeper arcs only generic bytes and term statistics exist.
    • The byte Flag is used to indicate which part of metadata exists on current arc. Specially the monotonic part is omitted when it is an array of 0s.
    • Since LongsSize is per-field fixed, it is only written once in field summary.