Class FSTTermsWriter

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:

• .tst: Term Dictionary

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 TermData
• TermData --> Flag, BytesSize?, LongDelta^LongsSize?, Byte^BytesSize?, < DocFreq[Same?], (TotalTermFreq-DocFreq) > ?
• Header --> CodecHeader (WriteHeader(DataOutput, String, Int32))
• DirOffset --> Uint64 (WriteInt64(Int64))
• DocFreq, LongsSize, BytesSize, NumFields, FieldNumber, DocCount --> VInt (WriteVInt32(Int32))
• TotalTermFreq, NumTerms, SumTotalTermFreq, SumDocFreq, LongDelta --> VLong (WriteVInt64(Int64))

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.

@lucene.experimental