org.apache.lucene.codecs.memory

Class FSTTermsWriter

java.lang.Object

FieldsConsumer

org.apache.lucene.codecs.memory.FSTTermsWriter

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:

• .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 --> FST<TermData>
• TermData --> Flag, BytesSize?, LongDelta^LongsSize?, Byte^BytesSize?, < 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.

Field Summary

Fields

Modifier and Type	Field and Description
static int	TERMS_VERSION_CURRENT
static int	TERMS_VERSION_START

Constructor Summary

Constructors

Constructor and Description
FSTTermsWriter(SegmentWriteState state, PostingsWriterBase postingsWriter)

Method Summary

Modifier and Type	Method and Description
void	close()
void	write(Fields fields, NormsProducer norms)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

TERMS_VERSION_START

public static final int TERMS_VERSION_START

See Also:

Constant Field Values

TERMS_VERSION_CURRENT

public static final int TERMS_VERSION_CURRENT

See Also:

Constant Field Values

Constructor Detail

FSTTermsWriter

public FSTTermsWriter(SegmentWriteState state,
                      PostingsWriterBase postingsWriter)
               throws java.io.IOException

Throws:

java.io.IOException

Method Detail

write

public void write(Fields fields,
                  NormsProducer norms)
           throws java.io.IOException

Throws:

java.io.IOException

close

public void close()
           throws java.io.IOException

Throws:

java.io.IOException