org.apache.lucene.codecs.memory

Class FSTTermsWriter

java.lang.Object

org.apache.lucene.codecs.FieldsConsumer

org.apache.lucene.codecs.memory.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:

• .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) Write all fields, terms and postings.

Methods inherited from class org.apache.lucene.codecs.FieldsConsumer

merge

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 IOException

Throws:

IOException

Method Detail

write

public void write(Fields fields)
           throws IOException

Description copied from class: FieldsConsumer

Write all fields, terms and postings. This the "pull" API, allowing you to iterate more than once over the postings, somewhat analogous to using a DOM API to traverse an XML tree.

Notes:

• You must compute index statistics, including each Term's docFreq and totalTermFreq, as well as the summary sumTotalTermFreq, sumTotalDocFreq and docCount.
• You must skip terms that have no docs and fields that have no terms, even though the provided Fields API will expose them; this typically requires lazily writing the field or term until you've actually seen the first term or document.
• The provided Fields instance is limited: you cannot call any methods that return statistics/counts; you cannot pass a non-null live docs when pulling docs/positions enums.

Specified by:

write in class FieldsConsumer

Throws:

IOException

close

public void close()
           throws IOException

Specified by:

close in interface Closeable

Specified by:

close in interface AutoCloseable

Specified by:

close in class FieldsConsumer

Throws:

IOException