Package 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?, 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.

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

## Field Summary

Fields
Modifier and Type
Field
Description
`static final int`
`TERMS_VERSION_CURRENT`
`static final int`
`TERMS_VERSION_START`

## Constructor Summary

Constructors
Constructor
Description
`FSTTermsWriter(SegmentWriteState state, PostingsWriterBase postingsWriter)`

## Method Summary

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

### 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 Details

### 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 Details

### FSTTermsWriter

public FSTTermsWriter(SegmentWriteState state, PostingsWriterBase postingsWriter) throws IOException

Throws:
  `IOException`

## Method Details

### write

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

Specified by:
  `write` in class `FieldsConsumer`
Throws:
  `IOException`

### close

public void close() throws IOException

Specified by:
  `close` in interface `AutoCloseable`
Specified by:
  `close` in interface `Closeable`
Specified by:
  `close` in class `FieldsConsumer`
Throws:
  `IOException`