org.apache.lucene.codecs.memory

Class FSTOrdTermsWriter

java.lang.Object

org.apache.lucene.codecs.FieldsConsumer

org.apache.lucene.codecs.memory.FSTOrdTermsWriter

All Implemented Interfaces:

Closeable, AutoCloseable

public class FSTOrdTermsWriter
extends FieldsConsumer

FST-based term dict, using ord as FST output. The FST holds the mapping between <term, ord>, and term's metadata is delta encoded into a single byte block. Typically the byte block consists of four 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 customized by postings base. 4. single-level skip list to speed up metadata decoding by ord.

Files:

• .tix: Term Index
• .tbk: Term Block

Term Index

The .tix contains a list of FSTs, one for each field. The FST maps a term to its corresponding order in current field.

• TermIndex(.tix) --> Header, TermFST^NumFields, Footer
• TermFST --> FST<long>
• Header --> IndexHeader
• Footer --> CodecFooter

Notes:

• Since terms are already sorted before writing to Term Block, their ords can directly used to seek term metadata from term block.

Term Block

The .tbk contains all the statistics and metadata for terms, along with field summary (e.g. per-field data like number of documents in current field). For each field, there are four blocks:

• statistics bytes block: contains term statistics;
• metadata longs block: delta-encodes monotonic part of metadata;
• metadata bytes block: encodes other parts of metadata;
• skip block: contains skip data, to speed up metadata seeking and decoding

File Format:

• TermBlock(.tbk) --> Header, PostingsHeader, FieldSummary, DirOffset
• FieldSummary --> NumFields, <FieldNumber, NumTerms, SumTotalTermFreq?, SumDocFreq, DocCount, LongsSize, DataBlock > ^NumFields, Footer
• DataBlock --> StatsBlockLength, MetaLongsBlockLength, MetaBytesBlockLength, SkipBlock, StatsBlock, MetaLongsBlock, MetaBytesBlock
• SkipBlock --> < StatsFPDelta, MetaLongsSkipFPDelta, MetaBytesSkipFPDelta, MetaLongsSkipDelta^LongsSize >^NumTerms
• StatsBlock --> < DocFreq[Same?], (TotalTermFreq-DocFreq) ? > ^NumTerms
• MetaLongsBlock --> < LongDelta^LongsSize, BytesSize > ^NumTerms
• MetaBytesBlock --> Byte ^{MetaBytesBlockLength}
• Header --> IndexHeader
• DirOffset --> Uint64
• NumFields, FieldNumber, DocCount, DocFreq, LongsSize, FieldNumber, DocCount --> VInt
• NumTerms, SumTotalTermFreq, SumDocFreq, StatsBlockLength, MetaLongsBlockLength, MetaBytesBlockLength, StatsFPDelta, MetaLongsSkipFPDelta, MetaBytesSkipFPDelta, MetaLongsSkipStart, TotalTermFreq, LongDelta,--> VLong
• Footer --> CodecFooter

Notes:

• The format of PostingsHeader and MetaBytes 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).
• During initialization the reader will load all the blocks into memory. SkipBlock will be decoded, so that during seek term dict can lookup file pointers directly. StatsFPDelta, MetaLongsSkipFPDelta, etc. are file offset for every SkipInterval's term. MetaLongsSkipDelta is the difference from previous one, which indicates the value of preceding metadata longs for every SkipInterval's term.
• DocFreq is the count of documents which contain the term. TotalTermFreq is the total number of occurrences of the term. Usually these two values are the same for long tail terms, therefore one bit is stole from DocFreq to check this case, so that encoding of TotalTermFreq may be omitted.

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

Field Summary

Fields

Modifier and Type	Field and Description
static int	SKIP_INTERVAL
static int	VERSION_CURRENT
static int	VERSION_START

Constructor Summary

Constructors

Constructor and Description
FSTOrdTermsWriter(SegmentWriteState state, PostingsWriterBase postingsWriter)

Method Summary

Methods

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

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

VERSION_START

public static final int VERSION_START

See Also:

Constant Field Values

VERSION_CURRENT

public static final int VERSION_CURRENT

See Also:

Constant Field Values

SKIP_INTERVAL

public static final int SKIP_INTERVAL

See Also:

Constant Field Values

Constructor Detail

FSTOrdTermsWriter

public FSTOrdTermsWriter(SegmentWriteState state,
                 PostingsWriterBase postingsWriter)
                  throws IOException

Throws:

IOException

Method Detail

write

public void write(Fields fields)
           throws IOException

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