C# (CSharp) Lucene.Net.Codecs.Memory Namespace

WARNING: This is exceptionally RAM intensive: it makes no effort to compress the postings data, storing terms as separate byte[] and postings as separate int[], but as a result it gives substantial increase in search performance.

This postings format supports TermsEnum#ord and TermsEnum#seekExact(long).

Because this holds all term bytes as a single byte[], you cannot have more than 2.1GB worth of term bytes in a single segment. @lucene.experimental

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
• Header --> CodecUtil#writeHeader CodecHeader
• Footer --> CodecUtil#writeFooter 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 --> CodecUtil#writeHeader CodecHeader
• DirOffset --> DataOutput#writeLong Uint64
• NumFields, FieldNumber, DocCount, DocFreq, LongsSize, FieldNumber, DocCount --> DataOutput#writeVInt VInt
• NumTerms, SumTotalTermFreq, SumDocFreq, StatsBlockLength, MetaLongsBlockLength, MetaBytesBlockLength, StatsFPDelta, MetaLongsSkipFPDelta, MetaBytesSkipFPDelta, MetaLongsSkipStart, TotalTermFreq, LongDelta,--> DataOutput#writeVLong VLong
• Footer --> CodecUtil#writeFooter 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.

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.

• 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 --> CodecUtil#writeHeader CodecHeader
• DirOffset --> DataOutput#writeLong Uint64
• DocFreq, LongsSize, BytesSize, NumFields, FieldNumber, DocCount --> DataOutput#writeVInt VInt
• TotalTermFreq, NumTerms, SumTotalTermFreq, SumDocFreq, LongDelta --> DataOutput#writeVLong 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.