Lucene 4.2
term vectors format
.
Very similarly to Lucene41StoredFieldsFormat
, this format is based
on compressed chunks of data, with documentlevel granularity so that a
document can never span across distinct chunks. Moreover, data is made as
compact as possible:
 textual data is compressed using the very light,
LZ4 compression algorithm,
 binary data is written using fixedsize blocks of
packed ints
.
Term vectors are stored using two files
 a data file where terms, frequencies, positions, offsets and payloads
are stored,
 an index file, loaded into memory, used to locate specific documents in
the data file.
Looking up term vectors for any document requires at most 1 disk seek.
File formats

A vector data file (extension .tvd). This file stores terms,
frequencies, positions, offsets and payloads for every document. Upon writing
a new segment, it accumulates data into memory until the buffer used to store
terms and payloads grows beyond 4KB. Then it flushes all metadata, terms
and positions to disk using LZ4
compression for terms and payloads and
blocks of packed ints
for positions.
Here is a more detailed description of the field data file format:
 VectorData (.tvd) > <Header>, PackedIntsVersion, ChunkSize, <Chunk>^{ChunkCount}, Footer
 Header >
CodecHeader
 PackedIntsVersion >
PackedInts.VERSION_CURRENT
as a VInt
 ChunkSize is the number of bytes of terms to accumulate before flushing, as a
VInt
 ChunkCount is not known in advance and is the number of chunks necessary to store all document of the segment
 Chunk > DocBase, ChunkDocs, < NumFields >, < FieldNums >, < FieldNumOffs >, < Flags >,
< NumTerms >, < TermLengths >, < TermFreqs >, < Positions >, < StartOffsets >, < Lengths >,
< PayloadLengths >, < TermAndPayloads >
 DocBase is the ID of the first doc of the chunk as a
VInt
 ChunkDocs is the number of documents in the chunk
 NumFields > DocNumFields^{ChunkDocs}
 DocNumFields is the number of fields for each doc, written as a
VInt
if ChunkDocs==1 and as a PackedInts
array otherwise
 FieldNums > FieldNumDelta^{TotalDistincFields}, a deltaencoded list of the sorted unique field numbers present in the chunk
 FieldNumOffs > FieldNumOff^{TotalFields}, as a
PackedInts
array
 FieldNumOff is the offset of the field number in FieldNums
 TotalFields is the total number of fields (sum of the values of NumFields)
 Flags > Bit < FieldFlags >
 Bit is a single bit which when true means that fields have the same options for every document in the chunk
 FieldFlags > if Bit==1: Flag^{TotalDistinctFields} else Flag^{TotalFields}
 Flag: a 3bits int where:
 the first bit means that the field has positions
 the second bit means that the field has offsets
 the third bit means that the field has payloads
 NumTerms > FieldNumTerms^{TotalFields}
 FieldNumTerms: the number of terms for each field, using
blocks of 64 packed ints
 TermLengths > PrefixLength^{TotalTerms} SuffixLength^{TotalTerms}
 TotalTerms: total number of terms (sum of NumTerms)
 PrefixLength: 0 for the first term of a field, the common prefix with the previous term otherwise using
blocks of 64 packed ints
 SuffixLength: length of the term minus PrefixLength for every term using
blocks of 64 packed ints
 TermFreqs > TermFreqMinus1^{TotalTerms}
 TermFreqMinus1: (frequency  1) for each term using
blocks of 64 packed ints
 Positions > PositionDelta^{TotalPositions}
 TotalPositions is the sum of frequencies of terms of all fields that have positions
 PositionDelta: the absolute position for the first position of a term, and the difference with the previous positions for following positions using
blocks of 64 packed ints
 StartOffsets > (AvgCharsPerTerm^{TotalDistinctFields}) StartOffsetDelta^{TotalOffsets}
 TotalOffsets is the sum of frequencies of terms of all fields that have offsets
 AvgCharsPerTerm: average number of chars per term, encoded as a float on 4 bytes. They are not present if no field has both positions and offsets enabled.
 StartOffsetDelta: (startOffset  previousStartOffset  AvgCharsPerTerm * PositionDelta). previousStartOffset is 0 for the first offset and AvgCharsPerTerm is 0 if the field has no positions using
blocks of 64 packed ints
 Lengths > LengthMinusTermLength^{TotalOffsets}
 LengthMinusTermLength: (endOffset  startOffset  termLength) using
blocks of 64 packed ints
 PayloadLengths > PayloadLength^{TotalPayloads}
 TotalPayloads is the sum of frequencies of terms of all fields that have payloads
 PayloadLength is the payload length encoded using
blocks of 64 packed ints
 TermAndPayloads > LZ4compressed representation of < FieldTermsAndPayLoads >^{TotalFields}
 FieldTermsAndPayLoads > Terms (Payloads)
 Terms: term bytes
 Payloads: payload bytes (if the field has payloads)
 Footer >
CodecFooter

An index file (extension .tvx).