Namespace Lucene.Net.Codecs.Lucene42

Classes

Lucene42Codec

Implements the Lucene 4.2 index format, with configurable per-field postings and docvalues formats.

If you want to reuse functionality of this codec in another codec, extend FilterCodec.

See Lucene.Net.Codecs.Lucene42 package documentation for file format details.

@lucene.experimental

Lucene42DocValuesFormat

Lucene 4.2 DocValues format.

Encodes the four per-document value types (Numeric,Binary,Sorted,SortedSet) with seven basic strategies.

  • Delta-compressed Numerics: per-document integers written in blocks of 4096. For each block the minimum value is encoded, and each entry is a delta from that minimum value.
  • Table-compressed Numerics: when the number of unique values is very small, a lookup table is written instead. Each per-document entry is instead the ordinal to this table.
  • Uncompressed Numerics: when all values would fit into a single byte, and the acceptableOverheadRatio would pack values into 8 bits per value anyway, they are written as absolute values (with no indirection or packing) for performance.
  • GCD-compressed Numerics: when all numbers share a common divisor, such as dates, the greatest common denominator (GCD) is computed, and quotients are stored using Delta-compressed Numerics.
  • Fixed-width Binary: one large concatenated byte[] is written, along with the fixed length. Each document's value can be addressed by maxDoc*length.
  • Variable-width Binary: one large concatenated byte[] is written, along with end addresses for each document. The addresses are written in blocks of 4096, with the current absolute start for the block, and the average (expected) delta per entry. For each document the deviation from the delta (actual - expected) is written.
  • Sorted: an FST mapping deduplicated terms to ordinals is written, along with the per-document ordinals written using one of the numeric strategies above.
  • SortedSet: an FST mapping deduplicated terms to ordinals is written, along with the per-document ordinal list written using one of the binary strategies above.

Files:

  1. .dvd: DocValues data
  2. .dvm: DocValues metadata

  1. The DocValues metadata or .dvm file.

    For DocValues field, this stores metadata, such as the offset into the DocValues data (.dvd)

    DocValues metadata (.dvm) --> Header,<FieldNumber,EntryType,Entry>NumFields,Footer

    Sorted fields have two entries: a SortedEntry with the FST metadata, and an ordinary NumericEntry for the document-to-ord metadata.

    SortedSet fields have two entries: a SortedEntry with the FST metadata, and an ordinary BinaryEntry for the document-to-ord-list metadata.

    FieldNumber of -1 indicates the end of metadata.

    EntryType is a 0 (NumericEntry), 1 (BinaryEntry, or 2 (SortedEntry)

    DataOffset is the pointer to the start of the data in the DocValues data (.dvd)

    CompressionType indicates how Numeric values will be compressed:

    • 0 --> delta-compressed. For each block of 4096 integers, every integer is delta-encoded from the minimum value within the block.
    • 1 --> table-compressed. When the number of unique numeric values is small and it would save space, a lookup table of unique values is written, followed by the ordinal for each document.
    • 2 --> uncompressed. When the acceptableOverheadRatio parameter would upgrade the number of bits required to 8, and all values fit in a byte, these are written as absolute binary values for performance.
    • 3 --> gcd-compressed. When all integers share a common divisor, only quotients are stored using blocks of delta-encoded ints.

    MinLength and MaxLength represent the min and max byte[] value lengths for Binary values. If they are equal, then all values are of a fixed size, and can be addressed as DataOffset + (docID * length). Otherwise, the binary values are of variable size, and packed integer metadata (PackedVersion,BlockSize) is written for the addresses.

  2. The DocValues data or .dvd file.

    For DocValues field, this stores the actual per-document data (the heavy-lifting)

    DocValues data (.dvd) --> Header,<NumericData | BinaryData | SortedData>NumFields,Footer

    SortedSet entries store the list of ordinals in their BinaryData as a sequences of increasing vLongs (WriteVInt64(Int64)), delta-encoded.

Limitations:

Lucene42FieldInfosFormat

Lucene 4.2 Field Infos format.

Field names are stored in the field info file, with suffix .fnm.

FieldInfos (.fnm) --> Header,FieldsCount, <FieldName,FieldNumber, FieldBits,DocValuesBits,Attributes> FieldsCount

Data types:

Field Descriptions:
  • FieldsCount: the number of fields in this file.
  • FieldName: name of the field as a UTF-8 String.
  • FieldNumber: the field's number. Note that unlike previous versions of Lucene, the fields are not numbered implicitly by their order in the file, instead explicitly.
  • FieldBits: a byte containing field options.
    • The low-order bit is one for indexed fields, and zero for non-indexed fields.
    • The second lowest-order bit is one for fields that have term vectors stored, and zero for fields without term vectors.
    • If the third lowest order-bit is set (0x4), offsets are stored into the postings list in addition to positions.
    • Fourth bit is unused.
    • If the fifth lowest-order bit is set (0x10), norms are omitted for the indexed field.
    • If the sixth lowest-order bit is set (0x20), payloads are stored for the indexed field.
    • If the seventh lowest-order bit is set (0x40), term frequencies and positions omitted for the indexed field.
    • If the eighth lowest-order bit is set (0x80), positions are omitted for the indexed field.
  • DocValuesBits: a byte containing per-document value types. The type recorded as two four-bit integers, with the high-order bits representing norms options, and the low-order bits representing DocValues options. Each four-bit integer can be decoded as such:
    • 0: no DocValues for this field.
    • 1: NumericDocValues. (NUMERIC)
    • 2: BinaryDocValues. (BINARY)
    • 3: SortedDocValues. (SORTED)
  • Attributes: a key-value map of codec-private attributes.

@lucene.experimental

Lucene42FieldInfosWriter

Lucene 4.2 FieldInfos writer.

Lucene42NormsFormat

Lucene 4.2 score normalization format.

NOTE: this uses the same format as Lucene42DocValuesFormat Numeric DocValues, but with different file extensions, and passing FASTEST for uncompressed encoding: trading off space for performance.

Files:

  • .nvd: DocValues data
  • .nvm: DocValues metadata

Lucene42RWCodec

Read-write version of for testing.

Lucene42RWDocValuesFormat

Read-write version of for testing.

Lucene42TermVectorsFormat

Lucene 4.2 term vectors format (TermVectorsFormat).

Very similarly to Lucene41StoredFieldsFormat, this format is based on compressed chunks of data, with document-level 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 fixed-size blocks of packed s (PackedInt32s).

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

  1. 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 s (BlockPackedWriter) for positions.

    Here is a more detailed description of the field data file format:

    • VectorData (.tvd) --> <Header>, PackedIntsVersion, ChunkSize, <Chunk>ChunkCount, Footer
    • Header --> CodecHeader (WriteHeader(DataOutput, String, Int32))
    • PackedIntsVersion --> VERSION_CURRENT as a VInt (WriteVInt32(Int32))
    • ChunkSize is the number of bytes of terms to accumulate before flushing, as a VInt (WriteVInt32(Int32))
    • 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 (WriteVInt32(Int32))
    • ChunkDocs is the number of documents in the chunk
    • NumFields --> DocNumFieldsChunkDocs
    • DocNumFields is the number of fields for each doc, written as a VInt (WriteVInt32(Int32)) if ChunkDocs==1 and as a PackedInt32s array otherwise
    • FieldNums --> FieldNumDeltaTotalDistincFields, a delta-encoded list of the sorted unique field numbers present in the chunk
    • FieldNumOffs --> FieldNumOffTotalFields, as a PackedInt32s 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: FlagTotalDistinctFields else FlagTotalFields
    • Flag: a 3-bits 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 --> FieldNumTermsTotalFields
    • FieldNumTerms: the number of terms for each field, using blocks of 64 packed s (BlockPackedWriter)
    • TermLengths --> PrefixLengthTotalTerms SuffixLengthTotalTerms
    • 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 s (BlockPackedWriter)
    • SuffixLength: length of the term minus PrefixLength for every term using blocks of 64 packed s (BlockPackedWriter)
    • TermFreqs --> TermFreqMinus1TotalTerms
    • TermFreqMinus1: (frequency - 1) for each term using blocks of 64 packed s (BlockPackedWriter)
    • Positions --> PositionDeltaTotalPositions
    • 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 s (BlockPackedWriter)
    • StartOffsets --> (AvgCharsPerTermTotalDistinctFields) StartOffsetDeltaTotalOffsets
    • 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 s (BlockPackedWriter)
    • Lengths --> LengthMinusTermLengthTotalOffsets
    • LengthMinusTermLength: (endOffset - startOffset - termLength) using blocks of 64 packed s (BlockPackedWriter)
    • PayloadLengths --> PayloadLengthTotalPayloads
    • 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 s (BlockPackedWriter)
    • TermAndPayloads --> LZ4-compressed representation of < FieldTermsAndPayLoads >TotalFields
    • FieldTermsAndPayLoads --> Terms (Payloads)
    • Terms: term bytes
    • Payloads: payload bytes (if the field has payloads)
    • Footer --> CodecFooter (WriteFooter(IndexOutput))

  2. An index file (extension .tvx).

@lucene.experimental