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, TermFSTNumFields, 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, MetaLongsSkipDeltaLongsSize >NumTerms
  - StatsBlock --> < DocFreq[Same?], (TotalTermFreq-DocFreq) ? > NumTerms
  - MetaLongsBlock --> < LongDeltaLongsSize, 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

    All Methods Instance Methods Concrete 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`

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