Package org.apache.lucene.backward_codecs.lucene80

# Class Lucene80DocValuesFormat

java.lang.Object
org.apache.lucene.codecs.DocValuesFormat
org.apache.lucene.backward_codecs.lucene80.Lucene80DocValuesFormat

All Implemented Interfaces:
  `NamedSPILoader.NamedSPI`

public final class Lucene80DocValuesFormat extends DocValuesFormat
Lucene 8.0 DocValues format.

Documents that have a value for the field are encoded in a way that it is always possible to know the ordinal of the current document in the set of documents that have a value. For instance, say the set of documents that have a value for the field is `{1, 5, 6, 11}`. When the iterator is on `6`, it knows that this is the 3rd item of the set. This way, values can be stored densely and accessed based on their index at search time. If all documents in a segment have a value for the field, the index is the same as the doc ID, so this case is encoded implicitly and is very fast at query time. On the other hand if some documents are missing a value for the field then the set of documents that have a value is encoded into blocks. All doc IDs that share the same upper 16 bits are encoded into the same block with the following strategies:

- SPARSE: This strategy is used when a block contains at most 4095 documents. The lower 16 bits of doc IDs are stored as `shorts` while the upper 16 bits are given by the block ID.
- DENSE: This strategy is used when a block contains between 4096 and 65535 documents. The lower bits of doc IDs are stored in a bit set. Advancing < 512 documents is performed using `ntz` operations while the index is computed by accumulating the `bit counts` of the visited longs. Advancing >= 512 documents is performed by skipping to the start of the needed 512 document sub-block and iterating to the specific document within that block. The index for the sub-block that is skipped to is retrieved from a rank-table positioned beforethe bit set. The rank-table holds the origo index numbers for all 512 documents sub-blocks, represented as an unsigned short for each 128 blocks.
- ALL: This strategy is used when a block contains exactly 65536 documents, meaning that the block is full. In that case doc IDs do not need to be stored explicitly. This is typically faster than both SPARSE and DENSE which is a reason why it is preferable to have all documents that have a value for a field using contiguous doc IDs, for instance by using `index sorting`.

Skipping blocks to arrive at a wanted document is either done on an iterative basis or by using the jump-table stored at the end of the chain of blocks. The jump-table holds the offset as well as the index for all blocks, packed in a single long per block.

Then the five per-document value types (Numeric,Binary,Sorted,SortedSet,SortedNumeric) are encoded using the following strategies:

`NUMERIC`:

- Delta-compressed: per-document integers written as deltas from the minimum value, compressed with bitpacking. For more information, see `LegacyDirectWriter`.
- Table-compressed: when the number of unique values is very small (< 256), and when there are unused "gaps" in the range of values used (such as `SmallFloat`), a lookup table is written instead. Each per-document entry is instead the ordinal to this table, and those ordinals are compressed with bitpacking (`LegacyDirectWriter`).
- GCD-compressed: 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.
- Monotonic-compressed: when all numbers are monotonically increasing offsets, they are written as blocks of bitpacked integers, encoding the deviation from the expected delta.
- Const-compressed: when there is only one possible value, no per-document data is needed and this value is encoded alone.

Depending on calculated gains, the numbers might be split into blocks of 16384 values. In that case, a jump-table with block offsets is appended to the blocks for O(1) access to the needed block.

`BINARY`:

- Fixed-width Binary: one large concatenated byte[] is written, along with the fixed length. Each document's value can be addressed directly with multiplication (`docID * length`).
- Variable-width Binary: one large concatenated byte[] is written, along with end addresses for each document. The addresses are written as Monotonic-compressed numerics.
- Prefix-compressed Binary: values are written in chunks of 16, with the first value written completely and other values sharing prefixes. chunk addresses are written as Monotonic-compressed numerics. A reverse lookup index is written from a portion of every 1024th term.

`SORTED`:

- Sorted: a mapping of ordinals to deduplicated terms is written as Prefix-compressed Binary, along with the per-document ordinals written using one of the numeric strategies above.

`SORTED_SET`:

- Single: if all documents have 0 or 1 value, then data are written like SORTED.
- SortedSet: a mapping of ordinals to deduplicated terms is written as Binary, an ordinal list and per-document index into this list are written using the numeric strategies above.

`SORTED_NUMERIC`:

- Single: if all documents have 0 or 1 value, then data are written like NUMERIC.
- SortedNumeric: a value list and per-document index into this list are written using the numeric strategies above.

Files:

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

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

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `static enum`
  `Lucene80DocValuesFormat.Mode`
  Configuration option for doc values.

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `static final String`
  `MODE_KEY`
  Attribute key for compression mode.

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `Lucene80DocValuesFormat()`
  Default constructor.
  `Lucene80DocValuesFormat(Lucene80DocValuesFormat.Mode mode)`
  Constructor

- ## Method Summary

  Modifier and Type
  Method
  Description
  `DocValuesConsumer`
  `fieldsConsumer(SegmentWriteState state)`
  Note: although this format is only used on older versions, we need to keep the write logic in addition to the read logic.
  `DocValuesProducer`
  `fieldsProducer(SegmentReadState state)`

  ### Methods inherited from class org.apache.lucene.codecs.DocValuesFormat

  `availableDocValuesFormats, forName, getName, reloadDocValuesFormats, toString`

  ### Methods inherited from class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Field Details

  - ### MODE_KEY

    public static final String MODE_KEY
    Attribute key for compression mode.

- ## Constructor Details

  - ### Lucene80DocValuesFormat

    public Lucene80DocValuesFormat()
    Default constructor.

  - ### Lucene80DocValuesFormat

    public Lucene80DocValuesFormat(Lucene80DocValuesFormat.Mode mode)
    Constructor

- ## Method Details

  - ### fieldsConsumer

    public DocValuesConsumer fieldsConsumer(SegmentWriteState state) throws IOException
    Note: although this format is only used on older versions, we need to keep the write logic in addition to the read logic. It's possible for doc values on older segments to be written to through doc values updates.
    Specified by:
      `fieldsConsumer` in class `DocValuesFormat`
    Throws:
      `IOException`

  - ### fieldsProducer

    public DocValuesProducer fieldsProducer(SegmentReadState state) throws IOException
    Specified by:
      `fieldsProducer` in class `DocValuesFormat`
    Throws:
      `IOException`