org.apache.lucene.codecs

## Class MultiLevelSkipListWriter

- java.lang.Object
- - org.apache.lucene.codecs.MultiLevelSkipListWriter

-     public abstract class MultiLevelSkipListWriter
      extends Object

  This abstract class writes skip lists with multiple levels.

       Example for skipInterval = 3:
                                                           c            (skip level 2)
                       c                 c                 c            (skip level 1)
           x     x     x     x     x     x     x     x     x     x      (skip level 0)
       d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d  (posting list)
           3     6     9     12    15    18    21    24    27    30     (df)

       d - document
       x - skip data
       c - skip data with child pointer

       Skip level i contains every skipInterval-th entry from skip level i-1.
       Therefore the number of entries on level i is: floor(df / ((skipInterval ^ (i + 1))).

       Each skip entry on a level i>0 contains a pointer to the corresponding skip entry in list i-1.
       This guarantees a logarithmic amount of skips to find the target document.

       While this class takes care of writing the different skip levels,
       subclasses must define the actual format of the skip data.

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

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected int` | `numberOfSkipLevels` number of levels in this skip list

  - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `MultiLevelSkipListWriter(int skipInterval, int maxSkipLevels, int df)` Creates a `MultiLevelSkipListWriter`, where `skipInterval` and `skipMultiplier` are the same.
    `protected` | `MultiLevelSkipListWriter(int skipInterval, int skipMultiplier, int maxSkipLevels, int df)` Creates a `MultiLevelSkipListWriter`.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `bufferSkip(int df)` Writes the current skip data to the buffers.
    `protected void` | `init()` Allocates internal skip buffers.
    `protected void` | `resetSkip()` Creates new buffers or empties the existing ones
    `long` | `writeSkip(IndexOutput output)` Writes the buffered skip lists to the given output.
    `protected abstract void` | `writeSkipData(int level, IndexOutput skipBuffer)` Subclasses must implement the actual skip data encoding in this method.

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

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

- - ### Field Detail

    - #### numberOfSkipLevels

          protected int numberOfSkipLevels

      number of levels in this skip list

  - ### Constructor Detail

    - #### MultiLevelSkipListWriter

          protected MultiLevelSkipListWriter(int skipInterval,
                                  int skipMultiplier,
                                  int maxSkipLevels,
                                  int df)

      Creates a `MultiLevelSkipListWriter`.

    - #### MultiLevelSkipListWriter

          protected MultiLevelSkipListWriter(int skipInterval,
                                  int maxSkipLevels,
                                  int df)

      Creates a `MultiLevelSkipListWriter`, where `skipInterval` and `skipMultiplier` are the same.

  - ### Method Detail

    - #### init

          protected void init()

      Allocates internal skip buffers.

    - #### resetSkip

          protected void resetSkip()

      Creates new buffers or empties the existing ones

    - #### writeSkipData

          protected abstract void writeSkipData(int level,
                           IndexOutput skipBuffer)
                                         throws IOException

      Subclasses must implement the actual skip data encoding in this method.
      Parameters:
        `level` - the level skip data shall be writing for
        `skipBuffer` - the skip buffer to write to
      Throws:
        `IOException`

    - #### bufferSkip

          public void bufferSkip(int df)
                          throws IOException

      Writes the current skip data to the buffers. The current document frequency determines the max level is skip data is to be written to.
      Parameters:
        `df` - the current document frequency
      Throws:
        `IOException` - If an I/O error occurs

    - #### writeSkip

          public long writeSkip(IndexOutput output)
                         throws IOException

      Writes the buffered skip lists to the given output.
      Parameters:
        `output` - the IndexOutput the skip lists shall be written to
      Returns:
        the pointer the skip list starts
      Throws:
        `IOException`

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