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
`protected int` `numberOfSkipLevels` number of levels in this skip list

### Constructor Summary

Constructors
`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

All Methods Instance Methods Abstract Methods Concrete Methods
`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 final 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`