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## org.apache.lucene.codecs
Class BlockTreeTermsReader

    java.lang.Object
      org.apache.lucene.index.Fields
          org.apache.lucene.codecs.FieldsProducer
              org.apache.lucene.codecs.BlockTreeTermsReader

All Implemented Interfaces:
  Closeable, Iterable<String>

    public class BlockTreeTermsReader

extends FieldsProducer

A block-based terms index and dictionary that assigns terms to variable length blocks according to how they share prefixes. The terms index is a prefix trie whose leaves are term blocks. The advantage of this approach is that seekExact is often able to determine a term cannot exist without doing any IO, and intersection with Automata is very fast. Note that this terms dictionary has it's own fixed terms index (ie, it does not support a pluggable terms index implementation).

NOTE: this terms dictionary does not support index divisor when opening an IndexReader. Instead, you can change the min/maxItemsPerBlock during indexing.

The data structure used by this implementation is very similar to a burst trie (http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.18.3499), but with added logic to break up too-large blocks of all terms sharing a given prefix into smaller ones.

Use `CheckIndex` with the `-verbose` option to see summary statistics on the blocks in the dictionary. See `BlockTreeTermsWriter`.

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

Nested Class Summary
`class` | `BlockTreeTermsReader.FieldReader` BlockTree's implementation of `Terms`.
`static class` | `BlockTreeTermsReader.Stats` BlockTree statistics for a single field returned by `BlockTreeTermsReader.FieldReader.computeStats()`.

Field Summary

Fields inherited from class org.apache.lucene.index.Fields
`EMPTY_ARRAY`

Constructor Summary
`BlockTreeTermsReader(Directory dir, FieldInfos fieldInfos, SegmentInfo info, PostingsReaderBase postingsReader, IOContext ioContext, String segmentSuffix, int indexDivisor)` Sole constructor.

Method Summary
`void` | `close()`
`Iterator<String>` | `iterator()` Returns an iterator that will step through all fields names.
`protected int` | `readHeader(IndexInput input)` Reads terms file header.
`protected int` | `readIndexHeader(IndexInput input)` Reads index file header.
`protected void` | `seekDir(IndexInput input, long dirOffset)` Seek `input` to the directory offset.
`int` | `size()` Returns the number of fields or -1 if the number of distinct field names is unknown.
`Terms` | `terms(String field)` Get the `Terms` for this field.

Methods inherited from class org.apache.lucene.index.Fields
`getUniqueTermCount`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### BlockTreeTermsReader

    public BlockTreeTermsReader(Directory dir,
                                FieldInfos fieldInfos,
                                SegmentInfo info,
                                PostingsReaderBase postingsReader,
                                IOContext ioContext,
                                String segmentSuffix,
                                int indexDivisor)
                         throws IOException

  Sole constructor.

  Throws:
    `IOException`

Method Detail

### readHeader

    protected int readHeader(IndexInput input)
                      throws IOException

  Reads terms file header.

  Throws:
    `IOException`

### readIndexHeader

    protected int readIndexHeader(IndexInput input)
                           throws IOException

  Reads index file header.

  Throws:
    `IOException`

### seekDir

    protected void seekDir(IndexInput input,
                           long dirOffset)
                    throws IOException

  Seek `input` to the directory offset.

  Throws:
    `IOException`

### close

    public void close()
               throws IOException

  Specified by:
    `close` in interface `Closeable`
  Specified by:
    `close` in class `FieldsProducer`
  Throws:
    `IOException`

### iterator

    public Iterator<String> iterator()

  Description copied from class: `Fields`
  Returns an iterator that will step through all fields names. This will not return null.

  Specified by:
    `iterator` in interface `Iterable<String>`
  Specified by:
    `iterator` in class `Fields`

### terms

    public Terms terms(String field)
                throws IOException

  Description copied from class: `Fields`
  Get the `Terms` for this field. This will return null if the field does not exist.

  Specified by:
    `terms` in class `Fields`
  Throws:
    `IOException`

### size

    public int size()

  Description copied from class: `Fields`
  Returns the number of fields or -1 if the number of distinct field names is unknown. If >= 0, `Fields.iterator()` will return as many field names.

  Specified by:
    `size` in class `Fields`

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