<?xml version="1.0"?>

<document>
    <header>
        <title>
            Apache Lucene - Index File Formats
        </title>
    </header>

    <body>
        <section id="Index File Formats"><title>Index File Formats</title>

            <p>
                This document defines the index file formats used
                in this version of Lucene. If you are using a different
                version of Lucene, please consult the copy of
                <code>docs/fileformats.html</code>
                that was distributed
                with the version you are using.
            </p>

            <p>
                Apache Lucene is written in Java, but several
                efforts are underway to write
                <a href="http://wiki.apache.org/lucene-java/LuceneImplementations">versions
                    of Lucene in other programming
                languages</a>.  If these versions are to remain compatible with Apache
                Lucene, then a language-independent definition of the Lucene index
                format is required.  This document thus attempts to provide a
                complete and independent definition of the Apache Lucene file
                formats.
            </p>

            <p>
                As Lucene evolves, this document should evolve.
                Versions of Lucene in different programming languages should endeavor
                to agree on file formats, and generate new versions of this document.
            </p>

            <p>
                Compatibility notes are provided in this document,
                describing how file formats have changed from prior versions.
            </p>

            <p>
                In version 2.1, the file format was changed to allow
                lock-less commits (ie, no more commit lock). The
                change is fully backwards compatible: you can open a
                pre-2.1 index for searching or adding/deleting of
                docs. When the new segments file is saved
                (committed), it will be written in the new file format
                (meaning no specific "upgrade" process is needed).
                But note that once a commit has occurred, pre-2.1
                Lucene will not be able to read the index.
            </p>

            <p>
                In version 2.3, the file format was changed to allow
		segments to share a single set of doc store (vectors &amp;
		stored fields) files.  This allows for faster indexing
		in certain cases.  The change is fully backwards
		compatible (in the same way as the lock-less commits
		change in 2.1).
            </p>

            <p>
	        In version 2.4, Strings are now written as true UTF-8
	        byte sequence, not Java's modified UTF-8.  See issue
	        LUCENE-510 for details.
            </p>

	    <p>
	        In version 2.9, an optional opaque Map&lt;String,String&gt;
	        CommitUserData may be passed to IndexWriter's commit
	        methods (and later retrieved), which is recorded in
	        the segments_N file.  See issue LUCENE-1382 for
	        details.  Also, diagnostics were added to each segment
	        written recording details about why it was written
	        (due to flush, merge; which OS/JRE was used; etc.).
	        See issue LUCENE-1654 for details.
            </p>
	    
	    <p>
	        In version 3.0, compressed fields are no longer
	        written to the index (they can still be read, but on
	        merge the new segment will write them,
	        uncompressed). See issue LUCENE-1960 for details.
            </p>

        <p>
            In version 3.1, segments records the code version
            that created them. See LUCENE-2720 for details.
            
            Additionally segments track explicitly whether or
            not they have term vectors. See LUCENE-2811 for details.
           </p>
        <p>
            In version 3.2, numeric fields are written as natively
            to stored fields file, previously they were stored in
            text format only.
           </p>
        <p>
            In version 3.4, fields can omit position data while
            still indexing term frequencies.
        </p>
        </section>

        <section id="Definitions"><title>Definitions</title>

            <p>
                The fundamental concepts in Lucene are index,
                document, field and term.
            </p>


            <p>
                An index contains a sequence of documents.
            </p>

            <ul>
                <li>
                    <p>
                        A document is a sequence of fields.
                    </p>
                </li>

                <li>
                    <p>
                        A field is a named sequence of terms.
                    </p>
                </li>

                <li>
                    A term is a string.
                </li>
            </ul>

            <p>
                The same string in two different fields is
                considered a different term.  Thus terms are represented as a pair of
                strings, the first naming the field, and the second naming text
                within the field.
            </p>

            <section id="Inverted Indexing"><title>Inverted Indexing</title>

                <p>
                    The index stores statistics about terms in order
                    to make term-based search more efficient.  Lucene's
                    index falls into the family of indexes known as an <i>inverted
                        index.</i> This is because it can list, for a term, the documents that contain
                    it.  This is the inverse of the natural relationship, in which
                    documents list terms.
                </p>
            </section>
            <section id="Types of Fields">
                <title>Types of Fields</title>
                <p>
                    In Lucene, fields may be <i>stored</i>, in which
                    case their text is stored in the index literally, in a non-inverted
                    manner.  Fields that are inverted are called <i>indexed</i>. A field
                    may be both stored and indexed.</p>

                <p>The text of a field may be <i>tokenized</i> into terms to be
                    indexed, or the text of a field may be used literally as a term to be indexed.
                    Most fields are
                    tokenized, but sometimes it is useful for certain identifier fields
                    to be indexed literally.
                </p>
                <p>See the <a href="api/core/org/apache/lucene/document/Field.html">Field</a> java docs for more information on Fields.</p>
            </section>

            <section id="Segments"><title>Segments</title>

                <p>
                    Lucene indexes may be composed of multiple sub-indexes, or
                    <i>segments</i>. Each segment is a fully independent index, which could be searched
                    separately. Indexes evolve by:
                </p>

                <ol>
                    <li>
                        <p>Creating new segments for newly added documents.</p>
                    </li>
                    <li>
                        <p>Merging existing segments.</p>
                    </li>
                </ol>

                <p>
                    Searches may involve multiple segments and/or multiple indexes, each
                    index potentially composed of a set of segments.
                </p>
            </section>

            <section id="Document Numbers"><title>Document Numbers</title>

                <p>
                    Internally, Lucene refers to documents by an integer <i>document
                        number</i>. The first document added to an index is numbered zero, and each
                    subsequent document added gets a number one greater than the previous.
                </p>

                <p>
                    <br/>
                </p>

                <p>
                    Note that a document's number may change, so caution should be taken
                    when storing these numbers outside of Lucene. In particular, numbers may
                    change in the following situations:
                </p>


                <ul>
                    <li>
                        <p>
                            The
                            numbers stored in each segment are unique only within the segment,
                            and must be converted before they can be used in a larger context.
                            The standard technique is to allocate each segment a range of
                            values, based on the range of numbers used in that segment.  To
                            convert a document number from a segment to an external value, the
                            segment's <i>base</i> document
                            number is added.  To convert an external value back to a
                            segment-specific value, the  segment is identified by the range that
                            the external value is in, and the segment's base value is
                            subtracted.  For example two five document segments might be
                            combined, so that the first segment has a base value of zero, and
                            the second of five.  Document three from the second segment would
                            have an external value of eight.
                        </p>
                    </li>
                    <li>
                        <p>
                            When documents are deleted, gaps are created
                            in the numbering. These are eventually removed as the index evolves
                            through merging. Deleted documents are dropped when segments are
                            merged. A freshly-merged segment thus has no gaps in its numbering.
                        </p>
                    </li>
                </ul>

            </section>

        </section>

        <section id="Overview"><title>Overview</title>

            <p>
                Each segment index maintains the following:
            </p>
            <ul>
                <li>
                    <p>Field names. This
                        contains the set of field names used in the index.

                    </p>
                </li>
                <li>
                    <p>Stored Field
                        values. This contains, for each document, a list of attribute-value
                        pairs, where the attributes are field names. These are used to
                        store auxiliary information about the document, such as its title,
                        url, or an identifier to access a
                        database. The set of stored fields are what is returned for each hit
                        when searching. This is keyed by document number.
                    </p>
                </li>
                <li>
                    <p>Term dictionary.
                        A dictionary containing all of the terms used in all of the indexed
                        fields of all of the documents. The dictionary also contains the
                        number of documents which contain the term, and pointers to the
                        term's frequency and proximity data.
                    </p>
                </li>

                <li>
                    <p>Term Frequency
                        data. For each term in the dictionary, the numbers of all the
                        documents that contain that term, and the frequency of the term in
                        that document, unless frequencies are omitted (IndexOptions.DOCS_ONLY)
                    </p>
                </li>

                <li>
                    <p>Term Proximity
                        data. For each term in the dictionary, the positions that the term
                        occurs in each document.  Note that this will
                        not exist if all fields in all documents omit position data.
                    </p>
                </li>

                <li>
                    <p>Normalization
                        factors. For each field in each document, a value is stored that is
                        multiplied into the score for hits on that field.
                    </p>
                </li>
                <li>
                    <p>Term Vectors. For each field in each document, the term vector
                        (sometimes called document vector) may be stored. A term vector consists
                        of term text and term frequency. To add Term Vectors to your index see the
                        <a href="api/core/org/apache/lucene/document/Field.html">Field</a>
                        constructors
                    </p>
                </li>
                <li>
                    <p>Deleted documents.
                        An optional file indicating which documents are deleted.
                    </p>
                </li>
            </ul>

            <p>Details on each of these are provided in subsequent sections.
            </p>
        </section>

        <section id="File Naming"><title>File Naming</title>

            <p>
                All files belonging to a segment have the same name with varying
                extensions. The extensions correspond to the different file formats
                described below. When using the Compound File format (default in 1.4 and greater) these files are
                collapsed into a single .cfs file (see below for details)
            </p>

            <p>
                Typically, all segments
                in an index are stored in a single directory, although this is not
                required.
            </p>

            <p>
                As of version 2.1 (lock-less commits), file names are
                never re-used (there is one exception, "segments.gen",
                see below). That is, when any file is saved to the
                Directory it is given a never before used filename.
                This is achieved using a simple generations approach.
                For example, the first segments file is segments_1,
                then segments_2, etc. The generation is a sequential
                long integer represented in alpha-numeric (base 36)
                form.
            </p>

        </section>
      <section id="file-names"><title>Summary of File Extensions</title>
        <p>The following table summarizes the names and extensions of the files in Lucene:
          <table>
            <tr>
              <th>Name</th>
              <th>Extension</th>
              <th>Brief Description</th>
            </tr>
            <tr>
              <td><a href="#Segments File">Segments File</a></td>
              <td>segments.gen, segments_N</td>
              <td>Stores information about segments</td>
            </tr>
            <tr>
              <td><a href="#Lock File">Lock File</a></td>
              <td>write.lock</td>
              <td>The Write lock prevents multiple IndexWriters from writing to the same file.</td>
            </tr>
            <tr>
              <td><a href="#Compound Files">Compound File</a></td>
              <td>.cfs</td>
              <td>An optional "virtual" file consisting of all the other index files for systems
              that frequently run out of file handles.</td>
            </tr>
              <tr>
              <td><a href="#Compound File">Compound File Entry table</a></td>
              <td>.cfe</td>
              <td>The "virtual" compound file's entry table holding all entries in the corresponding .cfs file (Since 3.4)</td>
            </tr>
            <tr>
              <td><a href="#Fields">Fields</a></td>
              <td>.fnm</td>
              <td>Stores information about the fields</td>
            </tr>
            <tr>
              <td><a href="#field_index">Field Index</a></td>
              <td>.fdx</td>
              <td>Contains pointers to field data</td>
            </tr>
            <tr>
              <td><a href="#field_data">Field Data</a></td>
              <td>.fdt</td>
              <td>The stored fields for documents</td>
            </tr>
            <tr>
              <td><a href="#tis">Term Infos</a></td>
              <td>.tis</td>
              <td>Part of the term dictionary, stores term info</td>
            </tr>
            <tr>
              <td><a href="#tii">Term Info Index</a></td>
              <td>.tii</td>
              <td>The index into the Term Infos file</td>
            </tr>
            <tr>
              <td><a href="#Frequencies">Frequencies</a></td>
              <td>.frq</td>
              <td>Contains the list of docs which contain each term along with frequency</td>
            </tr>
            <tr>
              <td><a href="#Positions">Positions</a></td>
              <td>.prx</td>
              <td>Stores position information about where a term occurs in the index</td>
            </tr>
            <tr>
              <td><a href="#Normalization Factors">Norms</a></td>
              <td>.nrm</td>
              <td>Encodes length and boost factors for docs and fields</td>
            </tr>
            <tr>
              <td><a href="#tvx">Term Vector Index</a></td>
              <td>.tvx</td>
              <td>Stores offset into the document data file</td>
            </tr>
            <tr>
              <td><a href="#tvd">Term Vector Documents</a></td>
              <td>.tvd</td>
              <td>Contains information about each document that has term vectors</td>
            </tr>
            <tr>
              <td><a href="#tvf">Term Vector Fields</a></td>
              <td>.tvf</td>
              <td>The field level info about term vectors</td>
            </tr>
            <tr>
              <td><a href="#Deleted Documents">Deleted Documents</a></td>
              <td>.del</td>
              <td>Info about what files are deleted</td>
            </tr>
          </table>

        </p>
      </section>

        <section id="Primitive Types"><title>Primitive Types</title>

            <section id="Byte"><title>Byte</title>

                <p>
                    The most primitive type
                    is an eight-bit byte. Files are accessed as sequences of bytes. All
                    other data types are defined as sequences
                    of bytes, so file formats are byte-order independent.
                </p>

            </section>

            <section id="UInt32"><title>UInt32</title>

                <p>
                    32-bit unsigned integers are written as four
                    bytes, high-order bytes first.
                </p>
                <p>
                    UInt32    --&gt; &lt;Byte&gt;<sup>4</sup>
                </p>

            </section>

            <section id="Uint64"><title>Uint64</title>

                <p>
                    64-bit unsigned integers are written as eight
                    bytes, high-order bytes first.
                </p>

                <p>UInt64    --&gt; &lt;Byte&gt;<sup>8</sup>
                </p>

            </section>

            <section id="VInt"><title>VInt</title>

                <p>
                    A variable-length format for positive integers is
                    defined where the high-order bit of each byte indicates whether more
                    bytes remain to be read. The low-order seven bits are appended as
                    increasingly more significant bits in the resulting integer value.
                    Thus values from zero to 127 may be stored in a single byte, values
                    from 128 to 16,383 may be stored in two bytes, and so on.
                </p>

                <p>
                    <b>VInt Encoding Example</b>
                </p>

                <table width="100%" border="0" cellpadding="4" cellspacing="0">
                    <col width="64*"/>
                    <col width="64*"/>
                    <col width="64*"/>
                    <col width="64*"/>
                    <tr valign="TOP">
                        <td width="25%">
                            <p align="RIGHT">
                                <b>Value</b>
                            </p>
                        </td>
                        <td width="25%">
                            <p align="RIGHT">
                                <b>First byte</b>
                            </p>
                        </td>
                        <td width="25%">
                            <p align="RIGHT">
                                <b>Second byte</b>
                            </p>
                        </td>
                        <td width="25%">
                            <p align="RIGHT">
                                <b>Third byte</b>
                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="0" sdnum="1033;0;#,##0">
                            <p align="RIGHT">0
                            </p>
                        </td>
                        <td width="25%" sdval="0" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                00000000
                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.07cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="1" sdnum="1033;0;#,##0">
                            <p align="RIGHT">1
                            </p>
                        </td>
                        <td width="25%" sdval="1" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                00000001
                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.07cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="2" sdnum="1033;0;#,##0">
                            <p align="RIGHT">2
                            </p>
                        </td>
                        <td width="25%" sdval="10" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                00000010
                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.07cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr>
                        <td width="25%" valign="TOP">
                            <p align="RIGHT">...
                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: 0.11cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.07cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="127" sdnum="1033;0;#,##0">
                            <p align="RIGHT">127
                            </p>
                        </td>
                        <td width="25%" sdval="1111111" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                01111111
                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.07cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="128" sdnum="1033;0;#,##0">
                            <p align="RIGHT">128
                            </p>
                        </td>
                        <td width="25%" sdval="10000000" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                10000000
                            </p>
                        </td>
                        <td width="25%" sdval="1" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.07cm;
                               margin-right: 0.01cm">
                                00000001
                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="129" sdnum="1033;0;#,##0">
                            <p align="RIGHT">129
                            </p>
                        </td>
                        <td width="25%" sdval="10000001" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                10000001
                            </p>
                        </td>
                        <td width="25%" sdval="1" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.07cm;
                               margin-right: 0.01cm">
                                00000001
                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="130" sdnum="1033;0;#,##0">
                            <p align="RIGHT">130
                            </p>
                        </td>
                        <td width="25%" sdval="10000010" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                10000010
                            </p>
                        </td>
                        <td width="25%" sdval="1" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.07cm;
                               margin-right: 0.01cm">
                                00000001
                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr>
                        <td width="25%" valign="TOP">
                            <p align="RIGHT">...
                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: 0.11cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.07cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="16383" sdnum="1033;0;#,##0">
                            <p align="RIGHT">16,383
                            </p>
                        </td>
                        <td width="25%" sdval="11111111" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                11111111
                            </p>
                        </td>
                        <td width="25%" sdval="1111111" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.07cm;
                               margin-right: 0.01cm">
                                01111111
                            </p>
                        </td>
                        <td width="25%" sdnum="1033;0;00000000">
                            <p align="RIGHT" style="margin-left: -0.47cm; margin-right:
                               0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="16384" sdnum="1033;0;#,##0">
                            <p align="RIGHT">16,384
                            </p>
                        </td>
                        <td width="25%" sdval="10000000" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                10000000
                            </p>
                        </td>
                        <td width="25%" sdval="10000000" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.07cm;
                               margin-right: 0.01cm">
                                10000000
                            </p>
                        </td>
                        <td width="25%" sdval="1" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.47cm;
                               margin-right: 0.01cm">
                                00000001
                            </p>
                        </td>
                    </tr>
                    <tr valign="BOTTOM">
                        <td width="25%" sdval="16385" sdnum="1033;0;#,##0">
                            <p align="RIGHT">16,385
                            </p>
                        </td>
                        <td width="25%" sdval="10000001" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                10000001
                            </p>
                        </td>
                        <td width="25%" sdval="10000000" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.07cm;
                               margin-right: 0.01cm">
                                10000000
                            </p>
                        </td>
                        <td width="25%" sdval="1" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.47cm;
                               margin-right: 0.01cm">
                                00000001
                            </p>
                        </td>
                    </tr>
                    <tr>
                        <td width="25%" valign="TOP">
                            <p align="RIGHT">...
                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: 0.11cm;
                               margin-right: 0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.07cm;
                               margin-right: 0.01cm">
                                <br/>

                            </p>
                        </td>
                        <td width="25%" valign="BOTTOM" sdnum="1033;0;00000000">
                            <p class="western" align="RIGHT" style="margin-left: -0.47cm;
                               margin-right: 0.01cm">
                                <br/>

                            </p>
                        </td>
                    </tr>
                </table>

                <p>
                    This provides compression while still being
                    efficient to decode.
                </p>

            </section>

            <section id="Chars"><title>Chars</title>

                <p>
                    Lucene writes unicode
                    character sequences as UTF-8 encoded bytes.
                </p>


            </section>

            <section id="String"><title>String</title>

                <p>
		    Lucene writes strings as UTF-8 encoded bytes.
                    First the length, in bytes, is written as a VInt,
                    followed by the bytes.
                </p>

                <p>
                    String --&gt; VInt, Chars
                </p>

            </section>
        </section>

        <section id="Compound Types"><title>Compound Types</title>
            <section id="MapStringString"><title>Map&lt;String,String&gt;</title>

                <p>
		    In a couple places Lucene stores a Map
                    String-&gt;String.
                </p>

                <p>
  		    Map&lt;String,String&gt; --&gt; Count&lt;String,String&gt;<sup>Count</sup>
                </p>

            </section>

        </section>

        <section id="Per-Index Files"><title>Per-Index Files</title>

            <p>
                The files in this section exist one-per-index.
            </p>

            <section id="Segments File"><title>Segments File</title>

                <p>
                    The active segments in the index are stored in the
                    segment info file,
                    <tt>segments_N</tt>.
                    There may
                    be one or more
                    <tt>segments_N</tt>
                    files in the
                    index; however, the one with the largest
                    generation is the active one (when older
                    segments_N files are present it's because they
                    temporarily cannot be deleted, or, a writer is in
                    the process of committing, or a custom
                    <a href="api/core/org/apache/lucene/index/IndexDeletionPolicy.html">IndexDeletionPolicy</a>
		    is in use). This file lists each
                    segment by name, has details about the separate
                    norms and deletion files, and also contains the
                    size of each segment.
                </p>

                <p>
                    As of 2.1, there is also a file
                    <tt>segments.gen</tt>.
                    This file contains the
                    current generation (the
                    <tt>_N</tt>
                    in
                    <tt>segments_N</tt>)
                    of the index. This is
                    used only as a fallback in case the current
                    generation cannot be accurately determined by
                    directory listing alone (as is the case for some
                    NFS clients with time-based directory cache
                    expiraation). This file simply contains an Int32
                    version header (SegmentInfos.FORMAT_LOCKLESS =
                    -2), followed by the generation recorded as Int64,
                    written twice.
                </p>
                <p>
                    <b>3.1</b>
                    Segments --&gt; Format, Version, NameCounter, SegCount, &lt;SegVersion, SegName, SegSize, DelGen, DocStoreOffset, [DocStoreSegment, DocStoreIsCompoundFile], HasSingleNormFile, NumField,
                    NormGen<sup>NumField</sup>,
                    IsCompoundFile, DeletionCount, HasProx, Diagnostics, HasVectors&gt;<sup>SegCount</sup>, CommitUserData, Checksum
                </p>

                <p>
                    Format, NameCounter, SegCount, SegSize, NumField,
                    DocStoreOffset, DeletionCount --&gt; Int32
                </p>

               <p>
                    Version, DelGen, NormGen, Checksum --&gt; Int64
                </p>

                <p>
                   SegVersion, SegName, DocStoreSegment --&gt; String
                </p>

		<p>
		   Diagnostics --&gt; Map&lt;String,String&gt;
		</p>

                <p>
                    IsCompoundFile, HasSingleNormFile,
                    DocStoreIsCompoundFile, HasProx, HasVectors --&gt; Int8
                </p>

		<p>
		    CommitUserData --&gt; Map&lt;String,String&gt;
                </p>

                <p>
                    Format is -9 (SegmentInfos.FORMAT_DIAGNOSTICS).
                </p>

                <p>
                    Version counts how often the index has been
                    changed by adding or deleting documents.
                </p>

                <p>
                    NameCounter is used to generate names for new segment files.
                </p>

                <p>
                    SegVersion is the code version that created the segment.
                </p>

                <p>
                    SegName is the name of the segment, and is used as the file name prefix
                    for all of the files that compose the segment's index.
                </p>

                <p>
                    SegSize is the number of documents contained in the segment index.
                </p>

                <p>
                    DelGen is the generation count of the separate
                    deletes file. If this is -1, there are no
                    separate deletes. If it is 0, this is a pre-2.1
                    segment and you must check filesystem for the
                    existence of _X.del. Anything above zero means
                    there are separate deletes (_X_N.del).
                </p>

                <p>
                    NumField is the size of the array for NormGen, or
                    -1 if there are no NormGens stored.
                </p>

                <p>
                    NormGen records the generation of the separate
                    norms files. If NumField is -1, there are no
                    normGens stored and they are all assumed to be 0
                    when the segment file was written pre-2.1 and all
                    assumed to be -1 when the segments file is 2.1 or
                    above. The generation then has the same meaning
                    as delGen (above).
                </p>

                <p>
                    IsCompoundFile records whether the segment is
                    written as a compound file or not. If this is -1,
                    the segment is not a compound file. If it is 1,
                    the segment is a compound file. Else it is 0,
                    which means we check filesystem to see if _X.cfs
                    exists.
                </p>

                <p>
                    If HasSingleNormFile is 1, then the field norms are
                    written as a single joined file (with extension
                    <tt>.nrm</tt>); if it is 0 then each field's norms
                    are stored as separate <tt>.fN</tt> files.  See
                    "Normalization Factors" below for details.
                </p>

                <p>
		    DocStoreOffset, DocStoreSegment,
                    DocStoreIsCompoundFile: If DocStoreOffset is -1,
                    this segment has its own doc store (stored fields
                    values and term vectors) files and DocStoreSegment
                    and DocStoreIsCompoundFile are not stored.  In
                    this case all files for stored field values
                    (<tt>*.fdt</tt> and <tt>*.fdx</tt>) and term
                    vectors (<tt>*.tvf</tt>, <tt>*.tvd</tt> and
                    <tt>*.tvx</tt>) will be stored with this segment.
                    Otherwise, DocStoreSegment is the name of the
                    segment that has the shared doc store files;
                    DocStoreIsCompoundFile is 1 if that segment is
                    stored in compound file format (as a <tt>.cfx</tt>
                    file); and DocStoreOffset is the starting document
                    in the shared doc store files where this segment's
                    documents begin.  In this case, this segment does
                    not store its own doc store files but instead
                    shares a single set of these files with other
                    segments.
                </p>

                <p>
		    Checksum contains the CRC32 checksum of all bytes
		    in the segments_N file up until the checksum.
		    This is used to verify integrity of the file on
		    opening the index.
		</p>

		<p>
		    DeletionCount records the number of deleted
		    documents in this segment.
		</p>

		<p>
		    HasProx is 1 if any fields in this segment have
		    position data (IndexOptions.DOCS_AND_FREQS_AND_POSITIONS); else, it's 0.
		</p>

		<p>
		    CommitUserData stores an optional user-supplied
		    opaque Map&lt;String,String&gt; that was passed to
		    IndexWriter's commit or prepareCommit, or
		    IndexReader's flush methods.
                </p>
		<p>
		    The Diagnostics Map is privately written by
		    IndexWriter, as a debugging aid, for each segment
		    it creates.  It includes metadata like the current
		    Lucene version, OS, Java version, why the segment
		    was created (merge, flush, addIndexes), etc.
                </p>
         
        <p> HasVectors is 1 if this segment stores term vectors,
            else it's 0.
                </p>

            </section>

            <section id="Lock File"><title>Lock File</title>

                <p>
                    The write lock, which is stored in the index
                    directory by default, is named "write.lock".  If
                    the lock directory is different from the index
                    directory then the write lock will be named
                    "XXXX-write.lock" where XXXX is a unique prefix
                    derived from the full path to the index directory.
                    When this file is present, a writer is currently
                    modifying the index (adding or removing
                    documents).  This lock file ensures that only one
                    writer is modifying the index at a time.
                </p>
            </section>

            <section id="Deletable File"><title>Deletable File</title>

                <p>
                    A writer dynamically computes
                    the files that are deletable, instead, so no file
                    is written.
                </p>

            </section>

            <section id="Compound Files"><title>Compound Files</title>

                <p>Starting with Lucene 1.4 the compound file format became default. This
                    is simply a container for all files described in the next section
					(except for the .del file).</p>
								<p>Compound Entry Table (.cfe) --&gt; Version,  FileCount, &lt;FileName, DataOffset, DataLength&gt;
                    <sup>FileCount</sup>
                </p>

                <p>Compound (.cfs) --&gt; FileData <sup>FileCount</sup>
                </p>
                
								<p>Version --&gt; Int</p>
								
                <p>FileCount --&gt; VInt</p>

                <p>DataOffset --&gt; Long</p>
                
                <p>DataLength --&gt; Long</p>

                <p>FileName --&gt; String</p>

                <p>FileData --&gt; raw file data</p>
                <p>The raw file data is the data from the individual files named above.</p>

		<p>Starting with Lucene 2.3, doc store files (stored
		field values and term vectors) can be shared in a
		single set of files for more than one segment.  When
		compound file is enabled, these shared files will be
		added into a single compound file (same format as
		above) but with the extension <tt>.cfx</tt>.
		</p>

            </section>

        </section>

        <section id="Per-Segment Files"><title>Per-Segment Files</title>

            <p>
                The remaining files are all per-segment, and are
                thus defined by suffix.
            </p>
            <section id="Fields"><title>Fields</title>
                <p>
                    <br/>
                    <b>Field Info</b>
                    <br/>
                </p>

                <p>
                    Field names are
                    stored in the field info file, with suffix .fnm.
                </p>
                <p>
                    FieldInfos
                    (.fnm) --&gt; FNMVersion,FieldsCount, &lt;FieldName,
                    FieldBits&gt;
                    <sup>FieldsCount</sup>
                </p>

                <p>
                    FNMVersion, FieldsCount --&gt; VInt
                </p>

                <p>
                    FieldName --&gt; String
                </p>

                <p>
                    FieldBits --&gt; Byte
                </p>

                <p>
                    <ul>
                        <li>
                            The low-order bit is one for
                            indexed fields, and zero for non-indexed fields.
                        </li>
                        <li>
                            The second lowest-order
                            bit is one for fields that have term vectors stored, and zero for fields
                            without term vectors.
                        </li>
                        <li>If the fifth lowest-order bit is set (0x10), norms are omitted for the indexed field.</li>
                        <li>If the sixth lowest-order bit is set (0x20), payloads are stored for the indexed field.</li>
                        <li>If the seventh lowest-order bit is set (0x40), term frequencies and positions omitted for the indexed field.</li>
                        <li>If the eighth lowest-order bit is set (0x80), positions are omitted for the indexed field.</li>
                    </ul>
                </p>

		<p>
		   FNMVersion (added in 2.9) is -2 for indexes from 2.9 - 3.3. It is -3 for indexes in Lucene 3.4+
		</p>

                <p>
                    Fields are numbered by their order in this file. Thus field zero is
                    the
                    first field in the file, field one the next, and so on. Note that,
                    like document numbers, field numbers are segment relative.
                </p>



                <p>
                    <br/>
                    <b>Stored Fields</b>
                    <br/>
                </p>

                <p>
                    Stored fields are represented by two files:
                </p>

                <ol>
                    <li><a name="field_index"/>
                        <p>
                            The field index, or .fdx file.
                        </p>

                        <p>
                            This contains, for each document, a pointer to
                            its field data, as follows:
                        </p>

                        <p>
                            FieldIndex
                            (.fdx) --&gt;
                            &lt;FieldValuesPosition&gt;
                            <sup>SegSize</sup>
                        </p>
                        <p>FieldValuesPosition
                            --&gt; Uint64
                        </p>
                        <p>This
                            is used to find the location within the field data file of the
                            fields of a particular document. Because it contains fixed-length
                            data, this file may be easily randomly accessed. The position of
                            document
                            <i>n</i>
                            's
                            <i></i>
                            field data is the Uint64 at
                            <i>n*8</i>
                            in
                            this file.
                        </p>
                    </li>
                    <li>
                        <p><a name="field_data"/>
                            The field data, or .fdt file.

                        </p>

                        <p>
                            This contains the stored fields of each document,
                            as follows:
                        </p>

                        <p>
                            FieldData (.fdt) --&gt;
                            &lt;DocFieldData&gt;
                            <sup>SegSize</sup>
                        </p>
                        <p>DocFieldData --&gt;
                            FieldCount, &lt;FieldNum, Bits, Value&gt;
                            <sup>FieldCount</sup>
                        </p>
                        <p>FieldCount --&gt;
                            VInt
                        </p>
                        <p>FieldNum --&gt;
                            VInt
                        </p>
                        <p>Bits --&gt;
                            Byte
                        </p>
                        <p>
                            <ul>
                                <li>low order bit is one for tokenized fields</li>
                                <li>second bit is one for fields containing binary data</li>
                                <li>third bit is one for fields with compression option enabled
                                    (if compression is enabled, the algorithm used is ZLIB),
                                    only available for indexes until Lucene version 2.9.x</li>
                                <li>4th to 6th bit (mask: 0x7&lt;&lt;3) define the type of a
                                numeric field: <ul>
                                  <li>all bits in mask are cleared if no numeric field at all</li>
                                  <li>1&lt;&lt;3: Value is Int</li>
                                  <li>2&lt;&lt;3: Value is Long</li>
                                  <li>3&lt;&lt;3: Value is Int as Float (as of Float.intBitsToFloat)</li>
                                  <li>4&lt;&lt;3: Value is Long as Double (as of Double.longBitsToDouble)</li>
                                </ul></li>
                            </ul>
                        </p>
                        <p>Value --&gt;
                            String | BinaryValue | Int | Long (depending on Bits)
                        </p>
                        <p>BinaryValue --&gt;
                            ValueSize, &lt;Byte&gt;^ValueSize
                        </p>
                        <p>ValueSize --&gt;
                            VInt
                        </p>

                    </li>
                </ol>

            </section>
            <section id="Term Dictionary"><title>Term Dictionary</title>

                <p>
                    The term dictionary is represented as two files:
                </p>
                <ol>
                    <li><a name="tis"/>
                        <p>
                            The term infos, or tis file.
                        </p>

                        <p>
                            TermInfoFile (.tis)--&gt;
                            TIVersion, TermCount, IndexInterval, SkipInterval, MaxSkipLevels, TermInfos
                        </p>
                        <p>TIVersion --&gt;
                            UInt32
                        </p>
                        <p>TermCount --&gt;
                            UInt64
                        </p>
                        <p>IndexInterval --&gt;
                            UInt32
                        </p>
                        <p>SkipInterval --&gt;
                            UInt32
                        </p>
                        <p>MaxSkipLevels --&gt;
                            UInt32
                        </p>
                        <p>TermInfos --&gt;
                            &lt;TermInfo&gt;
                            <sup>TermCount</sup>
                        </p>
                        <p>TermInfo --&gt;
                            &lt;Term, DocFreq, FreqDelta, ProxDelta, SkipDelta&gt;
                        </p>
                        <p>Term --&gt;
                            &lt;PrefixLength, Suffix, FieldNum&gt;
                        </p>
                        <p>Suffix --&gt;
                            String
                        </p>
                        <p>PrefixLength,
                            DocFreq, FreqDelta, ProxDelta, SkipDelta
                            <br/>
                            --&gt; VInt
                        </p>
                        <p>
			    This file is sorted by Term. Terms are
                            ordered first lexicographically (by UTF16
                            character code) by the term's field name,
                            and within that lexicographically (by
                            UTF16 character code) by the term's text.
                        </p>
                        <p>TIVersion names the version of the format
                            of this file and is equal to TermInfosWriter.FORMAT_CURRENT.
                        </p>
                        <p>Term
                            text prefixes are shared. The PrefixLength is the number of initial
                            characters from the previous term which must be pre-pended to a
                            term's suffix in order to form the term's text. Thus, if the
                            previous term's text was "bone" and the term is "boy",
                            the PrefixLength is two and the suffix is "y".
                        </p>
                        <p>FieldNumber
                            determines the term's field, whose name is stored in the .fdt file.
                        </p>
                        <p>DocFreq
                            is the count of documents which contain the term.
                        </p>
                        <p>FreqDelta
                            determines the position of this term's TermFreqs within the .frq
                            file. In particular, it is the difference between the position of
                            this term's data in that file and the position of the previous
                            term's data (or zero, for the first term in the file).
                        </p>
                        <p>ProxDelta
                            determines the position of this term's TermPositions within the .prx
                            file. In particular, it is the difference between the position of
                            this term's data in that file and the position of the previous
                            term's data (or zero, for the first term in the file.  For fields
			                that omit position data, this will be 0 since
                            prox information is not stored.
                        </p>
                        <p>SkipDelta determines the position of this
                            term's SkipData within the .frq file. In
                            particular, it is the number of bytes
                            after TermFreqs that the SkipData starts.
                            In other words, it is the length of the
                            TermFreq data. SkipDelta is only stored 
                            if DocFreq is not smaller than SkipInterval.
                        </p>
                    </li>
                    <li>
                        <p><a name="tii"/>
                            The term info index, or .tii file.
                        </p>

                        <p>
                            This contains every IndexInterval
                            <sup>th</sup>
                            entry from the .tis
                            file, along with its location in the &quot;tis&quot; file. This is
                            designed to be read entirely into memory and used to provide random
                            access to the &quot;tis&quot; file.
                        </p>

                        <p>
                            The structure of this file is very similar to the
                            .tis file, with the addition of one item per record, the IndexDelta.
                        </p>

                        <p>
                            TermInfoIndex (.tii)--&gt;
                            TIVersion, IndexTermCount, IndexInterval, SkipInterval, MaxSkipLevels, TermIndices
                        </p>
                        <p>TIVersion --&gt;
                            UInt32
                        </p>
                        <p>IndexTermCount --&gt;
                            UInt64
                        </p>
                        <p>IndexInterval --&gt;
                            UInt32
                        </p>
                        <p>SkipInterval --&gt;
                            UInt32
                        </p>
                        <p>TermIndices --&gt;
                            &lt;TermInfo, IndexDelta&gt;
                            <sup>IndexTermCount</sup>
                        </p>
                        <p>IndexDelta --&gt;
                            VLong
                        </p>
                        <p>IndexDelta
                            determines the position of this term's TermInfo within the .tis file. In
                            particular, it is the difference between the position of this term's
                            entry in that file and the position of the previous term's entry.
                        </p>
                        <p>SkipInterval is the fraction of TermDocs stored in skip tables. It is used to accelerate TermDocs.skipTo(int).
                            Larger values result in smaller indexes, greater acceleration, but fewer accelerable cases, while
                            smaller values result in bigger indexes, less acceleration (in case of a small value for MaxSkipLevels) and more
                            accelerable cases.</p>
                        <p>MaxSkipLevels is the max. number of skip levels stored for each term in the .frq file. A low value results in 
                           smaller indexes but less acceleration, a larger value results in slighly larger indexes but greater acceleration.
                           See format of .frq file for more information about skip levels.</p>
                    </li>
                </ol>
            </section>

            <section id="Frequencies"><title>Frequencies</title>

                <p>
                    The .frq file contains the lists of documents
                    which contain each term, along with the frequency of the term in that
                    document (except when frequencies are omitted: IndexOptions.DOCS_ONLY).
                </p>
                <p>FreqFile (.frq) --&gt;
                    &lt;TermFreqs, SkipData&gt;
                    <sup>TermCount</sup>
                </p>
                <p>TermFreqs --&gt;
                    &lt;TermFreq&gt;
                    <sup>DocFreq</sup>
                </p>
                <p>TermFreq --&gt;
                    DocDelta[, Freq?]
                </p>
                <p>SkipData --&gt;
                    &lt;&lt;SkipLevelLength, SkipLevel&gt;
                    <sup>NumSkipLevels-1</sup>, SkipLevel&gt;
                    &lt;SkipDatum&gt;
                </p>
                <p>SkipLevel --&gt;
                    &lt;SkipDatum&gt;
                    <sup>DocFreq/(SkipInterval^(Level + 1))</sup>
                </p>
                <p>SkipDatum --&gt;
                    DocSkip,PayloadLength?,FreqSkip,ProxSkip,SkipChildLevelPointer?
                </p>
                <p>DocDelta,Freq,DocSkip,PayloadLength,FreqSkip,ProxSkip --&gt;
                    VInt
                </p>
                <p>SkipChildLevelPointer --&gt;
                    VLong
                </p>
                <p>TermFreqs
                    are ordered by term (the term is implicit, from the .tis file).
                </p>
                <p>TermFreq
                    entries are ordered by increasing document number.
                </p>
                <p>DocDelta: if frequencies are indexed, this determines both
                    the document number and the frequency. In
                    particular, DocDelta/2 is the difference between
                    this document number and the previous document
                    number (or zero when this is the first document in
                    a TermFreqs). When DocDelta is odd, the frequency
                    is one. When DocDelta is even, the frequency is
                    read as another VInt.  If frequencies are omitted, DocDelta
                    contains the gap (not multiplied by 2) between
                    document numbers and no frequency information is
                    stored.
                </p>
                <p>For example, the TermFreqs for a term which occurs
                    once in document seven and three times in document
                    eleven, with frequencies indexed, would be the following
                    sequence of VInts:
                </p>
                <p>15, 8, 3
                </p>
		<p> If frequencies were omitted (IndexOptions.DOCS_ONLY) it would be this sequence
		of VInts instead:
		  </p>
		 <p>
		   7,4
                 </p>
                <p>DocSkip records the document number before every
                    SkipInterval
                    <sup>th</sup>
                    document in TermFreqs.
                    If payloads are disabled for the term's field,
                    then DocSkip represents the difference from the
                    previous value in the sequence.
                    If payloads are enabled for the term's field, 
                    then DocSkip/2 represents the difference from the
                    previous value in the sequence. If payloads are enabled
                    and DocSkip is odd,
                    then PayloadLength is stored indicating the length 
                    of the last payload before the SkipInterval<sup>th</sup>
                    document in TermPositions.
					FreqSkip and ProxSkip record the position of every
                    SkipInterval
                    <sup>th</sup>
                    entry in FreqFile and
                    ProxFile, respectively. File positions are
                    relative to the start of TermFreqs and Positions,
                    to the previous SkipDatum in the sequence.
                </p>
                <p>For example, if DocFreq=35 and SkipInterval=16,
                    then there are two SkipData entries, containing
                    the 15
                    <sup>th</sup>
                    and 31
                    <sup>st</sup>
                    document
                    numbers in TermFreqs. The first FreqSkip names
                    the number of bytes after the beginning of
                    TermFreqs that the 16
                    <sup>th</sup>
                    SkipDatum
                    starts, and the second the number of bytes after
                    that that the 32
                    <sup>nd</sup>
                    starts. The first
                    ProxSkip names the number of bytes after the
                    beginning of Positions that the 16
                    <sup>th</sup>
                    SkipDatum starts, and the second the number of
                    bytes after that that the 32
                    <sup>nd</sup>
                    starts.
                </p>
                <p>Each term can have multiple skip levels.
                   The amount of skip levels for a term is NumSkipLevels = Min(MaxSkipLevels, floor(log(DocFreq/log(SkipInterval)))).
                   The number of SkipData entries for a skip level is DocFreq/(SkipInterval^(Level + 1)), whereas the lowest skip
                   level is Level=0. <br></br>
                   Example: SkipInterval = 4, MaxSkipLevels = 2, DocFreq = 35. Then skip level 0 has 8 SkipData entries,
                   containing the 3<sup>rd</sup>, 7<sup>th</sup>, 11<sup>th</sup>, 15<sup>th</sup>, 19<sup>th</sup>, 23<sup>rd</sup>,
                   27<sup>th</sup>, and 31<sup>st</sup> document numbers in TermFreqs. Skip level 1 has 2 SkipData entries, containing the
                   15<sup>th</sup> and 31<sup>st</sup> document numbers in TermFreqs. <br></br>
                   The SkipData entries on all upper levels &gt; 0 contain a SkipChildLevelPointer referencing the corresponding SkipData
                   entry in level-1. In the example has entry 15 on level 1 a pointer to entry 15 on level 0 and entry 31 on level 1 a pointer
                   to entry 31 on level 0.                   
                </p>

            </section>
            <section id="Positions"><title>Positions</title>

                <p>
                    The .prx file contains the lists of positions that
                    each term occurs at within documents.  Note that
                    fields omitting positional data do not store
                    anything into this file, and if all fields in the
                    index omit positional data then the .prx file will not
                    exist.
                </p>
                <p>ProxFile (.prx) --&gt;
                    &lt;TermPositions&gt;
                    <sup>TermCount</sup>
                </p>
                <p>TermPositions --&gt;
                    &lt;Positions&gt;
                    <sup>DocFreq</sup>
                </p>
                <p>Positions --&gt;
                    &lt;PositionDelta,Payload?&gt;
                    <sup>Freq</sup>
                </p>
                <p>Payload --&gt;
                    &lt;PayloadLength?,PayloadData&gt;
                </p>
                <p>PositionDelta --&gt;
                    VInt
                </p>
                <p>PayloadLength --&gt;
                    VInt
                </p>
                <p>PayloadData --&gt;
                    byte<sup>PayloadLength</sup>
                </p>
                <p>TermPositions
                    are ordered by term (the term is implicit, from the .tis file).
                </p>
                <p>Positions
                    entries are ordered by increasing document number (the document
                    number is implicit from the .frq file).
                </p>
                <p>PositionDelta
                    is, if payloads are disabled for the term's field, the difference 
                    between the position of the current occurrence in
                    the document and the previous occurrence (or zero, if this is the
                    first occurrence in this document).
                    If payloads are enabled for the term's field, then PositionDelta/2
                    is the difference between the current and the previous position. If
                    payloads are enabled and PositionDelta is odd, then PayloadLength is 
                    stored, indicating the length of the payload at the current term position.
                </p>
                <p>
                    For example, the TermPositions for a
                    term which occurs as the fourth term in one document, and as the
                    fifth and ninth term in a subsequent document, would be the following
                    sequence of VInts (payloads disabled):
                </p>
                <p>4,
                    5, 4
                </p>
                <p>PayloadData
                    is metadata associated with the current term position. If PayloadLength
                    is stored at the current position, then it indicates the length of this 
                    Payload. If PayloadLength is not stored, then this Payload has the same
                    length as the Payload at the previous position.
                </p>
            </section>
            <section id="Normalization Factors"><title>Normalization Factors</title>

				        <p>There's a single .nrm file containing all norms:
                </p>
                <p>AllNorms
                    (.nrm) --&gt; NormsHeader,&lt;Norms&gt;
                    <sup>NumFieldsWithNorms</sup>
                </p>
                <p>Norms
                    --&gt; &lt;Byte&gt;
                    <sup>SegSize</sup>
                </p>
                <p>NormsHeader
                    --&gt; 'N','R','M',Version
                </p>
                <p>Version
                    --&gt; Byte
                </p>
                <p>NormsHeader 
					has 4 bytes, last of which is the format version for this file, currently -1.
                </p>
                <p>Each
                    byte encodes a floating point value. Bits 0-2 contain the 3-bit
                    mantissa, and bits 3-8 contain the 5-bit exponent.
                </p>
                <p>These
                    are converted to an IEEE single float value as follows:
                </p>
                <ol>
                    <li>
                        <p>If
                            the byte is zero, use a zero float.
                        </p>
                    </li>
                    <li>
                        <p>Otherwise,
                            set the sign bit of the float to zero;
                        </p>
                    </li>
                    <li>
                        <p>add
                            48 to the exponent and use this as the float's exponent;
                        </p>
                    </li>
                    <li>
                        <p>map
                            the mantissa to the high-order 3 bits of the float's mantissa; and

                        </p>
                    </li>
                    <li>
                        <p>set
                            the low-order 21 bits of the float's mantissa to zero.
                        </p>
                    </li>
                </ol>
                <p>A separate norm file is created when the norm values of an existing segment are modified. 
					When field <em>N</em> is modified, a separate norm file <em>.sN</em> 
					is created, to maintain the norm values for that field.
                </p>
				<p>Separate norm files are created (when adequate) for both compound and non compound segments.
                </p>

            </section>
            <section id="Term Vectors"><title>Term Vectors</title>
                <p>
		  Term Vector support is an optional on a field by
                  field basis. It consists of 3 files.
                </p>
                <ol>
                    <li><a name="tvx"/>
                        <p>The Document Index or .tvx file.</p>
                        <p>For each document, this stores the offset
                           into the document data (.tvd) and field
                           data (.tvf) files.
                        </p>
                        <p>DocumentIndex (.tvx) --&gt; TVXVersion&lt;DocumentPosition,FieldPosition&gt;
                            <sup>NumDocs</sup>
                        </p>
                        <p>TVXVersion --&gt; Int (TermVectorsReader.CURRENT)</p>
                        <p>DocumentPosition --&gt; UInt64 (offset in
                        the .tvd file)</p>
                        <p>FieldPosition --&gt; UInt64 (offset in the
                        .tvf file)</p>
                    </li>
                    <li><a name="tvd"/>
                        <p>The Document or .tvd file.</p>
                        <p>This contains, for each document, the number of fields, a list of the fields with
                            term vector info and finally a list of pointers to the field information in the .tvf
                            (Term Vector Fields) file.</p>
                        <p>
                            Document (.tvd) --&gt; TVDVersion&lt;NumFields, FieldNums, FieldPositions&gt;
                            <sup>NumDocs</sup>
                        </p>
                        <p>TVDVersion --&gt; Int (TermVectorsReader.FORMAT_CURRENT)</p>
                        <p>NumFields --&gt; VInt</p>
                        <p>FieldNums --&gt; &lt;FieldNumDelta&gt;
                            <sup>NumFields</sup>
                        </p>
                        <p>FieldNumDelta --&gt; VInt</p>
                        <p>FieldPositions --&gt; &lt;FieldPositionDelta&gt;
                            <sup>NumFields-1</sup>
                        </p>
                        <p>FieldPositionDelta --&gt; VLong</p>
                        <p>The .tvd file is used to map out the fields that have term vectors stored and
                            where the field information is in the .tvf file.</p>
                    </li>
                    <li><a name="tvf"/>
                        <p>The Field or .tvf file.</p>
                        <p>This file contains, for each field that has a term vector stored, a list of
                            the terms, their frequencies and, optionally, position and offest information.</p>
                        <p>Field (.tvf) --&gt; TVFVersion&lt;NumTerms, Position/Offset, TermFreqs&gt;
                            <sup>NumFields</sup>
                        </p>
                        <p>TVFVersion --&gt; Int (TermVectorsReader.FORMAT_CURRENT)</p>
                        <p>NumTerms --&gt; VInt</p>
                        <p>Position/Offset --&gt; Byte</p>
                        <p>TermFreqs --&gt; &lt;TermText, TermFreq, Positions?, Offsets?&gt;
                            <sup>NumTerms</sup>
                        </p>
                        <p>TermText --&gt; &lt;PrefixLength, Suffix&gt;</p>
                        <p>PrefixLength --&gt; VInt</p>
                        <p>Suffix --&gt; String</p>
                        <p>TermFreq --&gt; VInt</p>
                        <p>Positions --&gt; &lt;VInt&gt;<sup>TermFreq</sup></p>
                        <p>Offsets --&gt; &lt;VInt, VInt&gt;<sup>TermFreq</sup></p>
                        <br/>
                        <p>Notes:</p>
                        <ul>
                            <li>Position/Offset byte stores whether this term vector has position or offset information stored.</li>
                            <li>Term
                                text prefixes are shared. The PrefixLength is the number of initial
                                characters from the previous term which must be pre-pended to a
                                term's suffix in order to form the term's text. Thus, if the
                                previous term's text was "bone" and the term is "boy",
                                the PrefixLength is two and the suffix is "y".
                            </li>
                            <li>Positions are stored as delta encoded VInts. This means we only store the difference of the current position from the last position</li>
                            <li>Offsets are stored as delta encoded VInts. The first VInt is the startOffset, the second is the endOffset.</li>
                        </ul>


                    </li>
                </ol>
            </section>

            <section id="Deleted Documents"><title>Deleted Documents</title>

                <p>The .del file is
                    optional, and only exists when a segment contains deletions.
                </p>

                <p>Although per-segment, this file is maintained exterior to compound segment files.
                </p>
                <p>
                Deletions
                    (.del) --&gt; [Format],ByteCount,BitCount, Bits | DGaps (depending on Format)
                </p>

                <p>Format,ByteSize,BitCount --&gt;
                    Uint32
                </p>

                <p>Bits --&gt;
                    &lt;Byte&gt;
                    <sup>ByteCount</sup>
                </p>

				<p>DGaps --&gt;
                    &lt;DGap,NonzeroByte&gt;
                    <sup>NonzeroBytesCount</sup>
                </p>

                <p>DGap --&gt;
                    VInt
                </p>

                <p>NonzeroByte --&gt;
                    Byte
                </p>
				
                <p>Format
                    is Optional. -1 indicates DGaps. Non-negative value indicates Bits, and that Format is excluded.
                </p>

                <p>ByteCount
                    indicates the number of bytes in Bits. It is typically
                    (SegSize/8)+1.
                </p>

                <p>
                    BitCount
                    indicates the number of bits that are currently set in Bits.
                </p>

                <p>Bits
                    contains one bit for each document indexed. When the bit
                    corresponding to a document number is set, that document is marked as
                    deleted. Bit ordering is from least to most significant. Thus, if
                    Bits contains two bytes, 0x00 and 0x02, then document 9 is marked as
                    deleted.
                </p>

				<p>DGaps
                    represents sparse bit-vectors more efficiently than Bits.
                    It is made of DGaps on indexes of nonzero bytes in Bits,
                    and the nonzero bytes themselves. The number of nonzero bytes
                    in Bits (NonzeroBytesCount) is not stored.
                </p>
                <p>For example,
                    if there are 8000 bits and only bits 10,12,32 are set,
                    DGaps would be used:
                </p>
                <p>
                    (VInt) 1 , (byte) 20 , (VInt) 3 , (Byte) 1
                </p>
            </section>
        </section>

        <section id="Limitations"><title>Limitations</title>

            <p>
	      When referring to term numbers, Lucene's current
	      implementation uses a Java <code>int</code> to hold the
	      term index, which means the maximum number of unique
	      terms in any single index segment is ~2.1 billion times
	      the term index interval (default 128) = ~274 billion.
	      This is technically not a limitation of the index file
	      format, just of Lucene's current implementation.
	    </p>
	    <p>
	      Similarly, Lucene uses a Java <code>int</code> to refer
	      to document numbers, and the index file format uses an
	      <code>Int32</code> on-disk to store document numbers.
	      This is a limitation of both the index file format and
	      the current implementation.  Eventually these should be
	      replaced with either <code>UInt64</code> values, or
	      better yet, <code>VInt</code> values which have no
	      limit.
            </p>

        </section>

    </body>

</document>
