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/**
* Lucene 5.4 file format.
*
* <h1>Apache Lucene - Index File Formats</h1>
* <div>
* <ul>
* <li><a href="#Introduction">Introduction</a></li>
* <li><a href="#Definitions">Definitions</a>
* <ul>
* <li><a href="#Inverted_Indexing">Inverted Indexing</a></li>
* <li><a href="#Types_of_Fields">Types of Fields</a></li>
* <li><a href="#Segments">Segments</a></li>
* <li><a href="#Document_Numbers">Document Numbers</a></li>
* </ul>
* </li>
* <li><a href="#Overview">Index Structure Overview</a></li>
* <li><a href="#File_Naming">File Naming</a></li>
* <li><a href="#file-names">Summary of File Extensions</a>
* <ul>
* <li><a href="#Lock_File">Lock File</a></li>
* <li><a href="#History">History</a></li>
* <li><a href="#Limitations">Limitations</a></li>
* </ul>
* </li>
* </ul>
* </div>
* <a name="Introduction"></a>
* <h2>Introduction</h2>
* <div>
* <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/</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>
* </div>
* <a name="Definitions"></a>
* <h2>Definitions</h2>
* <div>
* <p>The fundamental concepts in Lucene are index, document, field and term.</p>
* <p>An index contains a sequence of documents.</p>
* <ul>
* <li>A document is a sequence of fields.</li>
* <li>A field is a named sequence of terms.</li>
* <li>A term is a sequence of bytes.</li>
* </ul>
* <p>The same sequence of bytes in two different fields is considered a different
* term. Thus terms are represented as a pair: the string naming the field, and the
* bytes within the field.</p>
* <a name="Inverted_Indexing"></a>
* <h3>Inverted Indexing</h3>
* <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>
* <a name="Types_of_Fields"></a>
* <h3>Types of Fields</h3>
* <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 {@link org.apache.lucene.document.Field Field}
* java docs for more information on Fields.</p>
* <a name="Segments"></a>
* <h3>Segments</h3>
* <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>Creating new segments for newly added documents.</li>
* <li>Merging existing segments.</li>
* </ol>
* <p>Searches may involve multiple segments and/or multiple indexes, each index
* potentially composed of a set of segments.</p>
* <a name="Document_Numbers"></a>
* <h3>Document Numbers</h3>
* <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>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>
* </div>
* <a name="Overview"></a>
* <h2>Index Structure Overview</h2>
* <div>
* <p>Each segment index maintains the following:</p>
* <ul>
* <li>
* {@link org.apache.lucene.codecs.lucene62.Lucene62SegmentInfoFormat Segment info}.
* This contains metadata about a segment, such as the number of documents,
* what files it uses,
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene50.Lucene50FieldInfosFormat Field names}.
* This contains the set of field names used in the index.
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene50.Lucene50StoredFieldsFormat 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.
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat 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.
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat 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)
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat 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.
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene53.Lucene53NormsFormat Normalization factors}.
* For each field in each document, a value is stored
* that is multiplied into the score for hits on that field.
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene50.Lucene50TermVectorsFormat 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
* {@link org.apache.lucene.document.Field Field} constructors
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene54.Lucene54DocValuesFormat Per-document values}.
* Like stored values, these are also keyed by document
* number, but are generally intended to be loaded into main memory for fast
* access. Whereas stored values are generally intended for summary results from
* searches, per-document values are useful for things like scoring factors.
* </li>
* <li>
* {@link org.apache.lucene.codecs.lucene50.Lucene50LiveDocsFormat Live documents}.
* An optional file indicating which documents are live.
* </li>
* </ul>
* <p>Details on each of these are provided in their linked pages.</p>
* </div>
* <a name="File_Naming"></a>
* <h2>File Naming</h2>
* <div>
* <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 (except
* for the Segment info file, the Lock file, and Deleted documents file) 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.
* 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>
* </div>
* <a name="file-names"></a>
* <h2>Summary of File Extensions</h2>
* <div>
* <p>The following table summarizes the names and extensions of the files in
* Lucene:</p>
* <table cellspacing="1" cellpadding="4" summary="lucene filenames by extension">
* <tr>
* <th>Name</th>
* <th>Extension</th>
* <th>Brief Description</th>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.index.SegmentInfos Segments File}</td>
* <td>segments_N</td>
* <td>Stores information about a commit point</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>{@link org.apache.lucene.codecs.lucene62.Lucene62SegmentInfoFormat Segment Info}</td>
* <td>.si</td>
* <td>Stores metadata about a segment</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50CompoundFormat Compound File}</td>
* <td>.cfs, .cfe</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>{@link org.apache.lucene.codecs.lucene50.Lucene50FieldInfosFormat Fields}</td>
* <td>.fnm</td>
* <td>Stores information about the fields</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50StoredFieldsFormat Field Index}</td>
* <td>.fdx</td>
* <td>Contains pointers to field data</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50StoredFieldsFormat Field Data}</td>
* <td>.fdt</td>
* <td>The stored fields for documents</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat Term Dictionary}</td>
* <td>.tim</td>
* <td>The term dictionary, stores term info</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat Term Index}</td>
* <td>.tip</td>
* <td>The index into the Term Dictionary</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat Frequencies}</td>
* <td>.doc</td>
* <td>Contains the list of docs which contain each term along with frequency</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat Positions}</td>
* <td>.pos</td>
* <td>Stores position information about where a term occurs in the index</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat Payloads}</td>
* <td>.pay</td>
* <td>Stores additional per-position metadata information such as character offsets and user payloads</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene53.Lucene53NormsFormat Norms}</td>
* <td>.nvd, .nvm</td>
* <td>Encodes length and boost factors for docs and fields</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene54.Lucene54DocValuesFormat Per-Document Values}</td>
* <td>.dvd, .dvm</td>
* <td>Encodes additional scoring factors or other per-document information.</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50TermVectorsFormat Term Vector Index}</td>
* <td>.tvx</td>
* <td>Stores offset into the document data file</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50TermVectorsFormat Term Vector Documents}</td>
* <td>.tvd</td>
* <td>Contains information about each document that has term vectors</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50TermVectorsFormat Term Vector Fields}</td>
* <td>.tvf</td>
* <td>The field level info about term vectors</td>
* </tr>
* <tr>
* <td>{@link org.apache.lucene.codecs.lucene50.Lucene50LiveDocsFormat Live Documents}</td>
* <td>.liv</td>
* <td>Info about what files are live</td>
* </tr>
* </table>
* </div>
* <a name="Lock_File"></a>
* <h2>Lock File</h2>
* 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.
* <a name="History"></a>
* <h2>History</h2>
* <p>Compatibility notes are provided in this document, describing how file
* formats have changed from prior versions:</p>
* <ul>
* <li>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.</li>
* <li>In version 2.3, the file format was changed to allow segments to share a
* single set of doc store (vectors & 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).</li>
* <li>In version 2.4, Strings are now written as true UTF-8 byte sequence, not
* Java's modified UTF-8. See <a href="http://issues.apache.org/jira/browse/LUCENE-510">
* LUCENE-510</a> for details.</li>
* <li>In version 2.9, an optional opaque Map<String,String> CommitUserData
* may be passed to IndexWriter's commit methods (and later retrieved), which is
* recorded in the segments_N file. See <a href="http://issues.apache.org/jira/browse/LUCENE-1382">
* LUCENE-1382</a> 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
* <a href="http://issues.apache.org/jira/browse/LUCENE-1654">LUCENE-1654</a> for details.</li>
* <li>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 <a href="http://issues.apache.org/jira/browse/LUCENE-1960">LUCENE-1960</a>
* for details.</li>
* <li>In version 3.1, segments records the code version that created them. See
* <a href="http://issues.apache.org/jira/browse/LUCENE-2720">LUCENE-2720</a> for details.
* Additionally segments track explicitly whether or not they have term vectors.
* See <a href="http://issues.apache.org/jira/browse/LUCENE-2811">LUCENE-2811</a>
* for details.</li>
* <li>In version 3.2, numeric fields are written as natively to stored fields
* file, previously they were stored in text format only.</li>
* <li>In version 3.4, fields can omit position data while still indexing term
* frequencies.</li>
* <li>In version 4.0, the format of the inverted index became extensible via
* the {@link org.apache.lucene.codecs.Codec Codec} api. Fast per-document storage
* ({@code DocValues}) was introduced. Normalization factors need no longer be a
* single byte, they can be any {@link org.apache.lucene.index.NumericDocValues NumericDocValues}.
* Terms need not be unicode strings, they can be any byte sequence. Term offsets
* can optionally be indexed into the postings lists. Payloads can be stored in the
* term vectors.</li>
* <li>In version 4.1, the format of the postings list changed to use either
* of FOR compression or variable-byte encoding, depending upon the frequency
* of the term. Terms appearing only once were changed to inline directly into
* the term dictionary. Stored fields are compressed by default. </li>
* <li>In version 4.2, term vectors are compressed by default. DocValues has
* a new multi-valued type (SortedSet), that can be used for faceting/grouping/joining
* on multi-valued fields.</li>
* <li>In version 4.5, DocValues were extended to explicitly represent missing values.</li>
* <li>In version 4.6, FieldInfos were extended to support per-field DocValues generation, to
* allow updating NumericDocValues fields.</li>
* <li>In version 4.8, checksum footers were added to the end of each index file
* for improved data integrity. Specifically, the last 8 bytes of every index file
* contain the zlib-crc32 checksum of the file.</li>
* <li>In version 4.9, DocValues has a new multi-valued numeric type (SortedNumeric)
* that is suitable for faceting/sorting/analytics.
* <li>In version 5.4, DocValues have been improved to store more information on disk:
* addresses for binary fields and ord indexes for multi-valued fields.
* </li>
* </ul>
* <a name="Limitations"></a>
* <h2>Limitations</h2>
* <div>
* <p>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, {@link org.apache.lucene.store.DataOutput#writeVInt VInt} values which have no limit.</p>
* </div>
*/
package org.apache.lucene.codecs.lucene54;