/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/**
* Code to maintain and access indices.
* <!-- TODO: add IndexWriter, IndexWriterConfig, DocValues, etc etc -->
* <h2>Table Of Contents</h2>
* <ol>
* <li><a href="#postings">Postings APIs</a>
* <ul>
* <li><a href="#fields">Fields</a></li>
* <li><a href="#terms">Terms</a></li>
* <li><a href="#documents">Documents</a></li>
* <li><a href="#positions">Positions</a></li>
* </ul>
* </li>
* <li><a href="#stats">Index Statistics</a>
* <ul>
* <li><a href="#termstats">Term-level</a></li>
* <li><a href="#fieldstats">Field-level</a></li>
* <li><a href="#segmentstats">Segment-level</a></li>
* <li><a href="#documentstats">Document-level</a></li>
* </ul>
* </li>
* </ol>
* <a name="postings"></a>
* <h2>Postings APIs</h2>
* <a name="fields"></a>
* <h3>
* Fields
* </h3>
* <p>
* {@link org.apache.lucene.index.Fields} is the initial entry point into the
* postings APIs, this can be obtained in several ways:
* <pre class="prettyprint">
* // access indexed fields for an index segment
* Fields fields = reader.fields();
* // access term vector fields for a specified document
* Fields fields = reader.getTermVectors(docid);
* </pre>
* Fields implements Java's Iterable interface, so it's easy to enumerate the
* list of fields:
* <pre class="prettyprint">
* // enumerate list of fields
* for (String field : fields) {
* // access the terms for this field
* Terms terms = fields.terms(field);
* }
* </pre>
* <a name="terms"></a>
* <h3>
* Terms
* </h3>
* <p>
* {@link org.apache.lucene.index.Terms} represents the collection of terms
* within a field, exposes some metadata and <a href="#fieldstats">statistics</a>,
* and an API for enumeration.
* <pre class="prettyprint">
* // metadata about the field
* System.out.println("positions? " + terms.hasPositions());
* System.out.println("offsets? " + terms.hasOffsets());
* System.out.println("payloads? " + terms.hasPayloads());
* // iterate through terms
* TermsEnum termsEnum = terms.iterator(null);
* BytesRef term = null;
* while ((term = termsEnum.next()) != null) {
* doSomethingWith(termsEnum.term());
* }
* </pre>
* {@link org.apache.lucene.index.TermsEnum} provides an iterator over the list
* of terms within a field, some <a href="#termstats">statistics</a> about the term,
* and methods to access the term's <a href="#documents">documents</a> and
* <a href="#positions">positions</a>.
* <pre class="prettyprint">
* // seek to a specific term
* boolean found = termsEnum.seekExact(new BytesRef("foobar"));
* if (found) {
* // get the document frequency
* System.out.println(termsEnum.docFreq());
* // enumerate through documents
* PostingsEnum docs = termsEnum.postings(null, null);
* // enumerate through documents and positions
* PostingsEnum docsAndPositions = termsEnum.postings(null, null, PostingsEnum.FLAG_POSITIONS);
* }
* </pre>
* <a name="documents"></a>
* <h3>
* Documents
* </h3>
* <p>
* {@link org.apache.lucene.index.PostingsEnum} is an extension of
* {@link org.apache.lucene.search.DocIdSetIterator}that iterates over the list of
* documents for a term, along with the term frequency within that document.
* <pre class="prettyprint">
* int docid;
* while ((docid = docsEnum.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
* System.out.println(docid);
* System.out.println(docsEnum.freq());
* }
* </pre>
* <a name="positions"></a>
* <h3>
* Positions
* </h3>
* <p>
* PostingsEnum also allows iteration
* of the positions a term occurred within the document, and any additional
* per-position information (offsets and payload). The information available
* is controlled by flags passed to TermsEnum#postings
* <pre class="prettyprint">
* int docid;
* PostingsEnum postings = termsEnum.postings(null, null, PostingsEnum.FLAG_PAYLOADS | PostingsEnum.FLAG_OFFSETS);
* while ((docid = postings.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
* System.out.println(docid);
* int freq = postings.freq();
* for (int i = 0; i < freq; i++) {
* System.out.println(postings.nextPosition());
* System.out.println(postings.startOffset());
* System.out.println(postings.endOffset());
* System.out.println(postings.getPayload());
* }
* }
* </pre>
* <a name="stats"></a>
* <h2>Index Statistics</h2>
* <a name="termstats"></a>
* <h3>
* Term statistics
* </h3>
* <ul>
* <li>{@link org.apache.lucene.index.TermsEnum#docFreq}: Returns the number of
* documents that contain at least one occurrence of the term. This statistic
* is always available for an indexed term. Note that it will also count
* deleted documents, when segments are merged the statistic is updated as
* those deleted documents are merged away.
* <li>{@link org.apache.lucene.index.TermsEnum#totalTermFreq}: Returns the number
* of occurrences of this term across all documents. Note that this statistic
* is unavailable (returns <code>-1</code>) if term frequencies were omitted
* from the index
* ({@link org.apache.lucene.index.IndexOptions#DOCS DOCS})
* for the field. Like docFreq(), it will also count occurrences that appear in
* deleted documents.
* </ul>
* <a name="fieldstats"></a>
* <h3>
* Field statistics
* </h3>
* <ul>
* <li>{@link org.apache.lucene.index.Terms#size}: Returns the number of
* unique terms in the field. This statistic may be unavailable
* (returns <code>-1</code>) for some Terms implementations such as
* {@link org.apache.lucene.index.MultiTerms}, where it cannot be efficiently
* computed. Note that this count also includes terms that appear only
* in deleted documents: when segments are merged such terms are also merged
* away and the statistic is then updated.
* <li>{@link org.apache.lucene.index.Terms#getDocCount}: Returns the number of
* documents that contain at least one occurrence of any term for this field.
* This can be thought of as a Field-level docFreq(). Like docFreq() it will
* also count deleted documents.
* <li>{@link org.apache.lucene.index.Terms#getSumDocFreq}: Returns the number of
* postings (term-document mappings in the inverted index) for the field. This
* can be thought of as the sum of {@link org.apache.lucene.index.TermsEnum#docFreq}
* across all terms in the field, and like docFreq() it will also count postings
* that appear in deleted documents.
* <li>{@link org.apache.lucene.index.Terms#getSumTotalTermFreq}: Returns the number
* of tokens for the field. This can be thought of as the sum of
* {@link org.apache.lucene.index.TermsEnum#totalTermFreq} across all terms in the
* field, and like totalTermFreq() it will also count occurrences that appear in
* deleted documents, and will be unavailable (returns <code>-1</code>) if term
* frequencies were omitted from the index
* ({@link org.apache.lucene.index.IndexOptions#DOCS DOCS})
* for the field.
* </ul>
* <a name="segmentstats"></a>
* <h3>
* Segment statistics
* </h3>
* <ul>
* <li>{@link org.apache.lucene.index.IndexReader#maxDoc}: Returns the number of
* documents (including deleted documents) in the index.
* <li>{@link org.apache.lucene.index.IndexReader#numDocs}: Returns the number
* of live documents (excluding deleted documents) in the index.
* <li>{@link org.apache.lucene.index.IndexReader#numDeletedDocs}: Returns the
* number of deleted documents in the index.
* <li>{@link org.apache.lucene.index.Fields#size}: Returns the number of indexed
* fields.
* </ul>
* <a name="documentstats"></a>
* <h3>
* Document statistics
* </h3>
* <p>
* Document statistics are available during the indexing process for an indexed field: typically
* a {@link org.apache.lucene.search.similarities.Similarity} implementation will store some
* of these values (possibly in a lossy way), into the normalization value for the document in
* its {@link org.apache.lucene.search.similarities.Similarity#computeNorm} method.
* <ul>
* <li>{@link org.apache.lucene.index.FieldInvertState#getLength}: Returns the number of
* tokens for this field in the document. Note that this is just the number
* of times that {@link org.apache.lucene.analysis.TokenStream#incrementToken} returned
* true, and is unrelated to the values in
* {@link org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute}.
* <li>{@link org.apache.lucene.index.FieldInvertState#getNumOverlap}: Returns the number
* of tokens for this field in the document that had a position increment of zero. This
* can be used to compute a document length that discounts artificial tokens
* such as synonyms.
* <li>{@link org.apache.lucene.index.FieldInvertState#getPosition}: Returns the accumulated
* position value for this field in the document: computed from the values of
* {@link org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute} and including
* {@link org.apache.lucene.analysis.Analyzer#getPositionIncrementGap}s across multivalued
* fields.
* <li>{@link org.apache.lucene.index.FieldInvertState#getOffset}: Returns the total
* character offset value for this field in the document: computed from the values of
* {@link org.apache.lucene.analysis.tokenattributes.OffsetAttribute} returned by
* {@link org.apache.lucene.analysis.TokenStream#end}, and including
* {@link org.apache.lucene.analysis.Analyzer#getOffsetGap}s across multivalued
* fields.
* <li>{@link org.apache.lucene.index.FieldInvertState#getUniqueTermCount}: Returns the number
* of unique terms encountered for this field in the document.
* <li>{@link org.apache.lucene.index.FieldInvertState#getMaxTermFrequency}: Returns the maximum
* frequency across all unique terms encountered for this field in the document.
* </ul>
* <p>
* Additional user-supplied statistics can be added to the document as DocValues fields and
* accessed via {@link org.apache.lucene.index.LeafReader#getNumericDocValues}.
*/
package org.apache.lucene.index;