slow-J commented on code in PR #12741: URL: https://github.com/apache/lucene/pull/12741#discussion_r1383744529
########## lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99PostingsFormat.java: ########## @@ -0,0 +1,518 @@ +/* + * 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. + */ +package org.apache.lucene.codecs.lucene99; + +import java.io.IOException; +import org.apache.lucene.codecs.BlockTermState; +import org.apache.lucene.codecs.CodecUtil; +import org.apache.lucene.codecs.FieldsConsumer; +import org.apache.lucene.codecs.FieldsProducer; +import org.apache.lucene.codecs.MultiLevelSkipListWriter; +import org.apache.lucene.codecs.PostingsFormat; +import org.apache.lucene.codecs.PostingsReaderBase; +import org.apache.lucene.codecs.PostingsWriterBase; +import org.apache.lucene.codecs.lucene90.blocktree.Lucene90BlockTreeTermsReader; +import org.apache.lucene.codecs.lucene90.blocktree.Lucene90BlockTreeTermsWriter; +import org.apache.lucene.index.IndexOptions; +import org.apache.lucene.index.SegmentReadState; +import org.apache.lucene.index.SegmentWriteState; +import org.apache.lucene.index.TermState; +import org.apache.lucene.store.DataOutput; +import org.apache.lucene.util.IOUtils; +import org.apache.lucene.util.packed.PackedInts; + +/** + * Lucene 9.9 postings format, which encodes postings in packed integer blocks for fast decode. + * + * <p>Basic idea: + * + * <ul> + * <li><b>Packed Blocks and VInt Blocks</b>: + * <p>In packed blocks, integers are encoded with the same bit width ({@link PackedInts packed + * format}): the block size (i.e. number of integers inside block) is fixed (currently 128). + * Additionally blocks that are all the same value are encoded in an optimized way. + * <p>In VInt blocks, integers are encoded as {@link DataOutput#writeVInt VInt}: the block + * size is variable. + * <li><b>Block structure</b>: + * <p>When the postings are long enough, Lucene99PostingsFormat will try to encode most + * integer data as a packed block. + * <p>Take a term with 259 documents as an example, the first 256 document ids are encoded as + * two packed blocks, while the remaining 3 are encoded as one VInt block. + * <p>Different kinds of data are always encoded separately into different packed blocks, but + * may possibly be interleaved into the same VInt block. + * <p>This strategy is applied to pairs: <document number, frequency>, <position, + * payload length>, <position, offset start, offset length>, and <position, + * payload length, offsetstart, offset length>. + * <li><b>Skipdata settings</b>: + * <p>The structure of skip table is quite similar to previous version of Lucene. Skip + * interval is the same as block size, and each skip entry points to the beginning of each + * block. However, for the first block, skip data is omitted. + * <li><b>Positions, Payloads, and Offsets</b>: + * <p>A position is an integer indicating where the term occurs within one document. A payload + * is a blob of metadata associated with current position. An offset is a pair of integers + * indicating the tokenized start/end offsets for given term in current position: it is + * essentially a specialized payload. + * <p>When payloads and offsets are not omitted, numPositions==numPayloads==numOffsets + * (assuming a null payload contributes one count). As mentioned in block structure, it is + * possible to encode these three either combined or separately. + * <p>In all cases, payloads and offsets are stored together. When encoded as a packed block, + * position data is separated out as .pos, while payloads and offsets are encoded in .pay + * (payload metadata will also be stored directly in .pay). When encoded as VInt blocks, all + * these three are stored interleaved into the .pos (so is payload metadata). + * <p>With this strategy, the majority of payload and offset data will be outside .pos file. + * So for queries that require only position data, running on a full index with payloads and + * offsets, this reduces disk pre-fetches. + * </ul> + * + * <p>Files and detailed format: + * + * <ul> + * <li><code>.tim</code>: <a href="#Termdictionary">Term Dictionary</a> + * <li><code>.tip</code>: <a href="#Termindex">Term Index</a> + * <li><code>.doc</code>: <a href="#Frequencies">Frequencies and Skip Data</a> + * <li><code>.pos</code>: <a href="#Positions">Positions</a> + * <li><code>.pay</code>: <a href="#Payloads">Payloads and Offsets</a> + * </ul> + * + * <a id="Termdictionary"></a> + * + * <dl> + * <dd><b>Term Dictionary</b> + * <p>The .tim file contains the list of terms in each field along with per-term statistics + * (such as docfreq) and pointers to the frequencies, positions, payload and skip data in the + * .doc, .pos, and .pay files. See {@link Lucene90BlockTreeTermsWriter} for more details on + * the format. + * <p>NOTE: The term dictionary can plug into different postings implementations: the postings + * writer/reader are actually responsible for encoding and decoding the PostingsHeader and + * TermMetadata sections described here: + * <ul> + * <li>PostingsHeader --> Header, PackedBlockSize + * <li>TermMetadata --> (DocFPDelta|SingletonDocID), PosFPDelta?, PosVIntBlockFPDelta?, + * PayFPDelta?, SkipFPDelta? + * <li>Header, --> {@link CodecUtil#writeIndexHeader IndexHeader} + * <li>PackedBlockSize, SingletonDocID --> {@link DataOutput#writeVInt VInt} + * <li>DocFPDelta, PosFPDelta, PayFPDelta, PosVIntBlockFPDelta, SkipFPDelta --> {@link + * DataOutput#writeVLong VLong} + * <li>Footer --> {@link CodecUtil#writeFooter CodecFooter} + * </ul> + * <p>Notes: + * <ul> + * <li>Header is a {@link CodecUtil#writeIndexHeader IndexHeader} storing the version + * information for the postings. + * <li>PackedBlockSize is the fixed block size for packed blocks. In packed block, bit width + * is determined by the largest integer. Smaller block size result in smaller variance + * among width of integers hence smaller indexes. Larger block size result in more + * efficient bulk i/o hence better acceleration. This value should always be a multiple + * of 64, currently fixed as 128 as a tradeoff. It is also the skip interval used to + * accelerate {@link org.apache.lucene.index.PostingsEnum#advance(int)}. + * <li>DocFPDelta determines the position of this term's TermFreqs within the .doc file. In + * particular, it is the difference of file offset between this term's data and previous + * term's data (or zero, for the first term in the block).On disk it is stored as the + * difference from previous value in sequence. + * <li>PosFPDelta determines the position of this term's TermPositions within the .pos file. + * While PayFPDelta determines the position of this term's <TermPayloads, + * TermOffsets?> within the .pay file. Similar to DocFPDelta, it is the difference + * between two file positions (or neglected, for fields that omit payloads and offsets). + * <li>PosVIntBlockFPDelta determines the position of this term's last TermPosition in last + * pos packed block within the .pos file. It is synonym for PayVIntBlockFPDelta or + * OffsetVIntBlockFPDelta. This is actually used to indicate whether it is necessary to + * load following payloads and offsets from .pos instead of .pay. Every time a new block + * of positions are to be loaded, the PostingsReader will use this value to check + * whether current block is packed format or VInt. When packed format, payloads and + * offsets are fetched from .pay, otherwise from .pos. (this value is neglected when + * total number of positions i.e. totalTermFreq is less or equal to PackedBlockSize). + * <li>SkipFPDelta determines the position of this term's SkipData within the .doc file. In + * particular, it is the length of the TermFreq data. SkipDelta is only stored if + * DocFreq is not smaller than SkipMinimum (i.e. 128 in Lucene99PostingsFormat). + * <li>SingletonDocID is an optimization when a term only appears in one document. In this + * case, instead of writing a file pointer to the .doc file (DocFPDelta), and then a + * VIntBlock at that location, the single document ID is written to the term dictionary. + * </ul> + * </dl> + * + * <a id="Termindex"></a> + * + * <dl> + * <dd><b>Term Index</b> + * <p>The .tip file contains an index into the term dictionary, so that it can be accessed + * randomly. See {@link Lucene90BlockTreeTermsWriter} for more details on the format. + * </dl> + * + * <a id="Frequencies"></a> + * + * <dl> + * <dd><b>Frequencies and Skip Data</b> + * <p>The .doc 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: {@link + * IndexOptions#DOCS}). It also saves skip data to the beginning of each packed or VInt block, Review Comment: Opened PR for the 2 javadoc comments: https://github.com/apache/lucene/pull/12776 -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: issues-unsubscr...@lucene.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org --------------------------------------------------------------------- To unsubscribe, e-mail: issues-unsubscr...@lucene.apache.org For additional commands, e-mail: issues-h...@lucene.apache.org
