jpountz commented on code in PR #14333:
URL: https://github.com/apache/lucene/pull/14333#discussion_r2027695983


##########
lucene/core/src/java/org/apache/lucene/codecs/lucene103/Lucene103PostingsFormat.java:
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@@ -0,0 +1,510 @@
+/*
+ * 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.lucene103;
+
+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.PostingsFormat;
+import org.apache.lucene.codecs.PostingsReaderBase;
+import org.apache.lucene.codecs.PostingsWriterBase;
+import 
org.apache.lucene.codecs.lucene103.blocktree.Lucene103BlockTreeTermsReader;
+import 
org.apache.lucene.codecs.lucene103.blocktree.Lucene103BlockTreeTermsWriter;
+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 10.3 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, Lucene103PostingsFormat 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: &lt;document number, 
frequency&gt;, &lt;position,
+ *       payload length&gt;, &lt;position, offset start, offset length&gt;, 
and &lt;position,
+ *       payload length, offsetstart, offset length&gt;.
+ *   <li><b>Skipdata</b>:
+ *       <p>Skipdata is interleaved with blocks on 2 levels. Level 0 skip data 
is interleaved
+ *       between every packed block. Level 1 skip data is interleaved between 
every 32 packed
+ *       blocks.
+ *   <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 Lucene103BlockTreeTermsWriter} 
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 --&gt; Header, PackedBlockSize
+ *         <li>TermMetadata --&gt; (DocFPDelta|SingletonDocID), PosFPDelta?, 
PosVIntBlockFPDelta?,
+ *             PayFPDelta?
+ *         <li>Header, --&gt; {@link CodecUtil#writeIndexHeader IndexHeader}
+ *         <li>PackedBlockSize, SingletonDocID --&gt; {@link 
DataOutput#writeVInt VInt}
+ *         <li>DocFPDelta, PosFPDelta, PayFPDelta, PosVIntBlockFPDelta --&gt; 
{@link
+ *             DataOutput#writeVLong VLong}
+ *         <li>Footer --&gt; {@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 
&lt;TermPayloads,
+ *             TermOffsets?&gt; 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>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 Lucene103BlockTreeTermsWriter} 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}). Skip data is saved at the end of each term's 
postings. The skip data
+ *       is saved once for the entire postings list.
+ *       <ul>
+ *         <li>docFile(.doc) --&gt; Header, 
&lt;TermFreqs&gt;<sup>TermCount</sup>, Footer
+ *         <li>Header --&gt; {@link CodecUtil#writeIndexHeader IndexHeader}
+ *         <li>TermFreqs --&gt; &lt;PackedBlock32&gt; 
<sup>PackedDocBlockNum/32</sup>, VIntBlock?
+ *         <li>PackedBlock32 --&gt; Level1SkipData, &lt;PackedBlock&gt; 
<sup>32</sup>
+ *         <li>PackedBlock --&gt; Level0SkipData, PackedDocDeltaBlock, 
PackedFreqBlock?
+ *         <li>VIntBlock --&gt;
+ *             
&lt;DocDelta[,Freq?]&gt;<sup>DocFreq-PackedBlockSize*PackedDocBlockNum</sup>
+ *         <li>Level1SkipData --&gt; DocDelta, DocFPDelta, Skip1NumBytes?, 
ImpactLength?, Impacts?,
+ *             PosFPDelta?, NextPosUpto?, PayFPDelta?, NextPayByteUpto?
+ *         <li>Level0SkipData --&gt; Skip0NumBytes, DocDelta, DocFPDelta, 
PackedBlockLength,
+ *             ImpactLength?, Impacts?, PosFPDelta?, NextPosUpto?, 
PayFPDelta?, NextPayByteUpto?
+ *         <li>PackedFreqBlock --&gt; {@link PackedInts PackedInts}, uses 
patching
+ *         <li>PackedDocDeltaBlock --&gt; {@link PackedInts PackedInts}, does 
not use patching
+ *         <li>Footer --&gt; {@link CodecUtil#writeFooter CodecFooter}
+ *       </ul>
+ *       <p>Notes:
+ *       <ul>
+ *         <li>PackedDocDeltaBlock is theoretically generated from two steps:
+ *             <ol>
+ *               <li>Calculate the difference between each document number and 
previous one, and get
+ *                   a d-gaps list (for the first document, use absolute 
value);
+ *               <li>For those d-gaps from first one to
+ *                   PackedDocBlockNum*PackedBlockSize<sup>th</sup>, 
separately encode as packed
+ *                   blocks.
+ *             </ol>
+ *             If frequencies are not omitted, PackedFreqBlock will be 
generated without d-gap step.
+ *         <li>VIntBlock stores remaining d-gaps (along with frequencies when 
possible) with a
+ *             format that encodes DocDelta and Freq:
+ *             <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>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>15, 8, 3
+ *             <p>If frequencies were omitted ({@link IndexOptions#DOCS}) it 
would be this sequence
+ *             of VInts instead:
+ *             <p>7,4
+ *         <li>PackedDocBlockNum is the number of packed blocks for current 
term's docids or
+ *             frequencies. In particular, PackedDocBlockNum = 
floor(DocFreq/PackedBlockSize)
+ *         <li>On skip data, DocDelta is the delta between the last doc of the 
previous block - or
+ *             -1 if there is no previous block - and the last doc of this 
block. This helps know by
+ *             how much the doc ID should be incremented in case the block 
gets skipped.
+ *         <li>Skip0Length is the length of skip data at level 0. Encoding it 
is useful when skip
+ *             data is never needed to quickly skip over skip data, e.g. if 
only using nextDoc(). It
+ *             is also used when only the first fields of skip data are 
needed, in order to skip
+ *             over remaining fields without reading them.
+ *         <li>ImpactLength and Impacts are only stored if frequencies are 
indexed.
+ *         <li>Since positions and payloads are also block encoded, the skip 
should skip to related
+ *             block first, then fetch the values according to in-block 
offset. PosFPSkip and
+ *             PayFPSkip record the file offsets of related block in .pos and 
.pay, respectively.
+ *             While PosBlockOffset indicates which value to fetch inside the 
related block
+ *             (PayBlockOffset is unnecessary since it is always equal to 
PosBlockOffset). Same as
+ *             DocFPSkip, the file offsets are relative to the start of 
current term's TermFreqs,
+ *             and stored as a difference sequence.
+ *         <li>PayByteUpto indicates the start offset of the current payload. 
It is equivalent to
+ *             the sum of the payload lengths in the current block up to 
PosBlockOffset
+ *         <li>ImpactLength is the total length of CompetitiveFreqDelta and 
CompetitiveNormDelta
+ *             pairs. CompetitiveFreqDelta and CompetitiveNormDelta are used 
to safely skip score
+ *             calculation for uncompetitive documents; See {@link
+ *             org.apache.lucene.codecs.CompetitiveImpactAccumulator} for more 
details.
+ *       </ul>
+ * </dl>
+ *
+ * <a id="Positions"></a>
+ *
+ * <dl>
+ *   <dd><b>Positions</b>
+ *       <p>The .pos file contains the lists of positions that each term 
occurs at within documents.
+ *       It also sometimes stores part of payloads and offsets for speedup.
+ *       <ul>
+ *         <li>PosFile(.pos) --&gt; Header, &lt;TermPositions&gt; 
<sup>TermCount</sup>, Footer
+ *         <li>Header --&gt; {@link CodecUtil#writeIndexHeader IndexHeader}
+ *         <li>TermPositions --&gt; &lt;PackedPosDeltaBlock&gt; 
<sup>PackedPosBlockNum</sup>,
+ *             VIntBlock?
+ *         <li>VIntBlock --&gt; &lt;PositionDelta[, PayloadLength?], 
PayloadData?, OffsetDelta?,
+ *             OffsetLength?&gt;<sup>PosVIntCount</sup>
+ *         <li>PackedPosDeltaBlock --&gt; {@link PackedInts PackedInts}
+ *         <li>PositionDelta, OffsetDelta, OffsetLength --&gt; {@link 
DataOutput#writeVInt VInt}
+ *         <li>PayloadData --&gt; {@link DataOutput#writeByte 
byte}<sup>PayLength</sup>
+ *         <li>Footer --&gt; {@link CodecUtil#writeFooter CodecFooter}
+ *       </ul>
+ *       <p>Notes:
+ *       <ul>
+ *         <li>TermPositions are order by term (terms are implicit, from the 
term dictionary), and
+ *             position values for each term document pair are incremental, 
and ordered by document
+ *             number.
+ *         <li>PackedPosBlockNum is the number of packed blocks for current 
term's positions,
+ *             payloads or offsets. In particular, PackedPosBlockNum =
+ *             floor(totalTermFreq/PackedBlockSize)
+ *         <li>PosVIntCount is the number of positions encoded as VInt format. 
In particular,
+ *             PosVIntCount = totalTermFreq - PackedPosBlockNum*PackedBlockSize
+ *         <li>The procedure how PackedPosDeltaBlock is generated is the same 
as PackedDocDeltaBlock
+ *             in chapter <a href="#Frequencies">Frequencies and Skip Data</a>.
+ *         <li>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.
+ *         <li>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>4, 5, 4
+ *         <li>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.
+ *         <li>OffsetDelta/2 is the difference between this position's 
startOffset from the previous
+ *             occurrence (or zero, if this is the first occurrence in this 
document). If
+ *             OffsetDelta is odd, then the length (endOffset-startOffset) 
differs from the previous
+ *             occurrence and an OffsetLength follows. Offset data is only 
written for {@link
+ *             IndexOptions#DOCS_AND_FREQS_AND_POSITIONS_AND_OFFSETS}.
+ *       </ul>
+ * </dl>
+ *
+ * <a id="Payloads"></a>
+ *
+ * <dl>
+ *   <dd><b>Payloads and Offsets</b>
+ *       <p>The .pay file will store payloads and offsets associated with 
certain term-document
+ *       positions. Some payloads and offsets will be separated out into .pos 
file, for performance
+ *       reasons.
+ *       <ul>
+ *         <li>PayFile(.pay): --&gt; Header, &lt;TermPayloads?, 
TermOffsets?&gt;
+ *             <sup>TermCount</sup>, Footer
+ *         <li>Header --&gt; {@link CodecUtil#writeIndexHeader IndexHeader}
+ *         <li>TermPayloads --&gt; &lt;PackedPayLengthBlock, SumPayLength, 
PayData&gt;
+ *             <sup>PackedPayBlockNum</sup>
+ *         <li>TermOffsets --&gt; &lt;PackedOffsetStartDeltaBlock, 
PackedOffsetLengthBlock&gt;
+ *             <sup>PackedPayBlockNum</sup>
+ *         <li>PackedPayLengthBlock, PackedOffsetStartDeltaBlock, 
PackedOffsetLengthBlock --&gt;
+ *             {@link PackedInts PackedInts}
+ *         <li>SumPayLength --&gt; {@link DataOutput#writeVInt VInt}
+ *         <li>PayData --&gt; {@link DataOutput#writeByte 
byte}<sup>SumPayLength</sup>
+ *         <li>Footer --&gt; {@link CodecUtil#writeFooter CodecFooter}
+ *       </ul>
+ *       <p>Notes:
+ *       <ul>
+ *         <li>The order of TermPayloads/TermOffsets will be the same as 
TermPositions, note that
+ *             part of payload/offsets are stored in .pos.
+ *         <li>The procedure how PackedPayLengthBlock and 
PackedOffsetLengthBlock are generated is
+ *             the same as PackedFreqBlock in chapter <a 
href="#Frequencies">Frequencies and Skip
+ *             Data</a>. While PackedStartDeltaBlock follows a same procedure 
as
+ *             PackedDocDeltaBlock.
+ *         <li>PackedPayBlockNum is always equal to PackedPosBlockNum, for the 
same term. It is also
+ *             synonym for PackedOffsetBlockNum.
+ *         <li>SumPayLength is the total length of payloads written within one 
block, should be the
+ *             sum of PayLengths in one packed block.
+ *         <li>PayLength in PackedPayLengthBlock is the length of each payload 
associated with the
+ *             current position.
+ *       </ul>
+ * </dl>
+ *
+ * @lucene.experimental
+ */
+public final class Lucene103PostingsFormat extends PostingsFormat {
+
+  /** Filename extension for some small metadata about how postings are 
encoded. */
+  public static final String META_EXTENSION = "psm";
+
+  /**
+   * Filename extension for document number, frequencies, and skip data. See 
chapter: <a
+   * href="#Frequencies">Frequencies and Skip Data</a>
+   */
+  public static final String DOC_EXTENSION = "doc";
+
+  /** Filename extension for positions. See chapter: <a 
href="#Positions">Positions</a> */
+  public static final String POS_EXTENSION = "pos";
+
+  /**
+   * Filename extension for payloads and offsets. See chapter: <a 
href="#Payloads">Payloads and
+   * Offsets</a>
+   */
+  public static final String PAY_EXTENSION = "pay";
+
+  /** Size of blocks. */
+  public static final int BLOCK_SIZE = ForUtil.BLOCK_SIZE;
+
+  public static final int BLOCK_MASK = BLOCK_SIZE - 1;
+
+  /** We insert skip data on every block and every SKIP_FACTOR=32 blocks. */
+  public static final int LEVEL1_FACTOR = 32;
+
+  /** Total number of docs covered by level 1 skip data: 32 * 128 = 4,096 */
+  public static final int LEVEL1_NUM_DOCS = LEVEL1_FACTOR * BLOCK_SIZE;
+
+  public static final int LEVEL1_MASK = LEVEL1_NUM_DOCS - 1;
+
+  static final String TERMS_CODEC = "Lucene103PostingsWriterTerms";
+  static final String META_CODEC = "Lucene103PostingsWriterMeta";
+  static final String DOC_CODEC = "Lucene103PostingsWriterDoc";
+  static final String POS_CODEC = "Lucene103PostingsWriterPos";
+  static final String PAY_CODEC = "Lucene103PostingsWriterPay";
+
+  static final int VERSION_START = 0;
+
+  /**
+   * Version that started encoding dense blocks as bit sets. Note: the old 
format is a subset of the
+   * new format, so Lucene103PostingsReader is able to read the old format 
without checking the
+   * version.
+   */
+  static final int VERSION_DENSE_BLOCKS_AS_BITSETS = 1;

Review Comment:
   We can remove this version constant now that you're forking the postings 
format, there is no bw compat to deal with.



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