LUCENE-3982: improve javadocs

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/pforcodec_3892@1374573 13f79535-47bb-0310-9956-ffa450edef68

lucene/core/src/java/org/apache/lucene/codecs/MultiLevelSkipListWriter.java

@@ -26,6 +26,8 @@ import org.apache.lucene.util.MathUtil;
 /**
  * This abstract class writes skip lists with multiple levels.
  * 
+ * <pre>
+ *
  * Example for skipInterval = 3:
  *                                                     c            (skip level 2)
  *                 c                 c                 c            (skip level 1) 
@@ -45,6 +47,7 @@ import org.apache.lucene.util.MathUtil;
  * 
  * While this class takes care of writing the different skip levels,
  * subclasses must define the actual format of the skip data.
+ * </pre>
  * @lucene.experimental
  */
 

lucene/core/src/java/org/apache/lucene/codecs/block/BlockPostingsFormat.java

@@ -30,10 +30,290 @@ import org.apache.lucene.index.SegmentReadState;
 import org.apache.lucene.index.SegmentWriteState;
 import org.apache.lucene.util.IOUtils;
 
+// javadocs
+import org.apache.lucene.codecs.MultiLevelSkipListWriter;
+import org.apache.lucene.codecs.CodecUtil;
+import org.apache.lucene.codecs.lucene40.Lucene40PostingsFormat;
+import org.apache.lucene.store.DataOutput;
+import org.apache.lucene.index.DocsEnum;
+import org.apache.lucene.index.FieldInfo.IndexOptions;
+import org.apache.lucene.index.FieldInfos;
+import org.apache.lucene.util.fst.FST;
+import org.apache.lucene.util.packed.PackedInts;
+
 /**
- * Encodes/decode postings in packed int blocks for faster
+ * Block postings format, which encodes postings in packed int blocks 
- * decode.
+ * for faster decode.
+ *
+ * <p>
+ * Basic idea:
+ * <ul>
+ *   <li>
+ *   <b>Packed Block and VInt Block</b>: 
+ *   <p>In packed block, integers are encoded with the same bit width ({@link PackedInts packed format}),
+ *      the block size (i.e. number of integers inside block) is fixed. </p>
+ *   <p>In VInt block, integers are encoded as {@link DataOutput#writeVInt VInt}, 
+ *      the block size is variable.</p>
+ *   </li>
+ *
+ *   <li> 
+ *   <b>Block structure</b>: 
+ *   <p>When the postings is long enough, BlockPostingsFormat will try to encode most integer data 
+ *      as packed block.</p> 
+ *   <p>Take a term with 259 documents as example, the first 256 document ids are encoded as two packed 
+ *      blocks, while the remaining 3 as one VInt block. </p>
+ *   <p>Different kinds of data are always encoded separately into different packed blocks, but may 
+ *      possible be encoded into a same VInt block. </p>
+ *   <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;.</p>
+ *   </li>
+ *
+ *   <li>
+ *   <b>Skipper setting</b>: 
+ *   <p>The structure of skip table is quite similar to Lucene40PostingsFormat. 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.</p>
+ *   </li>
+ *
+ *   <li>
+ *   <b>Positions, Payloads, and Offsets</b>: 
+ *   <p>A position is an integer indicating where the term occured in 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. </p>
+ *   <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 centralizedly or separately. 
+ *   <p>For all the cases, payloads and offsets are stored together. When encoded as 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 block, all these three are 
+ *      stored in .pos (so as payload metadata).</p>
+ *   </li>
+ * </ul>
+ * </p>
+ *
+ * <p>
+ * Files and detailed format:
+ * <ul>
+ *   <li><tt>.tim</tt>: <a href="#Termdictionary">Term Dictionary</a></li>
+ *   <li><tt>.tip</tt>: <a href="#Termindex">Term Index</a></li>
+ *   <li><tt>.doc</tt>: <a href="#Frequencies">Frequencies and Skip Data</a></li>
+ *   <li><tt>.pos</tt>: <a href="#Positions">Positions</a></li>
+ *   <li><tt>.pay</tt>: <a href="#Payloads">Payloads and Offsets</a></li>
+ * </ul>
+ * </p>
+ *
+ * <a name="Termdictionary" id="Termdictionary"></a>
+ * <dl>
+ * <dd>
+ * <b>Term Dictionary</b>
+ *
+ * <p>The .tim file format is quite similar to Lucene40PostingsFormat, 
+ *  with minor difference in MetadataBlock</p>
+ *
+ * <ul>
+ * <!-- TODO: expand on this, its not really correct and doesnt explain sub-blocks etc -->
+ *   <li>TermDictionary(.tim) --&gt; Header, DirOffset, PostingsHeader, PackedBlockSize, 
+ *                                   &lt;Block&gt;<sup>NumBlocks</sup>, FieldSummary</li>
+ *   <li>Block --&gt; SuffixBlock, StatsBlock, MetadataBlock</li>
+ *   <li>SuffixBlock --&gt; EntryCount, SuffixLength, Byte<sup>SuffixLength</sup></li>
+ *   <li>StatsBlock --&gt; StatsLength, &lt;DocFreq, TotalTermFreq&gt;<sup>EntryCount</sup></li>
+ *   <li>MetadataBlock --&gt; MetaLength, &lt;DocFPDelta, 
+ *                            &lt;PosFPDelta, PosBlockFPDelta?, PayFPDelta?&gt;?, 
+ *                            SkipFPDelta? &gt;<sup>EntryCount</sup></li>
+ *   <li>FieldSummary --&gt; NumFields, &lt;FieldNumber, NumTerms, RootCodeLength, 
+ *                           Byte<sup>RootCodeLength</sup>, SumDocFreq, DocCount&gt;
+ *                           <sup>NumFields</sup></li>
+ *   <li>Header, PostingsHeader --&gt; {@link CodecUtil#writeHeader CodecHeader}</li>
+ *   <li>DirOffset --&gt; {@link DataOutput#writeLong Uint64}</li>
+ *   <li>PackedBlockSize, EntryCount, SuffixLength, StatsLength, DocFreq, MetaLength, 
+ *       PosBlockFPDelta, SkipFPDelta, NumFields, FieldNumber, RootCodeLength, DocCount --&gt; 
+ *       {@link DataOutput#writeVInt VInt}</li>
+ *   <li>TotalTermFreq, DocFPDelta, PosFPDelta, NumTerms, SumTotalTermFreq, SumDocFreq --&gt; 
+ *       {@link DataOutput#writeVLong VLong}</li>
+ * </ul>
+ * <p>Notes:</p>
+ * <ul>
+ *    <li>Here explains MetadataBlock only, other fields are mentioned in 
+ *   <!--NOTE: change this manual html link, when directory structure is changed. -->
+ *   <a href="../../../../../org/apache/lucene/codecs/lucene40/Lucene40PostingsFormat.html#Termdictionary">Lucene40PostingsFormat:TermDictionary</a>
+ *    </li>
+ *    <li>PackedBlockSize is 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 accerlerate {@link DocsEnum#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).</li>
+ *    <li>PayFPDelta determines the position of this term's payload or offset data within the .pay file.
+ *        Similar to DocFPDelta, it is the difference between two file positions (or neglected, 
+ *        for fields that omit payloads and offsets, or for the first term in the block).</li>
+ *    <!--TODO: not quite sure, what is the difference?-->
+ *    <li>PosFPDelta and PosBlockFPDelta determine the position of this term's TermPositions within 
+ *        the .pos file. 
+ *    <li>PosBlockFPDelta determines the position of this term's TermPositions within the .pos file. 
+ *        Similar to DocFPDelta, it is the difference between two file positions (or neglected, 
+ *        for fields that omit position data, or for the first term in the block).</li>
+ *    <li>SkipFPDelta determines the position of this term's SkipData within the .doc
+ *        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 SkipMinimum, 
+ *        (i.e. 8 in BlockPostingsFormat).</li>
+ * </ul>
+ * </dd>
+ * </dl>
+ *
+ * <a name="Termindex" id="Termindex"></a>
+ * <dl>
+ * <dd>
+ * <b>Term Index</b>
+ * <p>The .tim file format is mentioned in
+ *   <!--NOTE: change this manual html link, when directory structure is changed. -->
+ *   <a href="../../../../../org/apache/lucene/codecs/lucene40/Lucene40PostingsFormat.html#Termindex">Lucene40PostingsFormat:TermIndex</a>
+ * </dd>
+ * </dl>
+ *
+ *
+ * <a name="Frequencies" 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_ONLY}). It also saves skip data to the beginning of 
+ * each packed or VInt block, when the length of document list is larger than packed block size.</p>
+ *
+ * <ul>
+ *   <li>docFile(.doc) --&gt; Header, &lt; TermFreqs, SkipData? &gt;<sup>TermCount</sup></li>
+ *   <li>Header --&gt; {@link CodecUtil#writeHeader CodecHeader}</li>
+ *   <li>TermFreqs --&gt; &lt; PackedBlock &gt; <sup>PackedDocBlockNum</sup>,  
+ *                        VIntBlock? </li>
+ *   <li>PackedBlock --&gt; PackedDocDeltaBlock, PackedFreqBlock?
+ *   <li>VIntBlock --&gt; &lt; DocDelta[, Freq?] &gt;<sup>DocFreq-PackedBlockSize*PackedDocBlockNum</sup>
+ *   <li>SkipData --&gt; &lt;&lt;SkipLevelLength, SkipLevel&gt;
+ *       <sup>NumSkipLevels-1</sup>, SkipLevel&gt; &lt;SkipDatum?&gt;</li>
+ *   <li>SkipLevel --&gt; &lt;SkipDatum&gt; <sup>TrimmedDocFreq/(PackedBlockSize^(Level + 1))</sup></li>
+ *   <li>SkipDatum --&gt; DocSkip, DocFPSkip, &lt; PosFPSkip, PosBlockOffset, PayLength?, 
+ *                        OffsetStart?, PayFPSkip? &gt;?, SkipChildLevelPointer?</li>
+ *   <li>PackedDocDeltaBlock, PackedFreqBlock --&gt; {@link PackedInts PackedInts}</li>
+ *   <li>DocDelta,Freq,DocSkip,DocFPSkip,PosFPSkip,PosBlockOffset,PayLength,OffsetStart,PayFPSkip --&gt; 
+ *   {@link DataOutput#writeVInt VInt}</li>
+ *   <li>SkipChildLevelPointer --&gt; {@link DataOutput#writeVLong VLong}</li>
+ * </ul>
+ * <p>Notes:</p>
+ * <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>
+ *       <li>For those d-gaps from first one to PackedDocBlockNum*PackedBlockSize<sup>th</sup>, 
+ *           seperately encode as packed blocks.</li>
+ *     </ol>
+ *     If frequencies are not omitted, PackedFreqBlock will be generated without d-gap step.
+ *   </li>
+ *   <li>VIntBlock stores remaining d-gaps (along with frequencies when possible) with a format 
+ *       mentioned in
+ *   <!--NOTE: change this manual html link, when directory structure is changed. -->
+ *   <a href="../../../../../org/apache/lucene/codecs/lucene40/Lucene40PostingsFormat.html#Frequencies">Lucene40PostingsFormat:Frequencies</a>
+ *   </li>
+ *   <li>PackedDocBlockNum is the number of packed blocks for current term's docids or frequencies. 
+ *       In particular, PackedDocBlockNum = floor(DocFreq/PackedBlockSize) </li>
+ *   <li>TrimmedDocFreq = DocFreq % PackedBlockSize == 0 ? DocFreq - 1 : DocFreq. 
+ *       We use this trick since the definition of skip entry is a little different from base interface.
+ *       In {@link MultiLevelSkipListWriter}, skip data is assumed to be saved for
+ *       skipInterval<sup>th</sup>, 2*skipInterval<sup>th</sup> ... posting in the list. However, 
+ *       in BlockPostingsFormat, the skip data is saved for skipInterval+1<sup>th</sup>, 
+ *       2*skipInterval+1<sup>th</sup> ... posting (skipInterval==PackedBlockSize in this case). 
+ *       When DocFreq is multiple of PackedBlockSize, MultiLevelSkipListWriter will expect one 
+ *       more skip data than BlockSkipWriter. </li>
+ *   <li>SkipDatum is the metadata of one skip entry.
+ *      For the first block (no matter packed or VInt), it is omitted.</li>
+ *   <li>DocSkip records the document number of every PackedBlockSize<sup>th</sup> document number in
+ *       the postings(i.e. last document number in each packed block). On disk it is stored as the 
+ *       difference from previous value in the sequence. </li>
+ *   <li>DocFPSkip records the file offsets of each block (excluding )posting at 
+ *       PackedBlockSize+1<sup>th</sup>, 2*PackedBlockSize+1<sup>th</sup> ... , in DocFile. 
+ *       The file offsets are relative to the start of current term's TermFreqs. 
+ *       On disk it is also stored as the difference from previous SkipDatum in the sequence.</li>
+ *   <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>
+ *   <li>PayLength indicates the length of last payload.</li>
+ *   <li>OffsetStart indicates the first value of last offset pair.</li>
+ * </ul>
+ * </dd>
+ * </dl>
+ *
+ * <a name="Positions" id="Positions"></a>
+ * <dl>
+ * <dd>
+ * <b>Positions</b>
+ * <ul>
+ *   <li>Pos(.prx) --&gt; Header, &lt;TermPositions&gt; <sup>TermCount</sup></li>
+ *   <li>Header --&gt; {@link CodecUtil#writeHeader CodecHeader}</li>
+ *   <li>TermPositions --&gt; &lt; PackedPosDeltaBlock &gt; <sup>PackedPosBlockNum</sup>,  
+ *                            VIntBlock? </li>
+ *   <li>VIntBlock --&gt; PosVIntCount &lt; PosDelta[, PayLength?], PayData?, 
+ *                        OffsetStartDelta?, OffsetLength? &gt;<sup>PosVIntCount</sup>
+ *   <li>PackedPosDeltaBlock --&gt; {@link PackedInts PackedInts}</li>
+ *   <li>PosVIntCount, PosDelta, OffsetStartDelta, OffsetLength --&gt; 
+ *       {@link DataOutput#writeVInt VInt}</li>
+ *   <li>PayData --&gt; {@link DataOutput#writeByte byte}<sup>PayLength</sup></li>
+ * </ul>
+ * <p>Notes:</p>
+ * <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>
+ *   <li>The procedure how PackedPosDeltaBlock is generated is the same as PackedDocDeltaBlock 
+ *       in chapter <a href="#Frequencies">Frequencies and Skip Data</a>.</li>
+ *   <li>PosDelta is the same as the format mentioned in 
+ *   <!--NOTE: change this manual html link, when directory structure is changed. -->
+ *   <a href="../../../../../org/apache/lucene/codecs/lucene40/Lucene40PostingsFormat.html#Positions">Lucene40PostingsFormat:Positions</a>
+ *   </li>
+ *   <li>OffsetStartDelta is the difference between this position's startOffset from the previous 
+ *       occurrence (or zero, if this is the first occurrence in this document).</li>
+ *   <li>OffsetLength indicates the length of the current offset (endOffset-startOffset).</li>
+ *   <li>PayloadData is the blob of metadata associated with current position.</li>
+ * </ul>
+ * </dd>
+ * </dl>
+ *
+ * <a name="Payloads" id="Payloads"></a>
+ * <dl>
+ * <dd>
+ * <b>Payloads and Offsets</b>
+ * <ul>
+ *   <li>PayFile(.pay): --&gt; Header, &lt;TermPayloads, TermOffsets?&gt; <sup>TermCount</sup></li>
+ *   <li>Header --&gt; {@link CodecUtil#writeHeader CodecHeader}</li>
+ *   <li>TermPayloads --&gt; &lt; PackedPayLengthBlock, PayBlockLength, PayData, PackedOffsetStartDeltaBlock?, PackedOffsetLengthBlock &gt; <sup>PackedPayBlockNum</sup>
+ *   <li>PackedPayLengthBlock, PackedOffsetStartDeltaBlock, PackedOffsetLengthBlock --&gt; {@link PackedInts PackedInts}</li>
+ *   <li>PayBlockLength --&gt; {@link DataOutput#writeVInt VInt}</li>
+ *   <li>PayData --&gt; {@link DataOutput#writeByte byte}<sup>PayBlockLength</sup></li>
+ * </ul>
+ * <p>Notes:</p>
+ * <ul>
+ *   <li>The order of TermPayloads/TermOffsets will be the same as TermPositions, note that part of 
+ *       payload/offsets are stored in .pos.</li>
+ *   <li>The procedure how PackedPayLengthBlock is generated is the same as PackedFreqBlock 
+ *       in chapter <a href="#Frequencies">Frequencies and Skip Data</a>.</li>
+ *   <li>PayBlockLength is the total length of payloads written within one block, should be the sum
+ *       of PayLengths in one packed block.</li>
+ *   <li>PayLength is the length of each payload, associated with current position.</li>
+ * </u>
+ * </dd>
+ * </dl>
+ * </p>
+ *
  */
+
 public final class BlockPostingsFormat extends PostingsFormat {
   public static final String DOC_EXTENSION = "doc";
   public static final String POS_EXTENSION = "pos";
@@ -42,7 +322,7 @@ public final class BlockPostingsFormat extends PostingsFormat {
   private final int minTermBlockSize;
   private final int maxTermBlockSize;
 
-  // NOTE: must be factor of 64 because of PackedInts long-aligned encoding/decoding
+  // NOTE: must be multiple of 64 because of PackedInts long-aligned encoding/decoding
   public final static int BLOCK_SIZE = 128;
 
   public BlockPostingsFormat() {

lucene/core/src/java/org/apache/lucene/codecs/block/BlockPostingsReader.java

@@ -52,7 +52,7 @@ import org.apache.lucene.util.IOUtils;
  * @see BlockSkipReader for details
  *
  */
-public final class BlockPostingsReader extends PostingsReaderBase {
+final class BlockPostingsReader extends PostingsReaderBase {
 
   private final IndexInput docIn;
   private final IndexInput posIn;

lucene/core/src/java/org/apache/lucene/codecs/lucene40/Lucene40PostingsFormat.java

@@ -159,7 +159,7 @@ import org.apache.lucene.util.fst.FST; // javadocs
  * with the frequency of the term in that document (except when frequencies are
  * omitted: {@link IndexOptions#DOCS_ONLY}).</p>
  * <ul>
- *   <li>FreqFile (.frq) --&gt; Header, &lt;TermFreqs, SkipData&gt; <sup>TermCount</sup></li>
+ *   <li>FreqFile (.frq) --&gt; Header, &lt;TermFreqs, SkipData?&gt; <sup>TermCount</sup></li>
  *   <li>Header --&gt; {@link CodecUtil#writeHeader CodecHeader}</li>
  *   <li>TermFreqs --&gt; &lt;TermFreq&gt; <sup>DocFreq</sup></li>
  *   <li>TermFreq --&gt; DocDelta[, Freq?]</li>