java.lang.Object
org.apache.lucene.search.DocIdSetIterator
org.apache.lucene.backward_codecs.lucene80.IndexedDISI

final class IndexedDISI extends DocIdSetIterator
Disk-based implementation of a DocIdSetIterator which can return the index of the current document, i.e. the ordinal of the current document among the list of documents that this iterator can return. This is useful to implement sparse doc values by only having to encode values for documents that actually have a value.

Implementation-wise, this DocIdSetIterator is inspired of roaring bitmaps and encodes ranges of 65536 documents independently and picks between 3 encodings depending on the density of the range:

  • ALL if the range contains 65536 documents exactly,
  • DENSE if the range contains 4096 documents or more; in that case documents are stored in a bit set,
  • SPARSE otherwise, and the lower 16 bits of the doc IDs are stored in a short.

Only ranges that contain at least one value are encoded.

This implementation uses 6 bytes per document in the worst-case, which happens in the case that all ranges contain exactly one document.

To avoid O(n) lookup time complexity, with n being the number of documents, two lookup tables are used: A lookup table for block offset and index, and a rank structure for DENSE block index lookups.

The lookup table is an array of int-pairs, with a pair for each block. It allows for direct jumping to the block, as opposed to iteration from the current position and forward one block at a time.

Each int-pair entry consists of 2 logical parts:

The first 32 bit int holds the index (number of set bits in the blocks) up to just before the wanted block. The maximum number of set bits is the maximum number of documents, which is < 2^31.

The next int holds the offset in bytes into the underlying slice. As there is a maximum of 2^16 blocks, it follows that the maximum size of any block must not exceed 2^15 bytes to avoid overflow (2^16 bytes if the int is treated as unsigned). This is currently the case, with the largest block being DENSE and using 2^13 + 36 bytes.

The cache overhead is numDocs/1024 bytes.

Note: There are 4 types of blocks: ALL, DENSE, SPARSE and non-existing (0 set bits). In the case of non-existing blocks, the entry in the lookup table has index equal to the previous entry and offset equal to the next non-empty block.

The block lookup table is stored at the end of the total block structure.

The rank structure for DENSE blocks is an array of byte-pairs with an entry for each sub-block (default 512 bits) out of the 65536 bits in the outer DENSE block.

Each rank-entry states the number of set bits within the block up to the bit before the bit positioned at the start of the sub-block. Note that that the rank entry of the first sub-block is always 0 and that the last entry can at most be 65536-2 = 65634 and thus will always fit into an byte-pair of 16 bits.

The rank structure for a given DENSE block is stored at the beginning of the DENSE block. This ensures locality and keeps logistics simple.

  • Field Details

    • BLOCK_SIZE

      private static final int BLOCK_SIZE
      See Also:
    • DENSE_BLOCK_LONGS

      private static final int DENSE_BLOCK_LONGS
      See Also:
    • DEFAULT_DENSE_RANK_POWER

      public static final byte DEFAULT_DENSE_RANK_POWER
      See Also:
    • MAX_ARRAY_LENGTH

      static final int MAX_ARRAY_LENGTH
      See Also:
    • slice

      final IndexInput slice
      The slice that stores the DocIdSetIterator.
    • jumpTableEntryCount

      final int jumpTableEntryCount
    • denseRankPower

      final byte denseRankPower
    • jumpTable

      final RandomAccessInput jumpTable
    • denseRankTable

      final byte[] denseRankTable
    • cost

      final long cost
    • block

      int block
    • blockEnd

      long blockEnd
    • denseBitmapOffset

      long denseBitmapOffset
    • nextBlockIndex

      int nextBlockIndex
    • method

    • doc

      int doc
    • index

      int index
    • exists

      boolean exists
    • word

      long word
    • wordIndex

      int wordIndex
    • numberOfOnes

      int numberOfOnes
    • denseOrigoIndex

      int denseOrigoIndex
    • gap

      int gap
  • Constructor Details

  • Method Details

    • flush

      private static void flush(int block, FixedBitSet buffer, int cardinality, byte denseRankPower, IndexOutput out) throws IOException
      Throws:
      IOException
    • createRank

      private static byte[] createRank(FixedBitSet buffer, byte denseRankPower)
    • writeBitSet

      static short writeBitSet(DocIdSetIterator it, IndexOutput out) throws IOException
      Writes the docIDs from it to out, in logical blocks, one for each 65536 docIDs in monotonically increasing gap-less order. DENSE blocks uses DEFAULT_DENSE_RANK_POWER of 9 (every 512 docIDs / 8 longs). The caller must keep track of the number of jump-table entries (returned by this method) as well as the denseRankPower (9 for this method) and provide them when constructing an IndexedDISI for reading.
      Parameters:
      it - the document IDs.
      out - destination for the blocks.
      Returns:
      the number of jump-table entries following the blocks, -1 for no entries. This should be stored in meta and used when creating an instance of IndexedDISI.
      Throws:
      IOException - if there was an error writing to out.
    • writeBitSet

      static short writeBitSet(DocIdSetIterator it, IndexOutput out, byte denseRankPower) throws IOException
      Writes the docIDs from it to out, in logical blocks, one for each 65536 docIDs in monotonically increasing gap-less order. The caller must keep track of the number of jump-table entries (returned by this method) as well as the denseRankPower and provide them when constructing an IndexedDISI for reading.
      Parameters:
      it - the document IDs.
      out - destination for the blocks.
      denseRankPower - for IndexedDISI.Method.DENSE blocks, a rank will be written every 2^denseRankPower docIDs. Values < 7 (every 128 docIDs) or > 15 (every 32768 docIDs) disables DENSE rank. Recommended values are 8-12: Every 256-4096 docIDs or 4-64 longs. DEFAULT_DENSE_RANK_POWER is 9: Every 512 docIDs. This should be stored in meta and used when creating an instance of IndexedDISI.
      Returns:
      the number of jump-table entries following the blocks, -1 for no entries. This should be stored in meta and used when creating an instance of IndexedDISI.
      Throws:
      IOException - if there was an error writing to out.
    • addJumps

      private static int[] addJumps(int[] jumps, long offset, int index, int startBlock, int endBlock)
    • flushBlockJumps

      private static short flushBlockJumps(int[] jumps, int blockCount, IndexOutput out) throws IOException
      Throws:
      IOException
    • createBlockSlice

      public static IndexInput createBlockSlice(IndexInput slice, String sliceDescription, long offset, long length, int jumpTableEntryCount) throws IOException
      Helper method for using IndexedDISI(IndexInput, RandomAccessInput, int, byte, long). Creates a disiSlice for the IndexedDISI data blocks, without the jump-table.
      Parameters:
      slice - backing data, holding both blocks and jump-table.
      sliceDescription - human readable slice designation.
      offset - relative to the backing data.
      length - full length of the IndexedDISI, including blocks and jump-table data.
      jumpTableEntryCount - the number of blocks covered by the jump-table.
      Returns:
      a jumpTable containing the block jump-data or null if no such table exists.
      Throws:
      IOException - if a RandomAccessInput could not be created from slice.
    • createJumpTable

      public static RandomAccessInput createJumpTable(IndexInput slice, long offset, long length, int jumpTableEntryCount) throws IOException
      Helper method for using IndexedDISI(IndexInput, RandomAccessInput, int, byte, long). Creates a RandomAccessInput covering only the jump-table data or null.
      Parameters:
      slice - backing data, holding both blocks and jump-table.
      offset - relative to the backing data.
      length - full length of the IndexedDISI, including blocks and jump-table data.
      jumpTableEntryCount - the number of blocks covered by the jump-table.
      Returns:
      a jumpTable containing the block jump-data or null if no such table exists.
      Throws:
      IOException - if a RandomAccessInput could not be created from slice.
    • docID

      public int docID()
      Description copied from class: DocIdSetIterator
      Returns the following:
      Specified by:
      docID in class DocIdSetIterator
    • advance

      public int advance(int target) throws IOException
      Description copied from class: DocIdSetIterator
      Advances to the first beyond the current whose document number is greater than or equal to target, and returns the document number itself. Exhausts the iterator and returns DocIdSetIterator.NO_MORE_DOCS if target is greater than the highest document number in the set.

      The behavior of this method is undefined when called with target ≤ current , or after the iterator has exhausted. Both cases may result in unpredicted behavior.

      When target > current it behaves as if written:

       int advance(int target) {
         int doc;
         while ((doc = nextDoc()) < target) {
         }
         return doc;
       }
       
      Some implementations are considerably more efficient than that.

      NOTE: this method may be called with DocIdSetIterator.NO_MORE_DOCS for efficiency by some Scorers. If your implementation cannot efficiently determine that it should exhaust, it is recommended that you check for that value in each call to this method.

      Specified by:
      advance in class DocIdSetIterator
      Throws:
      IOException
    • advanceExact

      public boolean advanceExact(int target) throws IOException
      Throws:
      IOException
    • advanceBlock

      private void advanceBlock(int targetBlock) throws IOException
      Throws:
      IOException
    • readBlockHeader

      private void readBlockHeader() throws IOException
      Throws:
      IOException
    • nextDoc

      public int nextDoc() throws IOException
      Description copied from class: DocIdSetIterator
      Advances to the next document in the set and returns the doc it is currently on, or DocIdSetIterator.NO_MORE_DOCS if there are no more docs in the set.
      NOTE: after the iterator has exhausted you should not call this method, as it may result in unpredicted behavior.
      Specified by:
      nextDoc in class DocIdSetIterator
      Throws:
      IOException
    • index

      public int index()
    • cost

      public long cost()
      Description copied from class: DocIdSetIterator
      Returns the estimated cost of this DocIdSetIterator.

      This is generally an upper bound of the number of documents this iterator might match, but may be a rough heuristic, hardcoded value, or otherwise completely inaccurate.

      Specified by:
      cost in class DocIdSetIterator
    • rankSkip

      private static void rankSkip(IndexedDISI disi, int targetInBlock) throws IOException
      If the distance between the current position and the target is > 8 words, the rank cache will be used to guarantee a worst-case of 1 rank-lookup and 7 word-read-and-count-bits operations. Note: This does not guarantee a skip up to target, only up to nearest rank boundary. It is the responsibility of the caller to iterate further to reach target.
      Parameters:
      disi - standard DISI.
      targetInBlock - lower 16 bits of the target
      Throws:
      IOException - if a DISI seek failed.