Package org.apache.lucene.codecs.lucene90

Class IndexedDISI

  • public 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 less than 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.

    NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    • Field Detail

      • DEFAULT_DENSE_RANK_POWER

        public static final byte DEFAULT_DENSE_RANK_POWER
        See Also:
        Constant Field Values

    • Constructor Detail

      • IndexedDISI

        public IndexedDISI​(IndexInput in,
                   long offset,
                   long length,
                   int jumpTableEntryCount,
                   byte denseRankPower,
                   long cost)
            throws IOException
        This constructor always creates a new blockSlice and a new jumpTable from in, to ensure that operations are independent from the caller. See IndexedDISI(IndexInput, RandomAccessInput, int, byte, long) for re-use of blockSlice and jumpTable.
        Parameters:
        in - backing data.
        offset - starting offset for blocks in the backing data.
        length - the number of bytes holding blocks and jump-table in the backing data.
        jumpTableEntryCount - the number of blocks covered by the jump-table. This must match the number returned by writeBitSet(DocIdSetIterator, IndexOutput, byte).
        denseRankPower - the number of docIDs covered by each rank entry in DENSE blocks, expressed as 2^denseRankPower. This must match the power given in writeBitSet(DocIdSetIterator, IndexOutput, byte)
        cost - normally the number of logical docIDs.
        Throws:
        IOException

    • Method Detail

      • writeBitSet

        public 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.

      • 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.

      • 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

      • 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