Github user tokee commented on a diff in the pull request: https://github.com/apache/lucene-solr/pull/525#discussion_r245040952 --- Diff: lucene/core/src/java/org/apache/lucene/codecs/lucene80/IndexedDISI.java --- @@ -0,0 +1,542 @@ +/* + * 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.lucene80; + +import java.io.DataInput; +import java.io.IOException; + +import org.apache.lucene.search.DocIdSetIterator; +import org.apache.lucene.store.IndexInput; +import org.apache.lucene.store.IndexOutput; +import org.apache.lucene.store.RandomAccessInput; +import org.apache.lucene.util.ArrayUtil; +import org.apache.lucene.util.BitSetIterator; +import org.apache.lucene.util.FixedBitSet; +import org.apache.lucene.util.RoaringDocIdSet; + +/** + * Disk-based implementation of a {@link 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. + * <p>Implementation-wise, this {@link DocIdSetIterator} is inspired of + * {@link RoaringDocIdSet roaring bitmaps} and encodes ranges of {@code 65536} + * documents independently and picks between 3 encodings depending on the + * density of the range:<ul> + * <li>{@code ALL} if the range contains 65536 documents exactly, + * <li>{@code DENSE} if the range contains 4096 documents or more; in that + * case documents are stored in a bit set, + * <li>{@code SPARSE} otherwise, and the lower 16 bits of the doc IDs are + * stored in a {@link DataInput#readShort() short}. + * </ul> + * <p>Only ranges that contain at least one value are encoded. + * <p>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 blockCache and index, and a rank structure + * for DENSE block lookups. + * + * The lookup table is an array of {@code long}s with an entry 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 long entry consists of 2 logical parts: + * + * The first 31 bits hold the index (number of set bits in the blocks) up to just before the + * wanted block. The next 33 bits 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 2^17 bits to avoid overflow. This is currently the case, with the largest + * block being DENSE and using 2^16 + 288 bits, and is likely to continue to hold as using + * more than double the amount of bits is unlikely to be an efficient representation. + * 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 unsigned {@code short}s with an entry + * for each sub-block of 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 65536-512 = 65024 and thus will always fit into an unsigned short. + * + * The rank structure for a given DENSE block is stored at the beginning of the DENSE block. + * This ensures locality and keeps logistics simple. + * + * @lucene.internal + */ +final class IndexedDISI extends DocIdSetIterator { + + // jump-table time/space trade-offs to consider: + // The block offsets and the block indexes could be stored in more compressed form with + // two PackedInts or two MonotonicDirectReaders. + // The DENSE ranks (128 shorts = 256 bytes) could likewise be compressed. As there is at least + // 4096 set bits in DENSE blocks, there will be at least one rank with 2^12 bits, so it is + // doubtful if there is much to gain here. + + private static final int BLOCK_SIZE = 65536; // The number of docIDs that a single block represents + static final int BLOCK_BITS = 16; + private static final long BLOCK_INDEX_SHIFT = 33; // Number of bits to shift a lookup entry to get the index + private static final long BLOCK_INDEX_MASK = ~0L << BLOCK_INDEX_SHIFT; // The index bits in a lookup entry + private static final long BLOCK_LOOKUP_MASK = ~BLOCK_INDEX_MASK; // The offset bits in a lookup entry + + private static final int DENSE_BLOCK_LONGS = BLOCK_SIZE/Long.SIZE; // 1024 + private static final int RANK_BLOCK_SIZE = 512; // The number of docIDs/bits in each rank-sub-block within a DENSE block + private static final int RANK_BLOCK_LONGS = RANK_BLOCK_SIZE/Long.SIZE; // The number of longs making up a rank-block (8) + private static final int RANK_BLOCK_BITS = 9; + private static final int RANKS_PER_BLOCK = BLOCK_SIZE / RANK_BLOCK_SIZE; // 128 + + static final int MAX_ARRAY_LENGTH = (1 << 12) - 1; + + private static void flush(int block, FixedBitSet buffer, int cardinality, IndexOutput out) throws IOException { + assert block >= 0 && block < 65536; + out.writeShort((short) block); + assert cardinality > 0 && cardinality <= 65536; + out.writeShort((short) (cardinality - 1)); + if (cardinality > MAX_ARRAY_LENGTH) { + if (cardinality != 65536) { // all docs are set + final byte[] rank = createRank(buffer); + out.writeBytes(rank, rank.length); + for (long word : buffer.getBits()) { + out.writeLong(word); + } + } + } else { + BitSetIterator it = new BitSetIterator(buffer, cardinality); + for (int doc = it.nextDoc(); doc != DocIdSetIterator.NO_MORE_DOCS; doc = it.nextDoc()) { + out.writeShort((short) doc); + } + } + } + + // Creates a DENSE rank-entry (the number of set bits up to a given point) for the buffer. + // One rank-entry for every 512 bits/8 longs for a total of 128 * 16 bits. + // Represented as a byte[] for fast flushing and mirroring of the retrieval representation. + private static byte[] createRank(FixedBitSet buffer) { + final byte[] rank = new byte[RANKS_PER_BLOCK*2]; + final long[] bits = buffer.getBits(); + int bitCount = 0; + for (int word = 0 ; word < DENSE_BLOCK_LONGS ; word++) { + if ((word & 0x07) == 0) { // Every 8 longs + rank[word >> 2] = (byte)(bitCount>>8); + rank[(word >> 2)+1] = (byte)(bitCount & 0xFF); + } + bitCount += Long.bitCount(bits[word]); + } + return rank; + } + + /** + * Writes the docIDs from it to out, in logical blocks, one for each 65536 docIDs in monotonically + * increasing gap-less order. + * @param it the document IDs. + * @param out destination for the blocks. + * @throws IOException if there was an error writing to out. + * @return the number of jump-table entries following the blocks, -1 for no entries. This should be stored in meta. + */ + static short writeBitSet(DocIdSetIterator it, IndexOutput out) throws IOException { + final long origo = out.getFilePointer(); // All jumps are relative to the origo + int totalCardinality = 0; + int blockCardinality = 0; + final FixedBitSet buffer = new FixedBitSet(1<<16); + long[] jumps = new long[ArrayUtil.oversize(1, Long.BYTES)]; + jumps[0] = out.getFilePointer()-origo; // First block starts at index 0 + int prevBlock = -1; + int jumpBlockIndex = 0; + + for (int doc = it.nextDoc(); doc != DocIdSetIterator.NO_MORE_DOCS; doc = it.nextDoc()) { + final int block = doc >>> 16; + if (prevBlock != -1 && block != prevBlock) { + // Track offset+index from previous block up to current + jumps = addJumps(jumps, out.getFilePointer()-origo, totalCardinality, jumpBlockIndex, prevBlock+1); + jumpBlockIndex = prevBlock+1; + // Flush block + flush(prevBlock, buffer, blockCardinality, out); + // Reset for next block + buffer.clear(0, buffer.length()); + totalCardinality += blockCardinality; + blockCardinality = 0; + } + buffer.set(doc & 0xFFFF); + blockCardinality++; + prevBlock = block; + } + if (blockCardinality > 0) { + jumps = addJumps(jumps, out.getFilePointer()-origo, totalCardinality, jumpBlockIndex, prevBlock+1); + flush(prevBlock, buffer, blockCardinality, out); + buffer.clear(0, buffer.length()); + prevBlock++; + } --- End diff -- Ah... The `writeBitset`-method takes an `DocIDSetIterator`, which has no concept of the range of documents, only the ones which are present. It is not possible to make jump-table entries for trailing blank blocks as there are none at write time. I have changed `IndexedDISI.advanceBlock` to handle jumps outside of the known range by jumping to the last known entry, which will always be the `NO_MORE_DOCS`-entry.
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