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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