Github user tokee commented on a diff in the pull request:
https://github.com/apache/lucene-solr/pull/525#discussion_r242070202
--- Diff:
lucene/core/src/java/org/apache/lucene/codecs/lucene80/IndexedDISI.java ---
@@ -0,0 +1,536 @@
+/*
+ * 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.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 holds the index (number of set bits in the blocks) up
to just before the
+ * wanted block. The next 33 bits holds the offset 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 + 32 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 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 = 512/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;
+
+ static final int MAX_ARRAY_LENGTH = (1 << 12) - 1;
+
+ private final long jumpTableOffset; // If -1, the use of jump-table is
disabled
+ private final long jumpTableEntryCount;
+
+ 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 short[] rank = createRank(buffer);
+ for (int i = 0 ; i < rank.length ; i++) {
+ out.writeShort(rank[i]);
+ }
+ 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 shorts.
+ private static short[] createRank(FixedBitSet buffer) {
+ final short[] rank = new short[128];
+ final long[] bits = buffer.getBits();
+ int bitCount = 0;
+ for (int word = 0 ; word < 1024 ; word++) {
+ if (word >> 3 << 3 == word) { // Every 8 longs
+ rank[word >> 3] = (short)bitCount;
+ }
+ 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.
+ */
+ static void 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(100, 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++;
+ }
+ final int lastBlock = prevBlock == -1 ? 0 : prevBlock;
+// jumps = addJumps(jumps, out.getFilePointer(), totalCardinality,
lastBlock, lastBlock+1);
+ // NO_MORE_DOCS is stored explicitly
+ buffer.set(DocIdSetIterator.NO_MORE_DOCS & 0xFFFF);
+ flush(DocIdSetIterator.NO_MORE_DOCS >>> 16, buffer, 1, out);
+ // offset+index jump-table stored at the end
+ flushBlockJumps(jumps, lastBlock, out, origo);
+ }
+
+ // Adds entries to the offset & index jump-table for blocks
+ private static long[] addJumps(long[] jumps, long offset, long index,
int startBlock, int endBlock) {
+ jumps = ArrayUtil.grow(jumps, endBlock +1);
+ final long jump = (index << BLOCK_INDEX_SHIFT) | offset;
+ for (int b = startBlock; b < endBlock; b++) {
+ jumps[b] = jump;
+ }
+ return jumps;
+ }
+
+ // Flushes the offet & index jump-table for blocks. This should be the
last data written to out
+ private static void flushBlockJumps(long[] jumps, int blockCount,
IndexOutput out, long origo) throws IOException {
+ if (blockCount == 1) { // A single jump is just wasted space so we
ignore that
+ blockCount = 0;
+ }
+ for (int i = 0 ; i < blockCount ; i++) {
+ out.writeLong(jumps[i]);
+ }
+ // The number of blocks are written as the last data in the
IndexedDISI structure.
+ // This makes it possible to infer the jumpTableOffset: lastPos -
#blocks * Long.BYTES
+ // As there are at most 32k blocks, the count is stored as a short
+ out.writeShort((short) blockCount);
+ }
+
+ /** The slice that stores the {@link DocIdSetIterator}. */
+ private final IndexInput slice;
+ private final long cost;
+
+ /**
+ * @param in backing data.
+ * @param offset starting offset for blocks in backing data.
+ * @param length the number of bytes in the backing data.
+ * @param cost normally the number of logical docIDs.
+ */
+ IndexedDISI(IndexInput in, long offset, long length, long cost) throws
IOException {
+ this(in.slice("docs", offset, length), cost);
+ }
+
+ /**
+ * This constructor allows to pass the slice directly in case it helps
reuse.
+ * see eg. Lucene80 norms producer's merge instance.
+ * @param slice backing data.
+ * @param cost normally the number of logical docIDs.
+ */
+ IndexedDISI(IndexInput slice, long cost) throws IOException {
+ this.slice = slice;
+ this.cost = cost;
+ origo = slice.getFilePointer();
+ slice.seek(slice.length()-Short.BYTES);
+ jumpTableEntryCount = slice.readShort();
+ jumpTableOffset = jumpTableEntryCount <= 1 ? -1 :
+ slice.getFilePointer()-Short.BYTES-jumpTableEntryCount*Long.BYTES;
+ slice.seek(origo);
+ }
+
+ private final long origo;
+ private int block = -1;
+ private long blockEnd;
+ private long rankOrigoOffset = -1; // Only used for DENSE blocks
+ private int nextBlockIndex = -1;
+ Method method;
+
+ private int doc = -1;
+ private int index = -1;
+
+ // SPARSE variables
+ boolean exists;
+
+ // DENSE variables
+ private long word;
+ private int wordIndex = -1;
+ // number of one bits encountered so far, including those of `word`
+ private int numberOfOnes;
+ // Used with rank for jumps inside of DENSE
+ private int denseOrigoIndex;
+
+ // ALL variables
+ private int gap;
+
+ @Override
+ public int docID() {
+ return doc;
+ }
+
+ @Override
+ public int advance(int target) throws IOException {
+ final int targetBlock = target & 0xFFFF0000;
+ if (block < targetBlock) {
+ advanceBlock(targetBlock);
+ }
+ if (block == targetBlock) {
+ if (method.advanceWithinBlock(this, target)) {
+ return doc;
+ }
+ readBlockHeader();
+ }
+ boolean found = method.advanceWithinBlock(this, block);
+ assert found;
+ return doc;
+ }
+
+ public boolean advanceExact(int target) throws IOException {
+ final int targetBlock = target & 0xFFFF0000;
+ if (block < targetBlock) {
+ advanceBlock(targetBlock);
+ }
+ boolean found = block == targetBlock &&
method.advanceExactWithinBlock(this, target);
+ this.doc = target;
+ return found;
+ }
+
+ private void advanceBlock(int targetBlock) throws IOException {
+ final int blockIndex = targetBlock >> 16;
+ // If the destination block is 2 blocks or more ahead, we use the
jump-table.
+ if (jumpTableOffset != -1L && blockIndex >= (block >> 16)+2 &&
blockIndex < jumpTableEntryCount) {
--- End diff --
I don't know. It is hard to measure such small things when the index size
is large. The check for `jumpTableOffset` should be JITted away and as the
address for the next block is already fetched as part of reading the block
headers, I estimated that skipping the lookup in the jump-table would be faster.
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