Add docIds writer for future vector format usage

server/src/main/java/org/elasticsearch/index/codec/vectors/DocIdsWriter.java

@@ -0,0 +1,376 @@
+/*
+ * @notice
+ * 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.
+ * Modifications copyright (C) 2025 Elasticsearch B.V.
+ */
+package org.elasticsearch.index.codec.vectors;
+
+import org.apache.lucene.index.PointValues.IntersectVisitor;
+import org.apache.lucene.search.DocIdSetIterator;
+import org.apache.lucene.store.DataOutput;
+import org.apache.lucene.store.IndexInput;
+import org.apache.lucene.util.ArrayUtil;
+import org.apache.lucene.util.DocBaseBitSetIterator;
+import org.apache.lucene.util.FixedBitSet;
+import org.apache.lucene.util.IntsRef;
+import org.apache.lucene.util.LongsRef;
+import org.apache.lucene.util.hnsw.IntToIntFunction;
+
+import java.io.IOException;
+import java.util.Arrays;
+
+/**
+ * This class is used to write and read the doc ids in a compressed format. The format is optimized
+ * for the number of bits per value (bpv) and the number of values.
+ *
+ * <p>It is copied from the BKD implementation.
+ */
+final class DocIdsWriter {
+    public static final int DEFAULT_MAX_POINTS_IN_LEAF_NODE = 512;
+
+    private static final byte CONTINUOUS_IDS = (byte) -2;
+    private static final byte BITSET_IDS = (byte) -1;
+    private static final byte DELTA_BPV_16 = (byte) 16;
+    private static final byte BPV_21 = (byte) 21;
+    private static final byte BPV_24 = (byte) 24;
+    private static final byte BPV_32 = (byte) 32;
+
+    private int[] scratch = new int[0];
+    private final LongsRef scratchLongs = new LongsRef();
+
+    /**
+     * IntsRef to be used to iterate over the scratch buffer. A single instance is reused to avoid
+     * re-allocating the object. The ints and length fields need to be reset each use.
+     *
+     * <p>The main reason for existing is to be able to call the {@link
+     * IntersectVisitor#visit(IntsRef)} method rather than the {@link IntersectVisitor#visit(int)}
+     * method. This seems to make a difference in performance, probably due to fewer virtual calls
+     * then happening (once per read call rather than once per doc).
+     */
+    private final IntsRef scratchIntsRef = new IntsRef();
+
+    {
+        // This is here to not rely on the default constructor of IntsRef to set offset to 0
+        scratchIntsRef.offset = 0;
+    }
+
+    DocIdsWriter() {}
+
+    void writeDocIds(IntToIntFunction docIds, int count, DataOutput out) throws IOException {
+        // docs can be sorted either when all docs in a block have the same value
+        // or when a segment is sorted
+        if (count == 0) {
+            out.writeByte(CONTINUOUS_IDS);
+            return;
+        }
+        if (count > scratch.length) {
+            scratch = new int[count];
+        }
+        boolean strictlySorted = true;
+        int min = docIds.apply(0);
+        int max = min;
+        for (int i = 1; i < count; ++i) {
+            int last = docIds.apply(i - 1);
+            int current = docIds.apply(i);
+            if (last >= current) {
+                strictlySorted = false;
+            }
+            min = Math.min(min, current);
+            max = Math.max(max, current);
+        }
+
+        int min2max = max - min + 1;
+        if (strictlySorted) {
+            if (min2max == count) {
+                // continuous ids, typically happens when segment is sorted
+                out.writeByte(CONTINUOUS_IDS);
+                out.writeVInt(docIds.apply(0));
+                return;
+            } else if (min2max <= (count << 4)) {
+                assert min2max > count : "min2max: " + min2max + ", count: " + count;
+                // Only trigger bitset optimization when max - min + 1 <= 16 * count in order to avoid
+                // expanding too much storage.
+                // A field with lower cardinality will have higher probability to trigger this optimization.
+                out.writeByte(BITSET_IDS);
+                writeIdsAsBitSet(docIds, count, out);
+                return;
+            }
+        }
+
+        if (min2max <= 0xFFFF) {
+            out.writeByte(DELTA_BPV_16);
+            for (int i = 0; i < count; i++) {
+                scratch[i] = docIds.apply(i) - min;
+            }
+            out.writeVInt(min);
+            final int halfLen = count >> 1;
+            for (int i = 0; i < halfLen; ++i) {
+                scratch[i] = scratch[halfLen + i] | (scratch[i] << 16);
+            }
+            for (int i = 0; i < halfLen; i++) {
+                out.writeInt(scratch[i]);
+            }
+            if ((count & 1) == 1) {
+                out.writeShort((short) scratch[count - 1]);
+            }
+        } else {
+            if (max <= 0x1FFFFF) {
+                out.writeByte(BPV_21);
+                final int oneThird = floorToMultipleOf16(count / 3);
+                final int numInts = oneThird * 2;
+                for (int i = 0; i < numInts; i++) {
+                    scratch[i] = docIds.apply(i) << 11;
+                }
+                for (int i = 0; i < oneThird; i++) {
+                    final int longIdx = i + numInts;
+                    scratch[i] |= docIds.apply(longIdx) & 0x7FF;
+                    scratch[i + oneThird] |= (docIds.apply(longIdx) >>> 11) & 0x7FF;
+                }
+                for (int i = 0; i < numInts; i++) {
+                    out.writeInt(scratch[i]);
+                }
+                int i = oneThird * 3;
+                for (; i < count - 2; i += 3) {
+                    out.writeLong(((long) docIds.apply(i)) | (((long) docIds.apply(i + 1)) << 21) | (((long) docIds.apply(i + 2)) << 42));
+                }
+                for (; i < count; ++i) {
+                    out.writeShort((short) docIds.apply(i));
+                    out.writeByte((byte) (docIds.apply(i) >>> 16));
+                }
+            } else if (max <= 0xFFFFFF) {
+                out.writeByte(BPV_24);
+
+                // encode the docs in the format that can be vectorized decoded.
+                final int quarter = count >> 2;
+                final int numInts = quarter * 3;
+                for (int i = 0; i < numInts; i++) {
+                    scratch[i] = docIds.apply(i) << 8;
+                }
+                for (int i = 0; i < quarter; i++) {
+                    final int longIdx = i + numInts;
+                    scratch[i] |= docIds.apply(longIdx) & 0xFF;
+                    scratch[i + quarter] |= (docIds.apply(longIdx) >>> 8) & 0xFF;
+                    scratch[i + quarter * 2] |= docIds.apply(longIdx) >>> 16;
+                }
+                for (int i = 0; i < numInts; i++) {
+                    out.writeInt(scratch[i]);
+                }
+                for (int i = quarter << 2; i < count; ++i) {
+                    out.writeShort((short) docIds.apply(i));
+                    out.writeByte((byte) (docIds.apply(i) >>> 16));
+                }
+            } else {
+                out.writeByte(BPV_32);
+                for (int i = 0; i < count; i++) {
+                    out.writeInt(docIds.apply(i));
+                }
+            }
+        }
+    }
+
+    private static void writeIdsAsBitSet(IntToIntFunction docIds, int count, DataOutput out) throws IOException {
+        int min = docIds.apply(0);
+        int max = docIds.apply(count - 1);
+
+        final int offsetWords = min >> 6;
+        final int offsetBits = offsetWords << 6;
+        final int totalWordCount = FixedBitSet.bits2words(max - offsetBits + 1);
+        long currentWord = 0;
+        int currentWordIndex = 0;
+
+        out.writeVInt(offsetWords);
+        out.writeVInt(totalWordCount);
+        // build bit set streaming
+        for (int i = 0; i < count; i++) {
+            final int index = docIds.apply(i) - offsetBits;
+            final int nextWordIndex = index >> 6;
+            assert currentWordIndex <= nextWordIndex;
+            if (currentWordIndex < nextWordIndex) {
+                out.writeLong(currentWord);
+                currentWord = 0L;
+                currentWordIndex++;
+                while (currentWordIndex < nextWordIndex) {
+                    currentWordIndex++;
+                    out.writeLong(0L);
+                }
+            }
+            currentWord |= 1L << index;
+        }
+        out.writeLong(currentWord);
+        assert currentWordIndex + 1 == totalWordCount;
+    }
+
+    /** Read {@code count} integers into {@code docIDs}. */
+    void readInts(IndexInput in, int count, int[] docIDs) throws IOException {
+        if (count > scratch.length) {
+            scratch = new int[count];
+        }
+        final int bpv = in.readByte();
+        switch (bpv) {
+            case CONTINUOUS_IDS:
+                readContinuousIds(in, count, docIDs);
+                break;
+            case BITSET_IDS:
+                readBitSet(in, count, docIDs);
+                break;
+            case DELTA_BPV_16:
+                readDelta16(in, count, docIDs);
+                break;
+            case BPV_21:
+                readInts21(in, count, docIDs);
+                break;
+            case BPV_24:
+                readInts24(in, count, docIDs);
+                break;
+            case BPV_32:
+                readInts32(in, count, docIDs);
+                break;
+            default:
+                throw new IOException("Unsupported number of bits per value: " + bpv);
+        }
+    }
+
+    private DocIdSetIterator readBitSetIterator(IndexInput in, int count) throws IOException {
+        int offsetWords = in.readVInt();
+        int longLen = in.readVInt();
+        scratchLongs.longs = ArrayUtil.growNoCopy(scratchLongs.longs, longLen);
+        in.readLongs(scratchLongs.longs, 0, longLen);
+        // make ghost bits clear for FixedBitSet.
+        if (longLen < scratchLongs.length) {
+            Arrays.fill(scratchLongs.longs, longLen, scratchLongs.longs.length, 0);
+        }
+        scratchLongs.length = longLen;
+        FixedBitSet bitSet = new FixedBitSet(scratchLongs.longs, longLen << 6);
+        return new DocBaseBitSetIterator(bitSet, count, offsetWords << 6);
+    }
+
+    private static void readContinuousIds(IndexInput in, int count, int[] docIDs) throws IOException {
+        int start = in.readVInt();
+        for (int i = 0; i < count; i++) {
+            docIDs[i] = start + i;
+        }
+    }
+
+    private void readBitSet(IndexInput in, int count, int[] docIDs) throws IOException {
+        DocIdSetIterator iterator = readBitSetIterator(in, count);
+        int docId, pos = 0;
+        while ((docId = iterator.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
+            docIDs[pos++] = docId;
+        }
+        assert pos == count : "pos: " + pos + ", count: " + count;
+    }
+
+    private static void readDelta16(IndexInput in, int count, int[] docIds) throws IOException {
+        final int min = in.readVInt();
+        final int half = count >> 1;
+        in.readInts(docIds, 0, half);
+        if (count == DEFAULT_MAX_POINTS_IN_LEAF_NODE) {
+            // Same format, but enabling the JVM to specialize the decoding logic for the default number
+            // of points per node proved to help on benchmarks
+            decode16(docIds, DEFAULT_MAX_POINTS_IN_LEAF_NODE / 2, min);
+        } else {
+            decode16(docIds, half, min);
+        }
+        // read the remaining doc if count is odd.
+        for (int i = half << 1; i < count; i++) {
+            docIds[i] = Short.toUnsignedInt(in.readShort()) + min;
+        }
+    }
+
+    private static void decode16(int[] docIDs, int half, int min) {
+        for (int i = 0; i < half; ++i) {
+            final int l = docIDs[i];
+            docIDs[i] = (l >>> 16) + min;
+            docIDs[i + half] = (l & 0xFFFF) + min;
+        }
+    }
+
+    private static int floorToMultipleOf16(int n) {
+        assert n >= 0;
+        return n & 0xFFFFFFF0;
+    }
+
+    private void readInts21(IndexInput in, int count, int[] docIDs) throws IOException {
+        int oneThird = floorToMultipleOf16(count / 3);
+        int numInts = oneThird << 1;
+        in.readInts(scratch, 0, numInts);
+        if (count == DEFAULT_MAX_POINTS_IN_LEAF_NODE) {
+            // Same format, but enabling the JVM to specialize the decoding logic for the default number
+            // of points per node proved to help on benchmarks
+            decode21(
+                docIDs,
+                scratch,
+                floorToMultipleOf16(DEFAULT_MAX_POINTS_IN_LEAF_NODE / 3),
+                floorToMultipleOf16(DEFAULT_MAX_POINTS_IN_LEAF_NODE / 3) * 2
+            );
+        } else {
+            decode21(docIDs, scratch, oneThird, numInts);
+        }
+        int i = oneThird * 3;
+        for (; i < count - 2; i += 3) {
+            long l = in.readLong();
+            docIDs[i] = (int) (l & 0x1FFFFFL);
+            docIDs[i + 1] = (int) ((l >>> 21) & 0x1FFFFFL);
+            docIDs[i + 2] = (int) (l >>> 42);
+        }
+        for (; i < count; ++i) {
+            docIDs[i] = (in.readShort() & 0xFFFF) | (in.readByte() & 0xFF) << 16;
+        }
+    }
+
+    private static void decode21(int[] docIds, int[] scratch, int oneThird, int numInts) {
+        for (int i = 0; i < numInts; ++i) {
+            docIds[i] = scratch[i] >>> 11;
+        }
+        for (int i = 0; i < oneThird; i++) {
+            docIds[i + numInts] = (scratch[i] & 0x7FF) | ((scratch[i + oneThird] & 0x7FF) << 11);
+        }
+    }
+
+    private void readInts24(IndexInput in, int count, int[] docIDs) throws IOException {
+        int quarter = count >> 2;
+        int numInts = quarter * 3;
+        in.readInts(scratch, 0, numInts);
+        if (count == DEFAULT_MAX_POINTS_IN_LEAF_NODE) {
+            // Same format, but enabling the JVM to specialize the decoding logic for the default number
+            // of points per node proved to help on benchmarks
+            assert floorToMultipleOf16(quarter) == quarter
+                : "We are relying on the fact that quarter of DEFAULT_MAX_POINTS_IN_LEAF_NODE"
+                    + " is a multiple of 16 to vectorize the decoding loop,"
+                    + " please check performance issue if you want to break this assumption.";
+            decode24(docIDs, scratch, DEFAULT_MAX_POINTS_IN_LEAF_NODE / 4, DEFAULT_MAX_POINTS_IN_LEAF_NODE / 4 * 3);
+        } else {
+            decode24(docIDs, scratch, quarter, numInts);
+        }
+        // Now read the remaining 0, 1, 2 or 3 values
+        for (int i = quarter << 2; i < count; ++i) {
+            docIDs[i] = (in.readShort() & 0xFFFF) | (in.readByte() & 0xFF) << 16;
+        }
+    }
+
+    private static void decode24(int[] docIDs, int[] scratch, int quarter, int numInts) {
+        for (int i = 0; i < numInts; ++i) {
+            docIDs[i] = scratch[i] >>> 8;
+        }
+        for (int i = 0; i < quarter; i++) {
+            docIDs[i + numInts] = (scratch[i] & 0xFF) | ((scratch[i + quarter] & 0xFF) << 8) | ((scratch[i + quarter * 2] & 0xFF) << 16);
+        }
+    }
+
+    private static void readInts32(IndexInput in, int count, int[] docIDs) throws IOException {
+        in.readInts(docIDs, 0, count);
+    }
+}