clintropolis commented on code in PR #12277: URL: https://github.com/apache/druid/pull/12277#discussion_r980399890
########## processing/src/main/java/org/apache/druid/segment/data/FrontCodedIndexed.java: ########## @@ -0,0 +1,393 @@ +/* + * 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.druid.segment.data; + +import com.google.common.base.Preconditions; +import org.apache.druid.common.config.NullHandling; +import org.apache.druid.java.util.common.ISE; +import org.apache.druid.query.monomorphicprocessing.RuntimeShapeInspector; + +import javax.annotation.Nullable; +import java.nio.ByteBuffer; +import java.nio.ByteOrder; +import java.util.Comparator; +import java.util.Iterator; +import java.util.NoSuchElementException; + +/** + * {@link Indexed} specialized for storing variable-width binary values (such as utf8 encoded strings), which must be + * sorted and unique, using 'front coding'. Front coding is a type of delta encoding for byte arrays, where sorted + * values are grouped into buckets. The first value of the bucket is written entirely, and remaining values are stored + * as a pair of an integer which indicates how much of the first byte array of the bucket to use as a prefix, followed + * by the remaining bytes after the prefix to complete the value. + * + * Getting a value first picks the appropriate bucket, finds its offset in the underlying buffer, then scans the bucket + * values to seek to the correct position of the value within the bucket in order to reconstruct it using the prefix + * length. + * + * Finding the index of a value involves binary searching the first values of each bucket to find the correct bucket, + * then a linear scan within the bucket to find the matching value (or negative insertion point -1 for values that + * are not present). + * + * The value iterator reads an entire bucket at a time, reconstructing the values into an array to iterate within the + * bucket before moving onto the next bucket as the iterator is consumed. + */ +public final class FrontCodedIndexed implements Indexed<ByteBuffer> +{ + public static FrontCodedIndexed read(ByteBuffer buffer, Comparator<ByteBuffer> comparator, ByteOrder ordering) + { + final ByteBuffer copy = buffer.asReadOnlyBuffer().order(ordering); + final byte version = copy.get(); + Preconditions.checkArgument(version == 0, "only V0 exists, encountered " + version); + final int bucketSize = copy.get(); + final boolean hasNull = NullHandling.IS_NULL_BYTE == copy.get(); + final int numValues = VByte.readInt(copy); + // size of offsets + values + final int size = VByte.readInt(copy); + // move position to end of buffer + buffer.position(copy.position() + size); + + final int numBuckets = (int) Math.ceil((double) numValues / (double) bucketSize); + final int adjustIndex = hasNull ? 1 : 0; + final int div = Integer.numberOfTrailingZeros(bucketSize); + final int rem = bucketSize - 1; + return new FrontCodedIndexed( + copy, + comparator, + bucketSize, + numBuckets, + (numValues & rem) == 0 ? bucketSize : numValues & rem, + hasNull, + numValues + adjustIndex, + adjustIndex, + div, + rem, + copy.position(), + copy.position() + ((numBuckets - 1) * Integer.BYTES) + ); + } + + private final ByteBuffer buffer; + private final int adjustedNumValues; + private final int adjustIndex; + private final int bucketSize; + private final int numBuckets; + private final int div; + private final int rem; + private final int offsetsPosition; + private final int bucketsPosition; + private final boolean hasNull; + private final int lastBucketNumValues; + private final Comparator<ByteBuffer> comparator; + + private FrontCodedIndexed( + ByteBuffer buffer, + Comparator<ByteBuffer> comparator, + int bucketSize, + int numBuckets, + int lastBucketNumValues, + boolean hasNull, + int adjustedNumValues, + int adjustIndex, + int div, + int rem, + int offsetsPosition, + int bucketsPosition + ) + { + if (Integer.bitCount(bucketSize) != 1) { + throw new ISE("bucketSize must be a power of two but was[%,d]", bucketSize); + } + this.buffer = buffer; + this.comparator = comparator; + this.bucketSize = bucketSize; + this.hasNull = hasNull; + this.adjustedNumValues = adjustedNumValues; + this.adjustIndex = adjustIndex; + this.div = div; + this.rem = rem; + this.numBuckets = numBuckets; + this.offsetsPosition = offsetsPosition; + this.bucketsPosition = bucketsPosition; + this.lastBucketNumValues = lastBucketNumValues; + } + + @Override + public int size() + { + return adjustedNumValues; + } + + @Nullable + @Override + public ByteBuffer get(int index) + { + final int adjustedIndex; + // due to vbyte encoding, the null value is not actually stored in the bucket (no negative values), so we adjust + // the index + if (hasNull) { + if (index == 0) { + return null; + } else { + adjustedIndex = index - 1; + } + } else { + adjustedIndex = index; + } + // find the bucket which contains the value with maths + final int offsetNum = adjustedIndex >> div; + final int bucketIndex = adjustedIndex & rem; + // get offset of that bucket in the value buffer + final int offset = offsetNum > 0 ? buffer.getInt(offsetsPosition + ((offsetNum - 1) * Integer.BYTES)) : 0; Review Comment: the offsets buffer stores the ending offsets of the bucket, so the start of the bucket is the previous end offset, or 0 if its the first offset -- This is an automated message from the Apache Git Service. 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