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| 1 | +/* |
| 2 | + * DedupCursor.java |
| 3 | + * |
| 4 | + * This source file is part of the FoundationDB open source project |
| 5 | + * |
| 6 | + * Copyright 2015-2025 Apple Inc. and the FoundationDB project authors |
| 7 | + * |
| 8 | + * Licensed under the Apache License, Version 2.0 (the "License"); |
| 9 | + * you may not use this file except in compliance with the License. |
| 10 | + * You may obtain a copy of the License at |
| 11 | + * |
| 12 | + * http://www.apache.org/licenses/LICENSE-2.0 |
| 13 | + * |
| 14 | + * Unless required by applicable law or agreed to in writing, software |
| 15 | + * distributed under the License is distributed on an "AS IS" BASIS, |
| 16 | + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 17 | + * See the License for the specific language governing permissions and |
| 18 | + * limitations under the License. |
| 19 | + */ |
| 20 | + |
| 21 | +package com.apple.foundationdb.record.cursors; |
| 22 | + |
| 23 | +import com.apple.foundationdb.annotation.API; |
| 24 | +import com.apple.foundationdb.async.AsyncUtil; |
| 25 | +import com.apple.foundationdb.record.RecordCoreException; |
| 26 | +import com.apple.foundationdb.record.RecordCursor; |
| 27 | +import com.apple.foundationdb.record.RecordCursorContinuation; |
| 28 | +import com.apple.foundationdb.record.RecordCursorProto; |
| 29 | +import com.apple.foundationdb.record.RecordCursorResult; |
| 30 | +import com.apple.foundationdb.record.RecordCursorVisitor; |
| 31 | +import com.apple.foundationdb.tuple.ByteArrayUtil2; |
| 32 | +import com.google.protobuf.ByteString; |
| 33 | +import com.google.protobuf.InvalidProtocolBufferException; |
| 34 | + |
| 35 | +import javax.annotation.Nonnull; |
| 36 | +import javax.annotation.Nullable; |
| 37 | +import java.util.Objects; |
| 38 | +import java.util.concurrent.CompletableFuture; |
| 39 | +import java.util.concurrent.Executor; |
| 40 | +import java.util.concurrent.atomic.AtomicReference; |
| 41 | +import java.util.function.Function; |
| 42 | + |
| 43 | +/** |
| 44 | + * A cursor that deduplicates adjacent elements in the input and only returns unique values. |
| 45 | + * An example usage for this kind of cursor is the iteration of KV pairs, returning the primary keys of the records. Since |
| 46 | + * there can be multiple Kvs per record, we need to filter out the redundancies. |
| 47 | + * This cursor takes in an <i>inner</i> cursor factory function and a pair of pack/unpack functions. Those are needed as |
| 48 | + * part of the continuation management, as the overall continuation for the cursor needs both the inner cursor continuation |
| 49 | + * and the last found value (so that can can compare it at the beginning of the next iteration). |
| 50 | + * <p> |
| 51 | + * This cursor also assumes that there is always some forward progress made in each iteration (unless the inner record |
| 52 | + * is exhausted), so that we will get some inner result during every iteration to feed into the continuation. |
| 53 | + * <p> |
| 54 | + * The cursor assumes that the inner cursor is sorted (the assumption is actually somewhat weaker: that the repeated elements are grouped) |
| 55 | + * such that all the elements of a certain repeated value appear in sequence, hence it can remove all but the first, and |
| 56 | + * the stored state can be kept to a minimum. |
| 57 | + * |
| 58 | + * @param <T> the type of elements of the cursor |
| 59 | + */ |
| 60 | +@API(API.Status.EXPERIMENTAL) |
| 61 | +public class DedupCursor<T> implements RecordCursor<T> { |
| 62 | + /** Inner cursor. */ |
| 63 | + @Nonnull |
| 64 | + private final RecordCursor<T> inner; |
| 65 | + /** The result returned from the cursor. */ |
| 66 | + @Nullable |
| 67 | + private RecordCursorResult<T> nextResult; |
| 68 | + /** The last value found (to be compared against the next value generated). */ |
| 69 | + @Nullable |
| 70 | + private T lastValue; |
| 71 | + /** The method that can pack a value into a byte[]. */ |
| 72 | + @Nonnull |
| 73 | + private final Function<T, byte[]> packValue; |
| 74 | + |
| 75 | + /** |
| 76 | + * Constructor. |
| 77 | + * @param innerCursorFactory factory method to create an inner cursor given a continuation |
| 78 | + * @param unpackValue a method that can unpack a value from byte array (used to deserialize from a continuation) |
| 79 | + * @param packValue a method that can pack a value into a byte array (used to serialize to a continuation). Note that |
| 80 | + * this will only be called if the value is NOT null. Null value will be converted internally to null byte[]. |
| 81 | + * {@code pack} itself should never return {@code null}. |
| 82 | + * @param continuation the cursor continuation (null if none) |
| 83 | + */ |
| 84 | + @API(API.Status.EXPERIMENTAL) |
| 85 | + public DedupCursor(@Nonnull Function<byte[], RecordCursor<T>> innerCursorFactory, |
| 86 | + @Nonnull Function<byte[], T> unpackValue, |
| 87 | + @Nonnull Function<T, byte[]> packValue, |
| 88 | + @Nullable byte[] continuation) { |
| 89 | + this.packValue = packValue; |
| 90 | + |
| 91 | + byte[] innerContinuation = null; |
| 92 | + if (continuation != null) { |
| 93 | + try { |
| 94 | + RecordCursorProto.DedupContinuation dedupContinuation = RecordCursorProto.DedupContinuation.parseFrom(continuation); |
| 95 | + // the inner is required in the continuation |
| 96 | + innerContinuation = dedupContinuation.getInnerContinuation().toByteArray(); |
| 97 | + if (dedupContinuation.hasLastValue()) { |
| 98 | + lastValue = unpackValue.apply(dedupContinuation.getLastValue().toByteArray()); |
| 99 | + } |
| 100 | + } catch (InvalidProtocolBufferException ex) { |
| 101 | + throw new RecordCoreException("Error parsing continuation", ex) |
| 102 | + .addLogInfo("raw_bytes", ByteArrayUtil2.loggable(continuation)); |
| 103 | + } |
| 104 | + } |
| 105 | + inner = innerCursorFactory.apply(innerContinuation); |
| 106 | + } |
| 107 | + |
| 108 | + @Nonnull |
| 109 | + @Override |
| 110 | + public CompletableFuture<RecordCursorResult<T>> onNext() { |
| 111 | + if (nextResult != null && !nextResult.hasNext()) { |
| 112 | + return CompletableFuture.completedFuture(nextResult); |
| 113 | + } |
| 114 | + |
| 115 | + AtomicReference<RecordCursorResult<T>> currentResult = new AtomicReference<>(); |
| 116 | + return AsyncUtil.whileTrue(() -> inner.onNext().thenApply(innerResult -> { |
| 117 | + currentResult.set(innerResult); |
| 118 | + final boolean hasNext = innerResult.hasNext(); |
| 119 | + // keep looping if we have more records and value is the same as before |
| 120 | + return hasNext && Objects.equals(innerResult.get(), lastValue); |
| 121 | + }), getExecutor()).thenApply(vignore -> applyResult(currentResult.get())); |
| 122 | + } |
| 123 | + |
| 124 | + @Nullable |
| 125 | + private RecordCursorResult<T> applyResult(final RecordCursorResult<T> currentResult) { |
| 126 | + if (currentResult.hasNext()) { |
| 127 | + lastValue = currentResult.get(); |
| 128 | + nextResult = RecordCursorResult.withNextValue(lastValue, |
| 129 | + new DedupCursorContinuation(currentResult.getContinuation(), lastValue)); |
| 130 | + } else { |
| 131 | + if (currentResult.getNoNextReason().isSourceExhausted()) { |
| 132 | + nextResult = RecordCursorResult.exhausted(); //continuation not valid here |
| 133 | + } else { |
| 134 | + nextResult = RecordCursorResult.withoutNextValue( |
| 135 | + new DedupCursorContinuation(currentResult.getContinuation(), lastValue), |
| 136 | + currentResult.getNoNextReason()); |
| 137 | + } |
| 138 | + } |
| 139 | + return nextResult; |
| 140 | + } |
| 141 | + |
| 142 | + @Override |
| 143 | + public void close() { |
| 144 | + inner.close(); |
| 145 | + } |
| 146 | + |
| 147 | + @Override |
| 148 | + public boolean isClosed() { |
| 149 | + return inner.isClosed(); |
| 150 | + } |
| 151 | + |
| 152 | + @Nonnull |
| 153 | + @Override |
| 154 | + public Executor getExecutor() { |
| 155 | + return inner.getExecutor(); |
| 156 | + } |
| 157 | + |
| 158 | + @Override |
| 159 | + public boolean accept(@Nonnull RecordCursorVisitor visitor) { |
| 160 | + if (visitor.visitEnter(this)) { |
| 161 | + inner.accept(visitor); |
| 162 | + } |
| 163 | + return visitor.visitLeave(this); |
| 164 | + } |
| 165 | + |
| 166 | + /** |
| 167 | + * Form a continuation that allows us to restart with the current cursor at its next position. |
| 168 | + * The continuation holds on to the state of the internal cursor as well as the last item seen (if any). |
| 169 | + */ |
| 170 | + private class DedupCursorContinuation implements RecordCursorContinuation { |
| 171 | + @Nonnull |
| 172 | + private final RecordCursorContinuation innerContinuation; |
| 173 | + @Nullable |
| 174 | + private final T lastValue; |
| 175 | + private byte[] cachedBytes; |
| 176 | + |
| 177 | + private DedupCursorContinuation(@Nonnull RecordCursorContinuation innerContinuation, @Nullable T lastValue) { |
| 178 | + this.innerContinuation = innerContinuation; |
| 179 | + this.lastValue = lastValue; |
| 180 | + } |
| 181 | + |
| 182 | + @Nullable |
| 183 | + @Override |
| 184 | + public byte[] toBytes() { |
| 185 | + if (isEnd()) { |
| 186 | + return null; |
| 187 | + } else { |
| 188 | + //form bytes exactly once |
| 189 | + if (cachedBytes == null) { |
| 190 | + byte[] lastValuePacked = pack(lastValue); |
| 191 | + final RecordCursorProto.DedupContinuation.Builder builder = RecordCursorProto.DedupContinuation.newBuilder() |
| 192 | + .setInnerContinuation(innerContinuation.toByteString()); |
| 193 | + if (lastValuePacked != null) { |
| 194 | + builder.setLastValue(ByteString.copyFrom(lastValuePacked)); |
| 195 | + } |
| 196 | + cachedBytes = builder.build().toByteArray(); |
| 197 | + } |
| 198 | + return cachedBytes; |
| 199 | + } |
| 200 | + } |
| 201 | + |
| 202 | + @Override |
| 203 | + public boolean isEnd() { |
| 204 | + return innerContinuation.isEnd(); |
| 205 | + } |
| 206 | + |
| 207 | + private byte[] pack(final T value) { |
| 208 | + return (value == null) ? null : packValue.apply(value); |
| 209 | + } |
| 210 | + } |
| 211 | +} |
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