Reduce unnecessary memory allocations in Cascades #2464
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,113 @@ | ||
| /* | ||
| * PreOrderIterator.java | ||
| * | ||
| * This source file is part of the FoundationDB open source project | ||
| * | ||
| * Copyright 2015-2024 Apple Inc. and the FoundationDB project authors | ||
| * | ||
| * Licensed 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. | ||
| */ | ||
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| package com.apple.foundationdb.record.query.plan.cascades; | ||
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| import com.google.common.base.Verify; | ||
| import com.google.common.collect.Iterables; | ||
| import org.apache.commons.lang3.tuple.MutablePair; | ||
| import org.apache.commons.lang3.tuple.Pair; | ||
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|
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| import javax.annotation.Nonnull; | ||
| import javax.annotation.concurrent.NotThreadSafe; | ||
| import java.util.ArrayDeque; | ||
| import java.util.Deque; | ||
| import java.util.Iterator; | ||
| import java.util.List; | ||
| import java.util.NoSuchElementException; | ||
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| /** | ||
| * An iterator that accesses all elements of a {@link TreeLike} object in pre-order fashion. | ||
| * It attempts to reduce the number of memory allocations while still being performant. It does so | ||
| * by implementing a mix of level-based traversal and pre-order traversal, where a {@link Deque} is used | ||
| * (as a stack) to enable depth first search, however instead of maintaining individual {@link TreeLike} node in | ||
| * each stack frame the entire list of node's children is stored instead in addition to an index pointing out | ||
| * to the current element that is returned by the stream. | ||
| * | ||
| * @param <T> The type of the iterator element. | ||
| */ | ||
| @NotThreadSafe | ||
| public final class PreOrderIterator<T extends TreeLike<T>> implements Iterator<T> { | ||
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| @Nonnull | ||
| private final Deque<Pair<Iterable<? extends T>, Integer>> stack; | ||
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| private static final int INITIAL_POSITION = -1; | ||
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| private PreOrderIterator(@Nonnull final T traversable) { | ||
| // initialize the stack with the {@link TreeLike}'s depth as capacity to avoid resizing. | ||
| stack = new ArrayDeque<>(traversable.height()); | ||
| // this is the only list allocation done to put the root in the stack. | ||
| // all the remaining lists added to the stack are references to children | ||
| // lists (copy by reference). | ||
| stack.push(MutablePair.of(List.of(traversable.getThis()), INITIAL_POSITION)); | ||
| } | ||
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| @Override | ||
| public boolean hasNext() { | ||
| while (true) { | ||
| if (stack.isEmpty()) { | ||
| return false; | ||
| } | ||
| final var top = Verify.verifyNotNull(stack.peekLast()); | ||
| final var currentLevelIndex = top.getRight(); | ||
| final var currentLevelItemsCount = Iterables.size(top.getLeft()); | ||
| if (currentLevelIndex + 1 < currentLevelItemsCount) { | ||
| return true; | ||
| } else { | ||
| // pop the stack, and continue doing so, until we either find a next element | ||
| // by looking into previous levels in reverse (stack) order, progressively | ||
| // do so until either an item is found, or the stack becomes empty, and no item | ||
| // is to be found meaning that the traversal is complete. | ||
| stack.removeLast(); | ||
| } | ||
| } | ||
| } | ||
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| @Override | ||
| public T next() { | ||
| if (!hasNext()) { | ||
| throw new NoSuchElementException("no more elements"); | ||
| } | ||
| final var top = Verify.verifyNotNull(stack.peekLast()); | ||
| final var currentIndexPosition = top.getRight(); | ||
| final var currentLevelItems = top.getLeft(); | ||
| final var nextItemIndex = currentIndexPosition + 1; | ||
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| // mark the next item as the current one in this stack frame, note that | ||
| // the position must be valid because hasNext() is called first, and hasNext() | ||
| // makes sure that, if there is a next element, it modifies the stack index such | ||
| // that incrementing it would lead to finding that next element correctly. | ||
| final var result = Iterables.get(currentLevelItems, nextItemIndex); | ||
| top.setValue(nextItemIndex); | ||
| final var resultChildren = result.getChildren(); | ||
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| // descend immediately to the children (if any) so to conform to pre-order DFS semantics. | ||
| if (!Iterables.isEmpty(resultChildren)) { | ||
| stack.add(MutablePair.of(resultChildren, INITIAL_POSITION)); | ||
| } | ||
| return result; | ||
| } | ||
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| @Nonnull | ||
| public static <T extends TreeLike<T>> PreOrderIterator<T> over(@Nonnull final T traversable) { | ||
| return new PreOrderIterator<>(traversable); | ||
| } | ||
| } | ||
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Can you not use
Pairbut define your own class? This is due to us removing commons stuff in the future.There was a problem hiding this comment.
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Right, but I don't want to do it in this PR to limit its scope, there are 80+ locations where we still use
Paircurrently, we can do this in a separate cleanup PR. (I am also usingMutablePairby the way, but we can also replace that in a cleanup PR). #2484