maedhroz commented on code in PR #1723: URL: https://github.com/apache/cassandra/pull/1723#discussion_r983096261
########## src/java/org/apache/cassandra/db/tries/Trie.java: ########## @@ -0,0 +1,596 @@ +/* + * 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.cassandra.db.tries; + +import java.util.Collection; +import java.util.Iterator; +import java.util.Map; +import java.util.function.BiConsumer; +import java.util.function.Consumer; +import java.util.function.Function; + +import com.google.common.collect.ImmutableList; + +import org.agrona.concurrent.UnsafeBuffer; +import org.apache.cassandra.utils.bytecomparable.ByteComparable; + +/** + * Base class for tries. + * + * Normal users of tries will only use the public methods, which provide various transformations of the trie, conversion + * of its content to other formats (e.g. iterable of values), and several forms of processing. + * + * For any unimplemented data extraction operations one can build on the {@link TrieEntriesWalker} (for-each processing) + * and {@link TrieEntriesIterator} (to iterator) base classes, which provide the necessary mechanisms to handle walking + * the trie. + * + * The internal representation of tries using this interface is defined in the {@link Cursor} interface. + * + * Cursors are a method of presenting the internal structure of a trie without representing nodes as objects, which is + * still useful for performing the basic operations on tries (iteration, slicing/intersection and merging). A cursor + * will list the nodes of a trie in order, together with information about the path that was taken to reach them. + * + * To begin traversal over a trie, one must retrieve a cursor by calling {@link #cursor()}. Because cursors are + * stateful, the traversal must always proceed from one thread. Should concurrent reads be required, separate calls to + * {@link #cursor()} must be made. Any modification that has completed before the construction of a cursor must be + * visible, but any later concurrent modifications may be presented fully, partially or not at all; this also means that + * if multiple are made, the cursor may see any part of any subset of them. + * + * Note: This model only supports depth-first traversals. We do not currently have a need for breadth-first walks. + * + * See Trie.md for further description of the trie representation model. + * + * @param <T> The content type of the trie. + */ +public abstract class Trie<T> +{ + /** + * A trie cursor. + * + * This is the internal representation of the trie, which enables efficient walks and basic operations (merge, + * slice) on tries. + * + * The cursor represents the state of a walk over the nodes of trie. It provides three main features: + * - the current "depth" or descend-depth in the trie; + * - the "incomingTransition", i.e. the byte that was used to reach the current point; + * - the "content" associated with the current node, + * and provides methods for advancing to the next position. This is enough information to extract all paths, and + * also to easily compare cursors over different tries that are advanced together. Advancing is always done in + * order; if one imagines the set of nodes in the trie with their associated paths, a cursor may only advance from a + * node with a lexicographically smaller path to one with bigger. The "advance" operation moves to the immediate + * next, it is also possible to skip over some items e.g. all children of the current node ("skipChildren"). + * + * Moving to the immediate next position in the lexicographic order is accomplished by: + * - if the current node has children, moving to its first child; + * - otherwise, ascend the parent chain and return the next child of the closest parent that still has any. + * As long as the trie is not exhausted, advancing always takes one step down, from the current node, or from a node + * on the parent chain. By comparing the new depth (which "advance" also returns) with the one before the advance, + * one can tell if the former was the case (if newDepth == oldDepth + 1) and how many steps up we had to take + * (oldDepth + 1 - newDepth). When following a path down, the cursor will stop on all prefixes. + * + * When it is created the cursor is placed on the root node with depth() = 0, incomingTransition() = -1. Since + * tries can have mappings for empty, content() can possibly be non-null. It is not allowed for a cursor to start + * in exhausted state (i.e. with depth() = -1). + * + * For example, the following trie: + * t + * r + * e + * e * + * i + * e * + * p * + * w + * i + * n * + * has nodes reachable with the paths + * "", t, tr, tre, tree*, tri, trie*, trip*, w, wi, win* + * and the cursor will list them with the following (depth, incomingTransition) pairs: + * (0, -1), (1, t), (2, r), (3, e), (4, e)*, (3, i), (4, e)*, (4, p)*, (1, w), (2, i), (3, n)* + * + * Because we exhaust transitions on bigger depths before we go the next transition on the smaller ones, when + * cursors are advanced together their positions can be easily compared using only the depth and incomingTransition: + * - one that is higher in depth is before one that is lower; + * - for equal depths, the one with smaller incomingTransition is first. + * + * If we consider walking the trie above in parallel with this: + * t + * r + * i + * c + * k * + * u + * p * + * the combined iteration will proceed as follows: + * (0, -1)+ (0, -1)+ cursors equal, advance both + * (1, t)+ (1, t)+ t cursors equal, advance both + * (2, r)+ (2, r)+ tr cursors equal, advance both + * (3, e)+ < (3, i) tre cursors not equal, advance smaller (3 = 3, e < i) + * (4, e)+ < (3, i) tree* cursors not equal, advance smaller (4 > 3) + * (3, i)+ (3, i)+ tri cursors equal, advance both + * (4, e) > (4, c)+ tric cursors not equal, advance smaller (4 = 4, e > c) + * (4, e) > (5, k)+ trick* cursors not equal, advance smaller (4 < 5) + * (4, e)+ < (1, u) trie* cursors not equal, advance smaller (4 > 1) + * (4, p)+ < (1, u) trip* cursors not equal, advance smaller (4 > 1) + * (1, w) > (1, u) u cursors not equal, advance smaller (1 = 1, w > u) + * (1, w) > (2, p) up* cursors not equal, advance smaller (1 = 1, w > u) + * (1, w)+ < (-1, -1) w cursors not equal, advance smaller (1 > -1) + * (2, i)+ < (-1, -1) wi cursors not equal, advance smaller (2 > -1) + * (3, n)+ < (-1, -1) win* cursors not equal, advance smaller (3 > -1) + * (-1, -1) (-1, -1) both exhasted + */ + interface Cursor<T> + { + + /** + * @return the current descend-depth; 0, if the cursor has just been created and is positioned on the root, + * and -1, if the trie has been exhausted. + */ + int depth(); + + /** + * @return the last transition taken; if positioned on the root, return -1 + */ + int incomingTransition(); + + /** + * @return the content associated with the current node. This may be non-null for any presented node, including + * the root. + */ + T content(); + + /** + * Advance one position to the node whose associated path is next lexicographically. + * This can be either: + * - descending one level to the first child of the current node, + * - ascending to the closest parent that has remaining children, and then descending one level to its next + * child. + * + * It is an error to call this after the trie has already been exhausted (i.e. when depth() == -1); + * for performance reasons we won't always check this. + * + * @return depth (can be prev+1 or <=prev), -1 means that the trie is exhausted + */ + int advance(); + + /** + * Advance, descending multiple levels if the cursor can do this for the current position without extra work + * (e.g. when positioned on a chain node in a memtable trie). If the current node does not have children this + * is exactly the same as advance(), otherwise it may take multiple steps down (but will not necessarily, even + * if they exist). + * + * Note that if any positions are skipped, their content must be null. + * + * This is an optional optimization; the default implementation falls back to calling advance. + * + * It is an error to call this after the trie has already been exhausted (i.e. when depth() == -1); + * for performance reasons we won't always check this. + * + * @param receiver object that will receive all transitions taken except the last; + * on ascend, or if only one step down was taken, it will not receive any + * @return the new depth, -1 if the trie is exhausted + */ + default int advanceMultiple(TransitionsReceiver receiver) + { + return advance(); + } + + /** + * Advance all the way to the next node with non-null content. + * + * It is an error to call this after the trie has already been exhausted (i.e. when depth() == -1); + * for performance reasons we won't always check this. + * + * @param receiver object that will receive all taken transitions + * @return the content, null if the trie is exhausted + */ + default T advanceToContent(ResettingTransitionsReceiver receiver) + { + int prevDepth = depth(); + while (true) + { + int currDepth = advanceMultiple(receiver); + if (currDepth <= 0) + return null; + if (receiver != null) + { + if (currDepth <= prevDepth) + receiver.resetPathLength(currDepth - 1); + receiver.addPathByte(incomingTransition()); + } + T content = content(); + if (content != null) + return content; + prevDepth = currDepth; + } + } + + /** + * Ignore the current node's children and advance to the next child of the closest node on the parent chain that + * has any. + * + * It is an error to call this after the trie has already been exhausted (i.e. when depth() == -1); + * for performance reasons we won't always check this. + * + * @return the new depth, always <= previous depth; -1 if the trie is exhausted + */ + int skipChildren(); + } + + protected abstract Cursor<T> cursor(); + + /** + * Used by {@link Cursor#advanceMultiple} to feed the transitions taken. + */ + protected interface TransitionsReceiver + { + /** Add a single byte to the path. */ + void addPathByte(int nextByte); + /** Add the count bytes from position pos in the given buffer. */ + void addPathBytes(UnsafeBuffer buffer, int pos, int count); + } + + /** + * Used by {@link Cursor#advanceToContent} to track the transitions and backtracking taken. + */ + interface ResettingTransitionsReceiver extends TransitionsReceiver + { + /** Delete all bytes beyond the given length. */ + void resetPathLength(int newLength); + } + + /** + * A push interface for walking over the trie. Builds upon TransitionsReceiver to be given the bytes of the + * path, and adds methods called on encountering content and completion. + * See {@link TrieDumper} for an example of how this can be used, and {@link TrieEntriesWalker} as a base class + * for other common usages. + */ + interface Walker<T, R> extends ResettingTransitionsReceiver + { + /** Called when content is found. */ + void content(T content); + + /** Called at the completion of the walk. */ + R complete(); + } + + // Version of the byte comparable conversion to use for all operations + static final ByteComparable.Version BYTE_COMPARABLE_VERSION = ByteComparable.Version.OSS42; + + /** + * Adapter interface providing the methods a {@link Walker} to a {@link Consumer}, so that the latter can be used + * with {@link #process}. + * + * This enables calls like + * trie.forEachEntry(x -> System.out.println(x)); + * to be mapped directly to a single call to {@link #process} without extra allocations. + */ + public interface ValueConsumer<T> extends Consumer<T>, Walker<T, Void> + { + default void content(T content) Review Comment: nit: `@Override`? 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