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HeavyKeeper 算法介绍与原理

原代码go语言实现出自Topk
HeavyKeeper是一种高效的流式TopK检测算法，专为识别大规模数据流中的频繁项（热点Key）而生，它基于Count-Min Sketch算法改进，主要通过以下组件实现：

1. 二维数组：算法维护一个二维数组，里面有 d 个数组，每个数组里有 w 个桶，桶里记录哈希指纹和计数值。

2. 计数衰减机制：核心创新点，当发生哈希冲突时，不是简单的覆盖，而是通过概率衰减原有计数。

3. 堆结构：维护一个大小为 k 的最小堆，用于记录当前观测到的TopK项。

当一个Key到达时：

1. 对Key应用d个哈希函数，映射到d个数组中的对应桶

2. 对每个桶：

• 如果桶为空或已存储的哈希指纹与当前哈希指纹相同，增加计数器

• 如果发生冲突，以概率P(decay) = 1/(b^C)衰减已有计数，b为衰减因子，C 为计数值

维护最小堆，保留最大的k个计数项

public class HeavyKeeper implements TopK {// 查找表大小，用于存放衰减概率private static final int LOOKUP_TABLE_SIZE = 256;private final int k;  // Top-K 的数量private final int width;  // 每层桶的宽度private final int depth;  // 总共的哈希层数private final double[] lookupTable;  // 衰减概率查找表private final Bucket[][] buckets;  // 哈希桶二维数组private final PriorityQueue<Node> minHeap;  // 最小堆用于维护前K个高频项private final BlockingQueue<Item> expelledQueue;  // 被移出 Top-K 的队列private final Random random;  // 用于概率衰减private long total;  // 总加入项的个数private final int minCount;  // 进入 Top-K 的最小频率门槛// 构造函数public HeavyKeeper(int k, int width, int depth, double decay, int minCount) {this.k = k;this.width = width;this.depth = depth;this.minCount = minCount;// 初始化查找表，存储每个 count 下的衰减概率this.lookupTable = new double[LOOKUP_TABLE_SIZE];for (int i = 0; i < LOOKUP_TABLE_SIZE; i++) {lookupTable[i] = Math.pow(decay, i);}// 初始化桶this.buckets = new Bucket[depth][width];for (int i = 0; i < depth; i++) {for (int j = 0; j < width; j++) {buckets[i][j] = new Bucket();}}// 初始化最小堆和其他结构this.minHeap = new PriorityQueue<>(Comparator.comparingInt(n -> n.count));this.expelledQueue = new LinkedBlockingQueue<>();this.random = new Random();this.total = 0;}// 返回当前 Top-K 列表@Overridepublic List<Item> list() {synchronized (minHeap) {List<Item> result = new ArrayList<>(minHeap.size());for (Node node : minHeap) {result.add(new Item(node.key, node.count));}// 按频率降序排序result.sort((a, b) -> Integer.compare(b.count(), a.count()));return result;}}// 返回被移出 Top-K 的项@Overridepublic BlockingQueue<Item> expelled() {return expelledQueue;}// 数据衰减操作（定期调用）@Overridepublic void fading() {// 所有桶的计数都右移一位（除以2）for (Bucket[] row : buckets) {for (Bucket bucket : row) {synchronized (bucket) {bucket.count = bucket.count >> 1;}}}// Top-K 小堆的值也同步衰减synchronized (minHeap) {PriorityQueue<Node> newHeap = new PriorityQueue<>(Comparator.comparingInt(n -> n.count));for (Node node : minHeap) {newHeap.add(new Node(node.key, node.count >> 1));}minHeap.clear();minHeap.addAll(newHeap);}total = total >> 1;}// 返回总项数@Overridepublic long total() {return total;}// 桶结构：记录指纹和频率private static class Bucket {long fingerprint;int count;}// 小堆节点private static class Node {final String key;final int count;Node(String key, int count) {this.key = key;this.count = count;}}// MurmurHash32 哈希函数private static int hash(byte[] data) {return HashUtil.murmur32(data);}// 添加元素逻辑@Overridepublic AddResult add(String key, int increment) {byte[] keyBytes = key.getBytes();long itemFingerprint = hash(keyBytes);int maxCount = 0;// 遍历每层哈希表for (int i = 0; i < depth; i++) {int bucketNumber = Math.abs(hash(keyBytes)) % width;Bucket bucket = buckets[i][bucketNumber];synchronized (bucket) {if (bucket.count == 0) {// 桶是空的，直接填入bucket.fingerprint = itemFingerprint;bucket.count = increment;maxCount = Math.max(maxCount, increment);} else if (bucket.fingerprint == itemFingerprint) {// 命中同一个元素，累加计数bucket.count += increment;maxCount = Math.max(maxCount, bucket.count);} else {// 不同元素，进行概率衰减for (int j = 0; j < increment; j++) {double decay = bucket.count < LOOKUP_TABLE_SIZE ?lookupTable[bucket.count] :lookupTable[LOOKUP_TABLE_SIZE - 1];// 随机衰减if (random.nextDouble() < decay) {bucket.count--;if (bucket.count == 0) {// 替换为当前项bucket.fingerprint = itemFingerprint;bucket.count = increment - j;maxCount = Math.max(maxCount, bucket.count);break;}}}}}}// 总计数累加total += increment;// 如果未达到最小门槛，不加入 Top-Kif (maxCount < minCount) {return new AddResult(null, false, null);}// 尝试加入 Top-K 小堆synchronized (minHeap) {boolean isHot = false;String expelled = null;// 如果已存在，更新它Optional<Node> existing = minHeap.stream().filter(n -> n.key.equals(key)).findFirst();if (existing.isPresent()) {minHeap.remove(existing.get());minHeap.add(new Node(key, maxCount));isHot = true;} else {// 不存在，则判断是否可以进入 Top-Kif (minHeap.size() < k || maxCount >= Objects.requireNonNull(minHeap.peek()).count) {Node newNode = new Node(key, maxCount);if (minHeap.size() >= k) {expelled = minHeap.poll().key;expelledQueue.offer(new Item(expelled, maxCount));}minHeap.add(newNode);isHot = true;}}return new AddResult(expelled, isHot, key);}}
}