用Java写一个布谷鸟过滤器

布谷鸟过滤器是一种基于哈希表的快速过滤算法，用于判断一个元素是否存在于一个集合中。下面是一个简单的 Java 实现：

import java.util.BitSet;

public class CuckooFilter {
    private final int capacity;
    private final int fingerprintSize;
    private final int maxNumKicks;
    private final BitSet bitSet;
    private final int[] fingerprints;

    public CuckooFilter(int capacity, int fingerprintSize, int maxNumKicks) {
        this.capacity = capacity;
        this.fingerprintSize = fingerprintSize;
        this.maxNumKicks = maxNumKicks;
        this.bitSet = new BitSet(capacity);
        this.fingerprints = new int[capacity];
    }

    private int getHash1(String key) {
        return key.hashCode() % capacity;
    }

    private int getHash2(int hash1, String key) {
        return (hash1 ^ key.hashCode()) % capacity;
    }

    private int getFingerprint(String key) {
        int hash = key.hashCode();
        hash ^= (hash >>> 20) ^ (hash >>> 12);
        return hash ^ (hash >>> 7) ^ (hash >>> 4) & ((1 << fingerprintSize) - 1);
    }

    private int getAlternativeIndex(int index, int fingerprint) {
        return (index ^ (fingerprint * 0x5bd1e995)) % capacity;
    }

    public boolean insert(String key) {
        int hash1 = getHash1(key);
        int hash2 = getHash2(hash1, key);
        int fingerprint = getFingerprint(key);
        for (int i = 0; i < maxNumKicks; i++) {
            int index = (i % 2 == 0) ? hash1 : hash2;
            int altIndex = getAlternativeIndex(index, fingerprint);
            if (bitSet.get(index)) {
                int oldFingerprint = fingerprints[index];
                fingerprints[index] = fingerprint;
                fingerprint = oldFingerprint;
                index = altIndex;
            } else {
                bitSet.set(index);
                fingerprints[index] = fingerprint;
                return true;
            }
        }
        return false;
    }

    public boolean contains(String key) {
        int hash1 = getHash1(key);
        int hash2 = getHash2(hash1, key);
        int fingerprint = getFingerprint(key);
        int index1 = hash1;
        int index2 = hash2;
        if (bitSet.get(index1) && fingerprints[index1] == fingerprint) {
            return true;
        } else if (bitSet.get(index2) && fingerprints[index2] == fingerprint) {
            return true;
        } else {
            int altIndex = getAlternativeIndex(index1, fingerprint);
            if (bitSet.get(altIndex) && fingerprints[altIndex] == fingerprint) {
                return true;
            } else {
                altIndex = getAlternativeIndex(index2, fingerprint);
                if (bitSet.get(altIndex) && fingerprints[altIndex] == fingerprint) {
                    return true;
                }
            }
        }
        return false;
    }
}

这个实现中使用了两个哈希函数来计算两个可能的索引位置。如果第一个位置已经被占用，就把对应的元素踢出去，然后将新元素放到对应的位置。如果第二个位置已经被占用，就重复这个过程，直到达到最大迭代次数为止。