C语言实现HyperLogLog

#include <stdio.h>
#include <stdlib.h>
#include <math.h> // gcc -lm for log() 

#define REGISTER_SIZE 16    // 寄存器的位数，默认为16位
#define ARRAY_SIZE (1 << REGISTER_SIZE)    // 寄存器数组的大小

typedef struct {
    unsigned int* registers;    // 寄存器数组
    int size;    // 寄存器数组的大小
    int b;    // 每个哈希值的前导零位数
} HyperLogLog;

// 使用djb2哈希算法
unsigned int hash_function(const char* data) {
    unsigned int hash = 5381;
    int c;

    while ((c = *data++)) {
        hash = ((hash << 5) + hash) + c; /* hash * 33 + c */
    }

    return hash;
}

// 创建并初始化HyperLogLog数据结构
HyperLogLog* hyperloglog_create() {
    HyperLogLog* hll = (HyperLogLog*)malloc(sizeof(HyperLogLog));
    hll->registers = (unsigned int*)calloc(ARRAY_SIZE, sizeof(unsigned int));
    hll->size = ARRAY_SIZE;
    hll->b = REGISTER_SIZE;

    return hll;
}

// 释放HyperLogLog数据结构的内存
void hyperloglog_destroy(HyperLogLog* hll) {
    free(hll->registers);
    free(hll);
}

// 计算哈希值的前导零位数
int leading_zeros(unsigned int value, int max_bits) {
    int count = 0;
    while ((value & 0x80000000) == 0 && count < max_bits) {
        value <<= 1;
        count++;
    }
    return count;
}

// 更新HyperLogLog数据结构的寄存器
void hyperloglog_update(HyperLogLog* hll, const char* data) {
    unsigned int hash = hash_function(data);    // 哈希函数需要根据实际情况实现
    unsigned int index = hash & (hll->size - 1);
    unsigned int value = hash >> REGISTER_SIZE;
    int count = leading_zeros(value, 32 - REGISTER_SIZE) + 1;
    if (count > hll->registers[index]) {
        hll->registers[index] = count;
    }
}

// 估计基数
double hyperloglog_estimate(HyperLogLog* hll) {
    double alpha = 0.7213 / (1 + 1.079 / hll->size);
    double estimate = 0;
    double sum = 0;

    for (int i = 0; i < hll->size; i++) {
        sum += 1.0 / (1 << hll->registers[i]);
    }

    estimate = alpha * hll->size * hll->size / sum;

    // 根据哈希函数的不同，可能需要进行修正
    if (estimate <= 2.5 * hll->size) {
        int zero_count = 0;
        for (int i = 0; i < hll->size; i++) {
            if (hll->registers[i] == 0) {
                zero_count++;
            }
        }
        if (zero_count != 0) {
            estimate = hll->size * log((double)hll->size / zero_count);
        }
    } else if (estimate > (1.0 / 30.0) * pow(2, 32)) {
        estimate = -pow(2, 32) * log(1 - estimate / pow(2, 32));
    }

    return estimate;
}

int main() {
    HyperLogLog* hll = hyperloglog_create();

    // 更新寄存器
    hyperloglog_update(hll, "data1");
    hyperloglog_update(hll, "data2");
    hyperloglog_update(hll, "data3");

    // 估计基数
    double estimate = hyperloglog_estimate(hll);
    printf("Estimated cardinality: %.0f\n", estimate);

    hyperloglog_destroy(hll);
    return 0;
}