A lightweight C library for implementing rate limiting in applications using the Count-Min Sketch algorithm. This library efficiently tracks IP addresses and determines when they exceed a specified request rate threshold.

The library uses a Count-Min Sketch data structure with the following approach:

1. IP addresses are hashed to two positions in a fixed-size array using SipHash-2-4
2. Each hit increments counters at both positions
3. When checking if an IP should be rate-limited, the minimum of the two counters is used
4. Counters automatically decay over time to prevent indefinite growth

Simply copy the ratelimit.h and ratelimit.c files into your project, or compile as a library.

#include "ratelimit.h"
#include <netinet/in.h>

int main() {
    RateLimiter rl;
    unsigned char key[16] = {0}; // Should be random in real usage

    // Initialize with 1024 slots and 2048 period
    if (ratelimiter_init(&rl, 1024, 2048, key) != 0) {
        return -1; // Error
    }

    struct sockaddr_in client_addr;
    client_addr.sin_family = AF_INET;
    client_addr.sin_addr.s_addr = inet_addr("192.168.1.100");

    // Check if client should be rate limited (more than 100 requests)
    int result = ratelimiter_hit(&rl, (struct sockaddr*)&client_addr, 100);

    if (result > 0) {
        // Client should be rate limited
        printf("Rate limiting this client\n");
    } else if (result == 0) {
        // Client is under the limit
        printf("Request allowed\n");
    }

    ratelimiter_free(&rl);
    return 0;
}

To compile with your application:

gcc -Wall -Wextra -o your_app your_app.c ratelimit.c

To compile and run the included test:

gcc -Wall -Wextra -o test_simple test_simple.c ratelimit.c
./test_simple

See ratelimit.h for full API documentation:

• ratelimiter_init(): Initialize a new rate limiter
• ratelimiter_free(): Free resources used by a rate limiter
• ratelimiter_hit(): Record a hit and check if rate limit is exceeded
• ratelimiter_rekey(): Change the hash key to redistribute entries