#include <stdio.h>
#include <sys/types.h>
#include <netinet/ip.h>
#include <sys/socket.h>
#include <unistd.h>

#define N 1000
int ports[N], listen_port = 1234;

/* find duplicate ports and show distance between them */
int analyze(void)
{
	int i, j, n = 0;

	for(i = 0; i < N; i++) {
		for(j = i + 1; j < N; j++) {
			if (ports[i] != ports[j])
				continue;
			printf(" port reused: %d: distance %d (%d,%d)\n", ports[i], j-i, j, i);
			n++;
		}
	}

	printf("%d ports were being reused\n", n);

	return 0;
}

int client(void)
{
	int sock, i = 0;
	size_t tmp;
	struct sockaddr_in addr_bind = {
		.sin_family = AF_INET,
		.sin_addr.s_addr = INADDR_ANY,
		.sin_port = 0,
	}, addr_connect = {
		.sin_family = AF_INET,
		.sin_addr.s_addr = htonl(0x7f000001),
		.sin_port = htons(listen_port),
	}, addr_sockname;

	sleep(1);

 loop:
	sock = socket(AF_INET, SOCK_STREAM, 0);
	if (!sock) {
		perror("client: failed to create socket");
		return -1;
	}

	if (bind(sock, (struct sockaddr*)&addr_bind, sizeof(addr_bind)) < 0) {
		perror("client: failed to bind");
		return -1;
	}

	tmp = sizeof(addr_sockname);
	if (getsockname(sock, (struct sockaddr*)&addr_sockname, &tmp) < 0) {
		perror("client: getsockname failed");
		return -1;
	}

	ports[i] = ntohs(addr_sockname.sin_port);

	if (connect(sock, (struct sockaddr*)&addr_connect, sizeof(addr_connect)) < 0) {
		perror("client: failed to connect");
		return -1;
	}
	

	if (read(sock, &tmp, sizeof(tmp)) != 0) {
		perror("read failed");
		return -1;
	}

	close(sock);

	if (++i < N)
		goto loop;


	return analyze();
}


int server(void)
{
	int sock = socket(AF_INET, SOCK_STREAM, 0), sock2;
	struct sockaddr_in addr = {
		.sin_family = AF_INET,
		.sin_addr.s_addr = INADDR_ANY,
	};

 again:
	addr.sin_port = htons(listen_port);

	if (!sock) {
		perror("server: failed to create socket");
		return -1;
	}

	if (bind(sock, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
		perror("server: failed to bind");
		listen_port++;
		goto again;
	}

	if (listen(sock, 128) < 0) {
		perror("server: failed to listen");
		return -1;
	}

	printf("listening (port %d)\n", listen_port);

	switch (fork()) {
	case -1:
		return -1;
	case 0:
		return client();
	}

 loop:
	sock2 = accept(sock, NULL, NULL);
	if (sock2 < 0) {
		perror("server: accept failed\n");
		return -1;
	}

	close(sock2);
	goto loop;
}


int main()
{

	return server();

}