#include <arpa/inet.h>
#include <errno.h>
#include <iostream>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <stdexcept>
#include <string.h>
#include <string>
#include <strings.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>

using namespace std;

#define throw_unix_error(fn) throw runtime_error(string(fn) + ": " + strerror(errno))

uint16_t in_cksum(uint16_t *addr, int len)
{
    int nleft = len;
    uint32_t sum = 0;
    uint16_t *w = addr;
    uint16_t answer = 0;

    /*
     * Our algorithm is simple, using a 32 bit accumulator (sum), we add
     * sequential 16 bit words to it, and at the end, fold back all the
     * carry bits from the top 16 bits into the lower 16 bits.
     */
    while (nleft > 1) {
        sum += *w++;
        nleft -= 2;
    }

    /* 4mop up an odd byte, if necessary */
    if (nleft == 1) {
        *(unsigned char *)(&answer) = *(unsigned char *)w;
        sum += answer;
    }

    /* 4add back carry outs from top 16 bits to low 16 bits */
    sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
    sum += (sum >> 16);                 /* add carry */
    answer = ~sum;                      /* truncate to 16 bits */
    return (answer);
}

long since_midnight()
{
    struct timeval tv;
    struct tm *tm;

    gettimeofday(&tv, NULL);
    tm = gmtime(&tv.tv_sec);
    return (tm->tm_hour * 60 * 60 + tm->tm_min * 60 + tm->tm_sec) * 1000 + tv.tv_usec / 1000;
}

void icmptime(const string &host)
{
    int sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
    if (sockfd < 0)
        throw_unix_error("socket");

    struct icmp icmp;
    bzero(&icmp, sizeof(icmp));
    icmp.icmp_type = ICMP_TIMESTAMP;
    icmp.icmp_code = 0;
    icmp.icmp_id = (unsigned short)getpid();
    icmp.icmp_seq = 0;
    icmp.icmp_otime = htonl(since_midnight());
    icmp.icmp_rtime = 0;
    icmp.icmp_ttime = 0;
    icmp.icmp_cksum = in_cksum((unsigned short *)&icmp, 20);

    struct sockaddr_in addr;
    bzero(&addr, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr(host.c_str());

    if (sendto(sockfd, &icmp, 20, 0, (struct sockaddr *)&addr, sizeof(addr)) < 0)
        throw_unix_error();

    struct __attribute__((__packed__)) {
        struct iphdr iphdr;
        struct icmp icmp;
    } reply;

    for (;;) {
        if (recvfrom(sockfd, &reply, sizeof(reply), 0, NULL, NULL) < 0)
            throw_unix_error();
        if (reply.iphdr.protocol == IPPROTO_ICMP &&
            reply.icmp.icmp_type == ICMP_TIMESTAMPREPLY &&
            reply.icmp.icmp_id == (unsigned short)getpid())
            break;
    }

    cout << "orig = " << ntohl(reply.icmp.icmp_otime)
         << ", recv = " << ntohl(reply.icmp.icmp_rtime)
         << ", xmit = " << ntohl(reply.icmp.icmp_ttime)
         << ", rtt = " << (since_midnight() - (long)ntohl(reply.icmp.icmp_otime)) << " ms"
         << "\ndifference = " << ((long)ntohl(reply.icmp.icmp_rtime) - (long)ntohl(reply.icmp.icmp_otime)) << " ms"
         << endl;
}

int main(int argc, char *argv[])
{
    if (argc != 2) {
        cout << "usage: icmptime host" << endl;
        return 0;
    }

    try {
        icmptime(argv[1]);
    } catch (const runtime_error &e) {
        cout << e.what() << endl;
    }
}