#include <getopt.h>
#include <netdb.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <sys/socket.h>

#define EPOCH 2208988800ull

// assumes either little or big endian. pdp-endian, youre out of luck
#define NLITTLE 0x41424344UL
#define NENDIAN  ('ABCD')

typedef struct {
#if NENDIAN == NLITTLE
  unsigned int mode: 3;
  unsigned int vn: 3;
  unsigned int li: 2;
#else
  unsigned int li: 2;
  unsigned int vn: 3;
  unsigned int mode: 3;
#endif
  uint8_t stratum;
  uint8_t poll;
  uint8_t prec;
  uint32_t delay;
  uint32_t dispersion;
  uint32_t refid;
  uint32_t rtms;
  uint32_t rtmf;
  uint32_t otms;
  uint32_t otmf;
  uint32_t rctms;
  uint32_t rctmf;
  uint32_t ttms;
  uint32_t ttmf;
} __attribute__((packed, aligned(1))) ntp;

typedef struct {
  const char* host;
  int port;
} ntp_info;

ntp_info NDEFAULT = {.host="europe.pool.ntp.org", .port = 123};

void err(char* msg) {
  fprintf(stderr, "Error %s.\n", msg);
  exit(1);
}

void usage(char* program, int code) {
  fprintf(stderr, "Usage: %s [options]\n", program);
  fputs("Options:\n", stderr);
  fputs("  -h HOST, --host=HOST  NTP server to use\n", stderr);
  fputs("             defaults to 'europe.pool.ntp.org'\n", stderr);
  fputs("  -p PORT, --port=PORT  NTP server port to use (must be numerical)\n", stderr);
  fputs("             defaults to '123'\n", stderr);
  fputs("  --help                Display this message and exit\n", stderr);
  exit(code);
}

ntp_info args(int argc, char** argv) {
  char c;
  ntp_info res = NDEFAULT;
  static struct option long_options[] = {
    {"host", required_argument, 0, 'h'},
    {"port", required_argument, 0, 'p'},
    {"help", no_argument, 0, 'u'},
    {NULL, 0, NULL, 0}
  };
  while ((c = getopt_long(argc, argv, "t:a:", long_options, 0)) != -1) {
    switch (c) {
      case 'h': res.host = optarg; break;
      case 'p': {
        char* end;
        res.port = strtol(optarg, &end, 10);
        if (end[0] != 0) {
          fputs("Port is not numerical.\n", stderr);
          usage(argv[0], 1);
        }
        break;
     }
     case 'u': usage(argv[0], 0);
     default: usage(argv[0], 1);
    }
  }
  return res;
}

int main(int argc, char** argv) {
  int sock;
  ntp n;
  struct sockaddr_in a;
  struct hostent* s;
  ntp_info i = args(argc, argv);

  memset(&n, 0, sizeof(ntp));
  n.vn = 3;
  n.mode = 3;

  sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
  if (sock < 0) err("opening socket");

  s = gethostbyname(i.host);
  if (!s) err("resolving host");

  bzero(&a, sizeof(a));
  a.sin_family = AF_INET;
  a.sin_port = htons(i.port);
  bcopy(s->h_addr, &a.sin_addr.s_addr, s->h_length);

  if (connect(sock, (struct sockaddr *) &a, sizeof(a)) < 0) err("connecting");
  if (write(sock, &n, sizeof(ntp)) < 0) err("writing to network");
  if (read(sock, &n, sizeof(ntp)) < 0) err("reading from network");

  n.ttms = ntohl(n.ttms);

  time_t tm = n.ttms - EPOCH;

  fputs(ctime(&tm), stdout);

  return 0;
}