/** Fully implemented server for the distributed mutual exclusion algorithm
 * @author Marko Mäkelä (Marko.Makela@HUT.FI)
 */

#include<stdio.h>
#include<stdlib.h>
#include<sys/socket.h>
#include<netinet/in.h>

#ifndef MSG_NOSIGNAL
# define MSG_NOSIGNAL 0
#endif
#if defined __osf__
typedef size_t socklen_t;
#endif
#if defined __sgi
typedef int socklen_t;
#endif

/** Messages originated by the clients */
enum client_msg { res_req, rel_req };
/** Messages originated by the server */
enum server_msg { res_rej, res_ack, rel_ack };

/** Display diagnostics of a client
 * 
 */
static void
diag (const struct sockaddr_in* addr,
      const char* msg)
{
  unsigned long ip = ntohl (addr->sin_addr.s_addr);
  unsigned short port = ntohs (addr->sin_port);
  fprintf (stderr, "client %u.%u.%u.%u:%u: %s\n",
	   (unsigned) (ip >> 24) & 0xff,
	   (unsigned) (ip >> 16) & 0xff,
	   (unsigned) (ip >> 8) & 0xff,
	   (unsigned) (ip) & 0xff,
	   port,
	   msg);
}

int
main (int argc, char** argv)
{
  /** the datagram socket */
  int s;
  /** the listening address */
  static struct sockaddr_in addr;
  /** the address of the active client */
  static struct sockaddr_in addr_active;

  addr_active.sin_family = 0;

  if (argc != 2) {
    fputs ("usage: server portnumber\n", stderr);
    return 1;
  }
  else {
    char* endp;
    unsigned long port = strtoul (argv[1], &endp, 0);
    if (!argv[1] || *endp || port > 65535) {
      fputs ("invalid port number \"", stderr);
      fputs (argv[1], stderr);
      fputs ("\"\n", stderr);
      return 1;
    }

    addr.sin_family = AF_INET;
    addr.sin_port = htons (port);
    addr.sin_addr.s_addr = INADDR_ANY;

    s = socket (PF_INET, SOCK_DGRAM, 0);
    if (s < 0) {
      perror ("socket");
      return 2;
    }
    if (bind (s, (struct sockaddr*) &addr, sizeof addr)) {
      perror ("bind");
      return 2;
    }
  }

  for (;;) {
    char c;
    static struct sockaddr_in addr_from;
    static socklen_t addrlen;
    int len;
    addrlen = sizeof addr_from;
    len = recvfrom (s, &c, 1, MSG_TRUNC | MSG_NOSIGNAL,
		    (struct sockaddr*) &addr_from, &addrlen);
    if (len < 0)
      perror ("recvfrom");
    else if (addrlen != sizeof addr_from || addr_from.sin_family != AF_INET ||
	     len != 1)
      fputs ("ignoring unexpected packet\n", stderr);
    else {
      switch (c) {
      default:
	diag (&addr_from, "ignoring unknown PDU");
	break;
      case res_req:
	if (!addr_active.sin_family) {
	  c = res_ack;
	  addr_active.sin_family = AF_INET;
	  addr_active.sin_addr.s_addr = addr_from.sin_addr.s_addr;
	  addr_active.sin_port = addr_from.sin_port;
	  diag (&addr_from, "res_req->res_ack");
	}
	else {
	  c = addr_from.sin_addr.s_addr != addr_active.sin_addr.s_addr ||
	      addr_from.sin_port != addr_active.sin_port
	    ? res_rej
	    : res_ack;
	  diag (&addr_from,
		c == res_ack ? "re-res_req->res_ack" : "res_req->res_rej");
	}
	sendto (s, &c, 1, MSG_NOSIGNAL,
		(struct sockaddr*) &addr_from, addrlen);
	break;
      case rel_req:
	if (addr_from.sin_addr.s_addr != addr_active.sin_addr.s_addr ||
	    addr_from.sin_port != addr_active.sin_port)
	  diag (&addr_from, "bogus rel_req");
	else {
	  addr_active.sin_family = 0;
	  diag (&addr_from, "rel_req");
	}
	c = rel_ack;
	sendto (s, &c, 1, MSG_NOSIGNAL,
		(struct sockaddr*) &addr_from, addrlen);
      }
    }
  }
}