#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> 
#include <string.h>
#include <strings.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/uio.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>

#define BUFFERLEN 255
#define SERV_TCP_PORT 9000
#define UNKNOWN_COMMAND "UNKNOWN COMMAND\n"
int panic(char *where)
{
  fprintf(stderr, "error at %s '%s'\n", where, strerror(errno));
  exit(127);
}

/* liest zeilenweise aus dem Socketdeskriptor sd und schreibt die
   Zeile in das char-Array buf. buflen gibt die Laenge von buf an.
   Rueckgabewert ist die Anzahl der gelesen Zeichen */
int readline(int sd, char *buf, int buflen)
{
  int n, rc;
  char c;

  for (n = 1; n < buflen; n++) {
    if ((rc = read(sd, &c, 1)) == 1) {
      *buf++ = c;
      if (c == '\n')
        break;
    } 
    else if (rc == 0) {
      if (n == 1)
        return 0;
      else
        break;
    } else
      return -1;
  }
  *buf = 0;
  return n;
}

void write_safe(int fd, const void *buf, size_t nbytes) 
{
  if (write(fd, buf, nbytes) < 0)
    panic("write()");
}

void process_requests(int fd, struct sockaddr_in *client_addr)
{
  char buf[BUFFERLEN];
  char answer[BUFFERLEN];
  int arg_1, arg_2;

  for(;;)
    {
      bzero((void *) buf, BUFFERLEN);
      bzero((void *) answer, BUFFERLEN);

      if (readline(fd, buf, BUFFERLEN) < 0)
        panic("readline()");
      
      if (sscanf(buf, "ADD %i %i", &arg_1, &arg_2) == 2)
        {
          sprintf(answer, "RESULT %i\n", arg_1 + arg_2);
          write_safe(fd, answer, strlen(answer));
        } else
      
      if (sscanf(buf, "MUL %i %i", &arg_1, &arg_2) == 2)
        {
          sprintf(answer, "RESULT %i\n", arg_1 * arg_2);
          write_safe(fd, answer, strlen(answer));
        } else

      if (strncmp("INFO", buf, 4) == 0)
        {
          sprintf(answer, "YOURIP %s\n", inet_ntoa(client_addr->sin_addr));
          write_safe(fd, answer, strlen(answer));
        } else
      
      if (strncmp("QUIT", buf, 4) == 0)
        {
          close(fd);
         break;
        }
      else  write_safe(fd, UNKNOWN_COMMAND, strlen(UNKNOWN_COMMAND));
    }
}

int main(int argc, char argv[])
{
  int sock_fd, client_fd, addr_len;
  struct sockaddr_in client_addr, server_addr; 
  
  if ((sock_fd = socket(AF_INET,SOCK_STREAM,0)) < 0)
    panic("socket()");
  
  bzero((void *) &server_addr, sizeof(struct sockaddr_in));
  server_addr.sin_family = AF_INET;
  server_addr.sin_port = htons(SERV_TCP_PORT);
  server_addr.sin_addr.s_addr =INADDR_ANY;
  
  if (bind(sock_fd, 
           (struct sockaddr *) &server_addr, 
           sizeof(struct sockaddr_in)) < 0)
    panic("bind()");
  
  if (listen(sock_fd, SOMAXCONN) < 0)
    panic("listen()");
  
  for(;;)
    {
      addr_len = sizeof(struct sockaddr_in);
      client_fd = 
        accept(sock_fd,
               (struct sockaddr *) &client_addr,
               &addr_len);
      if (client_fd < 0)
        panic("accept()");
      
      process_requests(client_fd, &client_addr);
    }
  close(sock_fd); /* close listening socket */
  return 0;
}