#include<X11/Xlib.h>
#include<X11/Xutil.h>
#include<stdio.h>
#include<stdlib.h>
struct point {
  int x, y;
};
struct segment {
  struct point p1, p2;
};
int turn(struct point q0, struct point q1, struct point q2)
{ int dx1, dx2, dy1, dy2;
  dx1=q1.x - q0.x; dy1 = q1.y - q0.y;
  dx2 = q2.x - q0.x; dy2 = q2.y - q0.y;
  if (dx1*dy2 > dy1*dx2) return 1;
  if (dx1*dy2 < dy1*dx2) return -1;
  if ((dx1*dx2 < 0)||(dy1*dy2<0)) return -1;
  if ((dx1*dx1+dy1*dy1) < (dx2*dx2+dy2*dy2)) return 1;
  return 0;
}
int intersect(struct segment s1, struct segment s2)
{
  return ((turn(s1.p1, s1.p2, s2.p1)*turn(s1.p1,s1.p2,s2.p2))<=0)
     &&  ((turn(s2.p1, s2.p2, s1.p1)*turn(s2.p1,s2.p2,s1.p2))<=0);
}
main()
{  char dummy;
   Display *d;
   Window w;
   GC gc;
   Font f;
   XEvent e;
   int i, black, white;

   struct segment test1, test2;
//   d=XOpenDisplay("tika:0");
   d=XOpenDisplay((getenv("DISPLAY") != NULL)?getenv( "DISPLAY"):":0");
   f=XLoadFont(d,"r14");
   black = BlackPixel(d, 0);
   white = WhitePixel(d, 0);

   w=XCreateSimpleWindow(d,DefaultRootWindow(d),5,5,400,400,5,
     black, white);
   XSelectInput(d,w,ButtonPressMask|ButtonReleaseMask);
   XMapWindow(d,w);
   gc=XCreateGC(d,w,0,0);
   XSetFont(d,gc,f);
   XFlush(d);
   while(1){
      for (i=1;i<=2;i++){
      XNextEvent(d,&e);
      switch(e.type){
	 case ButtonPress:
              test1.p1.x=e.xbutton.x;              
              test1.p1.y=e.xbutton.y;              
	      break;
	 case ButtonRelease:
              test1.p2.x=e.xbutton.x;              
              test1.p2.y=e.xbutton.y;              
              XDrawLine(d,w,gc,test1.p1.x,test1.p1.y,test1.p2.x,test1.p2.y);
	      break;
      }
      }
      for (i=1;i<=2;i++){
      XNextEvent(d,&e);
      switch(e.type){
	 case ButtonPress:
              test2.p1.x=e.xbutton.x;              
              test2.p1.y=e.xbutton.y;              
	      break;
	 case ButtonRelease:
              test2.p2.x=e.xbutton.x;              
              test2.p2.y=e.xbutton.y;              
              XDrawLine(d,w,gc,test2.p1.x,test2.p1.y,test2.p2.x,test2.p2.y);
	      break;
      }
      }
      if (intersect(test1, test2)) printf("intersect\n");
                           else printf("no intersection\n");
   }
}