is_cross.m

best routing protocol

function res =is_cross(pt1, pt2, pt3, pt4)
%check if line (pt1->pt2) and line (pt3->pt4) is across
res=0;

if pt3(1)==pt4(1) %horizontal line
  x=pt3(1);
  if (pt1(1)>=x) & (pt2(1)>=x) %above the h-line
	return
  elseif (pt1(1)<=x) & (pt2(1)<=x) % below the h-line
	return
  end
  b=(x-pt1(1))/(pt2(1)-pt1(1));
  cross_pt_y = pt1(2)+(pt2(2)-pt1(2))*b;
  if cross_pt_y>min(pt3(2),pt4(2)) & cross_pt_y<max(pt3(2),pt4(2))
	res=1;
  end
	
elseif pt3(2)==pt4(2) % vertical line
  y=pt3(2);
  if (pt1(2)>=y) & (pt2(2)>=y) % right to the v-line
	return
  elseif (pt1(2)<=y) & (pt2(2)<=y) % left to the v-line
	return
  end
  b=(y-pt1(2))/(pt2(2)-pt1(2));
  cross_pt_x = pt1(1)+(pt2(1)-pt1(1))*b;
  if cross_pt_x>min(pt3(1),pt4(1)) & cross_pt_x<max(pt3(1), pt4(1))
	res = 1;
  end
else
  error(['unsupported line cross computation--pt3->pt4 must be either' ...
		 ' horizontal or vertical']);
end