lines_intersect(unsigned int xl, unsigned int yl, unsigned int x2, unsigned int y2,
unsigned int x3, unsigned int y3, unsigned int x4, unsigned int y4)

{
 long al, a2, bl, b2, cl, c2; /* Coefficients of line eqns. */
 long rl, r2, r3, r4; /* 'Sign' values */
 long denom, offset, num; /* Intermediate values */

 //Check to see if the two points overlap
 if(xl==x2 && yl==y2) return DONTINTERSECT;

 /* Compute al, bl, cl, where line joining points 1 and 2
 *is"alx + bl y + cl = 0".
 */

 al = y2 yl;
 bl = xl -x2;
 cl =x2 yl -xl *y2;

 /* Compute r3 and r4.
 */


 r3 = al x3 +bl y3 + cl;
 r4=al *x4+bl *y4+cl;

 /* Check signs of r3 and r4. If both point 3 and point 4 lie on
 same side of line 1, the line segments do not intersect.
 */

 if(r3 != 0 &&
 r4!= 0 &&
 SAME_SIGNS( r3, r4 ))
 return ( DONTINTERSECT );

 /* Compute a2, b2, c2 */

 a2 = y4 y3;
 b2 = x3 x4;
 c2 = x4 y3 x3 y4;

 /* Compute rl and r2 */

 rl =a2 *xl +b2 yl + c2;
 r2 = a2 x2 + b2 y2 + c2;