#define EPSILON 0.0000001

// Given 2 lines such as line1 passes through P1 & P2, and line2 passes through P3 & P4, 
// find the segment intersecting the 2 lines passing through the point Pa on line1, and Pb on line2
// mua is the signed distance of the intersection point on line1 to P1
// mub is the signed distance of the intersection point on line2 to P3
// returns 1 in case of success, 0 otherwise (2 lines are parallel)
int LineLineIntersect(vector P1, P2, P3, P4, Pa, Pb; float mua, mub)
{
   vector p13 = P1 - P3;
   vector p43 = P4 - P3;
   if (length(p43) < EPSILON) return 0;

   vector p21 = P2 - P1;
   if (length(p21) < EPSILON) return 0;

   float d1343 = dot(p13, p43);
   float d4321 = dot(p43, p21);
   float d1321 = dot(p13, p21);
   float d4343 = dot(p43, p43);
   float d2121 = dot(p21, p21);

   float denom = d2121 * d4343 - d4321 * d4321;
   if (abs(denom) < EPSILON) return 0;

   float numer = d1343 * d4321 - d1321 * d4343;

   mua = numer / denom;
   mub = (d1343 + d4321 * mua) / d4343;

   Pa = P1 + (p21 * mua);
   Pb = P3 + (p43 * mub);

   return 1;
}