P = P0 + t * d

C = P0 + R * d

Len = AC * (AC x AB) / (|AC|*|AB|)
  SquaredLen = AC^2 * (AC x AB)^2 / (AC^2 * AB^2) = (AC x AB)^2 / AB^2

 AC x AB = (P0x+r*dx-Ax)*(By-Ay)-(P0y+r*dy-Ay)*(Bx-Ax) =
           r*(dx*(By-Ay)-dy*(Bx-Ax))+((P0x-Ax)*(By-Ay)(P0y-Ay)*(Bx-Ax)) =
           r * M + N
 But SquaredLen = r^2, so
 (r * M + N)^2 = r^2 * AB^2
 r^2 * (M^2 - AB^2) + r * 2 * M * N + N^2 = 0