line_line_intersection.rb

require 'matrix'

def line_intersection line1, line2
  l1 = line1.map{|v|v.map(&:to_f)}
  l2 = line2.map{|v|v.map(&:to_f)}
  a = l1[1]-l1[0]
  b = l2[1]-l2[0]
  ab = l1[0]-l2[0]
  cab = a.cross(b)
  nA = b.cross(ab).dot(cab)
  nB = a.cross(ab).dot(cab)
  d = a.cross(b).dot(cab)
  nAd = nA/d
  nBd = nB/d
  return nil if !(0..1).include?(nAd) || !(0..1).include?(nBd)
  a0 = l1[0]+a*nAd
  b0 = l2[0]+b*nBd
  if (a0-b0).magnitude < 0.001
    return a0 
  else
    nil
  end
end