tests if a point is inside a polygon (ported code from c example)

point_in_polygon.rb

polygon = [{:x=>0,:y=>0},{:x=>4,:y=>0},{:x=>2,:y=>2},{:x=>0,:y=>4}]
point0 = {:x=>0,:y=>0}
point1 = {:x=>0.00000000001,:y=>0.00000000001}
point2 = {:x=>1,:y=>1}
point3 = {:x=>2,:y=>2}
point4 = {:x=>1.99999999999,:y=>1.99999999999}

# http://www.visibone.com/inpoly/
# http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html
#
# int pnpoly(int nvert, float *vertx, float *verty, float testx, float testy)
# {
#   int i, j, c = 0;
#   for (i = 0, j = nvert-1; i < nvert; j = i++) {
#     if ( ((verty[i]>testy) != (verty[j]>testy)) &&
#    (testx < (vertx[j]-vertx[i]) * (testy-verty[i]) / (verty[j]-verty[i]) + vertx[i]) )
#        c = !c;
#   }
#   return c;
# }

def point_in_polygon?(point, polygon)
  inside = false
  (1...polygon.size).each do |index|
    if (((polygon[index-1][:y] > point[:y]) != (polygon[index][:y] > point[:y])) &&
      (point[:x] < (polygon[index-1][:x]-polygon[index][:x])*(point[:y]-polygon[index-1][:y])/(polygon[index][:y]-polygon[index-1][:y])+polygon[index-1][:x])) 
      inside = !inside
    end
  end
  inside
end