function rint(x)
  local two52 = 4503599627370496.0
  if x ~= x or x < -two52 or x > two52 then
      -- Already an Integer, or NaN, or Inf
  elseif x < 0 then
      x = ((x - two52) + two52)
  else
      x = ((x + two52) - two52)
  end
  return math.floor(x)
end

print("rint(0.50001) = "..rint(0.50001))

rint(0.50001) = 0