guess.lua

guess = function(a,b)
  return a==0 and b
      or b==0 and a
      or a>b  and guess(a-b,b)
      or          guess(a,b-a)
end

function guess2(a,b)
  if (a==0) then
    return b
  elseif (b==0) then
    return a
  elseif (a>b) then
    return guess2(a-b,b)
  else
    return guess2(a,b-a)
  end
end

function timed(fn)
  local starttime = os.clock()
  fn()
  local endtime = os.clock()
  print("It took " .. (endtime-starttime) .. "s!")
end

function repeated(fn, m, n)
  return
  function()
    for i = 1, m do
      for j = 1, n do
        fn(i, j)
      end
    end
  end
end

timed(repeated(guess,1000,1000))
timed(repeated(guess2,1000,1000))

results

It took 3.593s!
It took 3.407s!