Iterative Stein's (binary) GCD (greatest common divisor) algorithm in Ruby

gcd.rb

def gcd(a, b)
  return b if a.zero?
  return a if b.zero?

  k = 0
  while (a | b).even?
     a >>= 1; b >>= 1; k += 1
  end

  a >>= 1 while a.even?

  until b.zero?
    b >>= 1 while b.even?
    a, b = b, a if a > b
    b -= a
  end

  a << k
end