def sha1_ft(t, b, c, d)
      return (b & c) | ((~b) & d) if t < 20
      return b ^ c ^ d if t < 40
      return (b & c) | (b & d) | (c & d) if t < 60
      b ^ c ^ d
    end
def sha1_ft(t, b, c, d)
      return (b & c) | ((~b) & d) if t < 20
      return b ^ c ^ d if t < 40
      return (b & c) | (b & d) | (c & d) if t < 60
      b ^ c ^ d
    end
Perform the appropriate triplet combination function for the current
 iteration