code: func (totp *TOTP) ForPeriod(period int64) int32 {
  	data := int_to_bytestring(period)
  
  	hmacHash := hmac.New(sha1.New, totp.key)
  	hmacHash.Write(data)
  	digest := hmacHash.Sum(nil)
  	offset := int(digest[19] & 0xf)
  	code := int32(digest[offset]&0x7f)<<24 |
  		int32(digest[offset+1]&0xff)<<16 |
  		int32(digest[offset+2]&0xff)<<8 |
  		int32(digest[offset+3]&0xff)
  
  	code = int32(int64(code) % int64(math.Pow10(totp.Digits)))
  	return code
  }
docs: // Return the time-based OTP for the given period.