ethereum

func (s *ServiceETH) GetBalance(req *WalletBalanceRequest) (*big.Int, error) {
	client, err := ethclient.Dial(req.GatewayURL)
	if err != nil {
		return nil, err
	}

	rctx, cancel := context.WithDeadline(context.Background(), time.Now().Add(DefaultTimeout))
	defer cancel()

	walletAddress := common.HexToAddress(req.Address)

	if req.ContractAddress == "" {
		log.Infof("Get balance at %s", walletAddress.Hex())
		balance, err := client.BalanceAt(rctx, walletAddress, nil)
		if err != nil {
			return nil, err
		}

		return balance, nil
	}
	log.Infof("Call smart-contract %s, balanceOf(%s)", req.ContractAddress, walletAddress.Hex())
	// Prepare data
	hash := sha3.NewLegacyKeccak256()
	hash.Write([]byte("balanceOf(address)"))
	methodID := hash.Sum(nil)[:4]
	paddedAddress := common.LeftPadBytes(walletAddress.Bytes(), 32)

	var data []byte
	data = append(data, methodID...)
	data = append(data, paddedAddress...)

	contractAddress := common.HexToAddress(req.ContractAddress)
	msg := ethereum.CallMsg{To: &contractAddress, Data: data}
	balanceBytes, err := client.CallContract(rctx, msg, nil)
	if err != nil {
		return nil, err
	}

	balance := new(big.Int)
	balance.SetBytes(balanceBytes)
	return balance, nil
}