const baseRequest = {
  apiVersion: 2,
  apiVersionMinor: 0
};

const allowedCardNetworks = ["MASTERCARD", "VISA"];

const allowedCardAuthMethods = ["PAN_ONLY", "CRYPTOGRAM_3DS"];

const tokenizationSpecification = {
  type: 'PAYMENT_GATEWAY',
  parameters: {
    'gateway': 'liqpay',
    'gatewayMerchantId': 'your public key'
  }
};

const baseCardPaymentMethod = {
  type: 'CARD',
  parameters: {
    allowedAuthMethods: allowedCardAuthMethods,
    allowedCardNetworks: allowedCardNetworks
  }
};

const cardPaymentMethod = Object.assign(
  {},
  baseCardPaymentMethod,
  {
    tokenizationSpecification: tokenizationSpecification
  }
);

let paymentsClient = null;

function getGoogleIsReadyToPayRequest() {
  return Object.assign(
      {},
      baseRequest,
      {
        allowedPaymentMethods: [baseCardPaymentMethod]
      }
  );
}

function getGooglePaymentDataRequest() {
  const paymentDataRequest = Object.assign({}, baseRequest);
  paymentDataRequest.allowedPaymentMethods = [cardPaymentMethod];
  paymentDataRequest.transactionInfo = getGoogleTransactionInfo();
  paymentDataRequest.merchantInfo = {
    merchantId: '12345678901234567890',    
    merchantName: 'Example Merchant'
  };
  return paymentDataRequest;
}

function getGooglePaymentsClient() {
  if ( paymentsClient === null ) {
    paymentsClient = new google.payments.api.PaymentsClient({environment: 'PRODUCTION'});
  }
  return paymentsClient;
}

function onGooglePayLoaded() {
  const paymentsClient = getGooglePaymentsClient();
  paymentsClient.isReadyToPay(getGoogleIsReadyToPayRequest())
      .then(function(response) {
        if (response.result) {
          addGooglePayButton();
        }
      })
      .catch(function(err) {
        console.error(err);
      });
}

function addGooglePayButton() {
  const paymentsClient = getGooglePaymentsClient();
  const button = paymentsClient.createButton({onClick: onGooglePaymentButtonClicked});
  document.getElementById('container').appendChild(button);
}

function getGoogleTransactionInfo() {
  return {
    countryCode: 'US',
    currencyCode: 'USD',
    totalPriceStatus: 'FINAL',
    // set to cart total
    totalPrice: '1.00'
  };
}

function prefetchGooglePaymentData() {
  const paymentDataRequest = getGooglePaymentDataRequest();
  paymentDataRequest.transactionInfo = {
    totalPriceStatus: 'NOT_CURRENTLY_KNOWN',
    currencyCode: 'USD'
  };
  const paymentsClient = getGooglePaymentsClient();
  paymentsClient.prefetchPaymentData(paymentDataRequest);
}

function onGooglePaymentButtonClicked() {
  const paymentDataRequest = getGooglePaymentDataRequest();
  paymentDataRequest.transactionInfo = getGoogleTransactionInfo();

  const paymentsClient = getGooglePaymentsClient();
  paymentsClient.loadPaymentData(paymentDataRequest)
      .then(function(paymentData) {
        processPayment(paymentData);
      })
      .catch(function(err) {
        console.error(err);
      });
}

function processPayment(paymentData) {
  const gpay_token_object = paymentData.paymentMethodData.tokenizationData.token;
  const gpay_token = btoa(gpay_token_object); // base64 encode 
  // send gpay_token to liqpay api
}