Gasless Transactions

// Gasless Transaction via Fee Bump (Fee Sponsorship)
// The SPONSOR pays the XLM transaction fee on behalf of the user.

import * as StellarSdk from '@stellar/stellar-sdk';

export async function buildFeeBumpTransaction(
  innerTxXDR: string,
  sponsorSecretKey: string, // Platform's fee bot key
  feePerOp: number = 300   // stroops (~0.00003 XLM)
): Promise<string> {

  const server = new StellarSdk.Horizon.Server(
    'https://horizon-testnet.stellar.org'
  );

  // 1. Deserialize the user's signed inner transaction
  const innerTx = StellarSdk.TransactionBuilder
    .fromXDR(innerTxXDR, StellarSdk.Networks.TESTNET);

  // 2. Wrap it in a Fee Bump transaction
  const sponsorKeypair = StellarSdk.Keypair
    .fromSecret(sponsorSecretKey);

  const feeBumpTx = StellarSdk.TransactionBuilder
    .buildFeeBumpTransaction(
      sponsorKeypair,          // fee source account
      feePerOp,                // max fee per op in stroops
      innerTx,                 // wrapped inner tx
      StellarSdk.Networks.TESTNET
    );

  // 3. Sponsor signs the Fee Bump envelope
  feeBumpTx.sign(sponsorKeypair);

  // 4. Submit the Fee Bump to Horizon
  const result = await server.submitTransaction(feeBumpTx);
  return result.hash; // Returns the outer fee bump TX hash
}