USDT0 Omnichain Stablecoin on Hedera

struct SendParam {
    uint32 dstEid;       // destination chain LayerZero endpoint ID
    bytes32 to;          // recipient address, left-padded to bytes32
    uint256 amountLD;    // amount to send, in local decimals (6 for USDT0)
    uint256 minAmountLD; // minimum received after fees/slippage
    bytes extraOptions;  // execution options (e.g. gas for the destination)
    bytes composeMsg;    // optional composed message payload
    bytes oftCmd;        // OFT command bytes (empty for a standard send)
}

struct MessagingFee {
    uint256 nativeFee;   // fee paid in native gas (HBAR on Hedera)
    uint256 lzTokenFee;  // optional fee paid in the LayerZero token
}

import { ethers } from "ethers";

// Minimal ABI for the OFT send flow.
const OFT_ABI = [
  "function quoteSend((uint32 dstEid, bytes32 to, uint256 amountLD, uint256 minAmountLD, bytes extraOptions, bytes composeMsg, bytes oftCmd) sendParam, bool payInLzToken) view returns ((uint256 nativeFee, uint256 lzTokenFee) fee)",
  "function send((uint32 dstEid, bytes32 to, uint256 amountLD, uint256 minAmountLD, bytes extraOptions, bytes composeMsg, bytes oftCmd) sendParam, (uint256 nativeFee, uint256 lzTokenFee) fee, address refundAddress) payable returns ((bytes32 guid, uint64 nonce, (uint256 nativeFee, uint256 lzTokenFee) fee) msgReceipt, (uint256 amountSentLD, uint256 amountReceivedLD) oftReceipt)",
  "function token() view returns (address)",
];

const ERC20_ABI = [
  "function approve(address spender, uint256 amount) returns (bool)",
  "function allowance(address owner, address spender) view returns (uint256)",
];

// USDT0 OFT contract on Hedera mainnet.
const HEDERA_USDT0_OFT = "0xe3119e23fC2371d1E6b01775ba312035425A53d6";

// Left-pad a 20-byte EVM address to a 32-byte value.
function addressToBytes32(address) {
  return ethers.zeroPadValue(address, 32);
}

async function sendUsdt0({ provider, signer, dstEid, recipient, amount }) {
  const oft = new ethers.Contract(HEDERA_USDT0_OFT, OFT_ABI, signer);

  const sendParam = {
    dstEid,                          // e.g. 30110 for Arbitrum One
    to: addressToBytes32(recipient), // recipient on the destination chain
    amountLD: amount,                // 6-decimal amount, e.g. 100 USDT0 = 100_000000n
    minAmountLD: amount,             // adjust to allow for slippage if needed
    extraOptions: "0x",              // default execution options
    composeMsg: "0x",
    oftCmd: "0x",
  };

  // 1. Quote the LayerZero messaging fee (paid in HBAR when sending from Hedera).
  const fee = await oft.quoteSend(sendParam, false);

  // 2. Approve the OFT to pull the underlying token, if it is not already approved.
  const tokenAddress = await oft.token();
  const token = new ethers.Contract(tokenAddress, ERC20_ABI, signer);
  const owner = await signer.getAddress();
  const allowance = await token.allowance(owner, HEDERA_USDT0_OFT);
  if (allowance < amount) {
    await (await token.approve(HEDERA_USDT0_OFT, amount)).wait();
  }

  // 3. Send, attaching the native fee as msg.value.
  //    Hedera gotcha: quoteSend returns nativeFee in tinybars (8 decimals), but the
  //    JSON-RPC relay expects msg.value in weibars (18 decimals). Convert before sending.
  const nativeFeeWeibars = fee.nativeFee * 10n ** 10n;
  const tx = await oft.send(sendParam, fee, owner, { value: nativeFeeWeibars });
  return tx.wait();
}