How-to: Confirm a purchase onchain

import { createPublicClient, http, parseAbi } from "viem";
import { arc } from "./chains"; // your Arc chain config

const client = createPublicClient({
  chain: arc,
  transport: http(),
});

const USDC_EMITTER = "0xffffFFFfFFffffffffffffffFfFFFfffFFFfFFfE"; // native USDC, 18 decimals

const transferEvent = parseAbi([
  "event Transfer(address indexed from, address indexed to, uint256 value)",
])[0];

const latest = await client.getBlockNumber();
for (let from = lastCheckedBlock; from <= latest; from += 2_000n) {
  const to = from + 1_999n > latest ? latest : from + 1_999n;
  const logs = await client.getLogs({
    address: USDC_EMITTER,
    event: transferEvent,
    args: { to: destinationAddress },
    fromBlock: from,
    toBlock: to,
  });
  await processLogs(logs); // steps 2 and 3
  // Then persist `to` as lastCheckedBlock.
}

import { formatUnits } from "viem";

for (const log of logs) {
  const amount = log.args.value; // bigint, in token's smallest units
  const display = formatUnits(amount, 18); // native USDC on Arc
  // Record the delivered amount by `display` units.
}

for (const log of logs) {
  const key = { txHash: log.transactionHash, logIndex: log.logIndex };
  const seen = await db.deposits.find(key);
  if (seen) continue;

  const user = resolveUserByAddress(log.args.to);
  await db.deposits.insert({
    ...key,
    amount: log.args.value,
    user,
  });
  // Record the deposit for this user.
}