Redeem nUSD for USDC

import { BN } from "@coral-xyz/anchor";
import { PublicKey, Transaction } from "@solana/web3.js";
import { getAccount } from "@solana/spl-token";
import {
  addAtaIfMissing,
  ata,
  simulateSignSend,
  toBaseUnits,
  type NestClient,
} from "../nest-client";

export async function redeemNusdForUsdc(client: NestClient, userAmount: string) {
  const owner = client.wallet.publicKey;
  const amount = toBaseUnits(userAmount, 6);
  if (amount <= 0n) throw new Error("Redemption amount must be positive");

  const protocol = new PublicKey(client.deployment.accounts.protocol);
  const usdcMint = new PublicKey(client.deployment.mints.usdc);
  const nusdMint = new PublicKey(client.deployment.mints.nusd);
  const usdcTokenProgram = new PublicKey(client.deployment.tokenPrograms.usdc);
  const nusdTokenProgram = new PublicKey(client.deployment.tokenPrograms.nusd);
  const userNusdAccount = ata(nusdMint, owner, nusdTokenProgram);
  const psmUsdcVault = new PublicKey(client.deployment.vaults.psmUsdcVault);

  const [protocolBefore, sourceBefore, psmVaultBefore] = await Promise.all([
    client.core.account.protocol.fetch(protocol),
    getAccount(client.connection, userNusdAccount, "confirmed", nusdTokenProgram),
    getAccount(client.connection, psmUsdcVault, "confirmed", usdcTokenProgram),
  ]);
  if (protocolBefore.paused) throw new Error("Nest Core is paused");
  if (BigInt(protocolBefore.psmSwapOutFeeBps.toString()) !== 0n) {
    throw new Error("This integration expects the current zero-fee PSM configuration");
  }
  if (BigInt(protocolBefore.badDebtNusd.toString()) !== 0n) {
    throw new Error("PSM redemption is disabled while bad debt is outstanding");
  }
  if (sourceBefore.amount < amount) throw new Error("Insufficient nUSD balance");
  const idleBefore = BigInt(protocolBefore.psmIdleUsdc.toString());
  if (amount > idleBefore || amount > psmVaultBefore.amount) {
    throw new Error("Insufficient idle PSM USDC");
  }

  const transaction = new Transaction();
  const userUsdcAccount = await addAtaIfMissing(
    client,
    transaction,
    usdcMint,
    owner,
    usdcTokenProgram,
  );
  const destinationBefore = await client.connection.getAccountInfo(userUsdcAccount, "confirmed")
    ? (await getAccount(
        client.connection,
        userUsdcAccount,
        "confirmed",
        usdcTokenProgram,
      )).amount
    : 0n;

  transaction.add(await client.core.methods
    .psmSwapOut(new BN(amount.toString()))
    .accountsStrict({
      protocol,
      usdcMint,
      nusdMint,
      userNusdAccount,
      psmUsdcVault,
      userUsdcAccount,
      user: owner,
      nusdTokenProgram,
      usdcTokenProgram,
    })
    .instruction());

  const signature = await simulateSignSend(client, transaction);
  const [sourceAfter, destinationAfter, protocolAfter] = await Promise.all([
    getAccount(client.connection, userNusdAccount, "confirmed", nusdTokenProgram),
    getAccount(client.connection, userUsdcAccount, "confirmed", usdcTokenProgram),
    client.core.account.protocol.fetch(protocol),
  ]);
  if (
    sourceAfter.amount !== sourceBefore.amount - amount
    || destinationAfter.amount !== destinationBefore + amount
    || BigInt(protocolAfter.psmIdleUsdc.toString()) !== idleBefore - amount
  ) throw new Error(`PSM redemption verification failed for ${signature}`);

  return { signature, burnedNusd6: amount, receivedUsdc6: amount };
}