Repay debt

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

export async function repayDebt(
  client: NestClient,
  symbol: string,
  amount: string | "max",
) {
  const owner = client.wallet.publicKey;
  const market = marketBySymbol(client, symbol);
  const coreProgramId = new PublicKey(client.deployment.programs.nestCore);
  const protocol = new PublicKey(client.deployment.accounts.protocol);
  const collateralConfig = new PublicKey(market.collateralConfig);
  const vault = borrowVaultPda(coreProgramId, owner, collateralConfig);
  const nusdMint = new PublicKey(client.deployment.mints.nusd);
  const nusdTokenProgram = new PublicKey(client.deployment.tokenPrograms.nusd);
  const ownerNusdAccount = ata(nusdMint, owner, nusdTokenProgram);

  const [vaultBefore, walletNusd] = await Promise.all([
    client.core.account.vault.fetch(vault),
    getAccount(client.connection, ownerNusdAccount, "confirmed", nusdTokenProgram),
  ]);
  const debtBefore = BigInt(vaultBefore.principalDebt.toString())
    + BigInt(vaultBefore.accruedFee.toString());
  if (debtBefore === 0n) throw new Error(`${symbol} position has no debt`);

  // For max repayment, passing the wallet balance is safe: Nest only transfers
  // the final amount owed after accruing to the transaction timestamp.
  const requestedAmount = amount === "max"
    ? walletNusd.amount
    : toBaseUnits(amount, 6);
  if (requestedAmount <= 0n) throw new Error("Repayment must be positive");
  if (requestedAmount > walletNusd.amount) throw new Error("Insufficient nUSD balance");

  const transaction = new Transaction().add(await client.core.methods
    .repayNusd(new BN(requestedAmount.toString()))
    .accountsStrict({
      protocol,
      collateralConfig,
      vault,
      nusdMint,
      ownerNusdAccount,
      insuranceNusdVault: new PublicKey(client.deployment.vaults.insuranceNusdVault),
      stakerRevenueNusdVault: new PublicKey(client.deployment.vaults.stakerRevenueNusdVault),
      protocolRevenueNusdVault: new PublicKey(client.deployment.vaults.protocolRevenueNusdVault),
      stakingState: new PublicKey(client.deployment.accounts.stakingState),
      owner,
      nusdTokenProgram,
    })
    .instruction());

  const signature = await simulateSignSend(client, transaction);
  const [vaultAfter, walletAfter] = await Promise.all([
    client.core.account.vault.fetch(vault),
    getAccount(client.connection, ownerNusdAccount, "confirmed", nusdTokenProgram),
  ]);
  const principalAfter = BigInt(vaultAfter.principalDebt.toString());
  const feeAfter = BigInt(vaultAfter.accruedFee.toString());
  const debtAfter = principalAfter + feeAfter;
  const paidNusd6 = walletNusd.amount - walletAfter.amount;
  if (paidNusd6 <= 0n || paidNusd6 > requestedAmount) {
    throw new Error(`Repayment verification failed for ${signature}`);
  }

  return {
    signature,
    principalDebtNusd6: principalAfter,
    accruedFeeNusd6: feeAfter,
    paidNusd6,
    fullyRepaid: debtAfter === 0n,
  };
}