DyDxFlashLoan.sol

// SPDX-License-Identifier: AGPL-3.0-or-later
pragma solidity ^0.8.0;

import "./DyDx.sol";
import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import { SafeERC20 } from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";

contract DyDxFlashLoan is Structs {

    using SafeERC20 for IERC20;

    DyDxPool constant pool = DyDxPool(0x1E0447b19BB6EcFdAe1e4AE1694b0C3659614e4e);

    // DyDx tokens
    address constant WETH = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;
    address constant SAI = 0x89d24A6b4CcB1B6fAA2625fE562bDD9a23260359;

    // Both DyDx and mUSD tokens
    address constant USDC = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;
    address constant DAI = 0x6B175474E89094C44Da98b954EedeAC495271d0F;

    // mUSD tokens
    address constant sUSD = 0x57Ab1ec28D129707052df4dF418D58a2D46d5f51;
    address constant TUSD = 0x0000000000085d4780B73119b644AE5ecd22b376;
    address constant USDT = 0xdAC17F958D2ee523a2206206994597C13D831ec7;
    
    mapping(address => uint256) public currencies;

    constructor() public {
        currencies[WETH] = 1;
        currencies[SAI] = 2;
        currencies[USDC] = 3;
        currencies[DAI] = 4;
    }

    modifier onlyPool() {
        require(
            msg.sender == address(pool),
            "FlashLoan: only called by DyDx pool"
        );
        _;
    }

    function tokenToMarketId(address token) public view returns (uint256) {
        uint256 marketId = currencies[token];
        require(marketId != 0, "FlashLoan: Unsupported token");
        return marketId - 1;
    }

    // the DyDx will call `callFunction(address sender, Info memory accountInfo, bytes memory data) public` after during `operate` call
    function flashloan(address token, uint256 amount, bytes memory data)
        internal virtual
    {
        IERC20(token).approve(address(pool), amount + 1);
        Info[] memory infos = new Info[](1);
        ActionArgs[] memory args = new ActionArgs[](3);

        infos[0] = Info(address(this), 0);

        AssetAmount memory wamt = AssetAmount(
            false,
            AssetDenomination.Wei,
            AssetReference.Delta,
            amount
        );
        ActionArgs memory withdraw;
        withdraw.actionType = ActionType.Withdraw;
        withdraw.accountId = 0;
        withdraw.amount = wamt;
        withdraw.primaryMarketId = tokenToMarketId(token);
        withdraw.otherAddress = address(this);

        args[0] = withdraw;

        ActionArgs memory call;
        call.actionType = ActionType.Call;
        call.accountId = 0;
        call.otherAddress = address(this);
        call.data = data;

        args[1] = call;

        ActionArgs memory deposit;
        AssetAmount memory damt = AssetAmount(
            true,
            AssetDenomination.Wei,
            AssetReference.Delta,
            amount + 1
        );
        deposit.actionType = ActionType.Deposit;
        deposit.accountId = 0;
        deposit.amount = damt;
        deposit.primaryMarketId = tokenToMarketId(token);
        deposit.otherAddress = address(this);

        args[2] = deposit;

        pool.operate(infos, args);
    }
}