Flipper for zero slippage trading of OUSD for stablecoins (#558)

* Add flipper contract allow zero slippage trading of OUSD for stablecoins.

* Comments on methods, removing unneeded constant.

* Hide errors about tether not being ERC20 complient.

* Deploy script 15 + Prod flipper contract

* Fix deploy 15 logging

* Fix slither error

* Rinkeby deploy

* mainnet abi

* Fix comments in contract files

Co-authored-by: Franck Chastagnol <franck@originprotocol.com>

contracts/contracts/flipper/Flipper.sol

@@ -0,0 +1,119 @@
+pragma solidity 0.5.11;
+
+import "../governance/Governable.sol";
+import "../token/OUSD.sol";
+import "../interfaces/Tether.sol";
+import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
+import { SafeERC20 } from "@openzeppelin/contracts/token/ERC20/SafeERC20.sol";
+
+// Contract to exchange usdt, usdc, dai from and to ousd.
+//   - 1 to 1. No slippage
+//   - Optimized for low gas usage
+//   - No guarantee of availability
+
+
+contract Flipper is Governable {
+    using SafeERC20 for IERC20;
+
+    uint256 constant MAXIMUM_PER_TRADE = (25000 * 1e18);
+
+    // Saves approx 4K gas per swap by using hardcoded addresses.
+    IERC20 dai = IERC20(0x6B175474E89094C44Da98b954EedeAC495271d0F);
+    OUSD constant ousd = OUSD(0x2A8e1E676Ec238d8A992307B495b45B3fEAa5e86);
+    IERC20 usdc = IERC20(0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48);
+    Tether constant usdt = Tether(0xdAC17F958D2ee523a2206206994597C13D831ec7);
+
+    // -----------
+    // Constructor
+    // -----------
+    constructor() public {}
+
+    // -----------------
+    // Trading functions
+    // -----------------
+
+    /// @notice Purchase OUSD with Dai
+    /// @param amount Amount of OUSD to purchase, in 18 fixed decimals.
+    function buyOusdWithDai(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        require(dai.transferFrom(msg.sender, address(this), amount));
+        require(ousd.transfer(msg.sender, amount));
+    }
+
+    /// @notice Sell OUSD for Dai
+    /// @param amount Amount of OUSD to sell, in 18 fixed decimals.
+    function sellOusdForDai(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        require(dai.transfer(msg.sender, amount));
+        require(ousd.transferFrom(msg.sender, address(this), amount));
+    }
+
+    /// @notice Purchase OUSD with USDC
+    /// @param amount Amount of OUSD to purchase, in 18 fixed decimals.
+    function buyOusdWithUsdc(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        // Potential rounding error is an intentional tradeoff
+        require(usdc.transferFrom(msg.sender, address(this), amount / 1e12));
+        require(ousd.transfer(msg.sender, amount));
+    }
+
+    /// @notice Sell OUSD for USDC
+    /// @param amount Amount of OUSD to sell, in 18 fixed decimals.
+    function sellOusdForUsdc(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        require(usdc.transfer(msg.sender, amount / 1e12));
+        require(ousd.transferFrom(msg.sender, address(this), amount));
+    }
+
+    /// @notice Purchase OUSD with USDT
+    /// @param amount Amount of OUSD to purchase, in 18 fixed decimals.
+    function buyOusdWithUsdt(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        // Potential rounding error is an intentional tradeoff
+        // USDT does not return a boolean and reverts,
+        // so no need for a require.
+        usdt.transferFrom(msg.sender, address(this), amount / 1e12);
+        require(ousd.transfer(msg.sender, amount));
+    }
+
+    /// @notice Sell OUSD for USDT
+    /// @param amount Amount of OUSD to sell, in 18 fixed decimals.
+    function sellOusdForUsdt(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        // USDT does not return a boolean and reverts,
+        // so no need for a require.
+        usdt.transfer(msg.sender, amount / 1e12);
+        require(ousd.transferFrom(msg.sender, address(this), amount));
+    }
+
+    // --------------------
+    // Governance functions
+    // --------------------
+
+    /// @dev Opting into yield reduces the gas cost per transfer by about 4K, since
+    /// ousd needs to do less accounting and one less storage write.
+    function rebaseOptIn() external onlyGovernor nonReentrant {
+        ousd.rebaseOptIn();
+    }
+
+    /// @notice Owner function to withdraw a specific amount of a token
+    function withdraw(address token, uint256 amount)
+        external
+        onlyGovernor
+        nonReentrant
+    {
+        IERC20(token).safeTransfer(_governor(), amount);
+    }
+
+    /// @notice Owner function to withdraw all tradable tokens
+    /// @dev Equivalent to "pausing" the contract.
+    function withdrawAll() external onlyGovernor nonReentrant {
+        IERC20(dai).safeTransfer(_governor(), dai.balanceOf(address(this)));
+        IERC20(ousd).safeTransfer(_governor(), ousd.balanceOf(address(this)));
+        IERC20(address(usdt)).safeTransfer(
+            _governor(),
+            usdt.balanceOf(address(this))
+        );
+        IERC20(usdc).safeTransfer(_governor(), usdc.balanceOf(address(this)));
+    }
+}

contracts/contracts/flipper/FlipperDev.sol

@@ -0,0 +1,133 @@
+pragma solidity 0.5.11;
+
+import "../governance/Governable.sol";
+import "../token/OUSD.sol";
+import "../interfaces/Tether.sol";
+import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
+import { SafeERC20 } from "@openzeppelin/contracts/token/ERC20/SafeERC20.sol";
+
+// Contract to exchange usdt, usdc, dai from and to ousd.
+//   - 1 to 1. No slippage
+//   - Optimized for low gas usage
+//   - No guarantee of availability
+
+
+contract FlipperDev is Governable {
+    using SafeERC20 for IERC20;
+
+    uint256 constant MAXIMUM_PER_TRADE = (25000 * 1e18);
+
+    // Settable coin addresses allow easy testing and use of mock currencies.
+    IERC20 dai = IERC20(0);
+    OUSD ousd = OUSD(0);
+    IERC20 usdc = IERC20(0);
+    Tether usdt = Tether(0);
+
+    // ---------------------
+    // Dev constructor
+    // ---------------------
+    constructor(
+        address dai_,
+        address ousd_,
+        address usdc_,
+        address usdt_
+    ) public {
+        dai = IERC20(dai_);
+        ousd = OUSD(ousd_);
+        usdc = IERC20(usdc_);
+        usdt = Tether(usdt_);
+        require(address(ousd) != address(0));
+        require(address(dai) != address(0));
+        require(address(usdc) != address(0));
+        require(address(usdt) != address(0));
+    }
+
+    // -----------------
+    // Trading functions
+    // -----------------
+
+    /// @notice Purchase OUSD with Dai
+    /// @param amount Amount of OUSD to purchase, in 18 fixed decimals.
+    function buyOusdWithDai(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        require(dai.transferFrom(msg.sender, address(this), amount));
+        require(ousd.transfer(msg.sender, amount));
+    }
+
+    /// @notice Sell OUSD for Dai
+    /// @param amount Amount of OUSD to sell, in 18 fixed decimals.
+    function sellOusdForDai(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        require(dai.transfer(msg.sender, amount));
+        require(ousd.transferFrom(msg.sender, address(this), amount));
+    }
+
+    /// @notice Purchase OUSD with USDC
+    /// @param amount Amount of OUSD to purchase, in 18 fixed decimals.
+    function buyOusdWithUsdc(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        // Potential rounding error is an intentional tradeoff
+        require(usdc.transferFrom(msg.sender, address(this), amount / 1e12));
+        require(ousd.transfer(msg.sender, amount));
+    }
+
+    /// @notice Sell OUSD for USDC
+    /// @param amount Amount of OUSD to sell, in 18 fixed decimals.
+    function sellOusdForUsdc(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        require(usdc.transfer(msg.sender, amount / 1e12));
+        require(ousd.transferFrom(msg.sender, address(this), amount));
+    }
+
+    /// @notice Purchase OUSD with USDT
+    /// @param amount Amount of OUSD to purchase, in 18 fixed decimals.
+    function buyOusdWithUsdt(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        // Potential rounding error is an intentional tradeoff
+        // USDT does not return a boolean and reverts,
+        // so no need for a require.
+        usdt.transferFrom(msg.sender, address(this), amount / 1e12);
+        require(ousd.transfer(msg.sender, amount));
+    }
+
+    /// @notice Sell OUSD for USDT
+    /// @param amount Amount of OUSD to sell, in 18 fixed decimals.
+    function sellOusdForUsdt(uint256 amount) external {
+        require(amount <= MAXIMUM_PER_TRADE, "Amount too large");
+        // USDT does not return a boolean and reverts,
+        // so no need for a require.
+        usdt.transfer(msg.sender, amount / 1e12);
+        require(ousd.transferFrom(msg.sender, address(this), amount));
+    }
+
+    // --------------------
+    // Governance functions
+    // --------------------
+
+    /// @dev Opting into yield reduces the gas cost per transfer by about 4K, since
+    /// ousd needs to do less accounting and one less storage write.
+    function rebaseOptIn() external onlyGovernor nonReentrant {
+        ousd.rebaseOptIn();
+    }
+
+    /// @notice Owner function to withdraw a specific amount of a token
+    function withdraw(address token, uint256 amount)
+        external
+        onlyGovernor
+        nonReentrant
+    {
+        IERC20(token).safeTransfer(_governor(), amount);
+    }
+
+    /// @notice Owner function to withdraw all tradable tokens
+    /// @dev Equivalent to "pausing" the contract.
+    function withdrawAll() external onlyGovernor nonReentrant {
+        IERC20(dai).safeTransfer(_governor(), dai.balanceOf(address(this)));
+        IERC20(ousd).safeTransfer(_governor(), ousd.balanceOf(address(this)));
+        IERC20(address(usdt)).safeTransfer(
+            _governor(),
+            usdt.balanceOf(address(this))
+        );
+        IERC20(usdc).safeTransfer(_governor(), usdc.balanceOf(address(this)));
+    }
+}

contracts/contracts/interfaces/Tether.sol

@@ -0,0 +1,13 @@
+pragma solidity 0.5.11;
+
+interface Tether {
+    function transfer(address to, uint256 value) external;
+
+    function transferFrom(
+        address from,
+        address to,
+        uint256 value
+    ) external;
+
+    function balanceOf(address) external returns (uint256);
+}

contracts/deploy/001_core.js

@@ -427,6 +427,24 @@ const deployCore = async () => {
   log("Initialized OUSD");
 };
 
+// Deploy the Flipper trading contract
+const deployFlipper = async () => {
+  const assetAddresses = await getAssetAddresses(deployments);
+  const { governorAddr } = await hre.getNamedAccounts();
+  const sGovernor = await ethers.provider.getSigner(governorAddr);
+  const ousd = await ethers.getContract("OUSDProxy");
+
+  await deployWithConfirmation("FlipperDev", [
+    assetAddresses.DAI,
+    ousd.address,
+    assetAddresses.USDC,
+    assetAddresses.USDT,
+  ]);
+  const flipper = await ethers.getContract("FlipperDev");
+  await withConfirmation(flipper.transferGovernance(governorAddr));
+  await withConfirmation(flipper.connect(sGovernor).claimGovernance());
+};
+
 const main = async () => {
   console.log("Running 001_core deployment...");
   await deployOracles();
@@ -435,6 +453,7 @@ const main = async () => {
   await deployAaveStrategy();
   await deployThreePoolStrategy();
   await configureVault();
+  await deployFlipper();
   console.log("001_core deploy done.");
   return true;
 };