feat: add yield switching with signatures

src/contracts/Grateful.sol

@@ -3,6 +3,7 @@ pragma solidity 0.8.26;
 
 import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
 import {Ownable2Step} from "@openzeppelin/contracts/access/Ownable2Step.sol";
+import {ECDSA} from "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
 
 import {IERC20} from "@openzeppelin/contracts/interfaces/IERC20.sol";
 import {SafeERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
@@ -21,6 +22,7 @@ import {AaveV3ERC4626, IPool} from "yield-daddy/aave-v3/AaveV3ERC4626.sol";
 contract Grateful is IGrateful, Ownable2Step {
   using Bytes32AddressLib for bytes32;
   using SafeERC20 for IERC20;
+  using ECDSA for bytes32;
 
   /*//////////////////////////////////////////////////////////////
                                  STATE VARIABLES
@@ -29,6 +31,9 @@ contract Grateful is IGrateful, Ownable2Step {
   /// @inheritdoc IGrateful
   IPool public aavePool;
 
+  /// @inheritdoc IGrateful
+  string public constant SWITCH_YIELD_MESSAGE = unicode"🐧 At Grateful, we love penguins 🐧";
+
   /// @inheritdoc IGrateful
   mapping(address => bool) public tokensWhitelisted;
 
@@ -350,6 +355,17 @@ contract Grateful is IGrateful, Ownable2Step {
     yieldingFunds[msg.sender] = !yieldingFunds[msg.sender];
   }
 
+  /// @inheritdoc IGrateful
+  function switchYieldingFundsWithSig(bytes calldata _signature, address _merchant) external {
+    // TODO: Nonce pls
+    bytes32 signedMessageHash = keccak256(abi.encode(SWITCH_YIELD_MESSAGE));
+
+    if (signedMessageHash.recover(_signature) != _merchant) {
+      revert Grateful_InvalidSignature();
+    }
+    yieldingFunds[_merchant] = !yieldingFunds[_merchant];
+  }
+
   /// @inheritdoc IGrateful
   function setFee(
     uint256 _newFee

src/interfaces/IGrateful.sol

@@ -86,13 +86,19 @@ interface IGrateful {
   /// @notice Thrown when the user tries to withdraw more shares than they have.
   error Grateful_WithdrawExceedsShares();
 
+  /// @notice Thrown when the signature is not valid.
+  error Grateful_InvalidSignature();
+
   /*///////////////////////////////////////////////////////////////
                                  VARIABLES
     //////////////////////////////////////////////////////////////*/
 
   /// @notice Returns the owner of the contract.
   function owner() external view returns (address);
 
+  /// @notice Returns the message to sign to switch yield with signature
+  function SWITCH_YIELD_MESSAGE() external view returns (string memory);
+
   /// @notice Aave pool for yielding merchants' funds.
   function aavePool() external view returns (IPool);
 
@@ -342,6 +348,11 @@ interface IGrateful {
    */
   function switchYieldingFunds() external;
 
+  /**
+   * @notice Toggles the merchant's preference to yield funds using a signature.
+   */
+  function switchYieldingFundsWithSig(bytes calldata _signature, address _merchant) external;
+
   /**
    * @notice Calculates the ID of a payment.
    * @param _sender Address of the sender.

test/integration/Grateful.t.sol

@@ -2,6 +2,8 @@
 pragma solidity 0.8.26;
 
 import {OneTime} from "contracts/OneTime.sol";
+
+import {console} from "forge-std/console.sol";
 import {IntegrationBase} from "test/integration/IntegrationBase.sol";
 
 contract IntegrationGreeter is IntegrationBase {
@@ -268,4 +270,14 @@ contract IntegrationGreeter is IntegrationBase {
     uint256 feeAmount = _AMOUNT_USDC - amountAfterFee;
     assertEq(_usdc.balanceOf(_owner), feeAmount);
   }
+
+  function test_SwitchYieldingFundsWithSig() public {
+    string memory message = _grateful.SWITCH_YIELD_MESSAGE();
+
+    (uint8 v, bytes32 r, bytes32 s) = vm.sign(_userPk, keccak256(abi.encode(message)));
+
+    bytes memory signature = abi.encodePacked(r, s, v);
+
+    _grateful.switchYieldingFundsWithSig(signature, _user);
+  }
 }

test/integration/IntegrationBase.sol

@@ -19,7 +19,8 @@ contract IntegrationBase is Test {
   uint256 internal constant _FEE = 100;
 
   // EOAs
-  address internal _user = makeAddr("user");
+  uint256 internal _userPk = 0x4f9f153352796ab5f555d215a5cba1c2b0daef31de556ab65c522435fb23373c;
+  address internal _user = vm.addr(_userPk);
   address internal _merchant = makeAddr("merchant");
   address internal _owner = makeAddr("owner");
   address internal _gratefulAutomation = makeAddr("gratefulAutomation");