Staking limits (#68)

Closes: #59 

This PR implements staking limits. Here we add staking parameters that
define the maximum total assets(tBTC) that the Acre contract can hold
and the minimum deposit amount for a single deposit operation. Each
staking parameter can be updated by the owner.

core/contracts/Acre.sol

@@ -2,6 +2,7 @@
 pragma solidity ^0.8.21;
 
 import "@openzeppelin/contracts/token/ERC20/extensions/ERC4626.sol";
+import "@openzeppelin/contracts/access/Ownable.sol";
 
 /// @title Acre
 /// @notice This contract implements the ERC-4626 tokenized vault standard. By
@@ -14,12 +15,86 @@ import "@openzeppelin/contracts/token/ERC20/extensions/ERC4626.sol";
 ///      of yield-bearing vaults. This contract facilitates the minting and
 ///      burning of shares (stBTC), which are represented as standard ERC20
 ///      tokens, providing a seamless exchange with tBTC tokens.
-contract Acre is ERC4626 {
+contract Acre is ERC4626, Ownable {
+    // Minimum amount for a single deposit operation.
+    uint256 public minimumDepositAmount;
+    // Maximum total amount of tBTC token held by Acre.
+    uint256 public maximumTotalAssets;
+
     event StakeReferral(bytes32 indexed referral, uint256 assets);
+    event DepositParametersUpdated(
+        uint256 minimumDepositAmount,
+        uint256 maximumTotalAssets
+    );
+
+    error DepositAmountLessThanMin(uint256 amount, uint256 min);
 
     constructor(
         IERC20 tbtc
-    ) ERC4626(tbtc) ERC20("Acre Staked Bitcoin", "stBTC") {}
+    ) ERC4626(tbtc) ERC20("Acre Staked Bitcoin", "stBTC") Ownable(msg.sender) {
+        // TODO: Revisit the exact values closer to the launch.
+        minimumDepositAmount = 0.01 * 1e18; // 0.01 tBTC
+        maximumTotalAssets = 25 * 1e18; // 25 tBTC
+    }
+
+    /// @notice Updates deposit parameters.
+    /// @dev To disable the limit for deposits, set the maximum total assets to
+    ///      maximum (`type(uint256).max`).
+    /// @param _minimumDepositAmount New value of the minimum deposit amount. It
+    ///        is the minimum amount for a single deposit operation.
+    /// @param _maximumTotalAssets New value of the maximum total assets amount.
+    ///        It is the maximum amount of the tBTC token that the Acre can
+    ///        hold.
+    function updateDepositParameters(
+        uint256 _minimumDepositAmount,
+        uint256 _maximumTotalAssets
+    ) external onlyOwner {
+        // TODO: Introduce a parameters update process.
+        minimumDepositAmount = _minimumDepositAmount;
+        maximumTotalAssets = _maximumTotalAssets;
+
+        emit DepositParametersUpdated(
+            _minimumDepositAmount,
+            _maximumTotalAssets
+        );
+    }
+
+    /// @notice Mints shares to receiver by depositing exactly amount of
+    ///         tBTC tokens.
+    /// @dev Takes into account a deposit parameter, minimum deposit amount,
+    ///      which determines the minimum amount for a single deposit operation.
+    ///      The amount of the assets has to be pre-approved in the tBTC
+    ///      contract.
+    /// @param assets Approved amount of tBTC tokens to deposit.
+    /// @param receiver The address to which the shares will be minted.
+    /// @return Minted shares.
+    function deposit(
+        uint256 assets,
+        address receiver
+    ) public override returns (uint256) {
+        if (assets < minimumDepositAmount) {
+            revert DepositAmountLessThanMin(assets, minimumDepositAmount);
+        }
+
+        return super.deposit(assets, receiver);
+    }
+
+    /// @notice Mints shares to receiver by depositing tBTC tokens.
+    /// @dev Takes into account a deposit parameter, minimum deposit amount,
+    ///      which determines the minimum amount for a single deposit operation.
+    ///      The amount of the assets has to be pre-approved in the tBTC
+    ///      contract.
+    /// @param shares Amount of shares to mint. To get the amount of share use
+    ///        `previewMint`.
+    /// @param receiver The address to which the shares will be minted.
+    function mint(
+        uint256 shares,
+        address receiver
+    ) public override returns (uint256 assets) {
+        if ((assets = super.mint(shares, receiver)) < minimumDepositAmount) {
+            revert DepositAmountLessThanMin(assets, minimumDepositAmount);
+        }
+    }
 
     /// @notice Stakes a given amount of tBTC token and mints shares to a
     ///         receiver.
@@ -43,4 +118,43 @@ contract Acre is ERC4626 {
 
         return shares;
     }
+
+    /// @notice Returns the maximum amount of the tBTC token that can be
+    ///         deposited into the vault for the receiver, through a deposit
+    ///         call. It takes into account the deposit parameter, maximum total
+    ///         assets, which determines the total amount of tBTC token held by
+    ///         Acre.
+    /// @return The maximum amount of the tBTC token.
+    function maxDeposit(address) public view override returns (uint256) {
+        if (maximumTotalAssets == type(uint256).max) {
+            return type(uint256).max;
+        }
+
+        uint256 _totalAssets = totalAssets();
+
+        return
+            _totalAssets >= maximumTotalAssets
+                ? 0
+                : maximumTotalAssets - _totalAssets;
+    }
+
+    /// @notice Returns the maximum amount of the vault shares that can be
+    ///         minted for the receiver, through a mint call.
+    /// @dev Since the Acre contract limits the maximum total tBTC tokens this
+    ///      function converts the maximum deposit amount to shares.
+    /// @return The maximum amount of the vault shares.
+    function maxMint(address receiver) public view override returns (uint256) {
+        uint256 _maxDeposit = maxDeposit(receiver);
+
+        // slither-disable-next-line incorrect-equality
+        return
+            _maxDeposit == type(uint256).max
+                ? type(uint256).max
+                : convertToShares(_maxDeposit);
+    }
+
+    /// @return Returns deposit parameters.
+    function depositParameters() public view returns (uint256, uint256) {
+        return (minimumDepositAmount, maximumTotalAssets);
+    }
 }

core/deploy/21_transfer_ownership_acre.ts

@@ -19,5 +19,3 @@ const func: DeployFunction = async (hre: HardhatRuntimeEnvironment) => {
 export default func
 
 func.tags = ["TransferOwnershipAcre"]
-// TODO: Enable once Acre extends Ownable
-func.skip = async () => true