feat: add reserveBalanceOf

src/interfaces/IXToken.sol

@@ -53,6 +53,19 @@ interface IXToken {
      **/
     function pool() external view returns (address);
 
+     /**
+     * @dev Returns the reserve balance of the user that is backing the xTokens.
+     * @param user The user whose balance is calculated
+     * @return The reserve balance of the user
+     **/
+    function reserveBalanceOf(address user) external view returns (uint256);
+
+    /**
+     * @dev Returns the reserve total supply of the token.
+     * @return The reserve total supply
+     **/
+    function totalReserveSupply() external view returns (uint256);
+
     /**
      * @dev Mints `amount` xTokens to `account`
      * @param account The address receiving the minted tokens

src/protocol/xToken.sol

@@ -49,6 +49,24 @@ contract xToken is IXToken, ERC20, ERC20Permit {
         pool = msg.sender;
     }
 
+     /**
+     * @notice Returns the reserve balance of an account
+     * @dev This balance is independent of the asset price and represents the user's share of the reserve tokens in the pool
+     * @param account The address of the account
+     * @return The reserve balance of the account
+     */
+    function reserveBalanceOf(address account) public view returns (uint256) {
+        return _reserveBalances[account];
+    }
+
+    /**
+     * @notice Returns the total reserve supply
+     * @return The total reserve supply of tokens
+     */
+    function totalReserveSupply() public view returns (uint256) {
+        return _totalReserveSupply;
+    }
+
     /**
      * @notice Mints new tokens to an account
      * @dev Only callable by the pool contract