Fix logic of both contract

src/BankHub.sol

@@ -3,13 +3,14 @@
 pragma solidity 0.8.28;
 
 import {IDRCoin} from "./IDRCoin.sol";
-import {UUPSUpgradeable} from "@openzeppelin/contracts-upgradeable/proxy/utils/UUPSUpgradeable.sol";
 
-contract BankHub is UUPSUpgradeable {
+contract BankHub {
     // constants
     uint32 public constant MIN_INTEREST_RATE = 5;
+    uint32 public constant MAX_INTEREST_RATE = 100;
     uint32 public constant DENOMINATOR = 100;
     uint256 public constant MIN_LOAN_AMOUNT = 10e18;
+    uint256 public constant MAX_LOAN_AMOUNT = 100e18;
 
     // state variables
     address public owner;
@@ -47,12 +48,8 @@ contract BankHub is UUPSUpgradeable {
     event Withdraw(address indexed user, address indexed bank, uint256 amount);
     event Approved(address indexed bank);
 
-    constructor() {
+    constructor(address _owner) {
-        _disableInitializers();
+        owner = _owner;
-    }
-
-    function initialize() public initializer {
-        owner = msg.sender;
     }
 
     // user function
@@ -95,9 +92,9 @@ contract BankHub is UUPSUpgradeable {
         idrcoin.mint(msg.sender, interest);
 
         // transfer amount
-        idrcoin.transferFrom(address(_fromBank), msg.sender, _amount);
+        idrcoin.transferFrom(_fromBank, msg.sender, _amount);
 
-        emit Withdraw(msg.sender, _fromBank, _amount);
+        emit Withdraw(msg.sender, _fromBank, _amount + interest);
     }
 
     // bank function
@@ -110,6 +107,9 @@ contract BankHub is UUPSUpgradeable {
         if (_amount < MIN_LOAN_AMOUNT) {
             revert insufficientLoanAmount();
         }
+        if (_amount > MAX_LOAN_AMOUNT) {
+            revert insufficientLoanAmount();
+        }
 
         idrcoin.mint(_bank, _amount);
     }
@@ -126,6 +126,11 @@ contract BankHub is UUPSUpgradeable {
         owner = _newOwner;
     }
 
+    // set IDRCoin address
+    function setIDRCoin(address _idrcoin) public onlyOwner {
+        idrcoin = IDRCoin(_idrcoin);
+    }
+
     // whitelist partner bank, set interest rate and approve unlimited IDRCoin transfer by this contract
     function whiteList(address _bank) public onlyOwner {
         whiteListed[_bank] = true;
@@ -137,17 +142,13 @@ contract BankHub is UUPSUpgradeable {
 
     // revoke whitelist from partner bank
     // collect all IDRCoin from bank
+    // this is used to punish bank that misbehave
     function revokeWhiteList(address _bank) public onlyOwner {
         if (idrcoin.balanceOf(_bank) > 0) {
             idrcoin.transferFrom(_bank, owner, idrcoin.balanceOf(_bank));
         }
     }
 
-    // Implementation of the required UUPSUpgradeable function
-    function _authorizeUpgrade(address newImplementation) internal override {
-        require(msg.sender == owner, "only owner can authorize upgrades");
-    }
-
     // view function
     function isWhiteListed(address _bank) public view returns (bool) {
         return whiteListed[_bank];

src/IDRCoin.sol

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: MIT
 
 pragma solidity 0.8.28;
+
 import {ERC20} from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 import {IUSDT} from "./interfaces/IUSDT.sol";
 import {IBankHub} from "./interfaces/IBankHub.sol";
@@ -18,11 +19,12 @@ contract IDRCoin is ERC20 {
 
     // constant
     // convertion rate from USDT to IDR
-    uint256 constant public CONVERSION_RATE = 16000;
+    uint256 public constant CONVERSION_RATE = 16000;
+    uint8 public constant DECIMALS = 18;
 
     // enforce ppn 12% for ALL MINTING transaction involving IDRC token
-    uint256 constant public TAX = 12;
+    uint256 public constant TAX = 12;
-    uint256 constant public DENOMINATOR = 100;
+    uint256 public constant DENOMINATOR = 100;
 
     modifier onlyAdmin() {
         if (msg.sender != admin) {
@@ -47,8 +49,16 @@ contract IDRCoin is ERC20 {
     error BankCannotManualApprove();
 
     // event
-    event IDRC_Transfer(address indexed from, address indexed to, uint256 value);
+    event IDRC_Transfer(
-    event IDRC_Approval(address indexed owner, address indexed spender, uint256 value);
+        address indexed from,
+        address indexed to,
+        uint256 value
+    );
+    event IDRC_Approval(
+        address indexed owner,
+        address indexed spender,
+        uint256 value
+    );
     event IDRC_Mint(address indexed to, uint256 value);
     event IDRC_Burn(address indexed from, uint256 value);
 
@@ -81,9 +91,12 @@ contract IDRCoin is ERC20 {
     // external/public function
     // anyone can buy IDRC with USDT with fixed conversion rate
     function convertUSDtoIDR(uint256 amountInUSD) external {
-        usdt.transfer(address(this), amountInUSD);
+        usdt.transferFrom(msg.sender, address(this), amountInUSD);
-        uint256 amountInIDR = amountInUSD * CONVERSION_RATE * decimals();
+        // first we normalize the amount in usd by dividing it with its own decimals
-        _mint(msg.sender, amountInIDR);
+        // then we multiply it with the conversion rate and IDRC decimals
+        // result is the amount of IDRC to mint with the correct decimals
+        uint256 amountInIDR = (amountInUSD / 10 ** usdt.decimals()) *CONVERSION_RATE *  10 ** decimals();
+        mint_(msg.sender, amountInIDR);
 
         emit IDRC_Mint(msg.sender, amountInIDR);
     }
@@ -178,6 +191,10 @@ contract IDRCoin is ERC20 {
         return balances[_addr];
     }
 
+    function decimals() public pure override returns (uint8) {
+        return DECIMALS;
+    }
+
     // setter/admin function
     // set the bankHub address
     function setBankHub(address _bankHub) external onlyAdmin {

src/interfaces/IUSDT.sol

@@ -2,11 +2,13 @@
 pragma solidity 0.8.28;
 
 interface IUSDT {
-    function decimals() external returns (uint);
+    function decimals() external returns (uint256);
 
-    function balanceOf(address who) external returns (uint);
+    function balanceOf(address who) external returns (uint256);
 
-    function transfer(address to, uint value) external;
+    function transfer(address to, uint256 value) external;
 
-    function approve(address spender, uint value) external;
+    function transferFrom(address from, address to, uint256 value) external;
+
+    function approve(address spender, uint256 value) external;
 }