update all contracts

contracts/CubLearn.sol

@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "@openzeppelin/contracts/access/Ownable.sol";
+import "./DTLToken.sol";
+import "./NftCertificate.sol";
+
+contract CubLearn is Ownable {
+	DTLToken dtlContract;
+	NftCertificate cubcertContract;
+	
+	uint256 public cashbackPercentage;
+
+	// course id => price in DTL
+	mapping(string => uint256) private coursePrices;
+	
+	// course id => student address => is enrolled
+	mapping(string => mapping(address => bool)) private enrolledCourses;
+
+	// course id => student address => is certificate issued
+	mapping(string => mapping(address => bool)) private issuedCertificates;
+
+	constructor(address dtlContractAddress, address cubcertContractAddress) Ownable(msg.sender) {
+		dtlContract = DTLToken(dtlContractAddress);
+		cubcertContract = NftCertificate(cubcertContractAddress);
+
+        coursePrices["1"] = 300;
+        coursePrices["2"] = 500;
+        coursePrices["3"] = 200;
+        cashbackPercentage = 50;
+	}
+
+	function setCashbackPercentage(uint256 newValue) external onlyOwner {
+        require(newValue <= 100, "Cashback percentage cannot be more than 100.");
+
+		cashbackPercentage = newValue;
+	}
+
+	function enroll(string memory courseId) external {
+		require(coursePrices[courseId] != 0, "Course not found.");
+
+		dtlContract.transferFrom(msg.sender, address(this), coursePrices[courseId]);
+
+        enrolledCourses[courseId][msg.sender] = true;
+	}
+
+	function finishCourse(
+		string memory courseId,
+		string memory studentFullName,
+		string memory courseName,
+		string memory instructorFullName
+	) external {
+		require(enrolledCourses[courseId][msg.sender], "Must be enrolled in course.");
+		require(!issuedCertificates[courseId][msg.sender], "Certificate already issued.");
+
+		cubcertContract.mintCertificate(msg.sender, studentFullName, courseName, instructorFullName);
+
+		// Mark the certificate as issued
+		issuedCertificates[courseId][msg.sender] = true;
+
+        // Cashback
+        uint256 cashbackAmount = coursePrices[courseId] * cashbackPercentage / 100;
+        dtlContract.transfer(msg.sender, cashbackAmount);
+	}
+
+	function getDTLBalance() external view onlyOwner returns (uint256) {
+		return dtlContract.balanceOf(address(this));
+	}
+
+	function transferAllDTLBalanceToMe() external onlyOwner {
+		uint256 balance = dtlContract.balanceOf(address(this));
+		dtlContract.transfer(msg.sender, balance);
+	}
+}

contracts/DTLToken.sol

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+
+contract DTLToken is ERC20 {
+	constructor(address initialHolder) ERC20("Digital Turkish Lira", "DTL") {
+		_mint(initialHolder, 1000000000);
+	}
+
+    function decimals() public view virtual override returns (uint8) {
+        return 0;
+    }
+
+    function approveMax(address spender) external returns (bool) {
+        return approve(spender, totalSupply());
+    }
+}

contracts/NftCertificate.sol

@@ -12,9 +12,6 @@ contract NftCertificate is ERC721 {
 
 	mapping(uint256 => CertificateInfo) private certificateInfoMapping;
 
-    // course id => student address => is cert issued
-    mapping(string => mapping(address => bool)) private issuedCertificates;
-
 	struct CertificateInfo {
 		string studentFullName;
 		string courseName;
@@ -24,21 +21,16 @@ contract NftCertificate is ERC721 {
 	constructor() ERC721("CubLearn Certificate", "CUBCERT") {}
 
 	function mintCertificate(
-        string memory courseId,
+		address recipient,
 		string memory studentFullName,
 		string memory courseName,
 		string memory instructorFullName
 	) external {
-        require(!issuedCertificates[courseId][msg.sender], "Certificate already issued.");
-
 		uint256 tokenId = nextTokenId;
 		
 		_storeCertificateInfo(tokenId, studentFullName, courseName, instructorFullName);
 
-        _safeMint(msg.sender, tokenId);
-
-        // Mark the certificate as issued
-        issuedCertificates[courseId][msg.sender] = true;
+		_safeMint(recipient, tokenId);
 
 		nextTokenId++;
 	}
@@ -85,7 +77,7 @@ contract NftCertificate is ERC721 {
 	}
 
 	function tokenURI(uint256 tokenId)
-        external
+		public
 		view
 		override
 		returns (string memory)