Anatomi kontrak pintar

// Solidity example // Contoh Solidity
// Initializes the contract's data, setting the `owner` // Menginisialisasi data kontrak, mengatur `owner`
// to the address of the contract creator. // ke alamat pembuat kontrak.
constructor() public {
    // All smart contracts rely on external transactions to trigger its functions. // Semua kontrak pintar bergantung pada transaksi eksternal untuk memicu fungsinya.
    // `msg` is a global variable that includes relevant data on the given transaction, // `msg` adalah variabel global yang mencakup data relevan pada transaksi yang diberikan,
    // such as the address of the sender and the ETH value included in the transaction. // seperti alamat pengirim dan nilai ETH yang disertakan dalam transaksi.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties
    owner = msg.sender;
}

pragma solidity >=0.4.0 <=0.6.0;

contract ExampleDapp {
    string dapp_name; // state variable // variabel state

    // Called when the contract is deployed and initializes the value // Dipanggil saat kontrak disebarkan dan menginisialisasi nilai
    constructor() public {
        dapp_name = "My Example dapp";
    }

    // Get Function // Fungsi Get
    function read_name() public view returns(string) {
        return dapp_name;
    }

    // Set Function // Fungsi Set
    function update_name(string value) public {
        dapp_name = value;
    }
}

// Specifies the version of Solidity, using semantic versioning. // Menentukan versi Solidity, menggunakan pembuatan versi semantik.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#pragma // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#pragma
pragma solidity ^0.5.10;

// Defines a contract named `HelloWorld`. // Mendefinisikan kontrak bernama `HelloWorld`.
// A contract is a collection of functions and data (its state). // Kontrak adalah kumpulan fungsi dan data (state-nya).
// Once deployed, a contract resides at a specific address on the Ethereum blockchain. // Setelah disebarkan, kontrak berada di alamat tertentu di blockchain Ethereum.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html
contract HelloWorld {

    // Declares a state variable `message` of type `string`. // Mendeklarasikan variabel state `message` dengan tipe `string`.
    // State variables are variables whose values are permanently stored in contract storage. // Variabel state adalah variabel yang nilainya disimpan secara permanen di penyimpanan kontrak.
    // The keyword `public` makes variables accessible from outside a contract // Kata kunci `public` membuat variabel dapat diakses dari luar kontrak
    // and creates a function that other contracts or clients can call to access the value. // dan membuat fungsi yang dapat dipanggil oleh kontrak atau klien lain untuk mengakses nilainya.
    string public message;

    // Similar to many class-based object-oriented languages, a constructor is // Mirip dengan banyak bahasa berorientasi objek berbasis kelas, konstruktor adalah
    // a special function that is only executed upon contract creation. // fungsi khusus yang hanya dieksekusi saat pembuatan kontrak.
    // Constructors are used to initialize the contract's data. // Konstruktor digunakan untuk menginisialisasi data kontrak.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constructors // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constructors
    constructor(string memory initMessage) public {
        // Accepts a string argument `initMessage` and sets the value // Menerima argumen string `initMessage` dan mengatur nilainya
        // into the contract's `message` storage variable). // ke dalam variabel penyimpanan `message` kontrak).
        message = initMessage;
    }

    // A public function that accepts a string argument // Fungsi publik yang menerima argumen string
    // and updates the `message` storage variable. // dan memperbarui variabel penyimpanan `message`.
    function update(string memory newMessage) public {
        message = newMessage;
    }
}

pragma solidity ^0.5.10;

contract Token {
    // An `address` is comparable to an email address - it's used to identify an account on Ethereum. // Sebuah `address` sebanding dengan alamat email - ini digunakan untuk mengidentifikasi akun di Ethereum.
    // Addresses can represent a smart contract or an external (user) accounts. // Alamat dapat mewakili kontrak pintar atau akun (pengguna) eksternal.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#address // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/types.html#address
    address public owner;

    // A `mapping` is essentially a hash table data structure. // Sebuah `mapping` pada dasarnya adalah struktur data tabel hash.
    // This `mapping` assigns an unsigned integer (the token balance) to an address (the token holder). // `mapping` ini menetapkan bilangan bulat tak bertanda (saldo token) ke sebuah alamat (pemegang token).
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#mapping-types // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/types.html#mapping-types
    mapping (address => uint) public balances;

    // Events allow for logging of activity on the blockchain. // Event memungkinkan pencatatan aktivitas di blockchain.
    // Ethereum clients can listen for events in order to react to contract state changes. // Klien Ethereum dapat mendengarkan event untuk bereaksi terhadap perubahan state kontrak.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#events // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#events
    event Transfer(address from, address to, uint amount);

    // Initializes the contract's data, setting the `owner` // Menginisialisasi data kontrak, mengatur `owner`
    // to the address of the contract creator. // ke alamat pembuat kontrak.
    constructor() public {
        // All smart contracts rely on external transactions to trigger its functions. // Semua kontrak pintar bergantung pada transaksi eksternal untuk memicu fungsinya.
        // `msg` is a global variable that includes relevant data on the given transaction, // `msg` adalah variabel global yang mencakup data relevan pada transaksi yang diberikan,
        // such as the address of the sender and the ETH value included in the transaction. // seperti alamat pengirim dan nilai ETH yang disertakan dalam transaksi.
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties
        owner = msg.sender;
    }

    // Creates an amount of new tokens and sends them to an address. // Membuat sejumlah token baru dan mengirimkannya ke sebuah alamat.
    function mint(address receiver, uint amount) public {
        // `require` is a control structure used to enforce certain conditions. // `require` adalah struktur kontrol yang digunakan untuk memaksakan kondisi tertentu.
        // If a `require` statement evaluates to `false`, an exception is triggered, // Jika pernyataan `require` dievaluasi menjadi `false`, pengecualian akan dipicu,
        // which reverts all changes made to the state during the current call. // yang mengembalikan semua perubahan yang dibuat pada state selama panggilan saat ini.
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions

        // Only the contract owner can call this function // Hanya pemilik kontrak yang dapat memanggil fungsi ini
        require(msg.sender == owner, "You are not the owner.");

        // Enforces a maximum amount of tokens // Memaksakan jumlah maksimum token
        require(amount < 1e60, "Maximum issuance exceeded");

        // Increases the balance of `receiver` by `amount` // Meningkatkan saldo `receiver` sebesar `amount`
        balances[receiver] += amount;
    }

    // Sends an amount of existing tokens from any caller to an address. // Mengirim sejumlah token yang ada dari pemanggil mana pun ke sebuah alamat.
    function transfer(address receiver, uint amount) public {
        // The sender must have enough tokens to send // Pengirim harus memiliki cukup token untuk dikirim
        require(amount <= balances[msg.sender], "Insufficient balance.");

        // Adjusts token balances of the two addresses // Menyesuaikan saldo token dari kedua alamat
        balances[msg.sender] -= amount;
        balances[receiver] += amount;

        // Emits the event defined earlier // Memancarkan event yang didefinisikan sebelumnya
        emit Transfer(msg.sender, receiver, amount);
    }
}

pragma solidity ^0.5.10;

// Imports symbols from other files into the current contract. // Mengimpor simbol dari file lain ke dalam kontrak saat ini.
// In this case, a series of helper contracts from OpenZeppelin. // Dalam hal ini, serangkaian kontrak pembantu dari OpenZeppelin.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#importing-other-source-files // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#importing-other-source-files

import "../node_modules/@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "../node_modules/@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol";
import "../node_modules/@openzeppelin/contracts/introspection/ERC165.sol";
import "../node_modules/@openzeppelin/contracts/math/SafeMath.sol";

// The `is` keyword is used to inherit functions and keywords from external contracts. // Kata kunci `is` digunakan untuk mewarisi fungsi dan kata kunci dari kontrak eksternal.
// In this case, `CryptoPizza` inherits from the `IERC721` and `ERC165` contracts. // Dalam hal ini, `CryptoPizza` mewarisi dari kontrak `IERC721` dan `ERC165`.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#inheritance // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#inheritance
contract CryptoPizza is IERC721, ERC165 {
    // Uses OpenZeppelin's SafeMath library to perform arithmetic operations safely. // Menggunakan pustaka SafeMath OpenZeppelin untuk melakukan operasi aritmatika dengan aman.
    // Learn more: https://docs.openzeppelin.com/contracts/2.x/api/math#SafeMath // Pelajari lebih lanjut: https://docs.openzeppelin.com/contracts/2.x/api/math#SafeMath
    using SafeMath for uint256;

    // Constant state variables in Solidity are similar to other languages // Variabel state konstan di Solidity mirip dengan bahasa lain
    // but you must assign from an expression which is constant at compile time. // tetapi Anda harus menetapkan dari ekspresi yang konstan pada waktu kompilasi.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constant-state-variables // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constant-state-variables
    uint256 constant dnaDigits = 10;
    uint256 constant dnaModulus = 10 ** dnaDigits;
    bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;

    // Struct types let you define your own type // Tipe struct memungkinkan Anda mendefinisikan tipe Anda sendiri
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#structs // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/types.html#structs
    struct Pizza {
        string name;
        uint256 dna;
    }

    // Creates an empty array of Pizza structs // Membuat array kosong dari struct Pizza
    Pizza[] public pizzas;

    // Mapping from pizza ID to its owner's address // Pemetaan dari ID pizza ke alamat pemiliknya
    mapping(uint256 => address) public pizzaToOwner;

    // Mapping from owner's address to number of owned token // Pemetaan dari alamat pemilik ke jumlah token yang dimiliki
    mapping(address => uint256) public ownerPizzaCount;

    // Mapping from token ID to approved address // Pemetaan dari ID token ke alamat yang disetujui
    mapping(uint256 => address) pizzaApprovals;

    // You can nest mappings, this example maps owner to operator approvals // Anda dapat menyarangkan pemetaan, contoh ini memetakan pemilik ke persetujuan operator
    mapping(address => mapping(address => bool)) private operatorApprovals;

    // Internal function to create a random Pizza from string (name) and DNA // Fungsi internal untuk membuat Pizza acak dari string (nama) dan DNA
    function _createPizza(string memory _name, uint256 _dna)
        // The `internal` keyword means this function is only visible // Kata kunci `internal` berarti fungsi ini hanya terlihat
        // within this contract and contracts that derive this contract // di dalam kontrak ini dan kontrak yang diturunkan dari kontrak ini
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#visibility-and-getters // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#visibility-and-getters
        internal
        // `isUnique` is a function modifier that checks if the pizza already exists // `isUnique` adalah pengubah fungsi yang memeriksa apakah pizza sudah ada
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html#function-modifiers // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html#function-modifiers
        isUnique(_name, _dna)
    {
        // Adds Pizza to array of Pizzas and get id // Menambahkan Pizza ke array Pizza dan mendapatkan id
        uint256 id = SafeMath.sub(pizzas.push(Pizza(_name, _dna)), 1);

        // Checks that Pizza owner is the same as current user // Memeriksa bahwa pemilik Pizza sama dengan pengguna saat ini
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions

        // note that address(0) is the zero address, // perhatikan bahwa address(0) adalah alamat nol,
        // indicating that pizza[id] is not yet allocated to a particular user. // menunjukkan bahwa pizza[id] belum dialokasikan ke pengguna tertentu.

        assert(pizzaToOwner[id] == address(0));

        // Maps the Pizza to the owner // Memetakan Pizza ke pemilik
        pizzaToOwner[id] = msg.sender;
        ownerPizzaCount[msg.sender] = SafeMath.add(
            ownerPizzaCount[msg.sender],
            1
        );
    }

    // Creates a random Pizza from string (name) // Membuat Pizza acak dari string (nama)
    function createRandomPizza(string memory _name) public {
        uint256 randDna = generateRandomDna(_name, msg.sender);
        _createPizza(_name, randDna);
    }

    // Generates random DNA from string (name) and address of the owner (creator) // Menghasilkan DNA acak dari string (nama) dan alamat pemilik (pembuat)
    function generateRandomDna(string memory _str, address _owner)
        public
        // Functions marked as `pure` promise not to read from or modify the state // Fungsi yang ditandai sebagai `pure` berjanji untuk tidak membaca dari atau memodifikasi state
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#pure-functions // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#pure-functions
        pure
        returns (uint256)
    {
        // Generates random uint from string (name) + address (owner) // Menghasilkan uint acak dari string (nama) + alamat (pemilik)
        uint256 rand = uint256(keccak256(abi.encodePacked(_str))) +
            uint256(_owner);
        rand = rand % dnaModulus;
        return rand;
    }

    // Returns array of Pizzas found by owner // Mengembalikan array Pizza yang ditemukan oleh pemilik
    function getPizzasByOwner(address _owner)
        public
        // Functions marked as `view` promise not to modify state // Fungsi yang ditandai sebagai `view` berjanji untuk tidak memodifikasi state
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#view-functions // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#view-functions
        view
        returns (uint256[] memory)
    {
        // Uses the `memory` storage location to store values only for the // Menggunakan lokasi penyimpanan `memory` untuk menyimpan nilai hanya untuk
        // lifecycle of this function call. // siklus hidup panggilan fungsi ini.
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/introduction-to-smart-contracts.html#storage-memory-and-the-stack // Pelajari lebih lanjut: https://solidity.readthedocs.io/en/v0.5.10/introduction-to-smart-contracts.html#storage-memory-and-the-stack
        uint256[] memory result = new uint256[](ownerPizzaCount[_owner]);
        uint256 counter = 0;
        for (uint256 i = 0; i < pizzas.length; i++) {
            if (pizzaToOwner[i] == _owner) {
                result[counter] = i;
                counter++;
            }
        }
        return result;
    }

    // Transfers Pizza and ownership to other address // Mentransfer Pizza dan kepemilikan ke alamat lain
    function transferFrom(address _from, address _to, uint256 _pizzaId) public {
        require(_from != address(0) && _to != address(0), "Invalid address.");
        require(_exists(_pizzaId), "Pizza does not exist.");
        require(_from != _to, "Cannot transfer to the same address.");
        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");

        ownerPizzaCount[_to] = SafeMath.add(ownerPizzaCount[_to], 1);
        ownerPizzaCount[_from] = SafeMath.sub(ownerPizzaCount[_from], 1);
        pizzaToOwner[_pizzaId] = _to;

        // Emits event defined in the imported IERC721 contract // Memancarkan event yang didefinisikan dalam kontrak IERC721 yang diimpor
        emit Transfer(_from, _to, _pizzaId);
        _clearApproval(_to, _pizzaId);
    }

    /* *
     * Mentransfer kepemilikan ID token yang diberikan dengan aman ke alamat lain
     * Jika alamat target adalah kontrak, ia harus mengimplementasikan `onERC721Received`,
     * yang dipanggil saat transfer aman, dan mengembalikan nilai ajaib
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
     * jika tidak, transfer akan dikembalikan. */
    /**
     * Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
     * otherwise, the transfer is reverted.
    */
    function safeTransferFrom(address from, address to, uint256 pizzaId)
        public
    {
        // solium-disable-next-line arg-overflow // solium-disable-next-line arg-overflow
        this.safeTransferFrom(from, to, pizzaId, "");
    }

    /* *
     * Mentransfer kepemilikan ID token yang diberikan dengan aman ke alamat lain
     * Jika alamat target adalah kontrak, ia harus mengimplementasikan `onERC721Received`,
     * yang dipanggil saat transfer aman, dan mengembalikan nilai ajaib
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
     * jika tidak, transfer akan dikembalikan. */
    /**
     * Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
     * otherwise, the transfer is reverted.
     */
    function safeTransferFrom(
        address from,
        address to,
        uint256 pizzaId,
        bytes memory _data
    ) public {
        this.transferFrom(from, to, pizzaId);
        require(_checkOnERC721Received(from, to, pizzaId, _data), "Must implement onERC721Received.");
    }

    /* *
     * Fungsi internal untuk memanggil `onERC721Received` pada alamat target
     * Panggilan tidak dieksekusi jika alamat target bukan kontrak */
    /**
     * Internal function to invoke `onERC721Received` on a target address
     * The call is not executed if the target address is not a contract
     */
    function _checkOnERC721Received(
        address from,
        address to,
        uint256 pizzaId,
        bytes memory _data
    ) internal returns (bool) {
        if (!isContract(to)) {
            return true;
        }

        bytes4 retval = IERC721Receiver(to).onERC721Received(
            msg.sender,
            from,
            pizzaId,
            _data
        );
        return (retval == _ERC721_RECEIVED);
    }

    // Burns a Pizza - destroys Token completely // Membakar Pizza - menghancurkan Token sepenuhnya
    // The `external` function modifier means this function is // Pengubah fungsi `external` berarti fungsi ini adalah
    // part of the contract interface and other contracts can call it // bagian dari antarmuka kontrak dan kontrak lain dapat memanggilnya
    function burn(uint256 _pizzaId) external {
        require(msg.sender != address(0), "Invalid address.");
        require(_exists(_pizzaId), "Pizza does not exist.");
        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");

        ownerPizzaCount[msg.sender] = SafeMath.sub(
            ownerPizzaCount[msg.sender],
            1
        );
        pizzaToOwner[_pizzaId] = address(0);
    }

    // Returns count of Pizzas by address // Mengembalikan jumlah Pizza berdasarkan alamat
    function balanceOf(address _owner) public view returns (uint256 _balance) {
        return ownerPizzaCount[_owner];
    }

    // Returns owner of the Pizza found by id // Mengembalikan pemilik Pizza yang ditemukan berdasarkan id
    function ownerOf(uint256 _pizzaId) public view returns (address _owner) {
        address owner = pizzaToOwner[_pizzaId];
        require(owner != address(0), "Invalid Pizza ID.");
        return owner;
    }

    // Approves other address to transfer ownership of Pizza // Menyetujui alamat lain untuk mentransfer kepemilikan Pizza
    function approve(address _to, uint256 _pizzaId) public {
        require(msg.sender == pizzaToOwner[_pizzaId], "Must be the Pizza owner.");
        pizzaApprovals[_pizzaId] = _to;
        emit Approval(msg.sender, _to, _pizzaId);
    }

    // Returns approved address for specific Pizza // Mengembalikan alamat yang disetujui untuk Pizza tertentu
    function getApproved(uint256 _pizzaId)
        public
        view
        returns (address operator)
    {
        require(_exists(_pizzaId), "Pizza does not exist.");
        return pizzaApprovals[_pizzaId];
    }

    /* *
     * Fungsi privat untuk menghapus persetujuan saat ini dari ID token yang diberikan
     * Dikembalikan jika alamat yang diberikan memang bukan pemilik token */
    /**
     * Private function to clear current approval of a given token ID
     * Reverts if the given address is not indeed the owner of the token
     */
    function _clearApproval(address owner, uint256 _pizzaId) private {
        require(pizzaToOwner[_pizzaId] == owner, "Must be pizza owner.");
        require(_exists(_pizzaId), "Pizza does not exist.");
        if (pizzaApprovals[_pizzaId] != address(0)) {
            pizzaApprovals[_pizzaId] = address(0);
        }
    }

    /* * Mengatur atau membatalkan persetujuan dari operator yang diberikan
     * Operator diizinkan untuk mentransfer semua token pengirim atas nama mereka */
    /*
     * Sets or unsets the approval of a given operator
     * An operator is allowed to transfer all tokens of the sender on their behalf
     */
    function setApprovalForAll(address to, bool approved) public {
        require(to != msg.sender, "Cannot approve own address");
        operatorApprovals[msg.sender][to] = approved;
        emit ApprovalForAll(msg.sender, to, approved);
    }

    // Tells whether an operator is approved by a given owner // Memberi tahu apakah operator disetujui oleh pemilik yang diberikan
    function isApprovedForAll(address owner, address operator)
        public
        view
        returns (bool)
    {
        return operatorApprovals[owner][operator];
    }

    // Takes ownership of Pizza - only for approved users // Mengambil kepemilikan Pizza - hanya untuk pengguna yang disetujui
    function takeOwnership(uint256 _pizzaId) public {
        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");
        address owner = this.ownerOf(_pizzaId);
        this.transferFrom(owner, msg.sender, _pizzaId);
    }

    // Checks if Pizza exists // Memeriksa apakah Pizza ada
    function _exists(uint256 pizzaId) internal view returns (bool) {
        address owner = pizzaToOwner[pizzaId];
        return owner != address(0);
    }

    // Checks if address is owner or is approved to transfer Pizza // Memeriksa apakah alamat adalah pemilik atau disetujui untuk mentransfer Pizza
    function _isApprovedOrOwner(address spender, uint256 pizzaId)
        internal
        view
        returns (bool)
    {
        address owner = pizzaToOwner[pizzaId];
        // Disable solium check because of // Nonaktifkan pemeriksaan solium karena
        // https://github.com/duaraghav8/Solium/issues/175 // https://github.com/duaraghav8/Solium/issues/175
        // solium-disable-next-line operator-whitespace // solium-disable-next-line operator-whitespace
        return (spender == owner ||
            this.getApproved(pizzaId) == spender ||
            this.isApprovedForAll(owner, spender));
    }

    // Check if Pizza is unique and doesn't exist yet // Periksa apakah Pizza unik dan belum ada
    modifier isUnique(string memory _name, uint256 _dna) {
        bool result = true;
        for (uint256 i = 0; i < pizzas.length; i++) {
            if (
                keccak256(abi.encodePacked(pizzas[i].name)) ==
                keccak256(abi.encodePacked(_name)) &&
                pizzas[i].dna == _dna
            ) {
                result = false;
            }
        }
        require(result, "Pizza with such name already exists.");
        _;
    }

    // Returns whether the target address is a contract // Mengembalikan apakah alamat target adalah kontrak
    function isContract(address account) internal view returns (bool) {
        uint256 size;
        // Currently there is no better way to check if there is a contract in an address // Saat ini tidak ada cara yang lebih baik untuk memeriksa apakah ada kontrak di sebuah alamat
        // than to check the size of the code at that address. // daripada memeriksa ukuran kode di alamat tersebut.
        // See https://ethereum.stackexchange.com/a/14016/36603 // Lihat https://ethereum.stackexchange.com/a/14016/36603
        // for more details about how this works. // untuk detail lebih lanjut tentang cara kerjanya.
        // TODO Check this again before the Serenity release, because all addresses will be // TODO Periksa ini lagi sebelum rilis Serenity, karena semua alamat akan menjadi
        // contracts then. // kontrak pada saat itu.
        // solium-disable-next-line security/no-inline-assembly // solium-disable-next-line security/no-inline-assembly
        assembly {
            size := extcodesize(account)
        }
        return size > 0;
    }
}