// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract Mapping {
    // Mapping from address to uint
    mapping(address => uint) public myMap;

    function get(address _addr) public view returns (uint) {
        // Mapping always returns a value.
        // If the value was never set, it will return the default value.
        // The default value for uint is 0
        return myMap[_addr];
    }

    function set(address _addr, uint _i) public {
        // Update the value at this address
        myMap[_addr] = _i;
    }

    function remove(address _addr) public {
        // Reset the value to the default value.
        delete myMap[_addr];
    }
}

contract NestedMappings {
    // Mapping from address => (mapping from uint to bool)
    mapping(address => mapping(uint => bool)) public nestedMap;

    function get(address _addr1, uint _i) public view returns (bool) {
        // You can get values from a nested mapping
        // even when it is not initialized
        // The default value for a bool type is false
        return nestedMap[_addr1][_i];
    }

    function set(
        address _addr1,
        uint _i,
        bool _boo
    ) public {
        nestedMap[_addr1][_i] = _boo;
    }

    function remove(address _addr1, uint _i) public {
        delete nestedMap[_addr1][_i];
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract Enum {
    // Enum representing different possible shipping states
    enum Status {
        Pending,
        Shipped,
        Accepted,
        Rejected,
        Canceled
    }

    // Declare a variable of the type Status
    // This can only contain one of the predefined values
    Status public status;

    // Since enums are internally represented by uints
    // This function will always return a uint
    // Pending = 0
    // Shipped = 1
    // Accepted = 2
    // Rejected = 3
    // Canceled = 4
    // Value higher than 4 cannot be returned
    function get() public view returns (Status) {
        return status;
    }

    // Pass a uint for input to update the value
    function set(Status _status) public {
        status = _status;
    }

    // Update value to a specific enum members
    function cancel() public {
        status = Status.Canceled; // Will set status = 4
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract TodoList {
    // Declare a struct which groups together two data types
    struct TodoItem {
        string text;
        bool completed;
    }

    // Create an array of TodoItem structs
    TodoItem[] public todos;

    function createTodo(string memory _text) public {
        // There are multiple ways to initialize structs

        // Method 1 - Call it like a function
        todos.push(TodoItem(_text, false));

        // Method 2 - Explicitly set its keys
        todos.push(TodoItem({ text: _text, completed: false }));

        // Method 3 - Initialize an empty struct, then set individual properties
        TodoItem memory todo;
        todo.text = _text;
        todo.completed = false;
        todos.push(todo);
    }

    // Update a struct value
    function update(uint _index, string memory _text) public {
        todos[_index].text = _text;
    }

    // Update completed
    function toggleCompleted(uint _index) public {
        todos[_index].completed = !todos[_index].completed;
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract ViewAndPure {
    // Declare a state variable
    uint public x = 1;

    // Promise not to modify the state (but can read state)
    function addToX(uint y) public view returns (uint) {
        return x + y;
    }

    // Promise not to modify or read from state
    function add(uint i, uint j) public pure returns (uint) {
        return i + j;
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract Modifiers {
    address public owner;

    constructor() {
        // Set the contract deployer as the owner of the contract
        owner = msg.sender;
    }

    // Create a modifier that only allows a function to be called by the owner
    modifier onlyOwner() {
        require(msg.sender == owner, "You are not the owner");

        // Underscore is a special character used inside modifiers
        // Which tells Solidity to execute the function the modifier is used on
        // at this point
        // Therefore, this modifier will first perform the above check
        // Then run the rest of the code
        _;
    }

    // Create a function and apply the onlyOwner modifier on it
    function changeOwner(address _newOwner) public onlyOwner {
        // We will only reach this point if the modifier succeeds with its checks
        // So the caller of this transaction must be the current owner
        owner = _newOwner;
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract Events {
    // Declare an event which logs an address and a string
    event TestCalled(address sender, string message);

    function test() public {
        // Log an event 
        emit TestCalled(msg.sender, "Someone called test()!");
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract X {
    string public name;

    // You will need to provide a string argument when deploying the contract
    constructor(string memory _name) {
        // This will be set immediately when the contract is deployed
        name = _name;
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

/* Graph of inheritance
    A
   / 
  B   C
 /   /
D   E

*/

contract A {
    // Declare a virtual function foo() which can be overridden by children
    function foo() public pure virtual returns (string memory) {
        return "A";
    }
}

contract B is A {
    // Override A.foo();
    // But also allow this function to be overridden by further children
    // So we specify both keywords - virtual and override
    function foo() public pure virtual override returns (string memory) {
        return "B";
    }
}

contract C is A {
    // Similar to contract B above
    function foo() public pure virtual override returns (string memory) {
        return "C";
    }
}

// When inheriting from multiple contracts, if a function is defined multiple times, the right-most parent contract's function is used.
contract D is B, C {
    // D.foo() returns "C"
    // since C is the right-most parent with function foo();
    // override (B,C) means we want to override a method that exists in two parents
    function foo() public pure override (B, C) returns (string memory) {
        // super is a special keyword that is used to call functions
        // in the parent contract
        return super.foo();
    }
}

contract E is C, B {
    // E.foo() returns "B"
    // since B is the right-most parent with function foo();
    function foo() public pure override (C, B) returns (string memory) {
        return super.foo();
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract ReceiveEther {
    /*
    Which function is called, fallback() or receive()?

           send Ether
               |
         msg.data is empty?
              / 
            yes  no
            /     
receive() exists?  fallback()
         /   
        yes   no
        /      
    receive()   fallback()
    */

    // Function to receive Ether. msg.data must be empty
    receive() external payable {}

    // Fallback function is called when msg.data is not empty
    fallback() external payable {}

    function getBalance() public view returns (uint) {
        return address(this).balance;
    }
}

contract SendEther {
    function sendEth(address payable _to) public payable {
        // Just forward the ETH received in this payable function
        // to the given address
        uint amountToSend = msg.value;
        // call returns a bool value specifying success or failure
        (bool success, bytes memory data) = _to.call{value: msg.value}("");
        require(success == true, "Failed to send ETH");
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

interface MinimalERC20 {
    // Just include the functions we are interested in
    // in the interface
    function balanceOf(address account) external view returns (uint256);
}

contract MyContract {
    MinimalERC20 externalContract;

    constructor(address _externalContract) {
        // Initialize a MinimalERC20 contract instance
        externalContract = MinimalERC20(_externalContract);
    }

    function mustHaveSomeBalance() public {
        // Require that the caller of this transaction has a non-zero
        // balance of tokens in the external ERC20 contract
        uint balance = externalContract.balanceOf(msg.sender);
        require(balance > 0, "You don't own any tokens of external contract");
    }
}

├── Import.sol
└── Foo.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract Foo {
    string public name = "Foo";
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

// import Foo.sol from current directory
import "./Foo.sol";

contract Import {
    // Initialize Foo.sol
    Foo public foo = new Foo();

    // Test Foo.sol by getting it's name.
    function getFooName() public view returns (string memory) {
        return foo.name();
    }
}

// https://github.com/owner/repo/blob/branch/path/to/Contract.sol
import "https://github.com/owner/repo/blob/branch/path/to/Contract.sol";

// Example import ERC20.sol from openzeppelin-contract repo
// https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol";

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

library SafeMath {
    function add(uint x, uint y) internal pure returns (uint) {
        uint z = x + y;
        // If z overflowed, throw an error
        require(z >= x, "uint overflow");
        return z;
    }
}

contract TestSafeMath {
    function testAdd(uint x, uint y) public pure returns (uint) {
        return SafeMath.add(x, y);
    }
}