CHEAT.sol

//  note: code copied from Learn X in Y Minutes (Where X=Solidity) 
//    see   https://learnxinyminutes.com/docs/solidity/ and 
//          https://learnxinyminutes.com/docs/files/learnSolidity.sol




// First, a simple Bank contract
// Allows deposits, withdrawals, and balance checks

// simple_bank.sol (note .sol extension)
/* **** START EXAMPLE **** */

// Declare the source file compiler version
pragma solidity ^0.6.6;

// Start with Natspec comment (the three slashes)
// used for documentation - and as descriptive data for UI elements/actions

/// @title SimpleBank
/// @author nemild

/* 'contract' has similarities to 'class' in other languages (class variables,
inheritance, etc.) */
contract SimpleBank { // CapWords
    // Declare state variables outside function, persist through life of contract

    // dictionary that maps addresses to balances
    // always be careful about overflow attacks with numbers
    mapping (address => uint) private balances;

    // "private" means that other contracts can't directly query balances
    // but data is still viewable to other parties on blockchain

    address public owner;
    // 'public' makes externally readable (not writeable) by users or contracts

    // Events - publicize actions to external listeners
    event LogDepositMade(address accountAddress, uint amount);

    // Constructor, can receive one or many variables here; only one allowed
    constructor() public {
        // msg provides details about the message that's sent to the contract
        // msg.sender is contract caller (address of contract creator)
        owner = msg.sender;
    }

    /// @notice Deposit ether into bank
    /// @return The balance of the user after the deposit is made
    function deposit() public payable returns (uint) {
        // Use 'require' to test user inputs, 'assert' for internal invariants
        // Here we are making sure that there isn't an overflow issue
        require((balances[msg.sender] + msg.value) >= balances[msg.sender]);

        balances[msg.sender] += msg.value;
        // no "this." or "self." required with state variable
        // all values set to data type's initial value by default

        emit LogDepositMade(msg.sender, msg.value); // fire event

        return balances[msg.sender];
    }

    /// @notice Withdraw ether from bank
    /// @dev This does not return any excess ether sent to it
    /// @param withdrawAmount amount you want to withdraw
    /// @return remainingBal
    function withdraw(uint withdrawAmount) public returns (uint remainingBal) {
        require(withdrawAmount <= balances[msg.sender]);

        // Note the way we deduct the balance right away, before sending
        // Every .transfer/.send from this contract can call an external function
        // This may allow the caller to request an amount greater
        // than their balance using a recursive call
        // Aim to commit state before calling external functions, including .transfer/.send
        balances[msg.sender] -= withdrawAmount;

        // this automatically throws on a failure, which means the updated balance is reverted
        msg.sender.transfer(withdrawAmount);

        return balances[msg.sender];
    }

    /// @notice Get balance
    /// @return The balance of the user
    // 'view' (ex: constant) prevents function from editing state variables;
    // allows function to run locally/off blockchain
    function balance() view public returns (uint) {
        return balances[msg.sender];
    }
}
// ** END EXAMPLE **


// Now, the basics of Solidity

// 1. DATA TYPES AND ASSOCIATED METHODS
// uint used for currency amount (there are no doubles
//  or floats) and for dates (in unix time)
uint x;

// int of 256 bits, cannot be changed after instantiation
int constant a = 8;
int256 constant a = 8; // same effect as line above, here the 256 is explicit
uint constant VERSION_ID = 0x123A1; // A hex constant
// with 'constant', compiler replaces each occurrence with actual value

// All state variables (those outside a function)
// are by default 'internal' and accessible inside contract
// and in all contracts that inherit ONLY
// Need to explicitly set to 'public' to allow external contracts to access
int256 public a = 8;

// For int and uint, can explicitly set space in steps of 8 up to 256
// e.g., int8, int16, int24
uint8 b;
int64 c;
uint248 e;

// Be careful that you don't overflow, and protect against attacks that do
// For example, for an addition, you'd do:
uint256 c = a + b;
assert(c >= a); // assert tests for internal invariants; require is used for user inputs
// For more examples of common arithmetic issues, see Zeppelin's SafeMath library
// https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/math/SafeMath.sol


// No random functions built in, you can get a pseduo-random number by hashing the current blockhash, or get a truly random number using something like Chainlink VRF. 
// https://docs.chain.link/docs/get-a-random-number

// Type casting
int x = int(b);

bool b = true; // or do 'var b = true;' for inferred typing

// Addresses - holds 20 byte/160 bit Ethereum addresses
// No arithmetic allowed
address public owner;

// Types of accounts:
// Contract account: address set on create (func of creator address, num transactions sent)
// External Account: (person/external entity): address created from public key

// Add 'public' field to indicate publicly/externally accessible
// a getter is automatically created, but NOT a setter

// All addresses can be sent ether
owner.transfer(SOME_BALANCE); // fails and reverts on failure

// Can also do a lower level .send call, which returns a false if it failed
if (owner.send) {} // REMEMBER: wrap send in 'if', as contract addresses have
// functions executed on send and these can fail
// Also, make sure to deduct balances BEFORE attempting a send, as there is a risk of a recursive
// call that can drain the contract

// Can check balance
owner.balance; // the balance of the owner (user or contract)


// Bytes available from 1 to 32
byte a; // byte is same as bytes1
bytes2 b;
bytes32 c;

// Dynamically sized bytes
bytes m; // A special array, same as byte[] array (but packed tightly)
// More expensive than byte1-byte32, so use those when possible

// same as bytes, but does not allow length or index access (for now)
string n = "hello"; // stored in UTF8, note double quotes, not single
// string utility functions to be added in future
// prefer bytes32/bytes, as UTF8 uses more storage

// Type inference
// var does inferred typing based on first assignment,
// can't be used in functions parameters
var a = true;
// use carefully, inference may provide wrong type
// e.g., an int8, when a counter needs to be int16

// var can be used to assign function to variable
function a(uint x) returns (uint) {
    return x * 2;
}
var f = a;
f(22); // call

// by default, all values are set to 0 on instantiation

// Delete can be called on most types
// (does NOT destroy value, but sets value to 0, the initial value)
delete x;


// Destructuring/Tuples
(x, y) = (2, 7); // assign/swap multiple values


// 2. DATA STRUCTURES
// Arrays
bytes32[5] nicknames; // static array
bytes32[] names; // dynamic array
uint newLength = names.push("John"); // adding returns new length of the array
// Length
names.length; // get length
names.length = 1; // lengths can be set (for dynamic arrays in storage only)

// multidimensional array
uint[][5] x; // arr with 5 dynamic array elements (opp order of most languages)

// Dictionaries (any type to any other type)
mapping (string => uint) public balances;
balances["charles"] = 1;
// balances["ada"] result is 0, all non-set key values return zeroes
// 'public' allows following from another contract
contractName.balances("charles"); // returns 1
// 'public' created a getter (but not setter) like the following:
function balances(string _account) returns (uint balance) {
    return balances[_account];
}

// Nested mappings
mapping (address => mapping (address => uint)) public custodians;

// To delete
delete balances["John"];
delete balances; // sets all elements to 0

// Unlike other languages, CANNOT iterate through all elements in
// mapping, without knowing source keys - can build data structure
// on top to do this

// Structs
struct Bank {
    address owner;
    uint balance;
}
Bank b = Bank({
    owner: msg.sender,
    balance: 5
});
// or
Bank c = Bank(msg.sender, 5);

c.balance = 5; // set to new value
delete b;
// sets to initial value, set all variables in struct to 0, except mappings

// Enums
enum State { Created, Locked, Inactive }; // often used for state machine
State public state; // Declare variable from enum
state = State.Created;
// enums can be explicitly converted to ints
uint createdState = uint(State.Created); //  0

// Data locations: Memory vs. storage vs. calldata - all complex types (arrays,
// structs) have a data location
// 'memory' does not persist, 'storage' does
// Default is 'storage' for local and state variables; 'memory' for func params
// stack holds small local variables

// for most types, can explicitly set which data location to use


// 3. Simple operators
// Comparisons, bit operators and arithmetic operators are provided
// exponentiation: **
// exclusive or: ^
// bitwise negation: ~


// 4. Global Variables of note
// ** this **
this; // address of contract
// often used at end of contract life to transfer remaining balance to party
this.balance;
this.someFunction(); // calls func externally via call, not via internal jump

// ** msg - Current message received by the contract ** **
msg.sender; // address of sender
msg.value; // amount of ether provided to this contract in wei, the function should be marked "payable"
msg.data; // bytes, complete call data
msg.gas; // remaining gas

// ** tx - This transaction **
tx.origin; // address of sender of the transaction
tx.gasprice; // gas price of the transaction

// ** block - Information about current block **
now; // current time (approximately), alias for block.timestamp (uses Unix time)
// Note that this can be manipulated by miners, so use carefully

block.number; // current block number
block.difficulty; // current block difficulty
block.blockhash(1); // returns bytes32, only works for most recent 256 blocks
block.gasLimit();

// ** storage - Persistent storage hash **
storage['abc'] = 'def'; // maps 256 bit words to 256 bit words


// 5. FUNCTIONS AND MORE
// A. Functions
// Simple function
function increment(uint x) returns (uint) {
    x += 1;
    return x;
}

// Functions can return many arguments,
// and by specifying returned arguments name explicit return is not needed
function increment(uint x, uint y) returns (uint x, uint y) {
    x += 1;
    y += 1;
}
// Call previous function
uint (a,b) = increment(1,1);

// 'view' (alias for 'constant')
// indicates that function does not/cannot change persistent vars
// View function execute locally, not on blockchain
// Noted: constant keyword will soon be deprecated.
uint y = 1;

function increment(uint x) view returns (uint x) {
    x += 1;
    y += 1; // this line would fail
    // y is a state variable, and can't be changed in a view function
}

// 'pure' is more strict than 'view' or 'constant', and does not
// even allow reading of state vars
// The exact rules are more complicated, so see more about
// view/pure:
// http://solidity.readthedocs.io/en/develop/contracts.html#view-functions

// 'Function Visibility specifiers'
// These can be placed where 'view' is, including:
// public - visible externally and internally (default for function)
// external - only visible externally (including a call made with this.)
// private - only visible in the current contract
// internal - only visible in current contract, and those deriving from it

// Generally, a good idea to mark each function explicitly

// Functions hoisted - and can assign a function to a variable
function a() {
    var z = b;
    z();
}

function b() {

}

// All functions that receive ether must be marked 'payable'
function depositEther() public payable {
    balances[msg.sender] += msg.value;
}


// Prefer loops to recursion (max call stack depth is 1024)
// Also, don't setup loops that you haven't bounded,
// as this can hit the gas limit

// B. Events
// Events are notify external parties; easy to search and
// access events from outside blockchain (with lightweight clients)
// typically declare after contract parameters

// Typically, capitalized - and add Log in front to be explicit and prevent confusion
// with a function call

// Declare
event LogSent(address indexed from, address indexed to, uint amount); // note capital first letter

// Call
LogSent(from, to, amount);

/**

For an external party (a contract or external entity), to watch using
the Web3 Javascript library:

// The following is Javascript code, not Solidity code
Coin.LogSent().watch({}, '', function(error, result) {
    if (!error) {
        console.log("Coin transfer: " + result.args.amount +
            " coins were sent from " + result.args.from +
            " to " + result.args.to + ".");
        console.log("Balances now:\n" +
            "Sender: " + Coin.balances.call(result.args.from) +
            "Receiver: " + Coin.balances.call(result.args.to));
    }
}
**/

// Common paradigm for one contract to depend on another (e.g., a
// contract that depends on current exchange rate provided by another)

// C. Modifiers
// Modifiers validate inputs to functions such as minimal balance or user auth;
// similar to guard clause in other languages

// '_' (underscore) often included as last line in body, and indicates
// function being called should be placed there
modifier onlyAfter(uint _time) { require (now >= _time); _; }
modifier onlyOwner { require(msg.sender == owner); _; }
// commonly used with state machines
modifier onlyIfStateA (State currState) { require(currState == State.A); _; }

// Append right after function declaration
function changeOwner(newOwner)
onlyAfter(someTime)
onlyOwner()
onlyIfState(State.A)
{
    owner = newOwner;
}

// underscore can be included before end of body,
// but explicitly returning will skip, so use carefully
modifier checkValue(uint amount) {
    _;
    if (msg.value > amount) {
        uint amountToRefund = amount - msg.value;
        msg.sender.transfer(amountToRefund);
    }
}


// 6. BRANCHING AND LOOPS

// All basic logic blocks work - including if/else, for, while, break, continue
// return - but no switch

// Syntax same as javascript, but no type conversion from non-boolean
// to boolean (comparison operators must be used to get the boolean val)

// For loops that are determined by user behavior, be careful - as contracts have a maximal
// amount of gas for a block of code - and will fail if that is exceeded
// For example:
for(uint x = 0; x < refundAddressList.length; x++) {
    refundAddressList[x].transfer(SOME_AMOUNT);
}

// Two errors above:
// 1. A failure on transfer stops the loop from completing, tying up money
// 2. This loop could be arbitrarily long (based on the amount of users who need refunds), and
// therefore may always fail as it exceeds the max gas for a block
// Instead, you should let people withdraw individually from their subaccount, and mark withdrawn
// e.g., favor pull payments over push payments


// 7. OBJECTS/CONTRACTS

// A. Calling external contract
contract InfoFeed {
    function info() payable returns (uint ret)  { return 42; }
}

contract Consumer {
    InfoFeed feed; // points to contract on blockchain

    // Set feed to existing contract instance
    function setFeed(address addr) {
        // automatically cast, be careful; constructor is not called
        feed = InfoFeed(addr);
    }

    // Set feed to new instance of contract
    function createNewFeed() {
        feed = new InfoFeed(); // new instance created; constructor called
    }

    function callFeed() {
        // final parentheses call contract, can optionally add
        // custom ether value or gas
        feed.info.value(10).gas(800)();
    }
}

// B. Inheritance

// Order matters, last inherited contract (i.e., 'def') can override parts of
// previously inherited contracts
contract MyContract is abc, def("a custom argument to def") {

// Override function
    function z() {
        if (msg.sender == owner) {
            def.z(); // call overridden function from def
            super.z(); // call immediate parent overridden function
        }
    }
}

// abstract function
function someAbstractFunction(uint x);
// cannot be compiled, so used in base/abstract contracts
// that are then implemented

// C. Import

import "filename";
import "github.com/ethereum/dapp-bin/library/iterable_mapping.sol";


// 8. OTHER KEYWORDS

// A. Selfdestruct
// selfdestruct current contract, sending funds to address (often creator)
selfdestruct(SOME_ADDRESS);

// removes storage/code from current/future blocks
// helps thin clients, but previous data persists in blockchain

// Common pattern, lets owner end the contract and receive remaining funds
function remove() {
    if(msg.sender == creator) { // Only let the contract creator do this
        selfdestruct(creator); // Makes contract inactive, returns funds
    }
}

// May want to deactivate contract manually, rather than selfdestruct
// (ether sent to selfdestructed contract is lost)


// 9. CONTRACT DESIGN NOTES

// A. Obfuscation
// All variables are publicly viewable on blockchain, so anything
// that is private needs to be obfuscated (e.g., hashed w/secret)

// Steps: 1. Commit to something, 2. Reveal commitment
keccak256("some_bid_amount", "some secret"); // commit

// call contract's reveal function in the future
// showing bid plus secret that hashes to SHA3
reveal(100, "mySecret");

// B. Storage optimization
// Writing to blockchain can be expensive, as data stored forever; encourages
// smart ways to use memory (eventually, compilation will be better, but for now
// benefits to planning data structures - and storing min amount in blockchain)

// Cost can often be high for items like multidimensional arrays
// (cost is for storing data - not declaring unfilled variables)

// C. Data access in blockchain
// Cannot restrict human or computer from reading contents of
// transaction or transaction's state

// While 'private' prevents other *contracts* from reading data
// directly - any other party can still read data in blockchain

// All data to start of time is stored in blockchain, so
// anyone can observe all previous data and changes

// D. Oracles and External Data
// Oracles are ways to interact with your smart contracts outside the blockchain. 
// They are used to get data from the real world, send post requests, to the real world
// or vise versa.

// Time-based implementations of contracts are also done through oracles, as 
// contracts need to be directly called and can not "subscribe" to a time. 
// Due to smart contracts being decentralized, you also want to get your data
// in a decentralized manner, otherwise you run into the centralized risk that 
// smart contract design matter prevents. 

// To easiest way get and use pre-boxed decentralized data is with Chainlink Data Feeds
// https://docs.chain.link/docs/get-the-latest-price
// We can reference on-chain reference points that have already been aggregated by 
// multiple sources and delivered on-chain, and we can use it as a "data bank" 
// of sources. 

// You can see other examples making API calls here:
// https://docs.chain.link/docs/make-a-http-get-request

// And you can of course build your own oracle network, just be sure to know 
// how centralized vs decentralized your application is. 

// Setting up oracle networks yourself

// E. Cron Job
// Contracts must be manually called to handle time-based scheduling; can create external
// code to regularly ping, or provide incentives (ether) for others to
//

// F. Observer Pattern
// An Observer Pattern lets you register as a subscriber and
// register a function which is called by the oracle (note, the oracle pays
// for this action to be run)
// Some similarities to subscription in Pub/sub

// This is an abstract contract, both client and server classes import
// the client should implement
contract SomeOracleCallback {
    function oracleCallback(int _value, uint _time, bytes32 info) external;
}

contract SomeOracle {
    SomeOracleCallback[] callbacks; // array of all subscribers

    // Register subscriber
    function addSubscriber(SomeOracleCallback a) {
        callbacks.push(a);
    }

    function notify(value, time, info) private {
        for(uint i = 0;i < callbacks.length; i++) {
            // all called subscribers must implement the oracleCallback
            callbacks[i].oracleCallback(value, time, info);
        }
    }

    function doSomething() public {
        // Code to do something

        // Notify all subscribers
        notify(_value, _time, _info);
    }
}

// Now, your client contract can addSubscriber by importing SomeOracleCallback
// and registering with Some Oracle

// G. State machines
// see example below for State enum and inState modifier

// *** EXAMPLE: A crowdfunding example (broadly similar to Kickstarter) ***
// ** START EXAMPLE **

// CrowdFunder.sol
pragma solidity ^0.6.6;

/// @title CrowdFunder
/// @author nemild
contract CrowdFunder {
    // Variables set on create by creator
    address public creator;
    address payable public fundRecipient; // creator may be different than recipient, and must be payable
    uint public minimumToRaise; // required to tip, else everyone gets refund
    string campaignUrl;
    byte version = "1";

    // Data structures
    enum State {
        Fundraising,
        ExpiredRefund,
        Successful
    }
    struct Contribution {
        uint amount;
        address payable contributor;
    }

    // State variables
    State public state = State.Fundraising; // initialize on create
    uint public totalRaised;
    uint public raiseBy;
    uint public completeAt;
    Contribution[] contributions;

    event LogFundingReceived(address addr, uint amount, uint currentTotal);
    event LogWinnerPaid(address winnerAddress);

    modifier inState(State _state) {
        require(state == _state);
        _;
    }

    modifier isCreator() {
        require(msg.sender == creator);
        _;
    }

    // Wait 24 weeks after final contract state before allowing contract destruction
    modifier atEndOfLifecycle() {
    require(((state == State.ExpiredRefund || state == State.Successful) &&
        completeAt + 24 weeks < now));
        _;
    }

    function crowdFund(
        uint timeInHoursForFundraising,
        string memory _campaignUrl,
        address payable _fundRecipient,
        uint _minimumToRaise)
        public
    {
        creator = msg.sender;
        fundRecipient = _fundRecipient;
        campaignUrl = _campaignUrl;
        minimumToRaise = _minimumToRaise;
        raiseBy = now + (timeInHoursForFundraising * 1 hours);
    }

    function contribute()
    public
    payable
    inState(State.Fundraising)
    returns(uint256 id)
    {
        contributions.push(
            Contribution({
                amount: msg.value,
                contributor: msg.sender
            }) // use array, so can iterate
        );
        totalRaised += msg.value;

        emit LogFundingReceived(msg.sender, msg.value, totalRaised);

        checkIfFundingCompleteOrExpired();
        return contributions.length - 1; // return id
    }

    function checkIfFundingCompleteOrExpired()
    public
    {
        if (totalRaised > minimumToRaise) {
            state = State.Successful;
            payOut();

            // could incentivize sender who initiated state change here
        } else if ( now > raiseBy )  {
            state = State.ExpiredRefund; // backers can now collect refunds by calling getRefund(id)
        }
        completeAt = now;
    }

    function payOut()
    public
    inState(State.Successful)
    {
        fundRecipient.transfer(address(this).balance);
        LogWinnerPaid(fundRecipient);
    }

    function getRefund(uint256 id)
    inState(State.ExpiredRefund)
    public
    returns(bool)
    {
        require(contributions.length > id && id >= 0 && contributions[id].amount != 0 );

        uint256 amountToRefund = contributions[id].amount;
        contributions[id].amount = 0;

        contributions[id].contributor.transfer(amountToRefund);

        return true;
    }

    function removeContract()
    public
    isCreator()
    atEndOfLifecycle()
    {
        selfdestruct(msg.sender);
        // creator gets all money that hasn't be claimed
    }
}
// ** END EXAMPLE **


// 10. OTHER NATIVE FUNCTIONS

// Currency units
// Currency is defined using wei, smallest unit of Ether
uint minAmount = 1 wei;
uint a = 1 finney; // 1 ether == 1000 finney
// Other units, see: http://ether.fund/tool/converter

// Time units
1 == 1 second
1 minutes == 60 seconds

// Can multiply a variable times unit, as units are not stored in a variable
uint x = 5;
(x * 1 days); // 5 days

// Careful about leap seconds/years with equality statements for time
// (instead, prefer greater than/less than)

// Cryptography
// All strings passed are concatenated before hash action
sha3("ab", "cd");
ripemd160("abc");
sha256("def");

// 11. SECURITY

// Bugs can be disastrous in Ethereum contracts - and even popular patterns in Solidity,
// may be found to be antipatterns

// See security links at the end of this doc

// 12. LOW LEVEL FUNCTIONS
// call - low level, not often used, does not provide type safety
successBoolean = someContractAddress.call('function_name', 'arg1', 'arg2');

// callcode - Code at target address executed in *context* of calling contract
// provides library functionality
someContractAddress.callcode('function_name');


// 13. STYLE NOTES
// Based on Python's PEP8 style guide
// Full Style guide: http://solidity.readthedocs.io/en/develop/style-guide.html

// Quick summary:
// 4 spaces for indentation
// Two lines separate contract declarations (and other top level declarations)
// Avoid extraneous spaces in parentheses
// Can omit curly braces for one line statement (if, for, etc)
// else should be placed on own line


// 14. NATSPEC COMMENTS
// used for documentation, commenting, and external UIs

// Contract natspec - always above contract definition
/// @title Contract title
/// @author Author name

// Function natspec
/// @notice information about what function does; shown when function to execute
/// @dev Function documentation for developer

// Function parameter/return value natspec
/// @param someParam Some description of what the param does
/// @return Description of the return value