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+---
+language: Solidity
+filename: learnSolidity.sol
+contributors:
+  - ["Nemil Dalal", "https://www.nemil.com"]
+  - ["Joseph Chow", ""]
+---
+
+Solidity lets you program on [Ethereum](https://www.ethereum.org/), a
+blockchain-based virtual machine that allows the creation and
+execution of smart contracts, without needing centralized or trusted parties.
+
+Solidity is a statically typed, contract programming language that has
+similarities to Javascript and C. Like objects in OOP, each contract contains
+state variables, functions, and common data types. Contract-specific features
+include modifier (guard) clauses, event notifiers for listeners, and custom
+global variables.
+
+Some Ethereum contract examples include crowdfunding, voting, and blind auctions.
+
+As Solidity and Ethereum are under active development, experimental or beta
+features are explicitly marked, and subject to change. Pull requests welcome.
+
+```javascript
+// First, a simple Bank contract
+// Allows deposits, withdrawals, and balance checks
+
+// simple_bank.sol (note .sol extension)
+/* **** START EXAMPLE **** */
+
+// 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 { // CamelCase
+    // Declare state variables outside function, persist through life of contract
+
+    // dictionary that maps addresses to balances
+    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 DepositMade(address accountAddress, uint amount);
+
+    // Constructor, can receive one or many variables here; only one allowed
+    function AcmeBank() {
+        // 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 returns (uint) {
+        balances[msg.sender] += msg.value;
+        // no "this." or "self." required with state variable
+        // all values set to data type's initial value by default
+
+        DepositMade(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 The balance remaining for the user
+    function withdraw(uint withdrawAmount) public returns (uint remainingBal) {
+        if(balances[msg.sender] >= withdrawAmount) {
+            balances[msg.sender] -= withdrawAmount;
+
+            if (!msg.sender.send(withdrawAmount)) {
+                // to be safe, may be sending to contract that
+                // has overridden 'send' which may then fail
+                balances[msg.sender] += withdrawAmount;
+            }
+        }
+
+        return balances[msg.sender];
+    }
+
+    /// @notice Get balance
+    /// @return The balance of the user
+    // 'constant' prevents function from editing state variables;
+    // allows function to run locally/off blockchain
+    function balance() constant returns (uint) {
+        return balances[msg.sender];
+    }
+
+    // Fallback function - Called if other functions don't match call or
+    // sent ether without data
+    // Typically, called when invalid data is sent
+    // Added so ether sent to this contract is reverted if the contract fails
+    // otherwise, the sender's money is transferred to contract
+    function () {
+        throw; // throw reverts state to before call
+    }
+}
+// ** 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
+
+
+// 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
+
+// No random functions built in, use other contracts for randomness
+
+// 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.send(SOME_BALANCE); // returns false on failure
+if (owner.send) {} // typically wrap in 'if', as contract addresses have
+// functions have executed on send and can fail
+
+// can override send by defining your own
+
+// 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 inferrence
+// 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)
+uint x = 5;
+
+
+// Destructuring/Tuples
+(x, y) = (2, 7); // assign/swap multiple value
+
+
+// 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 x[][5]; // 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;
+console.log(balances["ada"]); // is 0, all non-set key values return zeroes
+// 'public' allows following from another contract
+contractName.balances("claude"); // returns 1
+// 'public' created a getter (but not setter) like the following:
+function balances(address _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 and enums
+struct Bank {
+    address owner;
+    uint balance;
+}
+Bank b = Bank({
+    owner: msg.sender,
+    balance: 5
+});
+// or
+Bank c = Bank(msg.sender, 5);
+
+c.amount = 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. stack - 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 send 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
+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)
+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
+
+
+// 4. 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 don't need to explicitly return
+function increment(uint x, uint y) returns (uint x, uint y) {
+    x += 1;
+    y += 1;
+}
+// Call previous functon
+uint (a,b) = increment(1,1);
+
+// 'constant' indicates that function does not/cannot change persistent vars
+// Constant function execute locally, not on blockchain
+uint y;
+
+function increment(uint x) constant returns (uint x) {
+    x += 1;
+    y += 1; // this line would fail
+    // y is a state variable, and can't be changed in a constant function
+}
+
+// 'Function Visibility specifiers'
+// These can be placed where 'constant' is, including:
+// public - visible externally and internally (default)
+// external
+// private - only visible in the current contract
+// internal - only visible in current contract, and those deriving from it
+
+// Functions hoisted - and can assign a function to a variable
+function a() {
+    var z = b;
+    b();
+}
+
+function b() {
+
+}
+
+
+// Prefer loops to recursion (max call stack depth is 1024)
+
+// B. Events
+// Events are notify external parties; easy to search and
+// access events from outside blockchain (with lightweight clients)
+// typically declare after contract parameters
+
+// Declare
+event Sent(address from, address to, uint amount); // note capital first letter
+
+// Call
+Sent(from, to, amount);
+
+// For an external party (a contract or external entity), to watch:
+Coin.Sent().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) { if (now <= _time) throw; _ }
+modifier onlyOwner { if (msg.sender == owner) _ }
+// commonly used with state machines
+modifier onlyIfState (State currState) { if (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) {
+        msg.sender.send(amount - msg.value);
+    }
+}
+
+
+// 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)
+
+
+// 7. OBJECTS/CONTRACTS
+
+// A. Calling external contract
+contract infoFeed {
+    function info() 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 overriden 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";
+
+// Importing under active development
+// Cannot currently be done at command line
+
+
+// 8. OTHER KEYWORDS
+
+// A. Throwing
+// Throwing
+throw; // reverts unused money to sender, state is reverted
+// Can't currently catch
+
+// Common design pattern is:
+if (!addr.send(123)) {
+    throw;
+}
+
+// B. 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
+sha3("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. 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
+
+// E. 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
+
+// F. State machines
+// see example below for State enum and inState modifier
+
+
+// *** EXAMPLE: A crowdfunding example (broadly similar to Kickstarter) ***
+// ** START EXAMPLE **
+
+// CrowdFunder.sol
+
+/// @title CrowdFunder
+/// @author nemild
+contract CrowdFunder {
+    // Variables set on create by creator
+    address public creator;
+    address public fundRecipient; // creator may be different than recipient
+    uint public minimumToRaise; // required to tip, else everyone gets refund
+    string campaignUrl;
+
+    // Data structures
+    enum State {
+        Fundraising,
+        ExpiredRefundPending,
+        Successful,
+        ExpiredRefundComplete
+    }
+    struct Contribution {
+        uint amount;
+        address contributor;
+    }
+
+    // State variables
+    State public state = State.Fundraising; // initialize on create
+    uint public totalRaised;
+    uint public raiseBy;
+    Contribution[] contributions;
+
+    event fundingReceived(address addr, uint amount, uint currentTotal);
+    event allRefundsSent();
+    event winnerPaid(address winnerAddress);
+
+    modifier inState(State _state) {
+        if (state != _state) throw;
+        _
+    }
+
+    modifier isCreator() {
+        if (msg.sender != creator) throw;
+        _
+    }
+
+    modifier atEndOfLifecycle() {
+        if(state != State.ExpiredRefundComplete && state != State.Successful) {
+            throw;
+        }
+    }
+
+    function CrowdFunder(
+        uint timeInHoursForFundraising,
+        string _campaignUrl,
+        address _fundRecipient,
+        uint _minimumToRaise)
+    {
+        creator = msg.sender;
+        fundRecipient = _fundRecipient;
+        campaignUrl = _campaignUrl;
+        minimumToRaise = _minimumToRaise;
+        raiseBy = now + (timeInHoursForFundraising * 1 hours);
+    }
+
+    function contribute()
+    public
+    inState(State.Fundraising)
+    {
+        contributions.push(
+            Contribution({
+                amount: msg.value,
+                contributor: msg.sender
+            }) // use array, so can iterate
+        );
+        totalRaised += msg.value;
+
+        fundingReceived(msg.sender, msg.value, totalRaised);
+
+        checkIfFundingCompleteOrExpired();
+    }
+
+    function checkIfFundingCompleteOrExpired() {
+        if (totalRaised > minimumToRaise) {
+            state = State.Successful;
+            payOut();
+
+            // could incentivize sender who initiated state change here
+        } else if ( now > raiseBy )  {
+            state = State.ExpiredRefundPending;
+            refundAll();
+        }
+    }
+
+    function payOut()
+    public
+    inState(State.Successful)
+    {
+        if(!fundRecipient.send(this.balance)) {
+            throw;
+        }
+
+        winnerPaid(fundRecipient);
+    }
+
+    function refundAll()
+    public
+    inState(State.ExpiredRefundPending)
+    {
+        uint length = contributions.length;
+        for (uint i = 0; i < length; i++) {
+            if(!contributions[i].contributor.send(contributions[i].amount)) {
+                throw;
+            }
+        }
+
+        allRefundsSent();
+        state = State.ExpiredRefundComplete;
+    }
+
+    function removeContract()
+    public
+    isCreator()
+    atEndOfLifecycle()
+    {
+        selfdestruct(msg.sender);
+    }
+
+    function () { throw; }
+}
+// ** 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. 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');
+
+
+// 12. STYLE NOTES
+// Based on Python's PEP8 style guide
+
+// 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
+
+
+// 13. NATSPEC COMENTS
+// 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
+```
+
+## Additional resources
+- [Solidity Docs](https://solidity.readthedocs.org/en/latest/)
+- [Solidity Style Guide](https://ethereum.github.io/solidity//docs/style-guide/): Ethereum's style guide is heavily derived from Python's [pep8](https://www.python.org/dev/peps/pep-0008/) style guide.
+- [Browser-based Solidity Editor](http://chriseth.github.io/browser-solidity/)
+- [Gitter Chat room](https://gitter.im/ethereum/solidity)
+- [Modular design strategies for Ethereum Contracts](https://docs.erisindustries.com/tutorials/solidity/)
+- Editor Snippets ([Ultisnips format](https://gist.github.com/nemild/98343ce6b16b747788bc))
+
+## Sample contracts
+- [Dapp Bin](https://github.com/ethereum/dapp-bin)
+- [Solidity Baby Step Contracts](https://github.com/fivedogit/solidity-baby-steps/tree/master/contracts)
+- [ConsenSys Contracts](https://github.com/ConsenSys/dapp-store-contracts)
+- [State of Dapps](http://dapps.ethercasts.com/)
+
+## Information purposefully excluded
+- Libraries
+
+## Style
+- Python's [PEP8](https://www.python.org/dev/peps/pep-0008/) is used as the baseline style guide, including its general philosophy
+
+## Future To Dos
+- New keywords: protected, inheritable
+
+Feel free to send a pull request with any edits - or email nemild -/at-/ gmail