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.gitignore vendored Normal file
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.DS_Store

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README.md
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## 🥷🏻🛵⛓️ playing pvp in the metaweb: solidity edition
## 🥷🏻🛵⛓️ web3 starting kit - solidity edition
<br>
#### 👉 start with [solidity tl; dr](solidity_tldr.md)
#### 🧰 dirs in this repo
* **start with [solidity tl; dr](basic_knowledge)**
* **[your workspace](basic_knowledge/workspace)**
* **[boilerplates](basic_knowledge/boilerplates)**
* **[tokens standard](basic_knowledge/token_standards)**
* **[bitwise](basic_knowledge/bitwise.md)**
* **[applications contracts](advanced_knowledge/applications_contracts)**
* **[merkle_trees](advanced_knowledge/merkle_trees)**
* **[saving gas](advanced_knowledge/saving_gas)**
* **[calldata](advanced_knowledge/calldata)**
* **[events](advanced_knowledge/events)**
* **[proxies](advanced_knowledge/proxies)**
* **[wallets](advanced_knowledge/wallets)**
<br>
* [set your workspace](workspace)
* [tokens standard](token_standards)
* [boilerplates](boilerplates)
* [saving gas](saving_gas)
* [abi encoding](abi_encoding)
* [calldata](calldata)
* [foundry](foundry)
* [events](events)
---
### related autistic symposium's resources
<br>
* **[ethernaut solutions and writeups in foundry](https://github.com/autistic-symposium/ethernaut-systematic-solutions-foundry-sol)**
* **[blockchain auditing](https://github.com/autistic-symposium/blockchains-security-toolkit)**
* **[amm-arb-toolkit-py](https://github.com/autistic-symposium/amm-arb-toolkit-py)**
* **[mev-toolkit](https://github.com/autistic-symposium/mev-toolkit)**
* **[searcher-cowswap-py](https://github.com/autistic-symposium/searcher-cowswap-py)**
* **[generative storytelling](https://github.com/autistic-symposium/generative-sol)**
<br>
---
@ -33,34 +39,29 @@
<br>
##### learning solidity
###### learning solidity
* [solidity docs](https://docs.soliditylang.org/en/v0.8.12/)
* [openzeppelin docs](https://docs.openzeppelin.com/)
* [solidity by example](https://solidity-by-example.org/)
* [smart contract programmer](https://www.youtube.com/channel/UCJWh7F3AFyQ_x01VKzr9eyA)
* [solidity vscode plugin](https://marketplace.visualstudio.com/items?itemName=tintinweb.solidity-visual-auditor)
* [learn solidity with examples, by jsagalli](https://github.com/James-Sangalli/learn-solidity-with-examples)
* **[solidity docs](https://docs.soliditylang.org/en/v0.8.12/)**
* **[openzeppelin docs](https://docs.openzeppelin.com/)**
* **[solidity by example](https://solidity-by-example.org/)**
* **[smart contract programmer](https://www.youtube.com/channel/UCJWh7F3AFyQ_x01VKzr9eyA)**
* **[learn solidity with examples, by jsagalli](https://github.com/James-Sangalli/learn-solidity-with-examples)**
* **[school of solana, by ackee](https://ackeeblockchain.com/school-of-solana)**
###### writing beautiful code
* **[solmate](https://github.com/transmissions11/solmate/)**
* **[dapp.tools](https://dapp.tools/)**
* **[solidity cheatsheet and best practices](https://github.com/manojpramesh/solidity-cheatsheet)**
<br>
###### tools
##### writing beautiful code
* [solmate](https://github.com/transmissions11/solmate/)
* [dapp.tools](https://dapp.tools/)
* [solidity cheatsheet and best practices](https://github.com/manojpramesh/solidity-cheatsheet)
* **[solidity vscode plugin](https://marketplace.visualstudio.com/items?itemName=tintinweb.solidity-visual-auditor)**
* **[another solidity vscode plugin](https://marketplace.visualstudio.com/items?itemName=JuanBlanco.solidity)**
###### advanced readings
<br>
##### tools
* [another solidity vscode plugin](https://marketplace.visualstudio.com/items?itemName=JuanBlanco.solidity)
<br>
##### advanced readings
* [reusable properties for ethereum contracts, by trailofbits](https://blog.trailofbits.com/2023/02/27/reusable-properties-ethereum-contracts-echidna/)
* **[on some of my favorite openzeppelin smart contracts](https://mirror.xyz/go-outside.eth/7Q5DK8cZNZ5CP6ThJjEithPvjgckA24D2wb-j0Ps5-I)**
* **[reusable properties for ethereum contracts, by trailofbits](https://blog.trailofbits.com/2023/02/27/reusable-properties-ethereum-contracts-echidna/)**

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abi_encoding/README.md
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## ABI encoding
<br>
### tl; dr
<br>
* the solidity built-in function `abi.encode` encodes solidity types into raw bytes, that can be interpreted directly by the EVM.
```
contract StringEncoding {
bytes public encodedString = abi.encode("hacking");
}
```
* this is what happens:
1. 1st (32 bytes) word = offset → indicates at which bytes index the string starts. If you count 32 from the beginning (= index 32), you will reach the starting point of where the actual encoded string starts.
2. 2nd (32 bytes) word = string length → in the case of the string, this indicates how many characters (including whitespaces) are included in the string.
3. 3rd (32 bytes) word = the actual utf8 encoded string → each individual bytes corresponds to hex notation of a letter / character encoded in utf8.
<br>
* other ABI Encodings
* address payable -> address
* contract -> address
* enum -> uint8
* struct -> tuple of elementry types
<br>
---
### resources
<br>

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## a place to dump application contracts
<br>
* disclaimer: these boilerplates might not be my code, but rather well-known references.

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
/*
Crowd fund ERC20 token
User creates a campaign.
Users can pledge, transferring their token to a campaign.
After the campaign ends, campaign creator can claim the funds if total amount pledged is more than the campaign goal.
Otherwise, campaign did not reach it's goal, users can withdraw their pledge.
*/
interface IERC20 {
function transfer(address, uint) external returns (bool);
function transferFrom(address, address, uint) external returns (bool);
}
contract CrowdFund {
event Launch(
uint id,
address indexed creator,
uint goal,
uint32 startAt,
uint32 endAt
);
event Cancel(uint id);
event Pledge(uint indexed id, address indexed caller, uint amount);
event Unpledge(uint indexed id, address indexed caller, uint amount);
event Claim(uint id);
event Refund(uint id, address indexed caller, uint amount);
struct Campaign {
// Creator of campaign
address creator;
// Amount of tokens to raise
uint goal;
// Total amount pledged
uint pledged;
// Timestamp of start of campaign
uint32 startAt;
// Timestamp of end of campaign
uint32 endAt;
// True if goal was reached and creator has claimed the tokens.
bool claimed;
}
IERC20 public immutable token;
// Total count of campaigns created.
// It is also used to generate id for new campaigns.
uint public count;
// Mapping from id to Campaign
mapping(uint => Campaign) public campaigns;
// Mapping from campaign id => pledger => amount pledged
mapping(uint => mapping(address => uint)) public pledgedAmount;
constructor(address _token) {
token = IERC20(_token);
}
function launch(uint _goal, uint32 _startAt, uint32 _endAt) external {
require(_startAt >= block.timestamp, "start at < now");
require(_endAt >= _startAt, "end at < start at");
require(_endAt <= block.timestamp + 90 days, "end at > max duration");
count += 1;
campaigns[count] = Campaign({
creator: msg.sender,
goal: _goal,
pledged: 0,
startAt: _startAt,
endAt: _endAt,
claimed: false
});
emit Launch(count, msg.sender, _goal, _startAt, _endAt);
}
function cancel(uint _id) external {
Campaign memory campaign = campaigns[_id];
require(campaign.creator == msg.sender, "not creator");
require(block.timestamp < campaign.startAt, "started");
delete campaigns[_id];
emit Cancel(_id);
}
function pledge(uint _id, uint _amount) external {
Campaign storage campaign = campaigns[_id];
require(block.timestamp >= campaign.startAt, "not started");
require(block.timestamp <= campaign.endAt, "ended");
campaign.pledged += _amount;
pledgedAmount[_id][msg.sender] += _amount;
token.transferFrom(msg.sender, address(this), _amount);
emit Pledge(_id, msg.sender, _amount);
}
function unpledge(uint _id, uint _amount) external {
Campaign storage campaign = campaigns[_id];
require(block.timestamp <= campaign.endAt, "ended");
campaign.pledged -= _amount;
pledgedAmount[_id][msg.sender] -= _amount;
token.transfer(msg.sender, _amount);
emit Unpledge(_id, msg.sender, _amount);
}
function claim(uint _id) external {
Campaign storage campaign = campaigns[_id];
require(campaign.creator == msg.sender, "not creator");
require(block.timestamp > campaign.endAt, "not ended");
require(campaign.pledged >= campaign.goal, "pledged < goal");
require(!campaign.claimed, "claimed");
campaign.claimed = true;
token.transfer(campaign.creator, campaign.pledged);
emit Claim(_id);
}
function refund(uint _id) external {
Campaign memory campaign = campaigns[_id];
require(block.timestamp > campaign.endAt, "not ended");
require(campaign.pledged < campaign.goal, "pledged >= goal");
uint bal = pledgedAmount[_id][msg.sender];
pledgedAmount[_id][msg.sender] = 0;
token.transfer(msg.sender, bal);
emit Refund(_id, msg.sender, bal);
}
}

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
/*
Dutch auction for NFT.
Auction
Seller of NFT deploys this contract setting a starting price for the NFT.
Auction lasts for 7 days.
Price of NFT decreases over time.
Participants can buy by depositing ETH greater than the current price computed by the smart contract.
Auction ends when a buyer buys the NFT.
*/
interface IERC721 {
function transferFrom(address _from, address _to, uint _nftId) external;
}
contract DutchAuction {
uint private constant DURATION = 7 days;
IERC721 public immutable nft;
uint public immutable nftId;
address payable public immutable seller;
uint public immutable startingPrice;
uint public immutable startAt;
uint public immutable expiresAt;
uint public immutable discountRate;
constructor(uint _startingPrice, uint _discountRate, address _nft, uint _nftId) {
seller = payable(msg.sender);
startingPrice = _startingPrice;
startAt = block.timestamp;
expiresAt = block.timestamp + DURATION;
discountRate = _discountRate;
require(_startingPrice >= _discountRate * DURATION, "starting price < min");
nft = IERC721(_nft);
nftId = _nftId;
}
function getPrice() public view returns (uint) {
uint timeElapsed = block.timestamp - startAt;
uint discount = discountRate * timeElapsed;
return startingPrice - discount;
}
function buy() external payable {
require(block.timestamp < expiresAt, "auction expired");
uint price = getPrice();
require(msg.value >= price, "ETH < price");
nft.transferFrom(seller, msg.sender, nftId);
uint refund = msg.value - price;
if (refund > 0) {
payable(msg.sender).transfer(refund);
}
selfdestruct(seller);
}
}

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17
/*
English auction for NFT.
Auction
Seller of NFT deploys this contract.
Auction lasts for 7 days.
Participants can bid by depositing ETH greater than the current highest bidder.
All bidders can withdraw their bid if it is not the current highest bid.
After the auction
Highest bidder becomes the new owner of NFT.
The seller receives the highest bid of ETH.
*/
interface IERC721 {
function safeTransferFrom(address from, address to, uint tokenId) external;
function transferFrom(address, address, uint) external;
}
contract EnglishAuction {
event Start();
event Bid(address indexed sender, uint amount);
event Withdraw(address indexed bidder, uint amount);
event End(address winner, uint amount);
IERC721 public nft;
uint public nftId;
address payable public seller;
uint public endAt;
bool public started;
bool public ended;
address public highestBidder;
uint public highestBid;
mapping(address => uint) public bids;
constructor(address _nft, uint _nftId, uint _startingBid) {
nft = IERC721(_nft);
nftId = _nftId;
seller = payable(msg.sender);
highestBid = _startingBid;
}
function start() external {
require(!started, "started");
require(msg.sender == seller, "not seller");
nft.transferFrom(msg.sender, address(this), nftId);
started = true;
endAt = block.timestamp + 7 days;
emit Start();
}
function bid() external payable {
require(started, "not started");
require(block.timestamp < endAt, "ended");
require(msg.value > highestBid, "value < highest");
if (highestBidder != address(0)) {
bids[highestBidder] += highestBid;
}
highestBidder = msg.sender;
highestBid = msg.value;
emit Bid(msg.sender, msg.value);
}
function withdraw() external {
uint bal = bids[msg.sender];
bids[msg.sender] = 0;
payable(msg.sender).transfer(bal);
emit Withdraw(msg.sender, bal);
}
function end() external {
require(started, "not started");
require(block.timestamp >= endAt, "not ended");
require(!ended, "ended");
ended = true;
if (highestBidder != address(0)) {
nft.safeTransferFrom(address(this), highestBidder, nftId);
seller.transfer(highestBid);
} else {
nft.safeTransferFrom(address(this), seller, nftId);
}
emit End(highestBidder, highestBid);
}
}

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// An example of calling multiple functions with a single transaction, using delegatecall.
contract MultiDelegatecall {
error DelegatecallFailed();
function multiDelegatecall(
bytes[] memory data
) external payable returns (bytes[] memory results) {
results = new bytes[](data.length);
for (uint i; i < data.length; i++) {
(bool ok, bytes memory res) = address(this).delegatecall(data[i]);
if (!ok) {
revert DelegatecallFailed();
}
results[i] = res;
}
}
}
// Why use multi delegatecall? Why not multi call?
// alice -> multi call --- call ---> test (msg.sender = multi call)
// alice -> test --- delegatecall ---> test (msg.sender = alice)
contract TestMultiDelegatecall is MultiDelegatecall {
event Log(address caller, string func, uint i);
function func1(uint x, uint y) external {
// msg.sender = alice
emit Log(msg.sender, "func1", x + y);
}
function func2() external returns (uint) {
// msg.sender = alice
emit Log(msg.sender, "func2", 2);
return 111;
}
mapping(address => uint) public balanceOf;
// WARNING: unsafe code when used in combination with multi-delegatecall
// user can mint multiple times for the price of msg.value
function mint() external payable {
balanceOf[msg.sender] += msg.value;
}
}
contract Helper {
function getFunc1Data(uint x, uint y) external pure returns (bytes memory) {
return abi.encodeWithSelector(TestMultiDelegatecall.func1.selector, x, y);
}
function getFunc2Data() external pure returns (bytes memory) {
return abi.encodeWithSelector(TestMultiDelegatecall.func2.selector);
}
function getMintData() external pure returns (bytes memory) {
return abi.encodeWithSelector(TestMultiDelegatecall.mint.selector);
}
}

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<br>
### tl; dr
<br>
* calldata is the encoded parameter(s) sent on functions by smart contracts to the evm (for example, through `abi.econde()`,`abi.econcdeWithSelector() for a particular interfaced function, or `abi.encodePacked()` for efficient dynamic variables).
* **function signature**: `selector()` generates the 4-bytes representing the method in the interface: this is how the evm knows which function is being interacted. function signatures are defined as the first four bytes of the Keccak hash of the canonical representation of the function signature.
@ -16,7 +12,7 @@
- static variables: the encoded representation of `uint`, `address`, `bool`, `bytes`, `tuples`
- dynamics variables: non-fixed-size types: `bytes`, `string`, dynamic and fixed arrays `<T>[]`
* opcodes are 1 byte in length, leading to 256 different possible opcodes. the EVM only uses 140 opcodes.
* opcodes are `1 byte` in length, leading to `256` different possible opcodes. the EVM only uses `140` opcodes.
<br>
@ -26,10 +22,10 @@
<br>
* [dedaub](https://library.dedaub.com/)
* [ethervm.io](https://ethervm.io/decompile)
* [panoramix](https://github.com/eveem-org/panoramix)
* [froundry's cast with --debug](https://book.getfoundry.sh/cast/index.html)
* **[dedaub](https://library.dedaub.com/)**
* **[ethervm.io](https://ethervm.io/decompile)**
* **[panoramix](https://github.com/eveem-org/panoramix)**
* **[froundry's cast with --debug](https://book.getfoundry.sh/cast/index.html)**
<br>
@ -41,5 +37,6 @@
<br>
* [mev memoirs in the arena part 1, by noxx](https://noxx.substack.com/p/mev-memoirs-into-the-arena-chapter?s=r)
* [mev memoirs in the arena part 2, by noxx](https://noxx.substack.com/p/mev-memoirs-into-the-arena-chapter-3e9)
* **[mev memoirs in the arena part 1, by noxx](https://noxx.substack.com/p/mev-memoirs-into-the-arena-chapter?s=r)**
* **[mev memoirs in the arena part 2, by noxx](https://noxx.substack.com/p/mev-memoirs-into-the-arena-chapter-3e9)**
* **[ethereum tx enconding and legacy encoding](https://hoangtrinhj.com/articles/ethereum-transaction-encoding)**

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// Contract address can be precomputed, before the contract is deployed, using create2
contract Factory {
// Returns the address of the newly deployed contract
function deploy(
address _owner,
uint _foo,
bytes32 _salt
) public payable returns (address) {
// This syntax is a newer way to invoke create2 without assembly, you just need to pass salt
// https://docs.soliditylang.org/en/latest/control-structures.html#salted-contract-creations-create2
return address(new TestContract{salt: _salt}(_owner, _foo));
}
}
// This is the older way of doing it using assembly
contract FactoryAssembly {
event Deployed(address addr, uint salt);
// 1. Get bytecode of contract to be deployed
// NOTE: _owner and _foo are arguments of the TestContract's constructor
function getBytecode(address _owner, uint _foo) public pure returns (bytes memory) {
bytes memory bytecode = type(TestContract).creationCode;
return abi.encodePacked(bytecode, abi.encode(_owner, _foo));
}
// 2. Compute the address of the contract to be deployed
// NOTE: _salt is a random number used to create an address
function getAddress(
bytes memory bytecode,
uint _salt
) public view returns (address) {
bytes32 hash = keccak256(
abi.encodePacked(bytes1(0xff), address(this), _salt, keccak256(bytecode))
);
// NOTE: cast last 20 bytes of hash to address
return address(uint160(uint(hash)));
}
// 3. Deploy the contract
// NOTE:
// Check the event log Deployed which contains the address of the deployed TestContract.
// The address in the log should equal the address computed from above.
function deploy(bytes memory bytecode, uint _salt) public payable {
address addr;
/*
NOTE: How to call create2
create2(v, p, n, s)
create new contract with code at memory p to p + n
and send v wei
and return the new address
where new address = first 20 bytes of keccak256(0xff + address(this) + s + keccak256(mem[p…(p+n)))
s = big-endian 256-bit value
*/
assembly {
addr := create2(
callvalue(), // wei sent with current call
// Actual code starts after skipping the first 32 bytes
add(bytecode, 0x20),
mload(bytecode), // Load the size of code contained in the first 32 bytes
_salt // Salt from function arguments
)
if iszero(extcodesize(addr)) {
revert(0, 0)
}
}
emit Deployed(addr, _salt);
}
}
contract TestContract {
address public owner;
uint public foo;
constructor(address _owner, uint _foo) payable {
owner = _owner;
foo = _foo;
}
function getBalance() public view returns (uint) {
return address(this).balance;
}
}

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## events and topics
## events, tx, topics
### tl; dr
<br>
* solidity provides two types of events:
- anonymous: 4 topics may be indexed, and there is not signature hash (no filter)
@ -8,6 +8,15 @@
<br>
---
### resources
### ethereum transactions
<br>
<img width="570" src="https://user-images.githubusercontent.com/126520850/227066769-507080bb-071a-45a7-b385-82b502a963a7.png">
<br>

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## a place to dump merkle trees examples
<br>
* disclaimer: these boilerplates might not be my code, but rather well-known references.

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// Merkle tree allows you to cryptographically prove that an element is contained
// in a set without revealing the entire set.
contract MerkleProof {
function verify(
bytes32[] memory proof,
bytes32 root,
bytes32 leaf,
uint index
) public pure returns (bool) {
bytes32 hash = leaf;
for (uint i = 0; i < proof.length; i++) {
bytes32 proofElement = proof[i];
if (index % 2 == 0) {
hash = keccak256(abi.encodePacked(hash, proofElement));
} else {
hash = keccak256(abi.encodePacked(proofElement, hash));
}
index = index / 2;
}
return hash == root;
}
}
contract TestMerkleProof is MerkleProof {
bytes32[] public hashes;
constructor() {
string[4] memory transactions = [
"alice -> bob",
"bob -> dave",
"carol -> alice",
"dave -> bob"
];
for (uint i = 0; i < transactions.length; i++) {
hashes.push(keccak256(abi.encodePacked(transactions[i])));
}
uint n = transactions.length;
uint offset = 0;
while (n > 0) {
for (uint i = 0; i < n - 1; i += 2) {
hashes.push(
keccak256(
abi.encodePacked(hashes[offset + i], hashes[offset + i + 1])
)
);
}
offset += n;
n = n / 2;
}
}
function getRoot() public view returns (bytes32) {
return hashes[hashes.length - 1];
}
/* verify
3rd leaf
0xdca3326ad7e8121bf9cf9c12333e6b2271abe823ec9edfe42f813b1e768fa57b
root
0xcc086fcc038189b4641db2cc4f1de3bb132aefbd65d510d817591550937818c7
index
2
proof
0x8da9e1c820f9dbd1589fd6585872bc1063588625729e7ab0797cfc63a00bd950
0x995788ffc103b987ad50f5e5707fd094419eb12d9552cc423bd0cd86a3861433
*/
}

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## a place to dump proxy examples
<br>
* disclaimer: these boilerplates might not be my code, but rather well-known references.

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
contract Proxy {
event Deploy(address);
receive() external payable {}
function deploy(bytes memory _code) external payable returns (address addr) {
assembly {
// create(v, p, n)
// v = amount of ETH to send
// p = pointer in memory to start of code
// n = size of code
addr := create(callvalue(), add(_code, 0x20), mload(_code))
}
// return address 0 on error
require(addr != address(0), "deploy failed");
emit Deploy(addr);
}
function execute(address _target, bytes memory _data) external payable {
(bool success, ) = _target.call{value: msg.value}(_data);
require(success, "failed");
}
}
contract TestContract1 {
address public owner = msg.sender;
function setOwner(address _owner) public {
require(msg.sender == owner, "not owner");
owner = _owner;
}
}
contract TestContract2 {
address public owner = msg.sender;
uint public value = msg.value;
uint public x;
uint public y;
constructor(uint _x, uint _y) payable {
x = _x;
y = _y;
}
}
contract Helper {
function getBytecode1() external pure returns (bytes memory) {
bytes memory bytecode = type(TestContract1).creationCode;
return bytecode;
}
function getBytecode2(uint _x, uint _y) external pure returns (bytes memory) {
bytes memory bytecode = type(TestContract2).creationCode;
return abi.encodePacked(bytecode, abi.encode(_x, _y));
}
function getCalldata(address _owner) external pure returns (bytes memory) {
return abi.encodeWithSignature("setOwner(address)", _owner);
}
}

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
contract MinimalProxy {
function clone(address target) external returns (address result) {
// convert address to 20 bytes
bytes20 targetBytes = bytes20(target);
// actual code //
// 3d602d80600a3d3981f3363d3d373d3d3d363d73bebebebebebebebebebebebebebebebebebebebe5af43d82803e903d91602b57fd5bf3
// creation code //
// copy runtime code into memory and return it
// 3d602d80600a3d3981f3
// runtime code //
// code to delegatecall to address
// 363d3d373d3d3d363d73 address 5af43d82803e903d91602b57fd5bf3
assembly {
/*
reads the 32 bytes of memory starting at pointer stored in 0x40
In solidity, the 0x40 slot in memory is special: it contains the "free memory pointer"
which points to the end of the currently allocated memory.
*/
let clone := mload(0x40)
// store 32 bytes to memory starting at "clone"
mstore(
clone,
0x3d602d80600a3d3981f3363d3d373d3d3d363d73000000000000000000000000
)
/*
| 20 bytes |
0x3d602d80600a3d3981f3363d3d373d3d3d363d73000000000000000000000000
^
pointer
*/
// store 32 bytes to memory starting at "clone" + 20 bytes
// 0x14 = 20
mstore(add(clone, 0x14), targetBytes)
/*
| 20 bytes | 20 bytes |
0x3d602d80600a3d3981f3363d3d373d3d3d363d73bebebebebebebebebebebebebebebebebebebebe
^
pointer
*/
// store 32 bytes to memory starting at "clone" + 40 bytes
// 0x28 = 40
mstore(
add(clone, 0x28),
0x5af43d82803e903d91602b57fd5bf30000000000000000000000000000000000
)
/*
| 20 bytes | 20 bytes | 15 bytes |
0x3d602d80600a3d3981f3363d3d373d3d3d363d73bebebebebebebebebebebebebebebebebebebebe5af43d82803e903d91602b57fd5bf3
*/
// create new contract
// send 0 Ether
// code starts at pointer stored in "clone"
// code size 0x37 (55 bytes)
result := create(0, clone, 0x37)
}
}
}

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// Transparent upgradeable proxy pattern
contract CounterV1 {
uint public count;
function inc() external {
count += 1;
}
}
contract CounterV2 {
uint public count;
function inc() external {
count += 1;
}
function dec() external {
count -= 1;
}
}
contract BuggyProxy {
address public implementation;
address public admin;
constructor() {
admin = msg.sender;
}
function _delegate() private {
(bool ok, ) = implementation.delegatecall(msg.data);
require(ok, "delegatecall failed");
}
fallback() external payable {
_delegate();
}
receive() external payable {
_delegate();
}
function upgradeTo(address _implementation) external {
require(msg.sender == admin, "not authorized");
implementation = _implementation;
}
}
contract Dev {
function selectors() external view returns (bytes4, bytes4, bytes4) {
return (
Proxy.admin.selector,
Proxy.implementation.selector,
Proxy.upgradeTo.selector
);
}
}
contract Proxy {
// All functions / variables should be private, forward all calls to fallback
// -1 for unknown preimage
// 0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc
bytes32 private constant IMPLEMENTATION_SLOT =
bytes32(uint(keccak256("eip1967.proxy.implementation")) - 1);
// 0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103
bytes32 private constant ADMIN_SLOT =
bytes32(uint(keccak256("eip1967.proxy.admin")) - 1);
constructor() {
_setAdmin(msg.sender);
}
modifier ifAdmin() {
if (msg.sender == _getAdmin()) {
_;
} else {
_fallback();
}
}
function _getAdmin() private view returns (address) {
return StorageSlot.getAddressSlot(ADMIN_SLOT).value;
}
function _setAdmin(address _admin) private {
require(_admin != address(0), "admin = zero address");
StorageSlot.getAddressSlot(ADMIN_SLOT).value = _admin;
}
function _getImplementation() private view returns (address) {
return StorageSlot.getAddressSlot(IMPLEMENTATION_SLOT).value;
}
function _setImplementation(address _implementation) private {
require(_implementation.code.length > 0, "implementation is not contract");
StorageSlot.getAddressSlot(IMPLEMENTATION_SLOT).value = _implementation;
}
// Admin interface //
function changeAdmin(address _admin) external ifAdmin {
_setAdmin(_admin);
}
// 0x3659cfe6
function upgradeTo(address _implementation) external ifAdmin {
_setImplementation(_implementation);
}
// 0xf851a440
function admin() external ifAdmin returns (address) {
return _getAdmin();
}
// 0x5c60da1b
function implementation() external ifAdmin returns (address) {
return _getImplementation();
}
// User interface //
function _delegate(address _implementation) internal virtual {
assembly {
// Copy msg.data. We take full control of memory in this inline assembly
// block because it will not return to Solidity code. We overwrite the
// Solidity scratch pad at memory position 0.
// calldatacopy(t, f, s) - copy s bytes from calldata at position f to mem at position t
// calldatasize() - size of call data in bytes
calldatacopy(0, 0, calldatasize())
// Call the implementation.
// out and outsize are 0 because we don't know the size yet.
// delegatecall(g, a, in, insize, out, outsize) -
// - call contract at address a
// - with input mem[in…(in+insize))
// - providing g gas
// - and output area mem[out…(out+outsize))
// - returning 0 on error (eg. out of gas) and 1 on success
let result := delegatecall(gas(), _implementation, 0, calldatasize(), 0, 0)
// Copy the returned data.
// returndatacopy(t, f, s) - copy s bytes from returndata at position f to mem at position t
// returndatasize() - size of the last returndata
returndatacopy(0, 0, returndatasize())
switch result
// delegatecall returns 0 on error.
case 0 {
// revert(p, s) - end execution, revert state changes, return data mem[p…(p+s))
revert(0, returndatasize())
}
default {
// return(p, s) - end execution, return data mem[p…(p+s))
return(0, returndatasize())
}
}
}
function _fallback() private {
_delegate(_getImplementation());
}
fallback() external payable {
_fallback();
}
receive() external payable {
_fallback();
}
}
contract ProxyAdmin {
address public owner;
constructor() {
owner = msg.sender;
}
modifier onlyOwner() {
require(msg.sender == owner, "not owner");
_;
}
function getProxyAdmin(address proxy) external view returns (address) {
(bool ok, bytes memory res) = proxy.staticcall(abi.encodeCall(Proxy.admin, ()));
require(ok, "call failed");
return abi.decode(res, (address));
}
function getProxyImplementation(address proxy) external view returns (address) {
(bool ok, bytes memory res) = proxy.staticcall(
abi.encodeCall(Proxy.implementation, ())
);
require(ok, "call failed");
return abi.decode(res, (address));
}
function changeProxyAdmin(address payable proxy, address admin) external onlyOwner {
Proxy(proxy).changeAdmin(admin);
}
function upgrade(address payable proxy, address implementation) external onlyOwner {
Proxy(proxy).upgradeTo(implementation);
}
}
library StorageSlot {
struct AddressSlot {
address value;
}
function getAddressSlot(
bytes32 slot
) internal pure returns (AddressSlot storage r) {
assembly {
r.slot := slot
}
}
}
contract TestSlot {
bytes32 public constant slot = keccak256("TEST_SLOT");
function getSlot() external view returns (address) {
return StorageSlot.getAddressSlot(slot).value;
}
function writeSlot(address _addr) external {
StorageSlot.getAddressSlot(slot).value = _addr;
}
}

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## gas optimization
<br>
- gas is the cost for on-chain computation and storage. examples:
- addition costs `3` gas, `keccak-256` costs `30 gas + 6 gas` for each `256 bits` of data being hashed.
- sending a transaction costs `21,000 gas` (intrinsic gas).
- creating a contract costs `32000 gas`.
- each **calldata** byte costs gas (gas per byte equal to `0`, and `16 gas` for the others), the larger the size of the transaction data, the higher the gas fees.
- each opcode has a [specific fixed cost to be paid upon execution](https://www.evm.codes/?fork=arrowGlacier).
- calculate gas for your code online at [remix](https://remix.ethereum.org/).
<br>
----
### general tricks to save gas
<br>
* replace `memory` with `calldata`
* load state variable to memory
* replace for loop `i++` with `++i`
* caching array elements
* short circuits
* brute force hashes of function names to find those that start `0000`, so this can save around `50 gas`.
* if you don’t need a variable anymore, you should delete it using the delete keyword provided by solidity or by setting it to its default value.
* avoid calls to other contracts.
<br>
<img width="300" src="https://user-images.githubusercontent.com/1130416/214452718-b051caed-49e4-45fb-b955-976d20e97cbd.png">
<br>
```
// gas golf
contract GasGolf {
// start - 50908 gas
// use calldata - 49163 gas
// load state variables to memory - 48952 gas
// short circuit - 48634 gas
// loop increments - 48244 gas
// cache array length - 48209 gas
// load array elements to memory - 48047 gas
// uncheck i overflow/underflow - 47309 gas
uint public total;
// start - not gas optimized
// function sumIfEvenAndLessThan99(uint[] memory nums) external {
// for (uint i = 0; i < nums.length; i += 1) {
// bool isEven = nums[i] % 2 == 0;
// bool isLessThan99 = nums[i] < 99;
// if (isEven && isLessThan99) {
// total += nums[i];
// }
// }
// }
// gas optimized
// [1, 2, 3, 4, 5, 100]
function sumIfEvenAndLessThan99(uint[] calldata nums) external {
uint _total = total;
uint len = nums.length;
for (uint i = 0; i < len; ) {
uint num = nums[i];
if (num % 2 == 0 && num < 99) {
_total += num;
}
unchecked {
++i;
}
}
total = _total;
}
}
```
<br>
---
### pack variables
<br>
* this code is an example of poor code and will consume 3 storage slot:
```
uint8 numberOne;
uint256 bigNumber;
uint8 numberTwo;
```
<br>
* a much more efficient way to do this in solidity will be:
```
uint8 numberOne;
uint8 numberTwo;
uint256 bigNumber;
```
<br>
---
### constant vs. immutable variables
<br>
* constant values can sometimes be cheaper than immutable values:
- for a constant variable, the expression assigned to it is copied to all the places where it is accessed and also re-evaluated each time, allowing local optimizations.
- immutable variables are evaluated once at construction time and their value is copied to all the places in the code where they are accessed. For these values, 32 bytes are reserved, even if they would fit in fewer bytes.
<br>
---
### mappings are cheaper than arrays
<br>
- avoid dynamically sized arrays
- an array is not stored sequentially in memory but as a mapping.
- you can pack Arrays but not Mappings.
- it’s cheaper to use arrays if you are using smaller elements like `uint8` which can be packed together.
- you can’t get the length of a mapping or parse through all its elements, so depending on your use case, you might be forced to use an Array even though it might cost you more gas.
<br>
---
### use bytes32 rather than string/bytes
<br>
- if you can fit your data in 32 bytes, then you should use `bytes32` datatype rather than bytes or strings as it is much cheaper in solidity.
- any fixed size variable in solidity is cheaper than variable size.
<br>
---
### modifiers
<br>
- for all the public functions, the input parameters are copied to memory automatically, and it costs gas.
- if your function is only called externally, then you should explicitly mark it as external.
- external function’s parameters are not copied into memory but are read from `calldata` directly.
- internal and private are both cheaper than public and external when called from inside the contract in some cases.
<br>
---
### no need to initialize variables with default values
<br>
- if a variable is not set/initialized, it is assumed to have the default value (0, false, 0x0 etc depending on the data type). If you explicitly initialize it with its default value, you are just wasting gas.
```
uint256 hello = 0; //bad, expensive
uint256 world; //good, cheap
```
<br>
---
### make use of single line swaps
<br>
- this is space-efficient:
```
(hello, world) = (world, hello)
```
<br>
---
### negative gas costs
<br>
- deleting a contract (SELFDESTRUCT) is worth a refund of 24,000 gas.
- changing a storage address from a nonzero value to zero (SSTORE[x] = 0) is worth a refund of 15,000 gas.
<br>
---
### [i ++](https://twitter.com/high_byte/status/1647080662094995457?s=20)
<br>
* instead of i++ you should write the ++ above the i.
```
while (true)
uint256 i = 0;
++
i
;
```
<br>
----
### unchecked math
<br>
* overflow and underflow of numbers in solidity 0.8 throw an error. this can be disabled with `unchecked`.
* disabling overflow / underflow check saves gas.
<br>
```
contract UncheckedMath {
function add(uint x, uint y) external pure returns (uint) {
// 22291 gas
// return x + y;
// 22103 gas
unchecked {
return x + y;
}
}
function sub(uint x, uint y) external pure returns (uint) {
// 22329 gas
// return x - y;
// 22147 gas
unchecked {
return x - y;
}
}
function sumOfCubes(uint x, uint y) external pure returns (uint) {
// Wrap complex math logic inside unchecked
unchecked {
uint x3 = x * x * x;
uint y3 = y * y * y;
return x3 + y3;
}
}
}
```
<br>
---
### resources
<br>
* **[truffle contract size](https://github.com/IoBuilders/truffle-contract-size)**
* **[foundry book on gas](https://book.getfoundry.sh/forge/gas-reports)**
* **[solidity gas optimizations](https://mirror.xyz/haruxe.eth/DW5verFv8KsYOBC0SxqWORYry17kPdeS94JqOVkgxAA)**
* **[hardhat on gas optimization](https://medium.com/@thelasthash/%EF%B8%8F-gas-optimization-with-hardhat-1e553eaea311)**
* **[resources for gas optimization](https://github.com/kadenzipfel/gas-optimizations)**
* **[awesome solidity gas optimization](https://github.com/iskdrews/awesome-solidity-gas-optimization)**
* **[mev-toolkit resources](https://github.com/go-outside-labs/mev-toolkit/tree/main/MEV_searchers/code_resources/gas_optimization)**
* **[how gas optimization can streamline smart contracts](https://medium.com/@ayomilk1/maximizing-efficiency-how-gas-optimization-can-streamline-your-smart-contracts-4bafcc6bf321)**
* **[math, solidity & gas optimizations | part 1/3](https://officercia.mirror.xyz/vtVVxbV35ETiBGxm-IpcFPcsK2_ZkL7vgiiGUkeSsP0)**

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;
// gaslless erc20 token transfer with meta transaction
interface IERC20Permit {
function totalSupply() external view returns (uint256);
function balanceOf(address account) external view returns (uint256);
function transfer(address recipient, uint256 amount) external returns (bool);
function allowance(address owner, address spender) external view returns (uint256);
function approve(address spender, uint256 amount) external returns (bool);
function transferFrom(
address sender,
address recipient,
uint256 amount
) external returns (bool);
function permit(
address owner,
address spender,
uint256 value,
uint256 deadline,
uint8 v,
bytes32 r,
bytes32 s
) external;
event Transfer(address indexed from, address indexed to, uint256 value);
event Approval(address indexed owner, address indexed spender, uint256 value);
}
contract GaslessTokenTransfer {
function send(
address token,
address sender,
address receiver,
uint256 amount,
uint256 fee,
uint256 deadline,
// Permit signature
uint8 v,
bytes32 r,
bytes32 s
) external {
// Permit
IERC20Permit(token).permit(
sender,
address(this),
amount + fee,
deadline,
v,
r,
s
);
// Send amount to receiver
IERC20Permit(token).transferFrom(sender, receiver, amount);
// Take fee - send fee to msg.sender
IERC20Permit(token).transferFrom(sender, msg.sender, fee);
}
}

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## a place to dump wallets examples
<br>
* disclaimer: these boilerplates might not be my code, but rather well-known references.

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
/*
Bi-directional payment channels allow participants Alice and Bob to repeatedly transfer Ether off chain.
Payments can go both ways, Alice pays Bob and Bob pays Alice.
Opening a channel
1. Alice and Bob fund a multi-sig wallet
2. Precompute payment channel address
3. Alice and Bob exchanges signatures of initial balances
4. Alice and Bob creates a transaction that can deploy a payment channel from
the multi-sig wallet
Update channel balances
1. Repeat steps 1 - 3 from opening a channel
2. From multi-sig wallet create a transaction that will
- delete the transaction that would have deployed the old payment channel
- and then create a transaction that can deploy a payment channel with the
new balances
Closing a channel when Alice and Bob agree on the final balance
1. From multi-sig wallet create a transaction that will
- send payments to Alice and Bob
- and then delete the transaction that would have created the payment channel
Closing a channel when Alice and Bob do not agree on the final balances
1. Deploy payment channel from multi-sig
2. call challengeExit() to start the process of closing a channel
3. Alice and Bob can withdraw funds once the channel is expired
*/
import "github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v4.5/contracts/utils/cryptography/ECDSA.sol";
contract BiDirectionalPaymentChannel {
using ECDSA for bytes32;
event ChallengeExit(address indexed sender, uint nonce);
event Withdraw(address indexed to, uint amount);
address payable[2] public users;
mapping(address => bool) public isUser;
mapping(address => uint) public balances;
uint public challengePeriod;
uint public expiresAt;
uint public nonce;
modifier checkBalances(uint[2] memory _balances) {
require(
address(this).balance >= _balances[0] + _balances[1],
"balance of contract must be >= to the total balance of users"
);
_;
}
// NOTE: deposit from multi-sig wallet
constructor(
address payable[2] memory _users,
uint[2] memory _balances,
uint _expiresAt,
uint _challengePeriod
) payable checkBalances(_balances) {
require(_expiresAt > block.timestamp, "Expiration must be > now");
require(_challengePeriod > 0, "Challenge period must be > 0");
for (uint i = 0; i < _users.length; i++) {
address payable user = _users[i];
require(!isUser[user], "user must be unique");
users[i] = user;
isUser[user] = true;
balances[user] = _balances[i];
}
expiresAt = _expiresAt;
challengePeriod = _challengePeriod;
}
function verify(
bytes[2] memory _signatures,
address _contract,
address[2] memory _signers,
uint[2] memory _balances,
uint _nonce
) public pure returns (bool) {
for (uint i = 0; i < _signatures.length; i++) {
/*
NOTE: sign with address of this contract to protect
agains replay attack on other contracts
*/
bool valid = _signers[i] ==
keccak256(abi.encodePacked(_contract, _balances, _nonce))
.toEthSignedMessageHash()
.recover(_signatures[i]);
if (!valid) {
return false;
}
}
return true;
}
modifier checkSignatures(
bytes[2] memory _signatures,
uint[2] memory _balances,
uint _nonce
) {
// Note: copy storage array to memory
address[2] memory signers;
for (uint i = 0; i < users.length; i++) {
signers[i] = users[i];
}
require(
verify(_signatures, address(this), signers, _balances, _nonce),
"Invalid signature"
);
_;
}
modifier onlyUser() {
require(isUser[msg.sender], "Not user");
_;
}
function challengeExit(
uint[2] memory _balances,
uint _nonce,
bytes[2] memory _signatures
)
public
onlyUser
checkSignatures(_signatures, _balances, _nonce)
checkBalances(_balances)
{
require(block.timestamp < expiresAt, "Expired challenge period");
require(_nonce > nonce, "Nonce must be greater than the current nonce");
for (uint i = 0; i < _balances.length; i++) {
balances[users[i]] = _balances[i];
}
nonce = _nonce;
expiresAt = block.timestamp + challengePeriod;
emit ChallengeExit(msg.sender, nonce);
}
function withdraw() public onlyUser {
require(block.timestamp >= expiresAt, "Challenge period has not expired yet");
uint amount = balances[msg.sender];
balances[msg.sender] = 0;
(bool sent, ) = msg.sender.call{value: amount}("");
require(sent, "Failed to send Ether");
emit Withdraw(msg.sender, amount);
}
}

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// Let's create an multi-sig wallet. Here are the specifications.
// The wallet owners can
// - submit a transaction
// - approve and revoke approval of pending transcations
// - anyone can execute a transcation after enough owners has approved it.
contract MultiSigWallet {
event Deposit(address indexed sender, uint amount, uint balance);
event SubmitTransaction(
address indexed owner,
uint indexed txIndex,
address indexed to,
uint value,
bytes data
);
event ConfirmTransaction(address indexed owner, uint indexed txIndex);
event RevokeConfirmation(address indexed owner, uint indexed txIndex);
event ExecuteTransaction(address indexed owner, uint indexed txIndex);
address[] public owners;
mapping(address => bool) public isOwner;
uint public numConfirmationsRequired;
struct Transaction {
address to;
uint value;
bytes data;
bool executed;
uint numConfirmations;
}
// mapping from tx index => owner => bool
mapping(uint => mapping(address => bool)) public isConfirmed;
Transaction[] public transactions;
modifier onlyOwner() {
require(isOwner[msg.sender], "not owner");
_;
}
modifier txExists(uint _txIndex) {
require(_txIndex < transactions.length, "tx does not exist");
_;
}
modifier notExecuted(uint _txIndex) {
require(!transactions[_txIndex].executed, "tx already executed");
_;
}
modifier notConfirmed(uint _txIndex) {
require(!isConfirmed[_txIndex][msg.sender], "tx already confirmed");
_;
}
constructor(address[] memory _owners, uint _numConfirmationsRequired) {
require(_owners.length > 0, "owners required");
require(
_numConfirmationsRequired > 0 &&
_numConfirmationsRequired <= _owners.length,
"invalid number of required confirmations"
);
for (uint i = 0; i < _owners.length; i++) {
address owner = _owners[i];
require(owner != address(0), "invalid owner");
require(!isOwner[owner], "owner not unique");
isOwner[owner] = true;
owners.push(owner);
}
numConfirmationsRequired = _numConfirmationsRequired;
}
receive() external payable {
emit Deposit(msg.sender, msg.value, address(this).balance);
}
function submitTransaction(
address _to,
uint _value,
bytes memory _data
) public onlyOwner {
uint txIndex = transactions.length;
transactions.push(
Transaction({
to: _to,
value: _value,
data: _data,
executed: false,
numConfirmations: 0
})
);
emit SubmitTransaction(msg.sender, txIndex, _to, _value, _data);
}
function confirmTransaction(
uint _txIndex
) public onlyOwner txExists(_txIndex) notExecuted(_txIndex) notConfirmed(_txIndex) {
Transaction storage transaction = transactions[_txIndex];
transaction.numConfirmations += 1;
isConfirmed[_txIndex][msg.sender] = true;
emit ConfirmTransaction(msg.sender, _txIndex);
}
function executeTransaction(
uint _txIndex
) public onlyOwner txExists(_txIndex) notExecuted(_txIndex) {
Transaction storage transaction = transactions[_txIndex];
require(
transaction.numConfirmations >= numConfirmationsRequired,
"cannot execute tx"
);
transaction.executed = true;
(bool success, ) = transaction.to.call{value: transaction.value}(
transaction.data
);
require(success, "tx failed");
emit ExecuteTransaction(msg.sender, _txIndex);
}
function revokeConfirmation(
uint _txIndex
) public onlyOwner txExists(_txIndex) notExecuted(_txIndex) {
Transaction storage transaction = transactions[_txIndex];
require(isConfirmed[_txIndex][msg.sender], "tx not confirmed");
transaction.numConfirmations -= 1;
isConfirmed[_txIndex][msg.sender] = false;
emit RevokeConfirmation(msg.sender, _txIndex);
}
function getOwners() public view returns (address[] memory) {
return owners;
}
function getTransactionCount() public view returns (uint) {
return transactions.length;
}
function getTransaction(
uint _txIndex
)
public
view
returns (
address to,
uint value,
bytes memory data,
bool executed,
uint numConfirmations
)
{
Transaction storage transaction = transactions[_txIndex];
return (
transaction.to,
transaction.value,
transaction.data,
transaction.executed,
transaction.numConfirmations
);
}
}

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
contract TestContract {
uint public i;
function callMe(uint j) public {
i += j;
}
function getData() public pure returns (bytes memory) {
return abi.encodeWithSignature("callMe(uint256)", 123);
}
}

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advanced_knowledge/wallets/unidirectional_payment_channel.sol Normal file
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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// Payment channels allow participants to repeatedly transfer Ether off chain.
// Here is how this contract is used:
// - Alice deploys the contract, funding it with some Ether.
// - Alice authorizes a payment by signing a message (off chain) and sends the signature to Bob.
// - Bob claims his payment by presenting the signed message to the smart contract.
// - If Bob does not claim his payment, Alice get her Ether back after the contract expires
// This is called a uni-directional payment channel since the payment can go only in a single direction from Alice to Bob.
import "github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v4.5/contracts/utils/cryptography/ECDSA.sol";
import "github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v4.5/contracts/security/ReentrancyGuard.sol";
contract UniDirectionalPaymentChannel is ReentrancyGuard {
using ECDSA for bytes32;
address payable public sender;
address payable public receiver;
uint private constant DURATION = 7 * 24 * 60 * 60;
uint public expiresAt;
constructor(address payable _receiver) payable {
require(_receiver != address(0), "receiver = zero address");
sender = payable(msg.sender);
receiver = _receiver;
expiresAt = block.timestamp + DURATION;
}
function _getHash(uint _amount) private view returns (bytes32) {
// NOTE: sign with address of this contract to protect agains
// replay attack on other contracts
return keccak256(abi.encodePacked(address(this), _amount));
}
function getHash(uint _amount) external view returns (bytes32) {
return _getHash(_amount);
}
function _getEthSignedHash(uint _amount) private view returns (bytes32) {
return _getHash(_amount).toEthSignedMessageHash();
}
function getEthSignedHash(uint _amount) external view returns (bytes32) {
return _getEthSignedHash(_amount);
}
function _verify(uint _amount, bytes memory _sig) private view returns (bool) {
return _getEthSignedHash(_amount).recover(_sig) == sender;
}
function verify(uint _amount, bytes memory _sig) external view returns (bool) {
return _verify(_amount, _sig);
}
function close(uint _amount, bytes memory _sig) external nonReentrant {
require(msg.sender == receiver, "!receiver");
require(_verify(_amount, _sig), "invalid sig");
(bool sent, ) = receiver.call{value: _amount}("");
require(sent, "Failed to send Ether");
selfdestruct(sender);
}
function cancel() external {
require(msg.sender == sender, "!sender");
require(block.timestamp >= expiresAt, "!expired");
selfdestruct(sender);
}
}

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200
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## bitwise operators
<br>
### example of bitwise operations
```
contract BitwiseOps {
// x = 1110 = 8 + 4 + 2 + 0 = 14
// y = 1011 = 8 + 0 + 2 + 1 = 11
// x & y = 1010 = 8 + 0 + 2 + 0 = 10
function and(uint x, uint y) external pure returns (uint) {
return x & y;
}
// x = 1100 = 8 + 4 + 0 + 0 = 12
// y = 1001 = 8 + 0 + 0 + 1 = 9
// x | y = 1101 = 8 + 4 + 0 + 1 = 13
function or(uint x, uint y) external pure returns (uint) {
return x | y;
}
// x = 1100 = 8 + 4 + 0 + 0 = 12
// y = 0101 = 0 + 4 + 0 + 1 = 5
// x ^ y = 1001 = 8 + 0 + 0 + 1 = 9
function xor(uint x, uint y) external pure returns (uint) {
return x ^ y;
}
// x = 00001100 = 0 + 0 + 0 + 0 + 8 + 4 + 0 + 0 = 12
// ~x = 11110011 = 128 + 64 + 32 + 16 + 0 + 0 + 2 + 1 = 243
function not(uint8 x) external pure returns (uint8) {
return ~x;
}
// 1 << 0 = 0001 --> 0001 = 1
// 1 << 1 = 0001 --> 0010 = 2
// 1 << 2 = 0001 --> 0100 = 4
// 1 << 3 = 0001 --> 1000 = 8
// 3 << 2 = 0011 --> 1100 = 12
function shiftLeft(uint x, uint bits) external pure returns (uint) {
return x << bits;
}
// 8 >> 0 = 1000 --> 1000 = 8
// 8 >> 1 = 1000 --> 0100 = 4
// 8 >> 2 = 1000 --> 0010 = 2
// 8 >> 3 = 1000 --> 0001 = 1
// 8 >> 4 = 1000 --> 0000 = 0
// 12 >> 1 = 1100 --> 0110 = 6
function shiftRight(uint x, uint bits) external pure returns (uint) {
return x >> bits;
}
// Get last n bits from x
function getLastNBits(uint x, uint n) external pure returns (uint) {
// Example, last 3 bits
// x = 1101 = 13
// mask = 0111 = 7
// x & mask = 0101 = 5
uint mask = (1 << n) - 1;
return x & mask;
}
// Get last n bits from x using mod operator
function getLastNBitsUsingMod(uint x, uint n) external pure returns (uint) {
// 1 << n = 2 ** n
return x % (1 << n);
}
// Get position of most significant bit
// x = 1100 = 10, most significant bit = 1000, so this function will return 3
function mostSignificantBit(uint x) external pure returns (uint) {
uint i = 0;
while ((x >>= 1) > 0) {
++i;
}
return i;
}
// Get first n bits from x
// len = length of bits in x = position of most significant bit of x, + 1
function getFirstNBits(uint x, uint n, uint len) external pure returns (uint) {
// Example
// x = 1110 = 14, n = 2, len = 4
// mask = 1100 = 12
// x & mask = 1100 = 12
uint mask = ((1 << n) - 1) << (len - n);
return x & mask;
}
}
```
<br>
----
### most significant bit
<br>
```
contract MostSignificantBitFunction {
// Find most significant bit using binary search
function mostSignificantBit(uint x) external pure returns (uint msb) {
// x >= 2 ** 128
if (x >= 0x100000000000000000000000000000000) {
x >>= 128;
msb += 128;
}
// x >= 2 ** 64
if (x >= 0x10000000000000000) {
x >>= 64;
msb += 64;
}
// x >= 2 ** 32
if (x >= 0x100000000) {
x >>= 32;
msb += 32;
}
// x >= 2 ** 16
if (x >= 0x10000) {
x >>= 16;
msb += 16;
}
// x >= 2 ** 8
if (x >= 0x100) {
x >>= 8;
msb += 8;
}
// x >= 2 ** 4
if (x >= 0x10) {
x >>= 4;
msb += 4;
}
// x >= 2 ** 2
if (x >= 0x4) {
x >>= 2;
msb += 2;
}
// x >= 2 ** 1
if (x >= 0x2) msb += 1;
}
}
```
<br>
----
### most significant bit in assembly
<br>
```
contract MostSignificantBitAssembly {
function mostSignificantBit(uint x) external pure returns (uint msb) {
assembly {
let f := shl(7, gt(x, 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF))
x := shr(f, x)
// or can be replaced with add
msb := or(msb, f)
}
assembly {
let f := shl(6, gt(x, 0xFFFFFFFFFFFFFFFF))
x := shr(f, x)
msb := or(msb, f)
}
assembly {
let f := shl(5, gt(x, 0xFFFFFFFF))
x := shr(f, x)
msb := or(msb, f)
}
assembly {
let f := shl(4, gt(x, 0xFFFF))
x := shr(f, x)
msb := or(msb, f)
}
assembly {
let f := shl(3, gt(x, 0xFF))
x := shr(f, x)
msb := or(msb, f)
}
assembly {
let f := shl(2, gt(x, 0xF))
x := shr(f, x)
msb := or(msb, f)
}
assembly {
let f := shl(1, gt(x, 0x3))
x := shr(f, x)
msb := or(msb, f)
}
assembly {
let f := gt(x, 0x1)
msb := or(msb, f)
}
}
}
```

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basic_knowledge/boilerplates/1_gm_world.sol Normal file
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pragma solidity ^0.8.10;
// Defines a contract named `GmWorld`.
// A contract is a collection of functions and data (its state).
// Once deployed, a contract resides at a specific address on the Ethereum blockchain.
contract GmWorld {
// Declares a state variable `happiness` of type `string`.
// State variables are variables whose values are permanently stored in contract storage.
// The keyword `public` makes variables accessible from outside a contract
// and creates a function that other contracts or clients can call to access the value.
string public happiness;
// Similar to many class-based object-oriented languages, a constructor is
// a special function that is only executed upon contract creation.
// Constructors are used to initialize the contract's data.
constructor(string memory warmGun) public {
// Accepts a string argument `warmGun` and sets the value
// into the contract's `happiness` storage variable).
happiness = warmGun;
}
// A public function that accepts a string argument
// and updates the `happiness` storage variable.
function update(string memory bobaTea) public {
happiness = bobaTea;
}
}

0
boilerplates/learning/token.sol → basic_knowledge/boilerplates/2_token.sol
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24
basic_knowledge/boilerplates/3_wallet.sol Normal file
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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// An example of a basic wallet.
// Anyone can send ETH and only the owner can withdraw.
contract EtherWallet {
address payable public owner;
constructor() {
owner = payable(msg.sender);
}
receive() external payable {}
function withdraw(uint _amount) external {
require(msg.sender == owner, "caller is not owner");
payable(msg.sender).transfer(_amount);
}
function getBalance() external view returns (uint) {
return address(this).balance;
}
}

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basic_knowledge/boilerplates/4_interable_mapping.sol Normal file
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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// You cannot iterate through a mapping.
// So here is an example of how to create an iterable mapping.
library IterableMapping {
// Iterable mapping from address to uint;
struct Map {
address[] keys;
mapping(address => uint) values;
mapping(address => uint) indexOf;
mapping(address => bool) inserted;
}
function get(Map storage map, address key) public view returns (uint) {
return map.values[key];
}
function getKeyAtIndex(Map storage map, uint index) public view returns (address) {
return map.keys[index];
}
function size(Map storage map) public view returns (uint) {
return map.keys.length;
}
function set(Map storage map, address key, uint val) public {
if (map.inserted[key]) {
map.values[key] = val;
} else {
map.inserted[key] = true;
map.values[key] = val;
map.indexOf[key] = map.keys.length;
map.keys.push(key);
}
}
function remove(Map storage map, address key) public {
if (!map.inserted[key]) {
return;
}
delete map.inserted[key];
delete map.values[key];
uint index = map.indexOf[key];
address lastKey = map.keys[map.keys.length - 1];
map.indexOf[lastKey] = index;
delete map.indexOf[key];
map.keys[index] = lastKey;
map.keys.pop();
}
}
contract TestIterableMap {
using IterableMapping for IterableMapping.Map;
IterableMapping.Map private map;
function testIterableMap() public {
map.set(address(0), 0);
map.set(address(1), 100);
map.set(address(2), 200); // insert
map.set(address(2), 200); // update
map.set(address(3), 300);
for (uint i = 0; i < map.size(); i++) {
address key = map.getKeyAtIndex(i);
assert(map.get(key) == i * 100);
}
map.remove(address(1));
// keys = [address(0), address(3), address(2)]
assert(map.size() == 3);
assert(map.getKeyAtIndex(0) == address(0));
assert(map.getKeyAtIndex(1) == address(3));
assert(map.getKeyAtIndex(2) == address(2));
}
}

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basic_knowledge/boilerplates/5_erc20_interface.sol Normal file
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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// ERC20 tokens provide functionalities to
// - transfer tokens
// - allow others to transfer tokens on behalf of the token holder
// https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.0.0/contracts/token/ERC20/IERC20.sol
interface IERC20 {
function totalSupply() external view returns (uint);
function balanceOf(address account) external view returns (uint);
function transfer(address recipient, uint amount) external returns (bool);
function allowance(address owner, address spender) external view returns (uint);
function approve(address spender, uint amount) external returns (bool);
function transferFrom(
address sender,
address recipient,
uint amount
) external returns (bool);
event Transfer(address indexed from, address indexed to, uint value);
event Approval(address indexed owner, address indexed spender, uint value);
}

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basic_knowledge/boilerplates/6_erc721_interface.sol Normal file
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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
interface IERC165 {
function supportsInterface(bytes4 interfaceID) external view returns (bool);
}
interface IERC721 is IERC165 {
function balanceOf(address owner) external view returns (uint balance);
function ownerOf(uint tokenId) external view returns (address owner);
function safeTransferFrom(address from, address to, uint tokenId) external;
function transferFrom(address from, address to, uint tokenId) external;
function approve(address to, uint tokenId) external;
function getApproved(uint tokenId) external view returns (address operator);
function setApprovalForAll(address operator, bool _approved) external;
function isApprovedForAll(
address owner,
address operator
) external view returns (bool);
}
interface IERC721Receiver {
function onERC721Received(
address operator,
address from,
uint tokenId,
bytes calldata data
) external returns (bytes4);
}
contract ERC721 is IERC721 {
event Transfer(address indexed from, address indexed to, uint indexed id);
event Approval(address indexed owner, address indexed spender, uint indexed id);
event ApprovalForAll(
address indexed owner,
address indexed operator,
bool approved
);
// Mapping from token ID to owner address
mapping(uint => address) internal _ownerOf;
// Mapping owner address to token count
mapping(address => uint) internal _balanceOf;
// Mapping from token ID to approved address
mapping(uint => address) internal _approvals;
// Mapping from owner to operator approvals
mapping(address => mapping(address => bool)) public isApprovedForAll;
function supportsInterface(bytes4 interfaceId) external pure returns (bool) {
return
interfaceId == type(IERC721).interfaceId ||
interfaceId == type(IERC165).interfaceId;
}
function ownerOf(uint id) external view returns (address owner) {
owner = _ownerOf[id];
require(owner != address(0), "token doesn't exist");
}
function balanceOf(address owner) external view returns (uint) {
require(owner != address(0), "owner = zero address");
return _balanceOf[owner];
}
function setApprovalForAll(address operator, bool approved) external {
isApprovedForAll[msg.sender][operator] = approved;
emit ApprovalForAll(msg.sender, operator, approved);
}
function approve(address spender, uint id) external {
address owner = _ownerOf[id];
require(
msg.sender == owner || isApprovedForAll[owner][msg.sender],
"not authorized"
);
_approvals[id] = spender;
emit Approval(owner, spender, id);
}
function getApproved(uint id) external view returns (address) {
require(_ownerOf[id] != address(0), "token doesn't exist");
return _approvals[id];
}
function _isApprovedOrOwner(
address owner,
address spender,
uint id
) internal view returns (bool) {
return (spender == owner ||
isApprovedForAll[owner][spender] ||
spender == _approvals[id]);
}
function transferFrom(address from, address to, uint id) public {
require(from == _ownerOf[id], "from != owner");
require(to != address(0), "transfer to zero address");
require(_isApprovedOrOwner(from, msg.sender, id), "not authorized");
_balanceOf[from]--;
_balanceOf[to]++;
_ownerOf[id] = to;
delete _approvals[id];
emit Transfer(from, to, id);
}
function safeTransferFrom(address from, address to, uint id) external {
transferFrom(from, to, id);
require(
to.code.length == 0 ||
IERC721Receiver(to).onERC721Received(msg.sender, from, id, "") ==
IERC721Receiver.onERC721Received.selector,
"unsafe recipient"
);
}
function safeTransferFrom(
address from,
address to,
uint id,
bytes calldata data
) external {
transferFrom(from, to, id);
require(
to.code.length == 0 ||
IERC721Receiver(to).onERC721Received(msg.sender, from, id, data) ==
IERC721Receiver.onERC721Received.selector,
"unsafe recipient"
);
}
function _mint(address to, uint id) internal {
require(to != address(0), "mint to zero address");
require(_ownerOf[id] == address(0), "already minted");
_balanceOf[to]++;
_ownerOf[id] = to;
emit Transfer(address(0), to, id);
}
function _burn(uint id) internal {
address owner = _ownerOf[id];
require(owner != address(0), "not minted");
_balanceOf[owner] -= 1;
delete _ownerOf[id];
delete _approvals[id];
emit Transfer(owner, address(0), id);
}
}
contract MyNFT is ERC721 {
function mint(address to, uint id) external {
_mint(to, id);
}
function burn(uint id) external {
require(msg.sender == _ownerOf[id], "not owner");
_burn(id);
}
}

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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
interface IERC1155 {
function safeTransferFrom(
address from,
address to,
uint256 id,
uint256 value,
bytes calldata data
) external;
function safeBatchTransferFrom(
address from,
address to,
uint256[] calldata ids,
uint256[] calldata values,
bytes calldata data
) external;
function balanceOf(address owner, uint256 id) external view returns (uint256);
function balanceOfBatch(
address[] calldata owners,
uint256[] calldata ids
) external view returns (uint256[] memory);
function setApprovalForAll(address operator, bool approved) external;
function isApprovedForAll(
address owner,
address operator
) external view returns (bool);
}
interface IERC1155TokenReceiver {
function onERC1155Received(
address operator,
address from,
uint256 id,
uint256 value,
bytes calldata data
) external returns (bytes4);
function onERC1155BatchReceived(
address operator,
address from,
uint256[] calldata ids,
uint256[] calldata values,
bytes calldata data
) external returns (bytes4);
}
contract ERC1155 is IERC1155 {
event TransferSingle(
address indexed operator,
address indexed from,
address indexed to,
uint256 id,
uint256 value
);
event TransferBatch(
address indexed operator,
address indexed from,
address indexed to,
uint256[] ids,
uint256[] values
);
event ApprovalForAll(
address indexed owner,
address indexed operator,
bool approved
);
event URI(string value, uint256 indexed id);
// owner => id => balance
mapping(address => mapping(uint256 => uint256)) public balanceOf;
// owner => operator => approved
mapping(address => mapping(address => bool)) public isApprovedForAll;
function balanceOfBatch(
address[] calldata owners,
uint256[] calldata ids
) external view returns (uint256[] memory balances) {
require(owners.length == ids.length, "owners length != ids length");
balances = new uint[](owners.length);
unchecked {
for (uint256 i = 0; i < owners.length; i++) {
balances[i] = balanceOf[owners[i]][ids[i]];
}
}
}
function setApprovalForAll(address operator, bool approved) external {
isApprovedForAll[msg.sender][operator] = approved;
emit ApprovalForAll(msg.sender, operator, approved);
}
function safeTransferFrom(
address from,
address to,
uint256 id,
uint256 value,
bytes calldata data
) external {
require(
msg.sender == from || isApprovedForAll[from][msg.sender],
"not approved"
);
require(to != address(0), "to = 0 address");
balanceOf[from][id] -= value;
balanceOf[to][id] += value;
emit TransferSingle(msg.sender, from, to, id, value);
if (to.code.length > 0) {
require(
IERC1155TokenReceiver(to).onERC1155Received(
msg.sender,
from,
id,
value,
data
) == IERC1155TokenReceiver.onERC1155Received.selector,
"unsafe transfer"
);
}
}
function safeBatchTransferFrom(
address from,
address to,
uint256[] calldata ids,
uint256[] calldata values,
bytes calldata data
) external {
require(
msg.sender == from || isApprovedForAll[from][msg.sender],
"not approved"
);
require(to != address(0), "to = 0 address");
require(ids.length == values.length, "ids length != values length");
for (uint256 i = 0; i < ids.length; i++) {
balanceOf[from][ids[i]] -= values[i];
balanceOf[to][ids[i]] += values[i];
}
emit TransferBatch(msg.sender, from, to, ids, values);
if (to.code.length > 0) {
require(
IERC1155TokenReceiver(to).onERC1155BatchReceived(
msg.sender,
from,
ids,
values,
data
) == IERC1155TokenReceiver.onERC1155BatchReceived.selector,
"unsafe transfer"
);
}
}
// ERC165
function supportsInterface(bytes4 interfaceId) external view returns (bool) {
return
interfaceId == 0x01ffc9a7 || // ERC165 Interface ID for ERC165
interfaceId == 0xd9b67a26 || // ERC165 Interface ID for ERC1155
interfaceId == 0x0e89341c; // ERC165 Interface ID for ERC1155MetadataURI
}
// ERC1155 Metadata URI
function uri(uint256 id) public view virtual returns (string memory) {}
// Internal functions
function _mint(address to, uint256 id, uint256 value, bytes memory data) internal {
require(to != address(0), "to = 0 address");
balanceOf[to][id] += value;
emit TransferSingle(msg.sender, address(0), to, id, value);
if (to.code.length > 0) {
require(
IERC1155TokenReceiver(to).onERC1155Received(
msg.sender,
address(0),
id,
value,
data
) == IERC1155TokenReceiver.onERC1155Received.selector,
"unsafe transfer"
);
}
}
function _batchMint(
address to,
uint256[] calldata ids,
uint256[] calldata values,
bytes calldata data
) internal {
require(to != address(0), "to = 0 address");
require(ids.length == values.length, "ids length != values length");
for (uint256 i = 0; i < ids.length; i++) {
balanceOf[to][ids[i]] += values[i];
}
emit TransferBatch(msg.sender, address(0), to, ids, values);
if (to.code.length > 0) {
require(
IERC1155TokenReceiver(to).onERC1155BatchReceived(
msg.sender,
address(0),
ids,
values,
data
) == IERC1155TokenReceiver.onERC1155BatchReceived.selector,
"unsafe transfer"
);
}
}
function _burn(address from, uint256 id, uint256 value) internal {
require(from != address(0), "from = 0 address");
balanceOf[from][id] -= value;
emit TransferSingle(msg.sender, from, address(0), id, value);
}
function _batchBurn(
address from,
uint256[] calldata ids,
uint256[] calldata values
) internal {
require(from != address(0), "from = 0 address");
require(ids.length == values.length, "ids length != values length");
for (uint256 i = 0; i < ids.length; i++) {
balanceOf[from][ids[i]] -= values[i];
}
emit TransferBatch(msg.sender, from, address(0), ids, values);
}
}
contract MyMultiToken is ERC1155 {
function mint(uint256 id, uint256 value, bytes memory data) external {
_mint(msg.sender, id, value, data);
}
function batchMint(
uint256[] calldata ids,
uint256[] calldata values,
bytes calldata data
) external {
_batchMint(msg.sender, ids, values, data);
}
function burn(uint256 id, uint256 value) external {
_burn(msg.sender, id, value);
}
function batchBurn(uint256[] calldata ids, uint256[] calldata values) external {
_batchBurn(msg.sender, ids, values);
}
}

58
basic_knowledge/boilerplates/8_bytecode_contract.sol Normal file
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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
// simple example of contract written in bytecode
contract Factory {
event Log(address addr);
// Deploys a contract that always returns 42
function deploy() external {
bytes memory bytecode = hex"69602a60005260206000f3600052600a6016f3";
address addr;
assembly {
// create(value, offset, size)
addr := create(0, add(bytecode, 0x20), 0x13)
}
require(addr != address(0));
emit Log(addr);
}
}
interface IContract {
function getMeaningOfLife() external view returns (uint);
}
// https://www.evm.codes/playground
/*
Run time code - return 42
602a60005260206000f3
// Store 42 to memory
mstore(p, v) - store v at memory p to p + 32
PUSH1 0x2a
PUSH1 0
MSTORE
// Return 32 bytes from memory
return(p, s) - end execution and return data from memory p to p + s
PUSH1 0x20
PUSH1 0
RETURN
Creation code - return runtime code
69602a60005260206000f3600052600a6016f3
// Store run time code to memory
PUSH10 0X602a60005260206000f3
PUSH1 0
MSTORE
// Return 10 bytes from memory starting at offset 22
PUSH1 0x0a
PUSH1 0x16
RETURN
*/

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## contracts boilerplates and examples to learn solidity
<br>
* **[gm_world.sol](1_gm_world.sol)**
* **[token.sol](2_token.sol)**
* **[wallet.sol](3_wallet.sol)**
* **[interable_mapping.sol](4_interable_mapping.sol)**
* **[erc20_interface.sol](5_erc20_interface.sol)**
* **[erc721_interface.sol](6_erc721_interface.sol)**
* **[erc1155.sol](7_erc1155.sol)**
* **[bytecode_contract.sol](8_bytecode_contract.sol)**

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## ethereum token standards
<br>
### tl; dr
<br>
* EIP stands for **ethereum improvement proposals**.
* ERC stands for **ethereum request for comments** (technical documents written by ethereum developers for ethereum community).
* each such document contains a set of rules required to implement tokens for the ethereum ecosystem.
<br>
---
### in this dir
<br>
* **[ERC20](erc20.md)**
* **[ERC777](erc777.md)**
* **[ERC721](erc721.md)**
<br>

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## ERC20
<br>
* defines a common interface for contracts implementing this token, such that any compatible token can be accessed and used in the same way.
* a transaction sending ether to an address changes the state of an address. a transaction transferring a token to an address only changes the state of the token contract, not the state of the recipient address.
* one of the main reasons for the success of EIP-20 tokens is in the interplay between `approve` and `transferFrom`, which allows for tokens to not
only be transferred between externally owned accounts (EOA).
- but to be used in other contracts under application specific conditions by abstracting away `msg.sender` as the mechanism for token access control.
* a limiting factor lies from the fact that the EIP-20 `approve` function is defined in terms of `msg.sender`.
- this means that user’s initial action involving EIP-20 tokens must be performed by an EOA.
- if the user needs to interact with a smart contract, then they need to make 2 transactions (`approve` and the smart contract internal call `transferFrom`), with gas costs.
<br>
---
### ERC20-compliant token contract
<br>
* `totalSupply`: Returns the total units of this token that currently exist. ERC20 tokens can have a fixed or a variable supply.
* `balanceOf`: Given an address, returns the token balance of that address.
* `transfer`: Given an address and amount, transfers that amount of tokens to that address, from the balance of the address that executed the transfer.
* `transferFrom`: Given a sender, recipient, and amount, transfers tokens from one account to another. Used in combination with approve.
* `approve`: given a recipient address and amount, authorizes that address to execute several transfers up to that amount, from the account that issued the approval.
* `allowance`: given an owner address and a spender address, returns the remaining amount that the spender is approved to withdraw from the owner.
* `Transfer`: event triggered upon a successful transfer (call to transfer or transferFrom) (even for zero-value transfers).
* `Approval`: event logged upon a successful call to approve.
<br>
---
### ERC20 optional functions
<br>
* in addition to the required functions listed in the previous section, the following optional functions are also defined by the standard:
- `name`: returns the human-readable name (e.g., "US Dollars") of the token.
- `symbol`: returns a human-readable symbol (e.g., "USD") for the token.
- `decimals`: returns the number of decimals used to divide token amounts. For example, if decimals is 2, then the token amount is divided by 100 to get its user representation.
<br>
---
### the ERC20 interface
<br>
```
contract ERC20 {
function totalSupply() constant returns (uint theTotalSupply);
function balanceOf(address _owner) constant returns (uint balance);
function transfer(address _to, uint _value) returns (bool success);
function transferFrom(address _from, address _to, uint _value) returns
(bool success);
function approve(address _spender, uint _value) returns (bool success);
function allowance(address _owner, address _spender) constant returns
(uint remaining);
event Transfer(address indexed _from, address indexed _to, uint _value);
event Approval(address indexed _owner, address indexed _spender, uint _value);
}
```

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<br>
To see the difference between ERC20 and ERC721, look at the internal data structure used in ERC721:
* to see the difference between ERC20 and ERC721, look at the internal data structure used in ERC721:
```
@ -12,14 +12,15 @@ mapping (uint256 => address) private deedOwner;
<br>
Whereas ERC20 tracks the balances that belong to each owner, with the owner being the primary key of the mapping,
ERC721 tracks each deed ID and who owns it, with the deed ID being the primary key of the mapping.
* whereas ERC20 tracks the balances that belong to each owner, with the owner being the primary key of the mapping, ERC721 tracks each deed ID and who owns it, with the deed ID being the primary key of the mapping.
<br>
---
### The ERC721 contract interface specification
### the ERC721 contract interface specification
<br>
```
interface ERC721 /* is ERC165 */ {

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<br>
* an ERC20-compatible interface
* transfer tokens using a send function, similar to ether transfers
* compatible with ERC820 for token contract registration
* allow contracts and addresses to control which tokens they send through a tokensToSend function that is called prior to sending
* enable contracts and addresses to be notified of the tokens' receipt by calling a tokensReceived function in the recipient, and to reduce the probability of tokens being locked into contracts by requiring contracts to provide a tokensReceived function
* allow existing contracts to use proxy contracts for the `tokensToSend and `tokensReceived` functions
* operate in the same way whether sending to a contract or an EOA
* provide specific events for the minting and burning of tokens
* enable operators (trusted third parties, intended to be verified contracts) to move tokens on behalf of a token holder
* provide metadata on token transfer transactions in userData and operatorData fields
<br>

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## workspaces
<br>
* **[i use foundry](foundry.md)**
* **[notes on using remix](remix.md)**
* **[a simple intro to hardhat with a `erc721` token](hardhat/)**
* **[old stuff: truffle](https://trufflesuite.com/)**

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## foundry and tests in solidity
<br>
### tl; dr
<br>
* **[foundry](https://book.getfoundry.sh/forge/differential-ffi-testing)** is a set of tools for evm-based smart contract development and tests, written in rust.
<br>
---
### resources
<br>
* **[ethernaut solutions systematically on foundry](https://github.com/go-outside-labs/ethernaut-foundry-detailed-solutions-sol)**
* **[go-outside-labs experiments on foundry](https://github.com/go-outside-labs/blockchain-science-rs#experiments-with-foundry)**
* **[how to test smart contracts](https://betterprogramming.pub/how-to-test-ethereum-smart-contracts-35abc8fa199d)**
* **[how to mock solidity contracts](https://ethereum.org/en/developers/tutorials/how-to-mock-solidity-contracts-for-testing/)**
* **[in-depth guide to testing ethereum smart contracts](https://iamdefinitelyahuman.medium.com/an-in-depth-guide-to-testing-ethereum-smart-contracts-2e41b2770297)**

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## get started with hardhat + erc721
<br>
1. Compile contract:
1. compile contract:
```
npx hardhat compile
```
<br>
2. Deploy contract:
2. deploy contract:
```
npx hardhat run scripts/deploy-contract.js --network rinkeby
```
3. Mint NFT:
<br>
3. mint NFT:
```
node scripts/mint-nft.js

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0
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@ -5,7 +5,7 @@
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1"
},
"author": "Mia von Steinkirch",
"author": "Mia Stein",
"license": "MIT",
"devDependencies": {
"@nomiclabs/hardhat-ethers": "^2.0.2",

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## remix IDE
<br>
* remix IDE is an open source web3 application and it's used for the entire journey of smart contract development.
<br>
<img width="414" alt="Screen Shot 2022-03-10 at 5 57 22 PM" src="https://user-images.githubusercontent.com/1130416/157715032-63dfbe5d-292d-48e3-8594-04902fb008f6.png">
<br>
* everything in Remix is a plugin. the plugin mamanger is the place to load functionalities and create your own plugins.
* by default, Remix stores files in workspaces, which are folders in the browser's local storage.
* you can publish all files from current workspace to a gist, using the gist API.
<br>
#### compiler (Solidity)
* you can compile (and deploy) contracts with versions of Solidity older than 0.4.12. however, the older compilers used a legacy AST.
* the "fork selection" dropdown list allows to compile code against a specific ehtereum hard fork.
<br>
#### optimization
* the optimizer tries to simplify complicated expressions, which reduces both code size and execution cost. It can reduce gas needed for contract deployment as well as for external calls made to the contract.
<br>
#### environment
* `JavaScript VM`: All transactions will be executed in a sandbox blockchain in the browser.
* `Injected Provider`: Metamaask is an example of a profiver that inject web3.
* `Web3 Provider`: Remix will connect to a remote node (you need to provide the URL to the selected provider: geth, parity or any ethereum client)
<br>
#### setup
* gas Limit: sets the amount of `ETH`, `WEI`, `GWEI` that is sent to ta contract or a payable function.
* deploy: sends a transaction that deplpys the selected contract.
* `atAdress`: used to access a contract whtat has already been deployed (does not cost gas).
* to interact with a contract using the ABI, create a new file in remix, with extension `.abi`.
* the Recorder is a tool used to save a bunch of transactions in a `json` file and rerun them later either in the same environment or in another.
* the Debugger shows the contract's state while stepping through a transaction.
* using generated sources will make it easier to audit your contracts.
* dtatic code analysis can be done by a plugin, so that you can examine the code for security vulnerabilities, bad development practices, etc.
* hardhat integration can be done with `hardhat.config.js` (Hardhat websocket listener should run at `65522`). hardhat provider is a plugin for Rrmix IDE.
<br>
#### generate artifacts
* when a compilation for a Solidity file succeeds, remix creates three `json` files for each compiled contract, that can be seen in the `File Explorers plugin`:
1. `artifacts/<contractName>.json`: contains links to libraries, the bytecode, gas estimation, the ABI.
2. `articfacts/<contractName_metadata>.json`: contains the metadata from the output of Solidity compilation.
3. `artifcats/build-info/<dynamic_hash>.json`: contains info about `solc` compiler version, compiler input and output.

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## boilerplates
<br>
* [learning](learning)
* [solidity 101](solidity_101)

32
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// Specifies the version of Solidity, using semantic versioning.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#pragma
pragma solidity ^0.5.10;
// Defines a contract named `HelloWorld`.
// A contract is a collection of functions and data (its state).
// Once deployed, a contract resides at a specific address on the Ethereum blockchain.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html
contract HelloWorld {
// Declares a state variable `message` of type `string`.
// State variables are variables whose values are permanently stored in contract storage.
// The keyword `public` makes variables accessible from outside a contract
// and creates a function that other contracts or clients can call to access the value.
string public message;
// Similar to many class-based object-oriented languages, a constructor is
// a special function that is only executed upon contract creation.
// Constructors are used to initialize the contract's data.
// Learn more: 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
// into the contract's `message` storage variable).
message = initMessage;
}
// A public function that accepts a string argument
// and updates the `message` storage variable.
function update(string memory newMessage) public {
message = newMessage;
}
}

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## foundry and tests in solidity
<br>
### tl; dr
<br>
* foundry is a set of tools for ethereum applications development, including tests in solidity
<br>
---
### resources
<br>
* [how to mock solidity contracts](https://ethereum.org/en/developers/tutorials/how-to-mock-solidity-contracts-for-testing/)
* [truffle smart contract test framework](https://ethereum.org/en/developers/tutorials/how-to-mock-solidity-contracts-for-testing/)
* [in-depth guide to testing ethereum smart contracts](https://iamdefinitelyahuman.medium.com/an-in-depth-guide-to-testing-ethereum-smart-contracts-2e41b2770297)
* [how to test smart contracts](https://betterprogramming.pub/how-to-test-ethereum-smart-contracts-35abc8fa199d)
* [foundry book](https://book.getfoundry.sh/forge/differential-ffi-testing)

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## basic solidity unity test
<br>
### assert vs. require
* Assert() should only be used to test for internal errors, and to check invariants.
* Require() should be used to ensure valid conditions are met that cannot be detected until execution time.
* You may optionally provide a message for require, but not for assert.
<br>
### functions
<br>
functions in a test file to make testing more structural:
* `beforeEach()` - Runs before each test
* `beforeAll()` - Runs before all tests
* `afterEach()` - Runs after each test
* `afterAll()` - Runs after all tests
<br>
a generic unit testing file looks like:
```
pragma solidity >=0.4.22 <0.8.0;
import "remix_tests.sol"; // this import is automatically injected by Remix.
import "remix_accounts.sol";
// Import here the file to test.
// File name has to end with '_test.sol', this file can contain more than one testSuite contracts
contract testSuite {
/// 'beforeAll' runs before all other tests
/// More special functions are: 'beforeEach', 'beforeAll', 'afterEach' & 'afterAll'
function beforeAll() public {
// Here should instantiate tested contract
Assert.equal(uint(1), uint(1), "1 should be equal to 1");
}
function checkSuccess() public {
// Use 'Assert' to test the contract,
// See documentation: https://remix-ide.readthedocs.io/en/latest/assert_library.html
Assert.equal(uint(2), uint(2), "2 should be equal to 2");
Assert.notEqual(uint(2), uint(3), "2 should not be equal to 3");
}
function checkSuccess2() public pure returns (bool) {
// Use the return value (true or false) to test the contract
return true;
}
function checkFailure() public {
Assert.equal(uint(1), uint(2), "1 is not equal to 2");
}
/// Custom Transaction Context
/// See more: https://remix-ide.readthedocs.io/en/latest/unittesting.html#customization
/// #sender: account-1
/// #value: 100
function checkSenderAndValue() public payable {
// account index varies 0-9, value is in wei
Assert.equal(msg.sender, TestsAccounts.getAccount(1), "Invalid sender");
Assert.equal(msg.value, 100, "Invalid value");
}
}
```
note that ine can input custom values for `msg.sender` & `msg.value` of transaction using NatSpec comments.
<br>
---
### resources
<br>
* [Solidity-Coverage](https://github.com/sc-forks/solidity-coverage)
* [Remix tests](https://github.com/ethereum/remix-project/tree/master/libs/remix-tests)
* [OpenZeppelin test helpers](https://github.com/OpenZeppelin/openzeppelin-test-helpers)
* [foundry forge tests](https://github.com/foundry-rs/foundry/tree/master/forge)
* [etheno](https://github.com/crytic/etheno)

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## gas
<br>
### gas costs
<br>
- gas is the cost for on-chain computation and storage. examples:
- addition costs 3 gas, keccak-256 costs 30 gas + 6 gas for each 256 bits of data being hashed
- sending a transaction costs 21,000 gas (intrinsic gas)
- creating a contract cost 32000 gas
- each **calldata** byte costs gas (gas per byte equal to 0, and 16 gas for the others), the larger the size of the transaction data, the higher the gas fees.
- each opcode has a [specific fixed cost to be paid upon execution](https://www.evm.codes/?fork=arrowGlacier)
<br>
----
### calculating gas for your code
<br>
* online: [remix](https://remix.ethereum.org/)
<br>
----
### tricks to save gas
<br>
#### small ones
<br>
- brute force hashes of function names to find those that start 0000, so this can save around 50 gas.
- if you don’t need a variable anymore, you should delete it using the delete keyword provided by solidity or by setting it to its default value.
- avoid calls to other contracts.
- ++i uses 5 gas less than i++.
<br>
<img width="450" alt="Screen Shot 2023-01-24 at 4 28 45 PM" src="https://user-images.githubusercontent.com/1130416/214452718-b051caed-49e4-45fb-b955-976d20e97cbd.png">
<br>
#### pack variables
The below code is an example of poor code and will consume 3 storage slot:
```
uint8 numberOne;
uint256 bigNumber;
uint8 numberTwo;
```
A much more efficient way to do this in solidity will be:
```
uint8 numberOne;
uint8 numberTwo;
uint256 bigNumber;
```
#### constant vs. immutable variables
Constant values can sometimes be cheaper than immutable values:
- For a constant variable, the expression assigned to it is copied to all the places where it is accessed and also re-evaluated each time, allowing local optimizations.
- Immutable variables are evaluated once at construction time and their value is copied to all the places in the code where they are accessed. For these values, 32 bytes are reserved, even if they would fit in fewer bytes.
#### mappings are cheaper than Arrays
- avoid dynamically sized arrays
- An array is not stored sequentially in memory but as a mapping.
- You can pack Arrays but not Mappings.
- It’s cheaper to use arrays if you are using smaller elements like `uint8` which can be packed together.
- You can’t get the length of a mapping or parse through all its elements, so depending on your use case, you might be forced to use an Array even though it might cost you more gas.
#### use bytes32 rather than string/bytes
- If you can fit your data in 32 bytes, then you should use bytes32 datatype rather than bytes or strings as it is much cheaper in solidity.
- Any fixed size variable in solidity is cheaper than variable size.
#### modifiers
- For all the public functions, the input parameters are copied to memory automatically, and it costs gas.
- If your function is only called externally, then you should explicitly mark it as external.
- External function’s parameters are not copied into memory but are read from `calldata` directly.
- internal and private are both cheaper than public and external when called from inside the contract in some cases.
#### no need to initialize variables with default values
- If a variable is not set/initialized, it is assumed to have the default value (0, false, 0x0 etc depending on the data type). If you explicitly initialize it with its default value, you are just wasting gas.
```
uint256 hello = 0; //bad, expensive
uint256 world; //good, cheap
```
#### make use of single line swaps
- This is space-efficient:
```
(hello, world) = (world, hello)
```
#### negative gas costs
- Deleting a contract (SELFDESTRUCT) is worth a refund of 24,000 gas.
- Changing a storage address from a nonzero value to zero (SSTORE[x] = 0) is worth a refund of 15,000 gas.
<br>
---
### resources
<br>
* [truffle contract size](https://github.com/IoBuilders/truffle-contract-size)
* [foundry book on gas](https://book.getfoundry.sh/forge/gas-reports)
* [solidity gas optimizations](https://mirror.xyz/haruxe.eth/DW5verFv8KsYOBC0SxqWORYry17kPdeS94JqOVkgxAA)
* [hardhat on gas optimization](https://medium.com/@thelasthash/%EF%B8%8F-gas-optimization-with-hardhat-1e553eaea311)
* [resources for gas optimization](https://github.com/kadenzipfel/gas-optimizations)
* [awesome solidity gas optimization](https://github.com/iskdrews/awesome-solidity-gas-optimization)

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## solidity tl;dr
<br>
### ethereum contracts
<br>
* until [account abstraction](https://github.com/go-outside-labs/mev-toolkit/tree/main/MEV_by_chains/MEV_on_Ethereum/account_abstraction) becomes a thing, there are two types of accounts, which are identified by an address of **160-bit length** (rightmost 160 bits of the **Keccak hash** of the RLP encoding of the structure with the sender and the nonce), and contain a **balance** (in wei), a **nonce** (number of tx made by the account), a **bytecode** (hash), and **stored data** (keccak hash of the root note od the storage trie). while **external accounts have a private key** and their code and storage are empty, **contract accounts store their bytecode** (and merkle root hash of the entire state tree).
<br>
<img width="516" src="https://user-images.githubusercontent.com/1130416/219830838-01ce01c8-e818-403e-8a7a-2dbcff68a7bc.png">
<br>
* the **creation of a contract** is a transaction where the **receiver address is empty** and its **data field contains compiled bytecode** (or calling `CREATE` opcode. the data sent is executed as bytecode, initializing the state variables in storage and determining the body of the contract being created. **contract memory** is a byte array, where data can be stored in 32 bytes (256 bit) or 1 byte (8 bit) chunks, reading in 32 bytes chunks (through `MSTORE`, `MLOAD`, `MSTORE8`).
<br>
<img width="516" src="https://user-images.githubusercontent.com/1130416/219829883-c94c0a80-1101-462e-99fa-afbf7feb2b57.png">
<br>
* contracts can call contracts through **message calls** (`CALL` opcode). every call has a **sender**, a **recipient**, a **payload** (data), a **value** (in wei), and some **gas**. a variant is `DELEGATECALL`, where target code is executed within the context of the calling contract, and `msg.sender` and `msg.value` do not change (the contract can dynamically load code - storage - from a different address at runtime - while current address and balance still refer to the calling contract).
<br>
------
### predefined global variables and functions
<br>
* when a contract is executed in the EVM, it has access to a small set of global objects: `block`, `msg`, and `tx` objects. in addition, solidity exposes a number of EVM opcodes as predefined functions:
<br>
##### msg
* `msg object`: the transaction that triggered the execution of the contract.
* `msg.sender`: sender address of the transaction.
* `msg.value`: ether sent with this call (in wei).
* `msg.data`: data payload of this call into our contract.
* `msg.sig`: first four bytes of the data payload, which is the function selector.
<br>
##### tx
* `tx.gasprice`: gas price in the calling transaction.
* `tx.origin`: address of the originating EOA for this transaction. WARNING: unsafe!
<br>
##### block
* `block.coinbase`: address of the recipient of the current block's fees and block reward.
* `block.gaslimit`: maximum amount of gas that can be spent across all transactions included in the current block.
* `block.number`: current block number (blockchain height).
* `block.timestamp`: timestamp placed in the current block by the miner (number of seconds since the Unix epoch).
<br>
##### address
* `address.balance`: balance of the address, in wei.
* `address.transfer(__amount__)`: transfers the amount (in wei) to this address, throwing an exception on any error.
* `address.send(__amount__)`: similar to transfer, only instead of throwing an exception, it returns false on error. WARNING: always check the return value of send.
* `address.call(__payload__)`: low-level `CALL` function—can construct an arbitrary message call with a data payload. Returns false on error. WARNING: unsafe.
* `address.delegatecall(__payload__)`: low-level `DELEGATECALL` function, like `callcode(...)` but with the full msg context seen by the current contract. Returns false on error. WARNING: advanced use only!
<br>
##### built-in functions
* `addmod`, `mulmod`: for modulo addition and multiplication. for example, `addmod(x,y,k)` calculates `(x + y) % k`.
* `keccak256`, `sha256`, `sha3`, `ripemd160`: calculate hashes with various standard hash algorithms.
* `ecrecover`: recovers the address used to sign a message from the signature.
* `selfdestruct(__recipient_address__)`: deletes the current contract, sending any remaining ether in the account to the recipient address.
* `this`: address of the currently executing contract account.
* [list of precompiled contracts](https://www.evm.codes/precompiled?fork=arrowGlacier)
<br>
##### what is considered modifying state
- writing to state variables
- emitting events
- creating other contracts
- sending ether via calls
- using selfdestruct
- using low-level calls
- calling any function not marked view or pure
- using inline assembly that contains certain opcodes
<br>
---
### solidity vs. python/js/c++
<br>
from python, we get:
- modifiers
- multiple inheritances
from js we get:
- function-level scoping
- the `var` keyword
from c/c++ we get:
- scoping: variables are visible from the point right after their declaration until the end of the smallest {}-block that contains the declaration.
- the good ol' value types (passed by value, so they are alway copied to the stack) and reference types (references to the same underlying variable).
- however, look how cool: a variable that is declared will have an initial default value whose byte-representation is all zeros.
- int and uint integers, with uint8 to uint256 in step of 8.
from being statically-typed:
- the type of each variable (local and state) needs to be specified at compile-time (as opposed to runtime).
<br>
you start files with the `SPDX License Identifier (`// SPDX-License-Identifier: MIT`)`. SPDX stands for software package data exchange. The compiler will include this in the bytecode metadata and make it machine readable.
<br>
**pragmas:** directives that are used to enable certain compiler features and checks.
version Pragma indicates the specific Solidity compiler version. It does not change the version of the compiler, though, so yeah, you will get an error if it does not match the compiler.
other types are Compiler version, ABI coder version, SMTCheker.
<br>
The best-practices for layout in a contract are:
1. state variables
2. events
3. modifiers
4. constructors
5. functions
<br>
**natspec comments**: Also known as the "ethereum natural language specification format". Written as triple slashes (`///`) or double asterisk block
`(/**...*/)`, directly above function declarations or statements to generate documentation in `JSON` format for developers and end-users. These are some tags:
* `@title`: describe the contract/interface
* `@author`
* `@notice`: explain to an end user what it does
* `@dev`: explain to a dev
* `@param`: document params
* `@return`: any returned variable
* `@inheritdoc`: copies missing tags from the base function (must be followed by contract name)
* `@custom`: anything application-defined
<br>
**events**: an abstraction on top of EVM's logging: emitting events cause the arguments to be stored in the transaction's log (which are associated with the address of the contract). events are emitted using **emit**.
events are especially useful for light clients and DApp services, which can "watch" for specific events and report them to the user interface, or make a change in the state of the application to reflect an event in an underlying contract.
<br>
---
### type of variables
<br>
**address types**. the address type comes in two types:
1. holds a 20 byte value (the size of an Ethereum address)
2. address payable: with additional members transfer and send. address payable is an address you can send Ether to (while plain address not).
explicit conversion from address to address payable can be done with `payable()`.
explicit conversion from or to address is allowed for `uint160`, integer literals, `byte20`, and contract types
the members of address type are pretty interesting: `.balance`, `.code`, `.codehash`, `.transfer`, `.send`, `.call`, `.delegatecall`, `.staticcall`.
<br>
**fixed-size Byte Arrays**: bytes1, bytes2, bytes3, …, bytes32 hold a sequence of bytes from one to up to 32. The type `byte[]` is an array of bytes, but due to padding rules, it wastes 31 bytes of space for each element, so it's better to use `bytes()`
<br>
**state variables**: variables that can be accessed by all functions of the contract and values are permanently stored in the contract storage.
**state visibility specifiers**: these are state variables that define how the methods will be accessed:
- `public`: part of the contract interface and can be accessed internally or via messages.
- `external`: like public functions, but cannot be called within the contract.
- `internal`: can only be accessed internally from within the current contracts (or contracts deriving from it).
- `private`: can only be accessed from the contract they are defined in and not in derived contracts.
- `pure`: neither reads nor writes any variables in storage. It can only operate on arguments and return data, without reference to any stored data. Pure functions are intended to encourage declarative-style programming without side effects or state.
- `payable`: can accept incoming payments. Functions not declared as payable will reject incoming payments. There are two exceptions, due to design decisions in the EVM: coinbase payments and `SELFDESTRUCT` inheritance will be paid even if the fallback function is not declared as payable.
**immutability**: state variables can be declared as constant or immutable, so they cannot be modified after the contract has been constructed. their difference is beautiful:
**for constant variables, the value is fixed at compile-time; for immutable variables, the value can still be assigned at construction time (in the constructor or point of declation)**
there is an entire gas cost thing too. For constant variables, the expression assigned is copied to all the places, and re-evaluated each time (local optimizations are possible). For immutable variables, the expression is evaluated once at constriction time and their value is copied to all the places in the code they are accessed, on a reserved 32 bytes, becoming usually more expensive than constant.
<br>
---
### functions
<br>
**functions modifiers**: used to change the behavior of functions in a declarative way, so that the function's control flow continues after the "_" in the preceding modifier. This symbol can appear in the modifier multiple times.
the underscore followed by a semicolon is a placeholder that is replaced by the code of the function that is being modified. Essentially, the modifier is "wrapped around" the modified function, placing its code in the location identified by the underscore character.
to apply a modifier, you add its name to the function declaration. More than one modifier can be applied to a function; they are applied in the sequence they are declared, as a space-separated list.
```
function destroy() public onlyOwner {
```
<br>
**function visibility specifiers**: these are how visibility works for functions:
- `public`: part of the contract interface and can be either called internally or via messages.
- `external`: part of the contract interface, and can be called from other contracts and via transactions. Here is the interesting part: an external function `func` cannot be called internally, so `func()` would not work. But `this.func()` does.
- `internal`: can only be accessed from within the current contract or contracts deriving from it.
- `private`: can only be accessed from the contract they are defined in and not even in derived contracts
<br>
**function mutability specifiers**:
- `view` functions can read the contract state but not modify it: enforced at runtime via STATICALL opcode.
- `pure` functions can neither read a contract nor modify it.
- only view can be enforced at the EVM level, not pure.
<br>
**overloading**: a contract can have multiple functions of the same name but with different parameter types. they are matched by the arguments supplied in the function call 😬.
<br>
---
### data structures
<br>
- `structs`: custom-defined types that can group several variables of same/different types together to create a custom data structure.
- `enums`: used to create custom types with a finite set of constants values. Cannot have more than 256 members.
<br>
**constructors**: when a contract is created, the function with *constructor* is executed once and then the final code of the contract is stored on the blockchain (all public and external functions, but not the constructor code or internal functions called by it).
<br>
**receive function**: a contract can have ONE *receive* function (*receive() external payable {...}*) without the function keyword, and no arguments and no return and... have `external` and `payable`. this is the function on plain ether transfers via `send()` or `transfer()`.
interesting facts:
- receive is executed on a call to the contract with empty calldata.
- receive might only rely on 2300 gas being available.
- a contract without Receive can actually receive Ether as a recipient of a coinbase transaction (miner block reward) or as a destination of `selfdestruct`.
- a contract cannot react to the Ether transfer above.
<br>
**falback function**: kinda in the same idea, a contract can have ONE *fallback* function, which must have external visibility.
- fallback is executed on a call to the contract if none of the other functions match the given function signature or no data was supplied and there is not receive Ether function.
<br>
**transfer:** the transfer function fails if the balance of the contract is not enough or if the transfer is rejected by the receiving account, revering on failure.
<br>
**send:** low-level counterpart of transfer, however, if the execution fails then send only returns false (return value must be checked by the caller).
<br>
----
### data management
<br>
* the evm manages different kinds of data depending on their context:
<br>
* **stack**: the evm operates on a virtual stack, which has a maximum size of 1024, stack items have a size of 256 bits (the evm is a 256-bit word machine, which facilitates keccak256 hash scheme and elliptic-curve). the opcodes to modify the stack are: `POP` (remove from stack), `PUSH n` (places the `n` butes item into the stack), `DUP n` (duplicates the `n`th stack item), `SWAP n` (exchanges the first and the `n`th stack item).
* **calldata**: read-only byte-addressable space where the data parameter of a tx or call is held. unlike the stack, to use this data, you have to specify an exact byte offset and number of bytes to read. the opcodes include: `CALLDATASIZE` (get size of tx data), `CALLDATALOAD` (loads 32 byte of tx data onto the stack), `CALLDATACOPY` (copies the number of bytes of the tx data to memory). there are also the inline assembly versions: `calldatasize`, `calldataload`, calldatacopy`. they can be called through:
```
assembly {
}
```
* **memory**: volatile read-write byte-addressable space (store data during execution) initiallized as zero. the evm opcodes are `MLOAD` (loads a word into the stack), `MSTORE` (saves a word to memory), `MSTORE8` (saves a byte to memory). gas costs since memory loads (MLOADs) are significantly cheaper in gas than SLOADs.
* **storage**: persistant read-write word-addressable space for contracts, addressed by words. it's a key-value mapping of 2**256 slots of 32 bytes each. gas to save data into storage is one of the highest operations. the evm opcodes are: `SLOAD` (loads a word from storage to stack), `SSTORE` (saves a word to storage).
* bitpack loading: storing multiple variables in a single 32-byts slot by ordering the byte size
* fixed-length arrays: takes a predetermined amount of slots.
* dynamic-length arrays: new elements assign slots after deployment (handled by the evm with keccak256 hashing)
* mappings: dynamic type with key hashes
<br>
----
### calling another contract
<br>
**call/delegatecall/ataticall**: ued to interface with contracts that do not adhere to ABI, or to give more direct control over encoding. they all take a single bytes memory parameter and return the success condition (as a bool) and the return data (byte memory).
with `DELEGATECALL`, only the code of the given address is used but all other aspects are taken from the current contract. The purpose is to use logic code that is stored in the callee contract but operates on the state of the caller contract.
with `STATCALL`, the execution will revert if the called function modifies the state in any way.
<br>
**creating a new instance**:
* the safest way to call another contract is if you create that other contract yourself.
* to do this, you can simply instantiate it, using the keyword `new`, as in other object-oriented languages. This keyword will create the contract on the blockchain and return an object that you can use to reference it.
```
contract Token is Mortal {
Faucet _faucet;
constructor() {
_faucet = new Faucet();
}
}
```
<br>
**addressing an existing instance**:
* another way you can call a contract is by casting the address of an existing instance of the contract.
* with this method, you apply a known interface to an existing instance.
* this is much riskier than the previous mechanism, because we don’t know for sure whether that address actually is a faucet object.
```
import "Faucet.sol";
contract Token is Mortal {
Faucet _faucet;
constructor(address _f) {
_faucet = Faucet(_f);
_faucet.withdraw(0.1 ether);
}
}
```
<br>
----
### block and tx properties**
- `blockhash`
- `block.chainid`
- `block.coinbase`
- `block.difficulty`
- `block.gaslimit`
- `block.number`
- `block.timestamp`
- `msg.data`
- `msg.sender`
- `msg.sig`
- `msg.value`
- `tx.gasprice`
- `gasleft`
- `tx.origin`
<br>
**randomness**. Not cute shit: you cannot rely on block.timestamp or blockhash as a source of randomness, as they can be influenced by miners to some degree.
<br>
---
### ABI encoding and decoding functions
<br>
- `abi.decode`
- `abi.encode`
- `abi.encodePacked`
- `abi.encodeWithSelector`
- `abi.encodeWithSignature`
<br>
----
### error handling
<br>
- `assert()`: causes a panic error and reverts if the condition is not met
- `require()`: reverts if the condition is not met
- `revert()`: abort execution and revert state changes

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## ethereum token standards
### tl; dr
* EIP stands for Ethereum Improvement Proposals.
* ERC stands for Ethereum request for comments (technical documents written by Ethereum developers for Ethereum community).
* Each such document contains a set of rules required to implement tokens for the Ethereum ecosystem.
<br>
---
### erc-20
* In the case of ERC20, a transaction sending ether to an address changes the state of an address.
- a transaction transferring a token to an address only changes the state of the token contract, not the state of the recipient address.
* one of the main reasons for the success of EIP-20 tokens is in the interplay between `approve` and `transferFrom`, which allows for tokens to not
only be transferred between externally owned accounts (EOA).
- but to be used in other contracts under application specific conditions by abstracting away `msg.sender` as the mechanism for token access control.
* a limiting factor lies from the fact that the EIP-20 `approve` function is defined in terms of `msg.sender`.
- this means that user’s initial action involving EIP-20 tokens must be performed by an EOA.
- if the user needs to interact with a smart contract, then they need to make 2 transactions (`approve` and the smart contract internal call `transferFrom`), with gas costs.
<br>
---
### in this dir
* [ERC20](erc20.md)
* [ERC777](erc777.md)
* [ERC721](erc721.md)
<br>
---
### resources

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## ERC20
<br>
* The ERC20 standard defines a common interface for contracts implementing this token, such that any compatible token can be accessed and used in the same way.
<br>
### ERC20-compliant token contract
* totalSupply
Returns the total units of this token that currently exist. ERC20 tokens can have a fixed or a variable supply.
* balanceOf
Given an address, returns the token balance of that address.
* transfer
Given an address and amount, transfers that amount of tokens to that address, from the balance of the address that executed the transfer.
* transferFrom
Given a sender, recipient, and amount, transfers tokens from one account to another. Used in combination with approve.
* approve
Given a recipient address and amount, authorizes that address to execute several transfers up to that amount, from the account that issued the approval.
* allowance
Given an owner address and a spender address, returns the remaining amount that the spender is approved to withdraw from the owner.
* Transfer
Event triggered upon a successful transfer (call to transfer or transferFrom) (even for zero-value transfers).
* Approval
Event logged upon a successful call to approve.
<br>
---
### ERC20 optional functions
In addition to the required functions listed in the previous section, the following optional functions are also defined by the standard:
* name
Returns the human-readable name (e.g., "US Dollars") of the token.
* symbol
Returns a human-readable symbol (e.g., "USD") for the token.
* decimals
Returns the number of decimals used to divide token amounts. For example, if decimals is 2, then the token amount is divided by 100 to get its user representation.
<br>
---
### The ERC20 interface defined in Solidity
```
contract ERC20 {
function totalSupply() constant returns (uint theTotalSupply);
function balanceOf(address _owner) constant returns (uint balance);
function transfer(address _to, uint _value) returns (bool success);
function transferFrom(address _from, address _to, uint _value) returns
(bool success);
function approve(address _spender, uint _value) returns (bool success);
function allowance(address _owner, address _spender) constant returns
(uint remaining);
event Transfer(address indexed _from, address indexed _to, uint _value);
event Approval(address indexed _owner, address indexed _spender, uint _value);
}
```

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## Remix IDE
<br>
Remix IDE is an open source web3 application and it's used for the entire journey of smart contract development.
<br>
<img width="414" alt="Screen Shot 2022-03-10 at 5 57 22 PM" src="https://user-images.githubusercontent.com/1130416/157715032-63dfbe5d-292d-48e3-8594-04902fb008f6.png">
<br>
* Everything in Remix is a plugin. The plugin mamanger is the place to load functionalities and create your own plugins.
* By default, Remix stores files in Workspaces, which are folders in the browser's local storage.
* You can publish all files from current workspace to a gist, using the Gist API.
<br>
#### Compiler (Solidity)
* You can compile (and deploy) contracts with versions of Solidity older than 0.4.12. However, the older compilers used a legacy AST.
* The "fork selection" dropdown list allows to compile code against a specific ehtereum hard fork.
<br>
#### Optimization
* The optimizer tries to simplify complicated expressions, which reduces both code size and execution cost. It can reduce gas needed for contract deployment as well as for external calls made to the contract.
<br>
#### Environment
* `JavaScript VM`: All transactions will be executed in a sandbox blockchain in the browser.
* `Injected Provider`: Metamaask is an example of a profiver that inject web3.
* `Web3 Provider`: Remix will connect to a remote node (you need to provide the URL to the selected provider: geth, parity or any ethereum client)
<br>
#### Setup
* Gas Limit: sets the amount of ETH, WEI, GWEI that is sent to ta contract or a payable function.
* Deploy: sends a transaction that deplpys the selected contract.
* atAdress: used to access a contract whtat has already been deployed (does not cost gas).
* To interact with a contract using the ABI, create a new file in Remix, with extension `.abi`.
* The Recorder is a tool used to save a bunch of transactions in a JSON file and rerun them later either in the same environment or in another.
* The Debugger shows the contract's state while stepping through a transaction.
* Using generated sources will make it easier to audit your contracts.
* Static code analysis can be done by a plugin, so that you can examine the code for security vulnerabilities, bad development practices, etc.
* Hardhat integration can be done with `hardhat.config.js` (Hardhat websocket listener should run at `65522`). Hardhat provider is a plugin for Remix IDE.
<br>
#### Generate artifacts
When a compilation for a Solidity file succeeds, Remix creates three Json files for each compiled contract, that can be seen in the `File Explorers plugin`:
1. `artifacts/<contractName>.json`: contains links to libraries, the bytecode, gas estimation, the ABI.
2. `articfacts/<contractName_metadata>.json`: contains the metadata from the output of Solidity compilation.
3. `artifcats/build-info/<dynamic_hash>.json`: contains info about `solc` compiler version, compiler input and output.