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move from invariant -> k (compute sqrt lazily)
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use getReserves more places

improve comment quality
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@NoahZinsmeister
NoahZinsmeister committed Jan 29, 2020
1 parent 452b531 commit 80ce1e4
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Showing 3 changed files with 45 additions and 48 deletions.
88 changes: 42 additions & 46 deletions contracts/UniswapV2Exchange.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -17,18 +17,13 @@ contract UniswapV2Exchange is IUniswapV2Exchange, UniswapV2ERC20 {
address public token0;
address public token1;

uint112 private reserve0; // single storage slot, (jointly) access via getReserves
uint112 private reserve1; // single storage slot, (jointly) access via getReserves
uint32 private blockTimestampLast; // single storage slot, (jointly) access via getReserves
uint112 private reserve0; // uses single storage slot, accessible via getReserves
uint112 private reserve1; // uses single storage slot, accessible via getReserves
uint32 private blockTimestampLast; // uses single storage slot, accessible via getReserves

uint public price0CumulativeLast;
uint public price1CumulativeLast;
uint public invariantLast;

event Mint(address indexed sender, uint amount0, uint amount1);
event Burn(address indexed sender, uint amount0, uint amount1, address indexed to);
event Swap(address indexed sender, address indexed tokenIn, uint amountIn, uint amountOut, address indexed to);
event Sync(uint112 reserve0, uint112 reserve1);
uint public kLast; // reserve0 * reserve1, as of immediately after the most recent liquidity event

bool private unlocked = true;
modifier lock() {
Expand All @@ -38,30 +33,35 @@ contract UniswapV2Exchange is IUniswapV2Exchange, UniswapV2ERC20 {
unlocked = true;
}

function getReserves() public view returns (uint112 _reserve0, uint112 _reserve1, uint32 _blockTimestampLast) {
_reserve0 = reserve0;
_reserve1 = reserve1;
_blockTimestampLast = blockTimestampLast;
}

function _safeTransfer(address token, address to, uint value) private {
(bool success, bytes memory data) = token.call(abi.encodeWithSelector(selector, to, value));
require(success && (data.length == 0 || abi.decode(data, (bool))), 'UniswapV2: TRANSFER_FAILED');
}

event Mint(address indexed sender, uint amount0, uint amount1);
event Burn(address indexed sender, uint amount0, uint amount1, address indexed to);
event Swap(address indexed sender, address indexed tokenIn, uint amountIn, uint amountOut, address indexed to);
event Sync(uint112 reserve0, uint112 reserve1);

constructor() public {
factory = msg.sender;
}

function initialize(address _token0, address _token1) external {
require(msg.sender == factory, 'UniswapV2: FORBIDDEN');
require(msg.sender == factory, 'UniswapV2: FORBIDDEN'); // sufficient check
token0 = _token0;
token1 = _token1;
}

function getReserves() external view returns (uint112 _reserve0, uint112 _reserve1, uint32 _blockTimestampLast) {
_reserve0 = reserve0;
_reserve1 = reserve1;
_blockTimestampLast = blockTimestampLast;
}

// update reserves and, on the first time this function is called per block, price accumulators
// update reserves and, on the first call per block, price accumulators
function _update(uint balance0, uint balance1, uint112 _reserve0, uint112 _reserve1) private {
require(balance0 <= uint112(-1) && balance1 <= uint112(-1), 'UniswapV2: BALANCE_OVERFLOW');
require(balance0 <= uint112(-1) && balance1 <= uint112(-1), 'UniswapV2: OVERFLOW');
uint32 blockTimestamp = uint32(block.timestamp % 2**32);
uint32 timeElapsed = blockTimestamp - blockTimestampLast; // overflow is desired
if (timeElapsed > 0 && _reserve0 != 0 && _reserve1 != 0) {
Expand All @@ -75,60 +75,57 @@ contract UniswapV2Exchange is IUniswapV2Exchange, UniswapV2ERC20 {
emit Sync(reserve0, reserve1);
}

// mint liquidity equivalent to 20% of newly accumulated fees
// if fee is on, mint liquidity equivalent to 20% of growth in sqrt(k)
function _mintFee(uint112 _reserve0, uint112 _reserve1) private returns (bool feeOn) {
address feeTo = IUniswapV2Factory(factory).feeTo();
feeOn = feeTo != address(0);
if (feeOn) {
uint _invariantLast = invariantLast; // gas savings
if (_invariantLast != 0) {
uint invariant = Math.sqrt(uint(_reserve0).mul(_reserve1));
if (invariant > _invariantLast) {
uint numerator = totalSupply.mul(invariant.sub(_invariantLast));
uint denominator = invariant.mul(4).add(_invariantLast);
uint _kLast = kLast; // gas savings
if (_kLast > 0) {
uint rootK = Math.sqrt(uint(_reserve0).mul(_reserve1));
uint rootKLast = Math.sqrt(_kLast);
if (rootK > rootKLast) {
uint numerator = totalSupply.mul(rootK.sub(rootKLast));
uint denominator = rootK.mul(4).add(rootKLast);
uint liquidity = numerator / denominator;
if (liquidity > 0) _mint(feeTo, liquidity);
}
}
}
}

// mint liquidity
function mint(address to) external lock returns (uint liquidity) {
uint112 _reserve0 = reserve0; // gas savings
uint112 _reserve1 = reserve1; // gas savings
(uint112 _reserve0, uint112 _reserve1,) = getReserves(); // gas savings
uint balance0 = IERC20(token0).balanceOf(address(this));
uint balance1 = IERC20(token1).balanceOf(address(this));
uint amount0 = balance0.sub(_reserve0);
uint amount1 = balance1.sub(_reserve1);

bool feeOn = _mintFee(_reserve0, _reserve1);
uint _totalSupply = totalSupply; // gas savings
uint _totalSupply = totalSupply; // gas savings, must be defined here since totalSupply can update in _mintFee
liquidity = _totalSupply == 0
? Math.sqrt(amount0.mul(amount1))
: Math.min(amount0.mul(_totalSupply) / _reserve0, amount1.mul(_totalSupply) / _reserve1);
require(liquidity > 0, 'UniswapV2: INSUFFICIENT_LIQUIDITY_MINTED');
_mint(to, liquidity);

_update(balance0, balance1, _reserve0, _reserve1);
if (feeOn) invariantLast = Math.sqrt(uint(reserve0).mul(reserve1));
if (feeOn) kLast = uint(reserve0).mul(reserve1);
emit Mint(msg.sender, amount0, amount1);
}

// burn liquidity
function burn(address to) external lock returns (uint amount0, uint amount1) {
uint112 _reserve0 = reserve0; // gas savings
uint112 _reserve1 = reserve1; // gas savings
address _token0 = token0; // gas savings
address _token1 = token1; // gas savings
(uint112 _reserve0, uint112 _reserve1,) = getReserves(); // gas savings
address _token0 = token0; // gas savings
address _token1 = token1; // gas savings
uint balance0 = IERC20(_token0).balanceOf(address(this));
uint balance1 = IERC20(_token1).balanceOf(address(this));
uint liquidity = balanceOf[address(this)];

bool feeOn = _mintFee(_reserve0, _reserve1);
uint _totalSupply = totalSupply; // gas savings
amount0 = liquidity.mul(balance0) / _totalSupply; // use balances instead of reserves to address force sends
amount1 = liquidity.mul(balance1) / _totalSupply; // use balances instead of reserves to address force sends
uint _totalSupply = totalSupply; // gas savings, must be defined here since totalSupply can update in _mintFee
amount0 = liquidity.mul(balance0) / _totalSupply; // using balances ensures pro-rata distribution
amount1 = liquidity.mul(balance1) / _totalSupply; // using balances ensures pro-rata distribution
require(amount0 > 0 && amount1 > 0, 'UniswapV2: INSUFFICIENT_LIQUIDITY_BURNED');
_burn(address(this), liquidity);
_safeTransfer(_token0, to, amount0);
Expand All @@ -137,22 +134,21 @@ contract UniswapV2Exchange is IUniswapV2Exchange, UniswapV2ERC20 {
balance1 = IERC20(_token1).balanceOf(address(this));

_update(balance0, balance1, _reserve0, _reserve1);
if (feeOn) invariantLast = Math.sqrt(uint(reserve0).mul(reserve1));
if (feeOn) kLast = uint(reserve0).mul(reserve1);
emit Burn(msg.sender, amount0, amount1, to);
}

// swap tokens
function swap(address tokenIn, uint amountOut, address to) external lock {
uint112 _reserve0 = reserve0; // gas savings
uint112 _reserve1 = reserve1; // gas savings
address _token0 = token0; // gas savings
address _token1 = token1; // gas savings
require(amountOut > 0, 'UniswapV2: INSUFFICIENT_OUTPUT_AMOUNT');
(uint112 _reserve0, uint112 _reserve1,) = getReserves(); // gas savings
address _token0 = token0; // gas savings
address _token1 = token1; // gas savings
uint balance0;
uint balance1;
uint amountIn;

if (tokenIn == _token0) {
require(0 < amountOut && amountOut < _reserve1, 'UniswapV2: INVALID_OUTPUT_AMOUNT');
require(amountOut < _reserve1, 'UniswapV2: INSUFFICIENT_LIQUIDITY');
balance0 = IERC20(_token0).balanceOf(address(this));
amountIn = balance0.sub(_reserve0);
require(amountIn > 0, 'UniswapV2: INSUFFICIENT_INPUT_AMOUNT');
Expand All @@ -161,7 +157,7 @@ contract UniswapV2Exchange is IUniswapV2Exchange, UniswapV2ERC20 {
balance1 = IERC20(_token1).balanceOf(address(this));
} else {
require(tokenIn == _token1, 'UniswapV2: INVALID_INPUT_TOKEN');
require(0 < amountOut && amountOut < _reserve0, 'UniswapV2: INVALID_OUTPUT_AMOUNT');
require(amountOut < _reserve0, 'UniswapV2: INSUFFICIENT_LIQUIDITY');
balance1 = IERC20(_token1).balanceOf(address(this));
amountIn = balance1.sub(_reserve1);
require(amountIn > 0, 'UniswapV2: INSUFFICIENT_INPUT_AMOUNT');
Expand Down
4 changes: 2 additions & 2 deletions contracts/UniswapV2Factory.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -25,15 +25,15 @@ contract UniswapV2Factory is IUniswapV2Factory {
require(tokenA != tokenB, 'UniswapV2: IDENTICAL_ADDRESSES');
(address token0, address token1) = tokenA < tokenB ? (tokenA, tokenB) : (tokenB, tokenA);
require(token0 != address(0), 'UniswapV2: ZERO_ADDRESS');
require(getExchange[token0][token1] == address(0), 'UniswapV2: EXCHANGE_EXISTS');
require(getExchange[token0][token1] == address(0), 'UniswapV2: EXCHANGE_EXISTS'); // single check is sufficient
bytes memory exchangeBytecode = type(UniswapV2Exchange).creationCode;
bytes32 salt = keccak256(abi.encodePacked(token0, token1));
assembly {
exchange := create2(0, add(exchangeBytecode, 32), mload(exchangeBytecode), salt)
}
IUniswapV2Exchange(exchange).initialize(token0, token1);
getExchange[token0][token1] = exchange;
getExchange[token1][token0] = exchange;
getExchange[token1][token0] = exchange; // populate mapping in the reverse direction
allExchanges.push(exchange);
emit ExchangeCreated(token0, token1, exchange, allExchanges.length);
}
Expand Down
1 change: 1 addition & 0 deletions contracts/interfaces/IUniswapV2Exchange.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@ interface IUniswapV2Exchange {
function getReserves() external view returns (uint112 reserve0, uint112 reserve1, uint32 blockTimestampLast);
function price0CumulativeLast() external view returns (uint);
function price1CumulativeLast() external view returns (uint);
function kLast() external view returns (uint);

function mint(address to) external returns (uint liquidity);
function burn(address to) external returns (uint amount0, uint amount1);
Expand Down

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