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pragma solidity =0.5.16;

import './interfaces/IUniswapV2ERC20.sol';
import './libraries/SafeMath.sol';

contract UniswapV2ERC20 is IUniswapV2ERC20 {
using SafeMath for uint;

string public constant name = 'Uniswap V2';
string public constant symbol = 'UNI-V2';
uint8 public constant decimals = 18;
uint  public totalSupply;
mapping(address => uint) public balanceOf;
mapping(address => mapping(address => uint)) public allowance;

bytes32 public DOMAIN_SEPARATOR;
// keccak256("Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)");
bytes32 public constant PERMIT_TYPEHASH = 0x6e71edae12b1b97f4d1f60370fef10105fa2faae0126114a169c64845d6126c9;
mapping(address => uint) public nonces;

event Approval(address indexed owner, address indexed spender, uint value);
event Transfer(address indexed from, address indexed to, uint value);

constructor() public {
    uint chainId;
    assembly {
        chainId := chainid
    }
    DOMAIN_SEPARATOR = keccak256(
        abi.encode(
            keccak256('EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)'),
            keccak256(bytes(name)),
            keccak256(bytes('1')),
            chainId,
            address(this)
        )
    );
}

function _mint(address to, uint value) internal {
    totalSupply = totalSupply.add(value);
    balanceOf[to] = balanceOf[to].add(value);
    emit Transfer(address(0), to, value);
}

function _burn(address from, uint value) internal {
    balanceOf[from] = balanceOf[from].sub(value);
    totalSupply = totalSupply.sub(value);
    emit Transfer(from, address(0), value);
}

function _approve(address owner, address spender, uint value) private {
    allowance[owner][spender] = value;
    emit Approval(owner, spender, value);
}

function _transfer(address from, address to, uint value) private {
    balanceOf[from] = balanceOf[from].sub(value);
    balanceOf[to] = balanceOf[to].add(value);
    emit Transfer(from, to, value);
}

function approve(address spender, uint value) external returns (bool) {
    _approve(msg.sender, spender, value);
    return true;
}

function transfer(address to, uint value) external returns (bool) {
    _transfer(msg.sender, to, value);
    return true;
}

function transferFrom(address from, address to, uint value) external returns (bool) {
    if (allowance[from][msg.sender] != uint(-1)) {
        allowance[from][msg.sender] = allowance[from][msg.sender].sub(value);
    }
    _transfer(from, to, value);
    return true;
}

function permit(address owner, address spender, uint value, uint deadline, uint8 v, bytes32 r, bytes32 s) external {
    require(deadline >= block.timestamp, 'UniswapV2: EXPIRED');
    bytes32 digest = keccak256(
        abi.encodePacked(
            '\x19\x01',
            DOMAIN_SEPARATOR,
            keccak256(abi.encode(PERMIT_TYPEHASH, owner, spender, value, nonces[owner]++, deadline))
        )
    );
    address recoveredAddress = ecrecover(digest, v, r, s);
    require(recoveredAddress != address(0) && recoveredAddress == owner, 'UniswapV2: INVALID_SIGNATURE');
    _approve(owner, spender, value);
}

}