pragma solidity 0.6.6;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

import "../interfaces/IERC20Permit.sol";

/// @dev https://eips.ethereum.org/EIPS/eip-2612
contract ERC20Permit is ERC20, IERC20Permit {
    /// @dev To make etherscan auto-verify new pool, this variable is not immutable
    bytes32 public domainSeparator;
    // keccak256("Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)");
    bytes32
        public constant PERMIT_TYPEHASH = 0x6e71edae12b1b97f4d1f60370fef10105fa2faae0126114a169c64845d6126c9;

    mapping(address => uint256) public nonces;

    constructor(
        string memory name,
        string memory symbol,
        string memory version
    ) public ERC20(name, symbol) {
        uint256 chainId;
        assembly {
            chainId := chainid()
        }
        domainSeparator = keccak256(
            abi.encode(
                keccak256(
                    "EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"
                ),
                keccak256(bytes(name)),
                keccak256(bytes(version)),
                chainId,
                address(this)
            )
        );
    }

    function permit(
        address owner,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external override {
        require(deadline >= block.timestamp, "ERC20Permit: EXPIRED");
        bytes32 digest = keccak256(
            abi.encodePacked(
                "\x19\x01",
                domainSeparator,
                keccak256(
                    abi.encode(PERMIT_TYPEHASH, owner, spender, value, nonces[owner]++, deadline)
                )
            )
        );
        address recoveredAddress = ecrecover(digest, v, r, s);
        require(
            recoveredAddress != address(0) && recoveredAddress == owner,
            "ERC20Permit: INVALID_SIGNATURE"
        );
        _approve(owner, spender, value);
    }
}