abstract contract ERC20 is IERC20, Domain {
    /// @notice owner > balance mapping.
    mapping(address => uint256) public override balanceOf;
    /// @notice owner > spender > allowance mapping.
    mapping(address => mapping(address => uint256)) public override allowance;
    /// @notice owner > nonce mapping. Used in `permit`.
    mapping(address => uint256) public nonces;

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

    /// @notice Transfers `amount` tokens from `msg.sender` to `to`.
    /// @param to The address to move the tokens.
    /// @param amount of the tokens to move.
    /// @return (bool) Returns True if succeeded.
    function transfer(address to, uint256 amount) public returns (bool) {
        // If `amount` is 0, or `msg.sender` is `to` nothing happens
        if (amount != 0 || msg.sender == to) {
            uint256 srcBalance = balanceOf[msg.sender];
            require(srcBalance >= amount, "ERC20: balance too low");
            if (msg.sender != to) {
                require(to != address(0), "ERC20: no zero address"); // Moved down so low balance calls safe some gas

                balanceOf[msg.sender] = srcBalance - amount; // Underflow is checked
                balanceOf[to] += amount;
            }
        }
        emit Transfer(msg.sender, to, amount);
        return true;
    }

    /// @notice Transfers `amount` tokens from `from` to `to`. Caller needs approval for `from`.
    /// @param from Address to draw tokens from.
    /// @param to The address to move the tokens.
    /// @param amount The token amount to move.
    /// @return (bool) Returns True if succeeded.
    function transferFrom(
        address from,
        address to,
        uint256 amount
    ) public returns (bool) {
        // If `amount` is 0, or `from` is `to` nothing happens
        if (amount != 0) {
            uint256 srcBalance = balanceOf[from];
            require(srcBalance >= amount, "ERC20: balance too low");

            if (from != to) {
                uint256 spenderAllowance = allowance[from][msg.sender];
                // If allowance is infinite, don't decrease it to save on gas (breaks with EIP-20).
                if (spenderAllowance != type(uint256).max) {
                    require(spenderAllowance >= amount, "ERC20: allowance too low");
                    allowance[from][msg.sender] = spenderAllowance - amount; // Underflow is checked
                }
                require(to != address(0), "ERC20: no zero address"); // Moved down so other failed calls safe some gas

                balanceOf[from] = srcBalance - amount; // Underflow is checked
                balanceOf[to] += amount;
            }
        }
        emit Transfer(from, to, amount);
        return true;
    }

    /// @notice Approves `amount` from sender to be spend by `spender`.
    /// @param spender Address of the party that can draw from msg.sender's account.
    /// @param amount The maximum collective amount that `spender` can draw.
    /// @return (bool) Returns True if approved.
    function approve(address spender, uint256 amount) public override returns (bool) {
        allowance[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;
    }

    // solhint-disable-next-line func-name-mixedcase
    function DOMAIN_SEPARATOR() external view returns (bytes32) {
        return _domainSeparator();
    }

    // keccak256("Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)");
    bytes32 private constant PERMIT_SIGNATURE_HASH = 0x6e71edae12b1b97f4d1f60370fef10105fa2faae0126114a169c64845d6126c9;

    /// @notice Approves `value` from `owner_` to be spend by `spender`.
    /// @param owner_ Address of the owner.
    /// @param spender The address of the spender that gets approved to draw from `owner_`.
    /// @param value The maximum collective amount that `spender` can draw.
    /// @param deadline This permit must be redeemed before this deadline (UTC timestamp in seconds).
    function permit(
        address owner_,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external override {
        require(owner_ != address(0), "ERC20: Owner cannot be 0");
        require(block.timestamp < deadline, "ERC20: Expired");
        require(
            ecrecover(_getDigest(keccak256(abi.encode(PERMIT_SIGNATURE_HASH, owner_, spender, value, nonces[owner_]++, deadline))), v, r, s) ==
                owner_,
            "ERC20: Invalid Signature"
        );
        allowance[owner_][spender] = value;
        emit Approval(owner_, spender, value);
    }
}
function approve(address spender, uint256 amount) public override returns (bool) {
    allowance[msg.sender][spender] = amount;
    emit Approval(msg.sender, spender, amount);
    return true;
}

/// @notice Approves `amount` from sender to be spend by `spender`.
/// @param spender Address of the party that can draw from msg.sender's account.
/// @param amount The maximum collective amount that `spender` can draw.
/// @return (bool) Returns True if approved.