DaiPermit.sol

// Copyright (C) 2017, 2018, 2019, 2020 dbrock, rain, mrchico, d-xo
// SPDX-License-Identifier: AGPL-3.0-only

pragma solidity >=0.6.12;

contract Math {
    // --- Math ---
    function add(uint x, uint y) internal pure returns (uint z) {
        require((z = x + y) >= x);
    }
    function sub(uint x, uint y) internal pure returns (uint z) {
        require((z = x - y) <= x);
    }
}

contract DaiPermit is Math {
    // --- ERC20 Data ---
    string  public constant name = "Token";
    string  public constant symbol = "TKN";
    uint8   public decimals = 18;
    uint256 public totalSupply;
	bytes32 public constant PERMIT_TYPEHASH = 0xea2aa0a1be11a07ed86d755c93467f4f82362b452371d1ba94d1715123511acb;

    mapping (address => uint)                      public balanceOf;
    mapping (address => mapping (address => uint)) public allowance;

    event Approval(address indexed src, address indexed guy, uint wad);
    event Transfer(address indexed src, address indexed dst, uint wad);

    // --- Init ---
    constructor(uint _totalSupply) public {
        totalSupply = _totalSupply;
        balanceOf[msg.sender] = _totalSupply;
        emit Transfer(address(0), msg.sender, _totalSupply);
    }

    // --- Token ---
    function transfer(address dst, uint wad) virtual public returns (bool) {
        return transferFrom(msg.sender, dst, wad);
    }
    function transferFrom(address src, address dst, uint wad) virtual public returns (bool) {
        require(balanceOf[src] >= wad, "insufficient-balance");
        if (src != msg.sender && allowance[src][msg.sender] != type(uint).max) {
            require(allowance[src][msg.sender] >= wad, "insufficient-allowance");
            allowance[src][msg.sender] = sub(allowance[src][msg.sender], wad);
        }
        balanceOf[src] = sub(balanceOf[src], wad);
        balanceOf[dst] = add(balanceOf[dst], wad);
        emit Transfer(src, dst, wad);
        return true;
    }
    function approve(address usr, uint wad) virtual public returns (bool) {
        allowance[msg.sender][usr] = wad;
        emit Approval(msg.sender, usr, wad);
        return true;
    }
	function permit(address holder, address spender, uint256 nonce, uint256 expiry,
                    bool allowed, uint8 v, bytes32 r, bytes32 s) external
    {
        bytes32 digest =
            keccak256(abi.encodePacked(
                "\x19\x01",
                DOMAIN_SEPARATOR,
                keccak256(abi.encode(PERMIT_TYPEHASH,
                                     holder,
                                     spender,
                                     nonce,
                                     expiry,
                                     allowed))
        ));

        require(holder != address(0), "Dai/invalid-address-0");
        require(holder == ecrecover(digest, v, r, s), "Dai/invalid-permit");
        require(expiry == 0 || now <= expiry, "Dai/permit-expired");
        require(nonce == nonces[holder]++, "Dai/invalid-nonce");
        uint wad = allowed ? uint(-1) : 0;
        allowance[holder][spender] = wad;
    }
}