// SPDX-License-Identifier: agpl-3.0
pragma solidity 0.6.12;

abstract contract PermitForERC20 {
  bytes32 public DOMAIN_SEPARATOR;
  bytes public constant EIP712_REVISION = bytes('1');
  bytes32 internal constant EIP712_DOMAIN =
    keccak256('EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)');
  bytes32 public constant PERMIT_TYPEHASH =
    keccak256('Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)');

  /// @dev owner => next valid nonce to submit with permit()
  mapping(address => uint256) public _nonces;

  constructor() public {
    _initializeDomainSeparator();
  }

  function _initializeDomainSeparator() internal {
    uint256 chainId;

    //solium-disable-next-line
    assembly {
      chainId := chainid()
    }

    DOMAIN_SEPARATOR = keccak256(
      abi.encode(
        EIP712_DOMAIN,
        keccak256(_getPermitDomainName()),
        keccak256(EIP712_REVISION),
        chainId,
        address(this)
      )
    );
  }

  /**
   * @dev implements the permit function as for https://github.com/ethereum/EIPs/blob/8a34d644aacf0f9f8f00815307fd7dd5da07655f/EIPS/eip-2612.md
   * @param owner the owner of the funds
   * @param spender the spender
   * @param value the amount
   * @param deadline the deadline timestamp, type(uint256).max for no deadline
   * @param v signature param
   * @param s signature param
   * @param r signature param
   */

  function permit(
    address owner,
    address spender,
    uint256 value,
    uint256 deadline,
    uint8 v,
    bytes32 r,
    bytes32 s
  ) external {
    require(owner != address(0), 'INVALID_OWNER');
    //solium-disable-next-line
    require(block.timestamp <= deadline, 'INVALID_EXPIRATION');
    uint256 currentValidNonce = _nonces[owner];
    bytes32 digest =
      keccak256(
        abi.encodePacked(
          '\x19\x01',
          DOMAIN_SEPARATOR,
          keccak256(abi.encode(PERMIT_TYPEHASH, owner, spender, value, currentValidNonce, deadline))
        )
      );

    require(owner == ecrecover(digest, v, r, s), 'INVALID_SIGNATURE');
    _nonces[owner] = currentValidNonce + 1;
    _approveByPermit(owner, spender, value);
  }

  function _approveByPermit(
    address owner,
    address spender,
    uint256 value
  ) internal virtual;

  function _getPermitDomainName() internal view virtual returns (bytes memory);
}