// SPDX-License-Identifier: MIT
// Adapted from https://github.com/OpenZeppelin/openzeppelin-contracts/blob/53516bc555a454862470e7860a9b5254db4d00f5/src/token/ERC20/ERC20Permit.sol
pragma solidity ^0.8.0;

import "./ERC20.sol";
import "./IERC2612.sol";

/**
 * @dev Extension of {ERC20} that allows token holders to use their tokens
 * without sending any transactions by setting {IERC20-allowance} with a
 * signature using the {permit} method, and then spend them via
 * {IERC20-transferFrom}.
 *
 * The {permit} signature mechanism conforms to the {IERC2612} interface.
 */
abstract contract ERC20Permit is ERC20, IERC2612 {
    mapping (address => uint256) public override nonces;

    bytes32 public immutable PERMIT_TYPEHASH = keccak256("Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)");
    bytes32 private immutable _DOMAIN_SEPARATOR;
    uint256 public immutable deploymentChainId;

    constructor(string memory name_, string memory symbol_, uint8 decimals_) ERC20(name_, symbol_, decimals_) {
        deploymentChainId = block.chainid;
        _DOMAIN_SEPARATOR = _calculateDomainSeparator(block.chainid);
    }

    /// @dev Calculate the DOMAIN_SEPARATOR.
    function _calculateDomainSeparator(uint256 chainId) private view returns (bytes32) {
        return keccak256(
            abi.encode(
                keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
                keccak256(bytes(name)),
                keccak256(bytes(version())),
                chainId,
                address(this)
            )
        );
    }

    /// @dev Return the DOMAIN_SEPARATOR.
    function DOMAIN_SEPARATOR() external view returns (bytes32) {
        return block.chainid == deploymentChainId ? _DOMAIN_SEPARATOR : _calculateDomainSeparator(block.chainid);
    }

    /// @dev Setting the version as a function so that it can be overriden
    function version() public pure virtual returns(string memory) { return "1"; }

    /**
     * @dev See {IERC2612-permit}.
     *
     * In cases where the free option is not a concern, deadline can simply be
     * set to uint(-1), so it should be seen as an optional parameter
     */
    function permit(address owner, address spender, uint256 amount, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external virtual override {
        require(deadline >= block.timestamp, "ERC20Permit: expired deadline");

        bytes32 hashStruct = keccak256(
            abi.encode(
                PERMIT_TYPEHASH,
                owner,
                spender,
                amount,
                nonces[owner]++,
                deadline
            )
        );

        bytes32 hash = keccak256(
            abi.encodePacked(
                "\x19\x01",
                block.chainid == deploymentChainId ? _DOMAIN_SEPARATOR : _calculateDomainSeparator(block.chainid),
                hashStruct
            )
        );

        address signer = ecrecover(hash, v, r, s);
        require(
            signer != address(0) && signer == owner,
            "ERC20Permit: invalid signature"
        );

        _setAllowance(owner, spender, amount);
    }
}