EIP712.sol

// SPDX-License-Identifier: MIT
pragma solidity 0.8.17;

/// @notice EIP712 helpers for permit2
/// @dev Maintains cross-chain replay protection in the event of a fork and properly handles delegated calls
/// @dev Reference: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/cryptography/EIP712.sol
contract EIP712 {
    // Cache the domain separator as an immutable value, but also store the chain id that it
    // corresponds to, in order to invalidate the cached domain separator if the chain id changes.
    bytes32 private immutable _CACHED_DOMAIN_SEPARATOR;
    uint256 private immutable _CACHED_CHAIN_ID;
    address private immutable _CACHED_THIS;

    bytes32 private constant _HASHED_NAME = keccak256("Permit2");
    bytes32 private constant _TYPE_HASH =
        keccak256("EIP712Domain(string name,uint256 chainId,address verifyingContract)");

    constructor() {
        _CACHED_THIS = address(this);
        _CACHED_CHAIN_ID = block.chainid;
        _CACHED_DOMAIN_SEPARATOR = _buildDomainSeparator(_TYPE_HASH, _HASHED_NAME);
    }

    /// @notice Returns the domain separator for the current chain.
    /// @dev Uses cached version if chainid and address are unchanged from construction.
    function DOMAIN_SEPARATOR() public view returns (bytes32) {
        return block.chainid == _CACHED_CHAIN_ID && address(this) == _CACHED_THIS
            ? _CACHED_DOMAIN_SEPARATOR
            : _buildDomainSeparator(_TYPE_HASH, _HASHED_NAME);
    }

    /// @notice Builds a domain separator using the current chainId and contract address.
    function _buildDomainSeparator(bytes32 typeHash, bytes32 nameHash) private view returns (bytes32) {
        return keccak256(abi.encode(typeHash, nameHash, block.chainid, address(this)));
    }

    /// @notice Creates an EIP-712 typed data hash
    function _hashTypedData(bytes32 dataHash) internal view returns (bytes32) {
        return keccak256(abi.encodePacked("\x19\x01", DOMAIN_SEPARATOR(), dataHash));
    }
}