add permit

contracts/DODOVendingMachine/impl/DVM.sol

@@ -53,6 +53,23 @@ contract DVM is DVMTrader, DVMFunding {
         name = string(abi.encodePacked(suffix, connect, addressToShortString(address(this))));
         symbol = "DLP";
         decimals = _BASE_TOKEN_.decimals();
+
+        // ============================== Permit ====================================
+        uint256 chainId;
+        assembly {
+            chainId := chainid()
+        }
+        DOMAIN_SEPARATOR = keccak256(
+            abi.encode(
+                // keccak256('EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)'),
+                0x8b73c3c69bb8fe3d512ecc4cf759cc79239f7b179b0ffacaa9a75d522b39400f,
+                keccak256(bytes(name)),
+                keccak256(bytes('1')),
+                chainId,
+                address(this)
+            )
+        );
+        // ==========================================================================
     }
 
     function addressToShortString(address _addr) public pure returns (string memory) {
@@ -68,7 +85,6 @@ contract DVM is DVMTrader, DVMFunding {
     }
 
     // ============ Version Control ============
-
     function version() external pure returns (string memory) {
         return "DVM 1.0.0";
     }