inline UUPSProxy with fixed license

contracts/PureSuperToken.sol

@@ -6,7 +6,7 @@ pragma solidity 0.8.19;
 // This abstract contract provides storage padding for the proxy
 import { CustomSuperTokenBase } from "@superfluid-finance/ethereum-contracts/contracts/interfaces/superfluid/CustomSuperTokenBase.sol";
 // Implementation of UUPSProxy (see https://eips.ethereum.org/EIPS/eip-1822)
-import { UUPSProxy } from "@superfluid-finance/ethereum-contracts/contracts/upgradability/UUPSProxy.sol";
+import { UUPSProxy } from "./base/UUPSProxy.sol";
 // Superfluid framework interfaces we need
 import { ISuperToken, ISuperTokenFactory, IERC20 } from "@superfluid-finance/ethereum-contracts/contracts/interfaces/superfluid/ISuperfluid.sol";
 

contracts/base/UUPSProxy.sol

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+pragma solidity ^0.8.0;
+
+import {Proxy} from "@openzeppelin/contracts/proxy/Proxy.sol";
+
+/// @title UUPS Proxy implementation contract
+/// @author jtriley.eth
+/// @notice Stores the logic contract's address at the _IMPLEMENTATION_SLOT
+/// @dev `initializeProxy(address)` is called by the Super Token Factory
+/// The call to the factory should be in the same transaction to avoid being
+/// front run
+contract UUPSProxy is Proxy {
+	/// @notice Thrown when the logic contract address is zero
+	error ZeroAddress();
+
+	/// @notice Thrown when the logic contract has been set
+	error Initialized();
+
+	/// @notice Precomputed from the following for gas savings
+	/// bytes32(uint256(keccak256("eip1967.proxy.implementation") - 1));
+	bytes32 internal constant _IMPLEMENTATION_SLOT =
+		0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc;
+
+	/// @notice Stores the logic contract address only once.
+	/// @dev Called by the SuperTokenFactory contract on upgrade
+	/// @param initialAddress logic contract address
+	function initializeProxy(address initialAddress) external {
+		if (initialAddress == address(0)) revert ZeroAddress();
+		if (_implementation() != address(0)) revert Initialized();
+		assembly {
+			sstore(_IMPLEMENTATION_SLOT, initialAddress)
+		}
+	}
+
+	/// @notice Reads logic contract from precomputed slot
+	/// @return impl Logic contract address
+	function _implementation()
+		internal
+		view
+		virtual
+		override
+		returns (address impl)
+	{
+		assembly {
+			impl := sload(_IMPLEMENTATION_SLOT)
+		}
+	}
+}