Contract Source Code:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import { LibDiamond } from "../libraries/LibDiamond.sol";
import { IERC173 } from "../interfaces/IERC173.sol";

contract OwnershipFacet is IERC173 {
    function transferOwnership(address _newOwner) external override {
        LibDiamond.enforceIsContractOwner();
        LibDiamond.setContractOwner(_newOwner);
    }

    function owner() external override view returns (address owner_) {
        owner_ = LibDiamond.contractOwner();
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/******************************************************************************\
* Author: Nick Mudge <[email protected]> (https://twitter.com/mudgen)
* EIP-2535 Diamonds: https://eips.ethereum.org/EIPS/eip-2535
/******************************************************************************/

interface IDiamondCut {
    enum FacetCutAction {Add, Replace, Remove}
    // Add=0, Replace=1, Remove=2

    struct FacetCut {
        address facetAddress;
        FacetCutAction action;
        bytes4[] functionSelectors;
    }

    /// @notice Add/replace/remove any number of functions and optionally execute
    ///         a function with delegatecall
    /// @param _diamondCut Contains the facet addresses and function selectors
    /// @param _init The address of the contract or facet to execute _calldata
    /// @param _calldata A function call, including function selector and arguments
    ///                  _calldata is executed with delegatecall on _init
    function diamondCut(
        FacetCut[] calldata _diamondCut,
        address _init,
        bytes calldata _calldata
    ) external;

    event DiamondCut(FacetCut[] _diamondCut, address _init, bytes _calldata);
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/// @title ERC-173 Contract Ownership Standard
///  Note: the ERC-165 identifier for this interface is 0x7f5828d0
/* is ERC165 */
interface IERC173 {
    /// @dev This emits when ownership of a contract changes.
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /// @notice Get the address of the owner
    /// @return owner_ The address of the owner.
    function owner() external view returns (address owner_);

    /// @notice Set the address of the new owner of the contract
    /// @dev Set _newOwner to address(0) to renounce any ownership.
    /// @param _newOwner The address of the new owner of the contract
    function transferOwnership(address _newOwner) external;
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/******************************************************************************\
* Author: Nick Mudge <[email protected]> (https://twitter.com/mudgen)
* EIP-2535 Diamonds: https://eips.ethereum.org/EIPS/eip-2535
/******************************************************************************/
import { IDiamondCut } from "../interfaces/IDiamondCut.sol";

// Remember to add the loupe functions from DiamondLoupeFacet to the diamond.
// The loupe functions are required by the EIP2535 Diamonds standard

error InitializationFunctionReverted(address _initializationContractAddress, bytes _calldata);

library LibDiamond {
    bytes32 constant DIAMOND_STORAGE_POSITION = keccak256("diamond.standard.diamond.storage");

    struct FacetAddressAndPosition {
        address facetAddress;
        uint96 functionSelectorPosition; // position in facetFunctionSelectors.functionSelectors array
    }

    struct FacetFunctionSelectors {
        bytes4[] functionSelectors;
        uint256 facetAddressPosition; // position of facetAddress in facetAddresses array
    }

    struct DiamondStorage {
        // maps function selector to the facet address and
        // the position of the selector in the facetFunctionSelectors.selectors array
        mapping(bytes4 => FacetAddressAndPosition) selectorToFacetAndPosition;
        // maps facet addresses to function selectors
        mapping(address => FacetFunctionSelectors) facetFunctionSelectors;
        // facet addresses
        address[] facetAddresses;
        // Used to query if a contract implements an interface.
        // Used to implement ERC-165.
        mapping(bytes4 => bool) supportedInterfaces;
        // owner of the contract
        address contractOwner;
    }

    function diamondStorage() internal pure returns (DiamondStorage storage ds) {
        bytes32 position = DIAMOND_STORAGE_POSITION;
        assembly {
            ds.slot := position
        }
    }

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    function setContractOwner(address _newOwner) internal {
        DiamondStorage storage ds = diamondStorage();
        address previousOwner = ds.contractOwner;
        ds.contractOwner = _newOwner;
        emit OwnershipTransferred(previousOwner, _newOwner);
    }

    function contractOwner() internal view returns (address contractOwner_) {
        contractOwner_ = diamondStorage().contractOwner;
    }

    function enforceIsContractOwner() internal view {
        require(msg.sender == diamondStorage().contractOwner, "LibDiamond: Must be contract owner");
    }

    event DiamondCut(IDiamondCut.FacetCut[] _diamondCut, address _init, bytes _calldata);

    // Internal function version of diamondCut
    function diamondCut(
        IDiamondCut.FacetCut[] memory _diamondCut,
        address _init,
        bytes memory _calldata
    ) internal {
        for (uint256 facetIndex; facetIndex < _diamondCut.length; facetIndex++) {
            IDiamondCut.FacetCutAction action = _diamondCut[facetIndex].action;
            if (action == IDiamondCut.FacetCutAction.Add) {
                addFunctions(_diamondCut[facetIndex].facetAddress, _diamondCut[facetIndex].functionSelectors);
            } else if (action == IDiamondCut.FacetCutAction.Replace) {
                replaceFunctions(_diamondCut[facetIndex].facetAddress, _diamondCut[facetIndex].functionSelectors);
            } else if (action == IDiamondCut.FacetCutAction.Remove) {
                removeFunctions(_diamondCut[facetIndex].facetAddress, _diamondCut[facetIndex].functionSelectors);
            } else {
                revert("LibDiamondCut: Incorrect FacetCutAction");
            }
        }
        emit DiamondCut(_diamondCut, _init, _calldata);
        initializeDiamondCut(_init, _calldata);
    }

    function addFunctions(address _facetAddress, bytes4[] memory _functionSelectors) internal {
        require(_functionSelectors.length > 0, "LibDiamondCut: No selectors in facet to cut");
        DiamondStorage storage ds = diamondStorage();        
        require(_facetAddress != address(0), "LibDiamondCut: Add facet can't be address(0)");
        uint96 selectorPosition = uint96(ds.facetFunctionSelectors[_facetAddress].functionSelectors.length);
        // add new facet address if it does not exist
        if (selectorPosition == 0) {
            addFacet(ds, _facetAddress);            
        }
        for (uint256 selectorIndex; selectorIndex < _functionSelectors.length; selectorIndex++) {
            bytes4 selector = _functionSelectors[selectorIndex];
            address oldFacetAddress = ds.selectorToFacetAndPosition[selector].facetAddress;
            require(oldFacetAddress == address(0), "LibDiamondCut: Can't add function that already exists");
            addFunction(ds, selector, selectorPosition, _facetAddress);
            selectorPosition++;
        }
    }

    function replaceFunctions(address _facetAddress, bytes4[] memory _functionSelectors) internal {
        require(_functionSelectors.length > 0, "LibDiamondCut: No selectors in facet to cut");
        DiamondStorage storage ds = diamondStorage();
        require(_facetAddress != address(0), "LibDiamondCut: Add facet can't be address(0)");
        uint96 selectorPosition = uint96(ds.facetFunctionSelectors[_facetAddress].functionSelectors.length);
        // add new facet address if it does not exist
        if (selectorPosition == 0) {
            addFacet(ds, _facetAddress);
        }
        for (uint256 selectorIndex; selectorIndex < _functionSelectors.length; selectorIndex++) {
            bytes4 selector = _functionSelectors[selectorIndex];
            address oldFacetAddress = ds.selectorToFacetAndPosition[selector].facetAddress;
            require(oldFacetAddress != _facetAddress, "LibDiamondCut: Can't replace function with same function");
            removeFunction(ds, oldFacetAddress, selector);
            addFunction(ds, selector, selectorPosition, _facetAddress);
            selectorPosition++;
        }
    }

    function removeFunctions(address _facetAddress, bytes4[] memory _functionSelectors) internal {
        require(_functionSelectors.length > 0, "LibDiamondCut: No selectors in facet to cut");
        DiamondStorage storage ds = diamondStorage();
        // if function does not exist then do nothing and return
        require(_facetAddress == address(0), "LibDiamondCut: Remove facet address must be address(0)");
        for (uint256 selectorIndex; selectorIndex < _functionSelectors.length; selectorIndex++) {
            bytes4 selector = _functionSelectors[selectorIndex];
            address oldFacetAddress = ds.selectorToFacetAndPosition[selector].facetAddress;
            removeFunction(ds, oldFacetAddress, selector);
        }
    }

    function addFacet(DiamondStorage storage ds, address _facetAddress) internal {
        enforceHasContractCode(_facetAddress, "LibDiamondCut: New facet has no code");
        ds.facetFunctionSelectors[_facetAddress].facetAddressPosition = ds.facetAddresses.length;
        ds.facetAddresses.push(_facetAddress);
    }    


    function addFunction(DiamondStorage storage ds, bytes4 _selector, uint96 _selectorPosition, address _facetAddress) internal {
        ds.selectorToFacetAndPosition[_selector].functionSelectorPosition = _selectorPosition;
        ds.facetFunctionSelectors[_facetAddress].functionSelectors.push(_selector);
        ds.selectorToFacetAndPosition[_selector].facetAddress = _facetAddress;
    }

    function removeFunction(DiamondStorage storage ds, address _facetAddress, bytes4 _selector) internal {        
        require(_facetAddress != address(0), "LibDiamondCut: Can't remove function that doesn't exist");
        // an immutable function is a function defined directly in a diamond
        require(_facetAddress != address(this), "LibDiamondCut: Can't remove immutable function");
        // replace selector with last selector, then delete last selector
        uint256 selectorPosition = ds.selectorToFacetAndPosition[_selector].functionSelectorPosition;
        uint256 lastSelectorPosition = ds.facetFunctionSelectors[_facetAddress].functionSelectors.length - 1;
        // if not the same then replace _selector with lastSelector
        if (selectorPosition != lastSelectorPosition) {
            bytes4 lastSelector = ds.facetFunctionSelectors[_facetAddress].functionSelectors[lastSelectorPosition];
            ds.facetFunctionSelectors[_facetAddress].functionSelectors[selectorPosition] = lastSelector;
            ds.selectorToFacetAndPosition[lastSelector].functionSelectorPosition = uint96(selectorPosition);
        }
        // delete the last selector
        ds.facetFunctionSelectors[_facetAddress].functionSelectors.pop();
        delete ds.selectorToFacetAndPosition[_selector];

        // if no more selectors for facet address then delete the facet address
        if (lastSelectorPosition == 0) {
            // replace facet address with last facet address and delete last facet address
            uint256 lastFacetAddressPosition = ds.facetAddresses.length - 1;
            uint256 facetAddressPosition = ds.facetFunctionSelectors[_facetAddress].facetAddressPosition;
            if (facetAddressPosition != lastFacetAddressPosition) {
                address lastFacetAddress = ds.facetAddresses[lastFacetAddressPosition];
                ds.facetAddresses[facetAddressPosition] = lastFacetAddress;
                ds.facetFunctionSelectors[lastFacetAddress].facetAddressPosition = facetAddressPosition;
            }
            ds.facetAddresses.pop();
            delete ds.facetFunctionSelectors[_facetAddress].facetAddressPosition;
        }
    }

    function initializeDiamondCut(address _init, bytes memory _calldata) internal {
        if (_init == address(0)) {
            return;
        }
        enforceHasContractCode(_init, "LibDiamondCut: _init address has no code");        
        (bool success, bytes memory error) = _init.delegatecall(_calldata);
        if (!success) {
            if (error.length > 0) {
                // bubble up error
                /// @solidity memory-safe-assembly
                assembly {
                    let returndata_size := mload(error)
                    revert(add(32, error), returndata_size)
                }
            } else {
                revert InitializationFunctionReverted(_init, _calldata);
            }
        }
    }

    function enforceHasContractCode(address _contract, string memory _errorMessage) internal view {
        uint256 contractSize;
        assembly {
            contractSize := extcodesize(_contract)
        }
        require(contractSize > 0, _errorMessage);
    }
}