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// Function that bridges taking amount from the t2bAddress where the user funds are parked.
function bridgeERC20(
    uint256 fees,
    uint256 nonce,
    bytes calldata bridgeData,
    bytes calldata signature
) external {
    // recovering signer.
    address recoveredSigner = ECDSA.recover(
        keccak256(
            abi.encodePacked(
                "\x19Ethereum Signed Message:\n32",
                keccak256(
                    abi.encode(
                        address(this),
                        nonce,
                        block.chainid, // uint256
                        fees,
                        bridgeData
                    )
                )
            )
        ),
        signature
    );

    if (signerAddress != recoveredSigner) revert SignerMismatch();
    // nonce is used by gated roles and we don't expect nonce to reach the max value of uint256
    unchecked {
        if (nonce != nextNonce[signerAddress]++) revert InvalidNonce();
    }

    if (bridgeVerifiers[uint32(bytes4(bridgeData[0:4]))] == address(0))
        revert UnsupportedBridge();

    // 解析数据
    (bool parseSuccess, bytes memory parsedData) = bridgeVerifiers[
        uint32(bytes4(bridgeData[0:4]))
    ].call(bridgeData[4:bridgeData.length - 1]);

    if (!parseSuccess) revert VerificationCallFailed();

    // 解析数据
    IT2BRequest.T2BRequest memory t2bRequest = abi.decode(
        parsedData,
        (IT2BRequest.T2BRequest)
    );

    // 获取派生地址
    address t2bAddress = getAddressFor(
        t2bRequest.recipient,
        t2bRequest.toChainId
    );

    // 判断 allowance
    if (
        ERC20(t2bRequest.token).allowance(t2bAddress, address(this)) <
        t2bRequest.amount
    ) {
        // 计算salt
        bytes32 uniqueSalt = keccak256(
            abi.encode(t2bRequest.recipient, t2bRequest.toChainId)
        );

        // 调用 CREATE2 创建临时地址
        new T2BApproval{salt: uniqueSalt}(address(this));
    }

    // 将派生地址的 ERC20代币转移
    ERC20(t2bRequest.token).safeTransferFrom(
        t2bAddress,
        address(this),
        t2bRequest.amount + fees
    );

    //... 其他代码, 略
}

// 部署派生地址
function deployApprovalContract(
    address receiver,
    uint256 toChainId
) public returns (address approvalAddress) {
    bytes32 uniqueSalt = keccak256(abi.encode(receiver, toChainId));
    approvalAddress = address(new T2BApproval{salt: uniqueSalt}(address(this)));
}

// 获取派生地址
function getAddressFor(
    address receiver,
    uint256 toChainId
) public view returns (address) {
    bytes32 salt = keccak256(abi.encode(receiver, toChainId));
    return
        address(
            uint160(
                uint256(
                    keccak256(

                        // 可以看下文的  CreateAddress2的实现
                        abi.encodePacked(
                            bytes1(0xff), // 固定的
                            address(this), // 本合约地址
                            salt, // salt

                            // 合约代码的 hash
                            keccak256(
                                abi.encodePacked(
                                    type(T2BApproval).creationCode, // 合约代码
                                    abi.encode(address(this)) // 合约
                                )
                            )
                        )
                    )
                )
            )
        );
}

contract T2BApproval {
    using SafeTransferLib for ERC20;

    error ZeroAddress();
    error InvalidTokenAddress();


    // Constructor
    constructor(address _t2bRouter) {
        // Set T2b Router.
        IT2BRouter t2bRouter = IT2BRouter(_t2bRouter);

        // Set Max Approvals for supported tokens.
        uint256 tokenIndex = 0;
        while (t2bRouter.supportedTokens(tokenIndex) != address(0)) {

            // 进行 approve操作
            ERC20(t2bRouter.supportedTokens(tokenIndex)).safeApprove(
                address(t2bRouter),
                type(uint256).max
            );
            unchecked {
                ++tokenIndex;
            }
        }

        // 销毁
        selfdestruct(payable(msg.sender));
    }
}