// SPDX-License-Identifier: Apache-2.0
// Copyright 2017 Loopring Technology Limited.
pragma solidity ^0.7.0;


/// @title Ownable
/// @author Brecht Devos - <brecht@loopring.org>
/// @dev The Ownable contract has an owner address, and provides basic
///      authorization control functions, this simplifies the implementation of
///      "user permissions".
contract Ownable
{
    address public owner;

    event OwnershipTransferred(
        address indexed previousOwner,
        address indexed newOwner
    );

    /// @dev The Ownable constructor sets the original `owner` of the contract
    ///      to the sender.
    constructor()
    {
        owner = msg.sender;
    }

    /// @dev Throws if called by any account other than the owner.
    modifier onlyOwner()
    {
        require(msg.sender == owner, "UNAUTHORIZED");
        _;
    }

    /// @dev Allows the current owner to transfer control of the contract to a
    ///      new owner.
    /// @param newOwner The address to transfer ownership to.
    function transferOwnership(
        address newOwner
        )
        public
        virtual
        onlyOwner
    {
        require(newOwner != address(0), "ZERO_ADDRESS");
        emit OwnershipTransferred(owner, newOwner);
        owner = newOwner;
    }

    function renounceOwnership()
        public
        onlyOwner
    {
        emit OwnershipTransferred(owner, address(0));
        owner = address(0);
    }
}

// Copyright 2017 Loopring Technology Limited.



/// @title Utility Functions for uint
/// @author Daniel Wang - <daniel@loopring.org>
library MathUint
{
    using MathUint for uint;

    function mul(
        uint a,
        uint b
        )
        internal
        pure
        returns (uint c)
    {
        c = a * b;
        require(a == 0 || c / a == b, "MUL_OVERFLOW");
    }

    function sub(
        uint a,
        uint b
        )
        internal
        pure
        returns (uint)
    {
        require(b <= a, "SUB_UNDERFLOW");
        return a - b;
    }

    function add(
        uint a,
        uint b
        )
        internal
        pure
        returns (uint c)
    {
        c = a + b;
        require(c >= a, "ADD_OVERFLOW");
    }

    function add64(
        uint64 a,
        uint64 b
        )
        internal
        pure
        returns (uint64 c)
    {
        c = a + b;
        require(c >= a, "ADD_OVERFLOW");
    }
}

// Copyright 2017 Loopring Technology Limited.



/// @title ERC20 Token Interface
/// @dev see https://github.com/ethereum/EIPs/issues/20
/// @author Daniel Wang - <daniel@loopring.org>
abstract contract ERC20
{
    function totalSupply()
        public
        virtual
        view
        returns (uint);

    function balanceOf(
        address who
        )
        public
        virtual
        view
        returns (uint);

    function allowance(
        address owner,
        address spender
        )
        public
        virtual
        view
        returns (uint);

    function transfer(
        address to,
        uint value
        )
        public
        virtual
        returns (bool);

    function transferFrom(
        address from,
        address to,
        uint    value
        )
        public
        virtual
        returns (bool);

    function approve(
        address spender,
        uint    value
        )
        public
        virtual
        returns (bool);
}

// Copyright 2017 Loopring Technology Limited.



// Copyright 2017 Loopring Technology Limited.



/// @title Utility Functions for addresses
/// @author Daniel Wang - <daniel@loopring.org>
/// @author Brecht Devos - <brecht@loopring.org>
library AddressUtil
{
    using AddressUtil for *;

    function isContract(
        address addr
        )
        internal
        view
        returns (bool)
    {
        // According to EIP-1052, 0x0 is the value returned for not-yet created accounts
        // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned
        // for accounts without code, i.e. `keccak256('')`
        bytes32 codehash;
        // solhint-disable-next-line no-inline-assembly
        assembly { codehash := extcodehash(addr) }
        return (codehash != 0x0 &&
                codehash != 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470);
    }

    function toPayable(
        address addr
        )
        internal
        pure
        returns (address payable)
    {
        return payable(addr);
    }

    // Works like address.send but with a customizable gas limit
    // Make sure your code is safe for reentrancy when using this function!
    function sendETH(
        address to,
        uint    amount,
        uint    gasLimit
        )
        internal
        returns (bool success)
    {
        if (amount == 0) {
            return true;
        }
        address payable recipient = to.toPayable();
        /* solium-disable-next-line */
        (success, ) = recipient.call{value: amount, gas: gasLimit}("");
    }

    // Works like address.transfer but with a customizable gas limit
    // Make sure your code is safe for reentrancy when using this function!
    function sendETHAndVerify(
        address to,
        uint    amount,
        uint    gasLimit
        )
        internal
        returns (bool success)
    {
        success = to.sendETH(amount, gasLimit);
        require(success, "TRANSFER_FAILURE");
    }

    // Works like call but is slightly more efficient when data
    // needs to be copied from memory to do the call.
    function fastCall(
        address to,
        uint    gasLimit,
        uint    value,
        bytes   memory data
        )
        internal
        returns (bool success, bytes memory returnData)
    {
        if (to != address(0)) {
            assembly {
                // Do the call
                success := call(gasLimit, to, value, add(data, 32), mload(data), 0, 0)
                // Copy the return data
                let size := returndatasize()
                returnData := mload(0x40)
                mstore(returnData, size)
                returndatacopy(add(returnData, 32), 0, size)
                // Update free memory pointer
                mstore(0x40, add(returnData, add(32, size)))
            }
        }
    }

    // Like fastCall, but throws when the call is unsuccessful.
    function fastCallAndVerify(
        address to,
        uint    gasLimit,
        uint    value,
        bytes   memory data
        )
        internal
        returns (bytes memory returnData)
    {
        bool success;
        (success, returnData) = fastCall(to, gasLimit, value, data);
        if (!success) {
            assembly {
                revert(add(returnData, 32), mload(returnData))
            }
        }
    }
}


// Copyright 2017 Loopring Technology Limited.





/// @title Claimable
/// @author Brecht Devos - <brecht@loopring.org>
/// @dev Extension for the Ownable contract, where the ownership needs
///      to be claimed. This allows the new owner to accept the transfer.
contract Claimable is Ownable
{
    address public pendingOwner;

    /// @dev Modifier throws if called by any account other than the pendingOwner.
    modifier onlyPendingOwner() {
        require(msg.sender == pendingOwner, "UNAUTHORIZED");
        _;
    }

    /// @dev Allows the current owner to set the pendingOwner address.
    /// @param newOwner The address to transfer ownership to.
    function transferOwnership(
        address newOwner
        )
        public
        override
        onlyOwner
    {
        require(newOwner != address(0) && newOwner != owner, "INVALID_ADDRESS");
        pendingOwner = newOwner;
    }

    /// @dev Allows the pendingOwner address to finalize the transfer.
    function claimOwnership()
        public
        onlyPendingOwner
    {
        emit OwnershipTransferred(owner, pendingOwner);
        owner = pendingOwner;
        pendingOwner = address(0);
    }
}



// Copyright 2017 Loopring Technology Limited.



/// @title ERC20 safe transfer
/// @dev see https://github.com/sec-bit/badERC20Fix
/// @author Brecht Devos - <brecht@loopring.org>
library ERC20SafeTransfer
{
    function safeTransferAndVerify(
        address token,
        address to,
        uint    value
        )
        internal
    {
        safeTransferWithGasLimitAndVerify(
            token,
            to,
            value,
            gasleft()
        );
    }

    function safeTransfer(
        address token,
        address to,
        uint    value
        )
        internal
        returns (bool)
    {
        return safeTransferWithGasLimit(
            token,
            to,
            value,
            gasleft()
        );
    }

    function safeTransferWithGasLimitAndVerify(
        address token,
        address to,
        uint    value,
        uint    gasLimit
        )
        internal
    {
        require(
            safeTransferWithGasLimit(token, to, value, gasLimit),
            "TRANSFER_FAILURE"
        );
    }

    function safeTransferWithGasLimit(
        address token,
        address to,
        uint    value,
        uint    gasLimit
        )
        internal
        returns (bool)
    {
        // A transfer is successful when 'call' is successful and depending on the token:
        // - No value is returned: we assume a revert when the transfer failed (i.e. 'call' returns false)
        // - A single boolean is returned: this boolean needs to be true (non-zero)

        // bytes4(keccak256("transfer(address,uint256)")) = 0xa9059cbb
        bytes memory callData = abi.encodeWithSelector(
            bytes4(0xa9059cbb),
            to,
            value
        );
        (bool success, ) = token.call{gas: gasLimit}(callData);
        return checkReturnValue(success);
    }

    function safeTransferFromAndVerify(
        address token,
        address from,
        address to,
        uint    value
        )
        internal
    {
        safeTransferFromWithGasLimitAndVerify(
            token,
            from,
            to,
            value,
            gasleft()
        );
    }

    function safeTransferFrom(
        address token,
        address from,
        address to,
        uint    value
        )
        internal
        returns (bool)
    {
        return safeTransferFromWithGasLimit(
            token,
            from,
            to,
            value,
            gasleft()
        );
    }

    function safeTransferFromWithGasLimitAndVerify(
        address token,
        address from,
        address to,
        uint    value,
        uint    gasLimit
        )
        internal
    {
        bool result = safeTransferFromWithGasLimit(
            token,
            from,
            to,
            value,
            gasLimit
        );
        require(result, "TRANSFER_FAILURE");
    }

    function safeTransferFromWithGasLimit(
        address token,
        address from,
        address to,
        uint    value,
        uint    gasLimit
        )
        internal
        returns (bool)
    {
        // A transferFrom is successful when 'call' is successful and depending on the token:
        // - No value is returned: we assume a revert when the transfer failed (i.e. 'call' returns false)
        // - A single boolean is returned: this boolean needs to be true (non-zero)

        // bytes4(keccak256("transferFrom(address,address,uint256)")) = 0x23b872dd
        bytes memory callData = abi.encodeWithSelector(
            bytes4(0x23b872dd),
            from,
            to,
            value
        );
        (bool success, ) = token.call{gas: gasLimit}(callData);
        return checkReturnValue(success);
    }

    function checkReturnValue(
        bool success
        )
        internal
        pure
        returns (bool)
    {
        // A transfer/transferFrom is successful when 'call' is successful and depending on the token:
        // - No value is returned: we assume a revert when the transfer failed (i.e. 'call' returns false)
        // - A single boolean is returned: this boolean needs to be true (non-zero)
        if (success) {
            assembly {
                switch returndatasize()
                // Non-standard ERC20: nothing is returned so if 'call' was successful we assume the transfer succeeded
                case 0 {
                    success := 1
                }
                // Standard ERC20: a single boolean value is returned which needs to be true
                case 32 {
                    returndatacopy(0, 0, 32)
                    success := mload(0)
                }
                // None of the above: not successful
                default {
                    success := 0
                }
            }
        }
        return success;
    }
}


// Taken from https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/SafeCast.sol




/**
 * @dev Wrappers over Solidity's uintXX/intXX casting operators with added overflow
 * checks.
 *
 * Downcasting from uint256/int256 in Solidity does not revert on overflow. This can
 * easily result in undesired exploitation or bugs, since developers usually
 * assume that overflows raise errors. `SafeCast` restores this intuition by
 * reverting the transaction when such an operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 *
 * Can be combined with {SafeMath} and {SignedSafeMath} to extend it to smaller types, by performing
 * all math on `uint256` and `int256` and then downcasting.
 */
library SafeCast {

    /**
     * @dev Returns the downcasted uint128 from uint256, reverting on
     * overflow (when the input is greater than largest uint128).
     *
     * Counterpart to Solidity's `uint128` operator.
     *
     * Requirements:
     *
     * - input must fit into 128 bits
     */
    function toUint128(uint256 value) internal pure returns (uint128) {
        require(value < 2**128, "SafeCast: value doesn\'t fit in 128 bits");
        return uint128(value);
    }

    /**
     * @dev Returns the downcasted uint96 from uint256, reverting on
     * overflow (when the input is greater than largest uint96).
     *
     * Counterpart to Solidity's `uint96` operator.
     *
     * Requirements:
     *
     * - input must fit into 96 bits
     */
    function toUint96(uint256 value) internal pure returns (uint96) {
        require(value < 2**96, "SafeCast: value doesn\'t fit in 96 bits");
        return uint96(value);
    }

    /**
     * @dev Returns the downcasted uint64 from uint256, reverting on
     * overflow (when the input is greater than largest uint64).
     *
     * Counterpart to Solidity's `uint64` operator.
     *
     * Requirements:
     *
     * - input must fit into 64 bits
     */
    function toUint64(uint256 value) internal pure returns (uint64) {
        require(value < 2**64, "SafeCast: value doesn\'t fit in 64 bits");
        return uint64(value);
    }

    /**
     * @dev Returns the downcasted uint32 from uint256, reverting on
     * overflow (when the input is greater than largest uint32).
     *
     * Counterpart to Solidity's `uint32` operator.
     *
     * Requirements:
     *
     * - input must fit into 32 bits
     */
    function toUint32(uint256 value) internal pure returns (uint32) {
        require(value < 2**32, "SafeCast: value doesn\'t fit in 32 bits");
        return uint32(value);
    }

    /**
     * @dev Returns the downcasted uint40 from uint256, reverting on
     * overflow (when the input is greater than largest uint40).
     *
     * Counterpart to Solidity's `uint32` operator.
     *
     * Requirements:
     *
     * - input must fit into 40 bits
     */
    function toUint40(uint256 value) internal pure returns (uint40) {
        require(value < 2**40, "SafeCast: value doesn\'t fit in 40 bits");
        return uint40(value);
    }

    /**
     * @dev Returns the downcasted uint16 from uint256, reverting on
     * overflow (when the input is greater than largest uint16).
     *
     * Counterpart to Solidity's `uint16` operator.
     *
     * Requirements:
     *
     * - input must fit into 16 bits
     */
    function toUint16(uint256 value) internal pure returns (uint16) {
        require(value < 2**16, "SafeCast: value doesn\'t fit in 16 bits");
        return uint16(value);
    }

    /**
     * @dev Returns the downcasted uint8 from uint256, reverting on
     * overflow (when the input is greater than largest uint8).
     *
     * Counterpart to Solidity's `uint8` operator.
     *
     * Requirements:
     *
     * - input must fit into 8 bits.
     */
    function toUint8(uint256 value) internal pure returns (uint8) {
        require(value < 2**8, "SafeCast: value doesn\'t fit in 8 bits");
        return uint8(value);
    }

    /**
     * @dev Converts a signed int256 into an unsigned uint256.
     *
     * Requirements:
     *
     * - input must be greater than or equal to 0.
     */
    function toUint256(int256 value) internal pure returns (uint256) {
        require(value >= 0, "SafeCast: value must be positive");
        return uint256(value);
    }

    /**
     * @dev Returns the downcasted int128 from int256, reverting on
     * overflow (when the input is less than smallest int128 or
     * greater than largest int128).
     *
     * Counterpart to Solidity's `int128` operator.
     *
     * Requirements:
     *
     * - input must fit into 128 bits
     *
     * _Available since v3.1._
     */
    function toInt128(int256 value) internal pure returns (int128) {
        require(value >= -2**127 && value < 2**127, "SafeCast: value doesn\'t fit in 128 bits");
        return int128(value);
    }

    /**
     * @dev Returns the downcasted int64 from int256, reverting on
     * overflow (when the input is less than smallest int64 or
     * greater than largest int64).
     *
     * Counterpart to Solidity's `int64` operator.
     *
     * Requirements:
     *
     * - input must fit into 64 bits
     *
     * _Available since v3.1._
     */
    function toInt64(int256 value) internal pure returns (int64) {
        require(value >= -2**63 && value < 2**63, "SafeCast: value doesn\'t fit in 64 bits");
        return int64(value);
    }

    /**
     * @dev Returns the downcasted int32 from int256, reverting on
     * overflow (when the input is less than smallest int32 or
     * greater than largest int32).
     *
     * Counterpart to Solidity's `int32` operator.
     *
     * Requirements:
     *
     * - input must fit into 32 bits
     *
     * _Available since v3.1._
     */
    function toInt32(int256 value) internal pure returns (int32) {
        require(value >= -2**31 && value < 2**31, "SafeCast: value doesn\'t fit in 32 bits");
        return int32(value);
    }

    /**
     * @dev Returns the downcasted int16 from int256, reverting on
     * overflow (when the input is less than smallest int16 or
     * greater than largest int16).
     *
     * Counterpart to Solidity's `int16` operator.
     *
     * Requirements:
     *
     * - input must fit into 16 bits
     *
     * _Available since v3.1._
     */
    function toInt16(int256 value) internal pure returns (int16) {
        require(value >= -2**15 && value < 2**15, "SafeCast: value doesn\'t fit in 16 bits");
        return int16(value);
    }

    /**
     * @dev Returns the downcasted int8 from int256, reverting on
     * overflow (when the input is less than smallest int8 or
     * greater than largest int8).
     *
     * Counterpart to Solidity's `int8` operator.
     *
     * Requirements:
     *
     * - input must fit into 8 bits.
     *
     * _Available since v3.1._
     */
    function toInt8(int256 value) internal pure returns (int8) {
        require(value >= -2**7 && value < 2**7, "SafeCast: value doesn\'t fit in 8 bits");
        return int8(value);
    }

    /**
     * @dev Converts an unsigned uint256 into a signed int256.
     *
     * Requirements:
     *
     * - input must be less than or equal to maxInt256.
     */
    function toInt256(uint256 value) internal pure returns (int256) {
        require(value < 2**255, "SafeCast: value doesn't fit in an int256");
        return int256(value);
    }
}

// Copyright 2017 Loopring Technology Limited.



/// @title IDepositContract.
/// @dev   Contract storing and transferring funds for an exchange.
///
///        ERC1155 tokens can be supported by registering pseudo token addresses calculated
///        as `address(keccak256(real_token_address, token_params))`. Then the custom
///        deposit contract can look up the real token address and paramsters with the
///        pseudo token address before doing the transfers.
/// @author Brecht Devos - <brecht@loopring.org>
interface IDepositContract
{
    /// @dev Returns if a token is suppoprted by this contract.
    function isTokenSupported(address token)
        external
        view
        returns (bool);

    /// @dev Transfers tokens from a user to the exchange. This function will
    ///      be called when a user deposits funds to the exchange.
    ///      In a simple implementation the funds are simply stored inside the
    ///      deposit contract directly. More advanced implementations may store the funds
    ///      in some DeFi application to earn interest, so this function could directly
    ///      call the necessary functions to store the funds there.
    ///
    ///      This function needs to throw when an error occurred!
    ///
    ///      This function can only be called by the exchange.
    ///
    /// @param from The address of the account that sends the tokens.
    /// @param token The address of the token to transfer (`0x0` for ETH).
    /// @param amount The amount of tokens to transfer.
    /// @param extraData Opaque data that can be used by the contract to handle the deposit
    /// @return amountReceived The amount to deposit to the user's account in the Merkle tree
    function deposit(
        address from,
        address token,
        uint96  amount,
        bytes   calldata extraData
        )
        external
        payable
        returns (uint96 amountReceived);

    /// @dev Transfers tokens from the exchange to a user. This function will
    ///      be called when a withdrawal is done for a user on the exchange.
    ///      In the simplest implementation the funds are simply stored inside the
    ///      deposit contract directly so this simply transfers the requested tokens back
    ///      to the user. More advanced implementations may store the funds
    ///      in some DeFi application to earn interest so the function would
    ///      need to get those tokens back from the DeFi application first before they
    ///      can be transferred to the user.
    ///
    ///      This function needs to throw when an error occurred!
    ///
    ///      This function can only be called by the exchange.
    ///
    /// @param from The address from which 'amount' tokens are transferred.
    /// @param to The address to which 'amount' tokens are transferred.
    /// @param token The address of the token to transfer (`0x0` for ETH).
    /// @param amount The amount of tokens transferred.
    /// @param extraData Opaque data that can be used by the contract to handle the withdrawal
    function withdraw(
        address from,
        address to,
        address token,
        uint    amount,
        bytes   calldata extraData
        )
        external
        payable;

    /// @dev Transfers tokens (ETH not supported) for a user using the allowance set
    ///      for the exchange. This way the approval can be used for all functionality (and
    ///      extended functionality) of the exchange.
    ///      Should NOT be used to deposit/withdraw user funds, `deposit`/`withdraw`
    ///      should be used for that as they will contain specialised logic for those operations.
    ///      This function can be called by the exchange to transfer onchain funds of users
    ///      necessary for Agent functionality.
    ///
    ///      This function needs to throw when an error occurred!
    ///
    ///      This function can only be called by the exchange.
    ///
    /// @param from The address of the account that sends the tokens.
    /// @param to The address to which 'amount' tokens are transferred.
    /// @param token The address of the token to transfer (ETH is and cannot be suppported).
    /// @param amount The amount of tokens transferred.
    function transfer(
        address from,
        address to,
        address token,
        uint    amount
        )
        external
        payable;

    /// @dev Checks if the given address is used for depositing ETH or not.
    ///      Is used while depositing to send the correct ETH amount to the deposit contract.
    ///
    ///      Note that 0x0 is always registered for deposting ETH when the exchange is created!
    ///      This function allows additional addresses to be used for depositing ETH, the deposit
    ///      contract can implement different behaviour based on the address value.
    ///
    /// @param addr The address to check
    /// @return True if the address is used for depositing ETH, else false.
    function isETH(address addr)
        external
        view
        returns (bool);
}


/// @title DefaultDepositContract
/// @dev   Default implementation of IDepositContract that just stores
///        all funds without doing anything with them.
///
///        Should be able to work with proxy contracts so the contract
///        can be updated easily (but with great caution and transparency!)
///        when necessary.
///
/// @author Brecht Devos - <brecht@loopring.org>
contract DefaultDepositContract is IDepositContract, Claimable
{
    using AddressUtil       for address;
    using ERC20SafeTransfer for address;
    using MathUint          for uint;
    using SafeCast          for uint;

    address public exchange;

    mapping (address => bool) needCheckBalance;

    modifier onlyExchange()
    {
        require(msg.sender == exchange, "UNAUTHORIZED");
        _;
    }

    modifier ifNotZero(uint amount)
    {
        if (amount == 0) return;
        else  _;
    }

    event CheckBalance(
        address indexed token,
        bool            checkBalance
    );

    function initialize(
        address _exchange
        )
        external
    {
        require(
            exchange == address(0) && _exchange != address(0),
            "INVALID_EXCHANGE"
        );
        owner = msg.sender;
        exchange = _exchange;
    }

    function setCheckBalance(
        address token,
        bool    checkBalance
        )
        external
        onlyOwner
    {
        require(needCheckBalance[token] != checkBalance, "INVALID_VALUE");

        needCheckBalance[token] = checkBalance;
        emit CheckBalance(token, checkBalance);
    }

    function isTokenSupported(address /*token*/)
        external
        override
        pure
        returns (bool)
    {
        return true;
    }

    function deposit(
        address from,
        address token,
        uint96  amount,
        bytes   calldata /*extraData*/
        )
        external
        override
        payable
        onlyExchange
        ifNotZero(amount)
        returns (uint96 amountReceived)
    {
        uint ethToReturn = 0;

        if (isETHInternal(token)) {
            require(msg.value >= amount, "INVALID_ETH_DEPOSIT");
            amountReceived = amount;
            ethToReturn = msg.value - amount;
        } else {
            bool checkBalance = needCheckBalance[token];
            uint balanceBefore = checkBalance ? ERC20(token).balanceOf(address(this)) : 0;

            token.safeTransferFromAndVerify(from, address(this), uint(amount));

            uint balanceAfter = checkBalance ? ERC20(token).balanceOf(address(this)) : amount;
            uint diff = balanceAfter.sub(balanceBefore);
            amountReceived = diff.toUint96();

            ethToReturn = msg.value;
        }

        if (ethToReturn > 0) {
            from.sendETHAndVerify(ethToReturn, gasleft());
        }
    }

    function withdraw(
        address /*from*/,
        address to,
        address token,
        uint    amount,
        bytes   calldata /*extraData*/
        )
        external
        override
        payable
        onlyExchange
        ifNotZero(amount)
    {
        if (isETHInternal(token)) {
            to.sendETHAndVerify(amount, gasleft());
        } else {
            if (!token.safeTransfer(to, amount)){
                uint amountPaid = ERC20(token).balanceOf(address(this));
                require(amountPaid < amount, "UNEXPECTED");
                token.safeTransferAndVerify(to, amountPaid);
            }
        }
    }

    function transfer(
        address from,
        address to,
        address token,
        uint    amount
        )
        external
        override
        payable
        onlyExchange
        ifNotZero(amount)
    {
        token.safeTransferFromAndVerify(from, to, amount);
    }

    function isETH(address addr)
        external
        override
        pure
        returns (bool)
    {
        return isETHInternal(addr);
    }

    // -- Internal --

    function isETHInternal(address addr)
        internal
        pure
        returns (bool)
    {
        return addr == address(0);
    }
}