Token

pragma solidity >=0.5.0;

interface IERC20Template {
    function initialize(
        string calldata name,
        string calldata symbol,
        address minter,
        uint256 cap,
        string calldata blob,
        address collector
    ) external returns (bool);

    function mint(address account, uint256 value) external;
    function minter() external view returns(address);    
    function name() external view returns (string memory);
    function symbol() external view returns (string memory);
    function decimals() external view returns (uint8);
    function cap() external view returns (uint256);
    function isMinter(address account) external view returns (bool);
    function isInitialized() external view returns (bool);
    function allowance(address owner, address spender)
        external
        view
        returns (uint256);
    function transferFrom(
        address from,
        address to,
        uint256 value
    ) external returns (bool);
    function balanceOf(address account) external view returns (uint256);
    function transfer(address to, uint256 value) external returns (bool);
    function proposeMinter(address newMinter) external;
    function approveMinter() external;
}

pragma solidity 0.5.7;
// Copyright BigchainDB GmbH and Ocean Protocol contributors
// SPDX-License-Identifier: (Apache-2.0 AND CC-BY-4.0)
// Code is Apache-2.0 and docs are CC-BY-4.0

import '../interfaces/IERC20Template.sol';
import 'openzeppelin-solidity/contracts/token/ERC20/ERC20.sol';


/**
* @title DataTokenTemplate
*  
* @dev DataTokenTemplate is an ERC20 compliant token template
*      Used by the factory contract as a bytecode reference to 
*      deploy new DataTokens.
*/
contract DataTokenTemplate is IERC20Template, ERC20 {
    using SafeMath for uint256;

    string  private _name;
    string  private _symbol;
    string  private _blob;
    uint256 private _cap;
    uint8 private constant _decimals = 18;
    address private _communityFeeCollector;
    bool    private initialized = false;
    address private _minter;
    address private _proposedMinter;
    uint256 public constant BASE = 10**18;
    uint256 public constant BASE_COMMUNITY_FEE_PERCENTAGE = BASE / 1000;
    uint256 public constant BASE_MARKET_FEE_PERCENTAGE = BASE / 1000;

    event OrderStarted(
            address indexed consumer,
            address indexed payer,
            uint256 amount, 
            uint256 serviceId, 
            uint256 timestamp,
            address indexed mrktFeeCollector,
            uint256 marketFee
    );

    event OrderFinished(
            bytes32 orderTxId, 
            address indexed consumer,
            uint256 amount, 
            uint256 serviceId, 
            address indexed provider,
            uint256 timestamp
    );

    event MinterProposed(
        address currentMinter,
        address newMinter
    );

    event MinterApproved(
        address currentMinter,
        address newMinter
    );

    modifier onlyNotInitialized() {
        require(
            !initialized,
            'DataTokenTemplate: token instance already initialized'
        );
        _;
    }
    
    modifier onlyMinter() {
        require(
            msg.sender == _minter,
            'DataTokenTemplate: invalid minter' 
        );
        _;
    }

    /**
     * @dev constructor
     *      Called prior contract deployment
     * @param name refers to a template DataToken name
     * @param symbol refers to a template DataToken symbol
     * @param minterAddress refers to an address that has minter role
     * @param cap the total ERC20 cap
     * @param blob data string refering to the resolver for the metadata
     * @param feeCollector it is the community fee collector address
     */
    constructor(
        string memory name,
        string memory symbol,
        address minterAddress,
        uint256 cap,
        string memory blob,
        address feeCollector
    )
        public
    {
        _initialize(
            name,
            symbol,
            minterAddress,
            cap,
            blob,
            feeCollector
        );
    }
    
    /**
     * @dev initialize
     *      Called prior contract initialization (e.g creating new DataToken instance)
     *      Calls private _initialize function. Only if contract is not initialized.
     * @param name refers to a new DataToken name
     * @param symbol refers to a nea DataToken symbol
     * @param minterAddress refers to an address that has minter rights
     * @param cap the total ERC20 cap
     * @param blob data string refering to the resolver for the metadata
     * @param feeCollector it is the community fee collector address
     */
    function initialize(
        string calldata name,
        string calldata symbol,
        address minterAddress,
        uint256 cap,
        string calldata blob,
        address feeCollector
    ) 
        external
        onlyNotInitialized
        returns(bool)
    {
        return _initialize(
            name,
            symbol,
            minterAddress,
            cap,
            blob,
            feeCollector
        );
    }

    /**
     * @dev _initialize
     *      Private function called on contract initialization.
     * @param name refers to a new DataToken name
     * @param symbol refers to a nea DataToken symbol
     * @param minterAddress refers to an address that has minter rights
     * @param cap the total ERC20 cap
     * @param blob data string refering to the resolver for the metadata
     * @param feeCollector it is the community fee collector address
     */
    function _initialize(
        string memory name,
        string memory symbol,
        address minterAddress,
        uint256 cap,
        string memory blob,
        address feeCollector
    )
        private
        returns(bool)
    {
        require(
            minterAddress != address(0), 
            'DataTokenTemplate: Invalid minter,  zero address'
        );

        require(
            _minter == address(0), 
            'DataTokenTemplate: Invalid minter, zero address'
        );

        require(
            feeCollector != address(0),
            'DataTokenTemplate: Invalid community fee collector, zero address'
        );

        require(
            cap != 0,
            'DataTokenTemplate: Invalid cap value'
        );
        _cap = cap;
        _name = name;
        _blob = blob;
        _symbol = symbol;
        _minter = minterAddress;
        _communityFeeCollector = feeCollector;
        initialized = true;
        return initialized;
    }

    /**
     * @dev mint
     *      Only the minter address can call it.
     *      msg.value should be higher than zero and gt or eq minting fee
     * @param account refers to an address that token is going to be minted to.
     * @param value refers to amount of tokens that is going to be minted.
     */
    function mint(
        address account,
        uint256 value
    ) 
        external  
        onlyMinter 
    {
        require(
            totalSupply().add(value) <= _cap, 
            'DataTokenTemplate: cap exceeded'
        );
        _mint(account, value);
    }

    /**
     * @dev startOrder
     *      called by payer or consumer prior ordering a service consume on a marketplace.
     * @param consumer is the consumer address (payer could be different address)
     * @param amount refers to amount of tokens that is going to be transfered.
     * @param serviceId service index in the metadata
     * @param mrktFeeCollector marketplace fee collector
     */
    function startOrder(
        address consumer,
        uint256 amount,
        uint256 serviceId,
        address mrktFeeCollector
    )
        external
    {
        uint256 marketFee = 0;
        uint256 communityFee = calculateFee(
            amount, 
            BASE_COMMUNITY_FEE_PERCENTAGE
        );
        transfer(_communityFeeCollector, communityFee);
        if(mrktFeeCollector != address(0)){
            marketFee = calculateFee(
                amount, 
                BASE_MARKET_FEE_PERCENTAGE
            );
            transfer(mrktFeeCollector, marketFee);
        }
        uint256 totalFee = communityFee.add(marketFee);
        transfer(_minter, amount.sub(totalFee));

        emit OrderStarted(
            consumer,
            msg.sender,
            amount,
            serviceId,
            block.timestamp,
            mrktFeeCollector,
            marketFee
        );
    }

    /**
     * @dev finishOrder
     *      called by provider prior completing service delivery only
     *      if there is a partial or full refund.
     * @param orderTxId refers to the transaction Id  of startOrder acts 
     *                  as a payment reference.
     * @param consumer refers to an address that has consumed that service.
     * @param amount refers to amount of tokens that is going to be transfered.
     * @param serviceId service index in the metadata.
     */
    function finishOrder(
        bytes32 orderTxId, 
        address consumer, 
        uint256 amount,
        uint256 serviceId
    )
        external
    {
        if ( amount != 0 )  
            require(
                transfer(consumer, amount),
                'DataTokenTemplate: failed to finish order'
            );

        emit OrderFinished(
            orderTxId, 
            consumer, 
            amount, 
            serviceId, 
            msg.sender,
            block.timestamp
        );
    }

    /**
     * @dev proposeMinter
     *      It proposes a new token minter address.
     *      Only the current minter can call it.
     * @param newMinter refers to a new token minter address.
     */
    function proposeMinter(address newMinter) 
        external 
        onlyMinter 
    {
        _proposedMinter = newMinter;
        emit MinterProposed(
            msg.sender,
            _proposedMinter
        );
    }

    /**
     * @dev approveMinter
     *      It approves a new token minter address.
     *      Only the current minter can call it.
     */
    function approveMinter()
        external
    {
        require(
            msg.sender == _proposedMinter,
            'DataTokenTemplate: invalid proposed minter address'
        );
        emit MinterApproved(
            _minter,
            _proposedMinter
        );
        _minter = _proposedMinter;
        _proposedMinter = address(0);
    }

    /**
     * @dev name
     *      It returns the token name.
     * @return DataToken name.
     */
    function name() external view returns(string memory) {
        return _name;
    }

    /**
     * @dev symbol
     *      It returns the token symbol.
     * @return DataToken symbol.
     */
    function symbol() external view returns(string memory) {
        return _symbol;
    }

    /**
     * @dev blob
     *      It returns the blob (e.g https://123.com).
     * @return DataToken blob.
     */
    function blob() external view returns(string memory) {
        return _blob;
    }

    /**
     * @dev decimals
     *      It returns the token decimals.
     *      how many supported decimal points
     * @return DataToken decimals.
     */
    function decimals() external view returns(uint8) {
        return _decimals;
    }

    /**
     * @dev cap
     *      it returns the capital.
     * @return DataToken cap.
     */
    function cap() external view returns (uint256) {
        return _cap;
    }

    /**
     * @dev isMinter
     *      It takes the address and checks whether it has a minter role.
     * @param account refers to the address.
     * @return true if account has a minter role.
     */
    function isMinter(address account) external view returns(bool) {
        return (_minter == account);
    } 

    /**
     * @dev minter
     * @return minter's address.
     */
    function minter()
        external
        view 
        returns(address)
    {
        return _minter;
    }

    /**
     * @dev isInitialized
     *      It checks whether the contract is initialized.
     * @return true if the contract is initialized.
     */ 
    function isInitialized() external view returns(bool) {
        return initialized;
    }

    /**
     * @dev calculateFee
     *      giving a fee percentage, and amount it calculates the actual fee
     * @param amount the amount of token
     * @param feePercentage the fee percentage 
     * @return the token fee.
     */ 
    function calculateFee(
        uint256 amount,
        uint256 feePercentage
    )
        public
        pure
        returns(uint256)
    {
        if(amount == 0) return 0;
        if(feePercentage == 0) return 0;
        return amount.mul(feePercentage).div(BASE);
    }
}

pragma solidity ^0.5.0;

/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with GSN meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
contract Context {
    // Empty internal constructor, to prevent people from mistakenly deploying
    // an instance of this contract, which should be used via inheritance.
    constructor () internal { }
    // solhint-disable-previous-line no-empty-blocks

    function _msgSender() internal view returns (address payable) {
        return msg.sender;
    }

    function _msgData() internal view returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

pragma solidity ^0.5.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when 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.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     * - Subtraction cannot overflow.
     *
     * _Available since v2.4.0._
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     *
     * _Available since v2.4.0._
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     *
     * _Available since v2.4.0._
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

pragma solidity ^0.5.0;

import "../../GSN/Context.sol";
import "./IERC20.sol";
import "../../math/SafeMath.sol";

/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20Mintable}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20 is Context, IERC20 {
    using SafeMath for uint256;

    mapping (address => uint256) private _balances;

    mapping (address => mapping (address => uint256)) private _allowances;

    uint256 private _totalSupply;

    /**
     * @dev See {IERC20-totalSupply}.
     */
    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See {IERC20-transfer}.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount) public returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

    /**
     * @dev See {IERC20-allowance}.
     */
    function allowance(address owner, address spender) public view returns (uint256) {
        return _allowances[owner][spender];
    }

    /**
     * @dev See {IERC20-approve}.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 amount) public returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }

    /**
     * @dev See {IERC20-transferFrom}.
     *
     * Emits an {Approval} event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of {ERC20};
     *
     * Requirements:
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     * - the caller must have allowance for `sender`'s tokens of at least
     * `amount`.
     */
    function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to {transfer}, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a {Transfer} event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(address sender, address recipient, uint256 amount) internal {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a {Transfer} event with `from` set to the zero address.
     *
     * Requirements
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal {
        require(account != address(0), "ERC20: mint to the zero address");

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a {Transfer} event with `to` set to the zero address.
     *
     * Requirements
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 amount) internal {
        require(account != address(0), "ERC20: burn from the zero address");

        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner`s tokens.
     *
     * This is internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(address owner, address spender, uint256 amount) internal {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`.`amount` is then deducted
     * from the caller's allowance.
     *
     * See {_burn} and {_approve}.
     */
    function _burnFrom(address account, uint256 amount) internal {
        _burn(account, amount);
        _approve(account, _msgSender(), _allowances[account][_msgSender()].sub(amount, "ERC20: burn amount exceeds allowance"));
    }
}

pragma solidity ^0.5.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP. Does not include
 * the optional functions; to access them see {ERC20Detailed}.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

{
  "remappings": [],
  "optimizer": {
    "enabled": true,
    "runs": 200
  },
  "evmVersion": "byzantium",
  "libraries": {
    "": {}
  },
  "outputSelection": {
    "*": {
      "*": [
        "evm.bytecode",
        "evm.deployedBytecode",
        "abi"
      ]
    }
  }
}

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