// Created By BitDNS.vip
// contact : Presale Pool
// SPDX-License-Identifier: MIT

pragma solidity ^0.5.8;

// File: @openzeppelin/contracts/math/SafeMath.sol
/**
 * @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;
    }
}

// File: @openzeppelin/contracts/token/ERC20/IERC20.sol
/**
 * @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);
}

interface IUSDT {
    function totalSupply() external view returns (uint);
    function balanceOf(address who) external view returns (uint);
    function transfer(address to, uint value) external;
    function allowance(address owner, address spender) external view returns (uint);
    function transferFrom(address from, address to, uint value) external;
    function approve(address spender, uint value) external;
    event Transfer(address indexed from, address indexed to, uint value);
    event Approval(address indexed owner, address indexed spender, uint value);
}

// File: @openzeppelin/contracts/utils/Address.sol
/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * This test is non-exhaustive, and there may be false-negatives: during the
     * execution of a contract's constructor, its address will be reported as
     * not containing a contract.
     *
     * IMPORTANT: It is unsafe to assume that an address for which this
     * function returns false is an externally-owned account (EOA) and not a
     * contract.
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies in extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        // solhint-disable-next-line no-inline-assembly
        assembly { size := extcodesize(account) }
        return size > 0;
    }
}

// File: @openzeppelin/contracts/token/ERC20/SafeERC20.sol
/**
 * @title SafeERC20
 * @dev Wrappers around ERC20 operations that throw on failure (when the token
 * contract returns false). Tokens that return no value (and instead revert or
 * throw on failure) are also supported, non-reverting calls are assumed to be
 * successful.
 * To use this library you can add a `using SafeERC20 for ERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
    using SafeMath for uint256;
    using Address for address;

    function safeTransfer(IERC20 token, address to, uint256 value) internal {
        callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(IERC20 token, address from, address to, uint256 value) internal {
        callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function callOptionalReturn(IERC20 token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves.

        // A Solidity high level call has three parts:
        //  1. The target address is checked to verify it contains contract code
        //  2. The call itself is made, and success asserted
        //  3. The return value is decoded, which in turn checks the size of the returned data.
        // solhint-disable-next-line max-line-length
        require(address(token).isContract(), "SafeERC20: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = address(token).call(data);
      
        require(success, "SafeERC20: low-level call failed");

        if (returndata.length > 0) { // Return data is optional
            // solhint-disable-next-line max-line-length
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

library SafeUSDT {
    using SafeMath for uint256;
    using Address for address;

    function safeTransfer(IUSDT token, address to, uint256 value) internal {
        callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(IUSDT token, address from, address to, uint256 value) internal {
        callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function callOptionalReturn(IUSDT token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves.

        // A Solidity high level call has three parts:
        //  1. The target address is checked to verify it contains contract code
        //  2. The call itself is made, and success asserted
        //  3. The return value is decoded, which in turn checks the size of the returned data.
        // solhint-disable-next-line max-line-length
        require(address(token).isContract(), "SafeERC20: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = address(token).call(data);
      
        require(success, "SafeERC20: low-level call failed");

        if (returndata.length > 0) { // Return data is optional
            // solhint-disable-next-line max-line-length
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

contract ExchangePool {
    using SafeMath for uint256;
    using SafeERC20 for IERC20;
    using SafeUSDT for IUSDT;
    using Address for address;

    IERC20 public presale_token;
    IUSDT  public currency_token;
    uint256 public presale_decimals;
    uint256 public currency_decimals;
    
    uint256 public totalSupply;
    uint256 public price;
    bool public canBuy;
    bool public canSell;
    uint256 public minBuyAmount = 1000 * 1e18;
    uint256 constant PRICE_UNIT = 1e8;   
    mapping(address => uint256) public balanceOf;
    address private governance;

    event Buy(address indexed user, uint256 token_amount, uint256 currency_amount, bool send);
    event Sell(address indexed user, uint256 token_amount, uint256 currency_amount);
    
    constructor () public {
        governance = msg.sender;
    }

    modifier onlyOwner() {
        require(msg.sender == governance, "!governance");
        _;
    }

    function start(address _presale_token, address _currency_token, 
        uint256 _presale_decimals, uint256 _currency_decimals, uint256 _price) public onlyOwner {
        require(_presale_token != address(0), "Token is non-contract");
        require(_presale_token != address(0) && _presale_token.isContract(), "Subscribe stoken is non-contract");
        require(_currency_token != address(0), "Token is non-contract");
        require(_currency_token != address(0) && _currency_token.isContract(), "Currency token is non-contract");

        presale_token = IERC20(_presale_token);
        currency_token = IUSDT(_currency_token);
        presale_decimals = _presale_decimals;
        currency_decimals = _currency_decimals;
        price = _price;
        canBuy = true;
        canSell = false;
    }

    function buy(uint256 token_amount) public {
        require(canBuy, "Buy not start yet, please wait...");
        require(token_amount >= minBuyAmount, "Subscribe amount must be larger than 1000");
        require(balanceOf[msg.sender] == 0, "Subscribe only once");
        
        uint256 currency_amount = token_amount * currency_decimals * price / (PRICE_UNIT * presale_decimals);
        uint256 total = presale_token.balanceOf(address(this));
        currency_token.safeTransferFrom(msg.sender, address(this), currency_amount);
        totalSupply = totalSupply.add(token_amount);
        balanceOf[msg.sender] = balanceOf[msg.sender].add(token_amount);
        if (token_amount <= total)
        {
            presale_token.safeTransfer(msg.sender, token_amount);
        }
        
        //emit Buy(msg.sender, token_amount, currency_amount, false);    
        emit Buy(msg.sender, token_amount, currency_amount, token_amount <= total);
    }

    function sell(uint256 token_amount) public {
        require(canSell, "Not end yet, please wait...");
        require(token_amount > 0, "Sell amount must be larger than 0");
        require(token_amount <= balanceOf[msg.sender], "Token balance is not enough");
        require(token_amount <= totalSupply, "Token balance is larger than totalSupply");
        
        uint256 currency_amount = token_amount * currency_decimals * price / (PRICE_UNIT * presale_decimals);
        currency_token.safeTransfer(msg.sender, currency_amount);
        presale_token.safeTransferFrom(msg.sender, address(this), token_amount);
        totalSupply = totalSupply.sub(token_amount);
        balanceOf[msg.sender] = balanceOf[msg.sender].sub(token_amount);
        
        emit Sell(msg.sender, token_amount, currency_amount);
    }

    function end() public onlyOwner {
        canBuy = false;
        canSell = true;
    }

    function finish() public onlyOwner {
        canBuy = false;
        canSell = false;
    }

    function getback(address account) public onlyOwner {
        uint256 leftPresale = presale_token.balanceOf(address(this));
        if (leftPresale > 0) {
            presale_token.safeTransfer(account, leftPresale);
        }
        uint256 leftCurrency = currency_token.balanceOf(address(this));
        if (leftCurrency > 0) {
            currency_token.safeTransfer(account, leftCurrency);
        }
    }

    function setMinBuyAmount(uint256 amount) public onlyOwner {
        minBuyAmount = amount;
    }
}