pragma solidity ^0.4.25;


/**
 * VNET Token Pre-Sale Contract
 * 
 * Send ETH here, and you will receive the VNET Tokens immediately.
 * 
 * https://vision.network/
 */


/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/179
 */
contract ERC20Basic {
    function totalSupply() public view returns (uint256);
    function balanceOf(address _who) public view returns (uint256);
    function transfer(address _to, uint256 _value) public returns (bool);
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
}


/**
 * @title Ownable
 * @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
     * account.
     */
    constructor() public {
        owner = msg.sender;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }

    /**
     * @dev Allows the current owner to transfer control of the contract to a newOwner.
     * @param _newOwner The address to transfer ownership to.
     */
    function transferOwnership(address _newOwner) public onlyOwner {
        require(_newOwner != address(0));
        emit OwnershipTransferred(owner, _newOwner);
        owner = _newOwner;
    }

    /**
     * @dev Rescue compatible ERC20Basic Token
     *
     * @param _token ERC20Basic The address of the token contract
     */
    function rescueTokens(ERC20Basic _token, address _receiver) external onlyOwner {
        uint256 balance = _token.balanceOf(this);
        assert(_token.transfer(_receiver, balance));
    }
}


/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {

    /**
     * @dev Multiplies two numbers, throws on overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
        if (a == 0) {
            return 0;
        }
        c = a * b;
        assert(c / a == b);
        return c;
    }

    /**
     * @dev Integer division of two numbers, truncating the quotient.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        // assert(b > 0); // Solidity automatically throws when dividing by 0
        // uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold
        return a / b;
    }

    /**
     * @dev Adds two numbers, throws on overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256 c) {
        c = a + b;
        assert(c >= a);
        return c;
    }

    /**
     * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;
    }
}


/**
 * @title VNET Token Pre-Sale
 */
contract VNETTokenPreSale is Ownable {
    using SafeMath for uint256;

    string public description = "VNET Token Pre-Sale Contract";
    
    ERC20Basic public vnetToken;
    address wallet;
    uint256 public ratioNext; // with 6 decimals
    uint256 public ethPrice; // with 8 decimals
    uint256 public vnetSold; // with 8 decimals
    uint256 public vnetSupply = 30 * (10 ** 8) * (10 ** 6); // 30 billion supply
    uint256 public vnetPriceStart = 0.0026 * (10 ** 8); // 0.0026 USD
    uint256 public vnetPriceTarget = 0.0065 * (10 ** 8); // 0.0065 USD
    uint256 public weiMinimum = 1 * (10 ** 18); // 1 Ether
    uint256 public weiMaximum = 100 * (10 ** 18); // 100 Ether
    uint256 public weiWelfare = 10 * (10 ** 18); // 10 Ether

    mapping(address => bool) public welfare;

    event Welfare(address indexed _buyer);
    event BuyVNET(address indexed _buyer, uint256 _ratio, uint256 _vnetAmount, uint256 _weiAmount);
    event EthPrice(uint256 _ethPrice);


    /**
     * @dev Constructor
     */
    constructor(ERC20Basic _vnetToken, uint256 _ethPrice) public {
        vnetToken = _vnetToken;
        wallet = owner;
        calcRatioNext();
        updateEthPrice(_ethPrice);
    }

    /**
     * @dev receive ETH and send tokens
     */
    function () public payable {
        // Make sure token balance > 0
        uint256 vnetBalance = vnetToken.balanceOf(address(this));
        require(vnetBalance > 0);
        require(vnetSold < vnetSupply);
        
        // Minimum & Maximum Limit
        uint256 weiAmount = msg.value;
        require(weiAmount >= weiMinimum);
        require(weiAmount <= weiMaximum);

        // VNET Token Amount to be transfer
        uint256 vnetAmount = weiAmount.mul(ratioNext).div(10 ** 18);

        // Transfer VNET
        if (vnetBalance >= vnetAmount) {
            assert(vnetToken.transfer(msg.sender, vnetAmount));
            emit BuyVNET(msg.sender, ratioNext, vnetAmount, weiAmount);
            vnetSold = vnetSold.add(vnetAmount);
            if (weiAmount >= weiWelfare) {
                welfare[msg.sender] = true;
                emit Welfare(msg.sender);
            }
        } else {
            uint256 weiExpend = vnetBalance.mul(10 ** 18).div(ratioNext);
            assert(vnetToken.transfer(msg.sender, vnetBalance));
            emit BuyVNET(msg.sender, ratioNext, vnetBalance, weiExpend);
            vnetSold = vnetSold.add(vnetBalance);
            msg.sender.transfer(weiAmount.sub(weiExpend));
            if (weiExpend >= weiWelfare) {
                welfare[msg.sender] = true;
                emit Welfare(msg.sender);
            }
        }

        // Calculate: ratioNext
        calcRatioNext();

        // transfer Ether
        uint256 etherBalance = address(this).balance;
        wallet.transfer(etherBalance);
    }

    /**
     * @dev calculate ration next
     */
    function calcRatioNext() private {
        ratioNext = ethPrice.mul(10 ** 6).div(vnetPriceStart.add(vnetPriceTarget.sub(vnetPriceStart).mul(vnetSold).div(vnetSupply)));
    }

    /**
     * @dev update wallet
     */
    function updateWallet(address _wallet) onlyOwner public {
        wallet = _wallet;
    }

    /**
     * @dev update ETH Price
     */
    function updateEthPrice(uint256 _ethPrice) onlyOwner public {
        ethPrice = _ethPrice;
        emit EthPrice(_ethPrice);
        calcRatioNext();
    }
}