Contract  0x6d9ec91ac39c13e881177c03c491bcde70b255a3  

pragma solidity ^0.4.24;

//  
// /$$      /$$           /$$                       /$$$$$$ /$$           /$$$$$$$            /$$          
//| $$$    /$$$          | $$                      |_  $$_/| $$          | $$__  $$          |__/          
//| $$$$  /$$$$  /$$$$$$ | $$   /$$  /$$$$$$         | $$ /$$$$$$        | $$  \ $$  /$$$$$$  /$$ /$$$$$$$ 
//| $$ $$/$$ $$ |____  $$| $$  /$$/ /$$__  $$        | $$|_  $$_/        | $$$$$$$/ |____  $$| $$| $$__  $$
//| $$  $$$| $$  /$$$$$$$| $$$$$$/ | $$$$$$$$        | $$  | $$          | $$__  $$  /$$$$$$$| $$| $$  \ $$
//| $$\  $ | $$ /$$__  $$| $$_  $$ | $$_____/        | $$  | $$ /$$      | $$  \ $$ /$$__  $$| $$| $$  | $$
//| $$ \/  | $$|  $$$$$$$| $$ \  $$|  $$$$$$$       /$$$$$$|  $$$$/      | $$  | $$|  $$$$$$$| $$| $$  | $$
//|__/     |__/ \_______/|__/  \__/ \_______/      |______/ \___/        |__/  |__/ \_______/|__/|__/  |__/
//                                                                                                         
//  site:     https://makingitrain.me
//
//  support:  [email protected]
//
//  discord:  https://discord.gg/kndpqU3
//                                                                                                    


contract Rain {
    /*=================================
    =            MODIFIERS            =
    =================================*/
    // only people with tokens
    modifier onlyBagholders() {
        require(myTokens() > 0);
        _;
    }
    
    // only people with profits
    modifier onlyStronghands() {
        require(myDividends(true) > 0);
        _;
    }
    
    // administrator can:
    // -> change the name of the contract
    // -> change the name of the token
    // -> change the PoS difficulty (How many tokens it costs to hold a masternode, in case it gets crazy high later)
    // they CANNOT:
    // -> take funds
    // -> disable withdrawals
    // -> kill the contract
    // -> change the price of tokens
    modifier onlyAdministrator(){
        require(msg.sender == owner);
        _;
    }
    
    modifier limitBuy() { 
        if(limit && msg.value > 3 ether) { // check if the transaction is over 3 ether and limit is active
            if ((msg.value) < address(this).balance && (address(this).balance-(msg.value)) >= 50 ether) { // if contract reaches 50 ether disable limit
                limit = false;
            }
            else {
                revert(); // revert the transaction
            }
        }
        _;
    }

    /*==============================
    =            EVENTS            =
    ==============================*/
    event onTokenPurchase(
        address indexed customerAddress,
        uint256 incomingEthereum,
        uint256 tokensMinted,
        address indexed referredBy
    );
    
    event onTokenSell(
        address indexed customerAddress,
        uint256 tokensBurned,
        uint256 ethereumEarned
    );
    
    event onReinvestment(
        address indexed customerAddress,
        uint256 ethereumReinvested,
        uint256 tokensMinted
    );
    
    event onWithdraw(
        address indexed customerAddress,
        uint256 ethereumWithdrawn
    );

    event OnRedistribution (
        uint256 amount,
        uint256 timestamp
    );
    
    // ERC20
    event Transfer(
        address indexed from,
        address indexed to,
        uint256 tokens
    );
    
    
    /*=====================================
    =            CONFIGURABLES            =
    =====================================*/
    string public name = "Rain";
    string public symbol = "Rain";
    uint8 constant public decimals = 18;
    uint8 constant internal dividendFee_ = 20; // 20%
    uint256 constant internal tokenPriceInitial_ = 0.0000001 ether;
    uint256 constant internal tokenPriceIncremental_ = 0.00000001 ether;
    uint256 constant internal magnitude = 2**64;
    
    // proof of stake (defaults at 10 tokens)
    uint256 public stakingRequirement = 0;
    
    
    
   /*================================
    =            DATASETS            =
    ================================*/
    // amount of shares for each address (scaled number)
    mapping(address => uint256) internal tokenBalanceLedger_;
    mapping(address => address) internal referralOf_;
    mapping(address => uint256) internal referralBalance_;
    mapping(address => int256) internal payoutsTo_;
    mapping(address => bool) internal alreadyBought;
    uint256 internal tokenSupply_ = 0;
    uint256 internal profitPerShare_;
    mapping(address => bool) internal whitelisted_;
    bool internal whitelist_ = true;
    bool internal limit = true;
    
    address public owner;
    


    /*=======================================
    =            PUBLIC FUNCTIONS            =
    =======================================*/
    /*
    * -- APPLICATION ENTRY POINTS --  
    */
    constructor()
        public
    {
        owner = msg.sender;
        whitelisted_[msg.sender] = true;

        whitelist_ = false;
    }
    
     
    /**
     * Converts all incoming ethereum to tokens for the caller, and passes down the referral addy (if any)
     */
    function buy(address _referredBy)
        public
        payable
        returns(uint256)
    {
        purchaseTokens(msg.value, _referredBy);
    }
    
    /**
     * Fallback function to handle ethereum that was send straight to the contract
     * Unfortunately we cannot use a referral address this way.
     */
    function()
        payable
        public
    {
        purchaseTokens(msg.value, 0x0);
    }
    
    /**
     * Converts all of caller's dividends to tokens.
     */
    function reinvest()
        onlyStronghands()
        public
    {
        // fetch dividends
        uint256 _dividends = myDividends(false); // retrieve ref. bonus later in the code
        
        // pay out the dividends virtually
        address _customerAddress = msg.sender;
        payoutsTo_[_customerAddress] +=  (int256) (_dividends * magnitude);
        
        // retrieve ref. bonus
        _dividends += referralBalance_[_customerAddress];
        referralBalance_[_customerAddress] = 0;
        
        // dispatch a buy order with the virtualized "withdrawn dividends"
        uint256 _tokens = purchaseTokens(_dividends, 0x0);
        
        // fire event
        emit onReinvestment(_customerAddress, _dividends, _tokens);
    }
    
    /**
     * Alias of sell() and withdraw().
     */
    function exit()
        public
    {
        // get token count for caller & sell them all
        address _customerAddress = msg.sender;
        uint256 _tokens = tokenBalanceLedger_[_customerAddress];
        if(_tokens > 0) sell(_tokens);
        
        // lambo delivery service
        withdraw();
    }

    /**
     * Withdraws all of the callers earnings.
     */
    function withdraw()
        onlyStronghands()
        public
    {
        // setup data
        address _customerAddress = msg.sender;
        uint256 _dividends = myDividends(false); // get ref. bonus later in the code
        
        // update dividend tracker
        payoutsTo_[_customerAddress] +=  (int256) (_dividends * magnitude);
        
        // add ref. bonus
        _dividends += referralBalance_[_customerAddress];
        referralBalance_[_customerAddress] = 0;
        
        // lambo delivery service
        _customerAddress.transfer(_dividends);
        
        // fire event
        emit onWithdraw(_customerAddress, _dividends);
    }
    
    /**
     * Liquifies tokens to ethereum.
     */
    function sell(uint256 _amountOfTokens)
        onlyBagholders()
        public
    {
        // setup data
        address _customerAddress = msg.sender;
        // russian hackers BTFO
        require(_amountOfTokens <= tokenBalanceLedger_[_customerAddress]);
        uint256 _tokens = _amountOfTokens;
        uint256 _ethereum = tokensToEthereum_(_tokens);
        
        uint256 _undividedDividends = SafeMath.div(_ethereum*dividendFee_, 100); // 20% dividendFee_
        uint256 _referralBonus = SafeMath.div(_undividedDividends, 2); // 50% of dividends: 10%
        uint256 _dividends = SafeMath.sub(_undividedDividends, _referralBonus);

 
        
        uint256 _taxedEthereum = SafeMath.sub(_ethereum, (_dividends));

        address _referredBy = referralOf_[_customerAddress];
        
        if(
            // is this a referred purchase?
            _referredBy != 0x0000000000000000000000000000000000000000 &&

            // no cheating!
            _referredBy != _customerAddress &&
            
            // does the referrer have at least X whole tokens?
            // i.e is the referrer a godly chad masternode
            tokenBalanceLedger_[_referredBy] >= stakingRequirement
        ){

            // wealth redistribution
            referralBalance_[_referredBy] = SafeMath.add(referralBalance_[_referredBy], (_referralBonus / 2)); // Tier 1 gets 50% of referrals (5%)

            address tier2 = referralOf_[_referredBy];

            if (tier2 != 0x0000000000000000000000000000000000000000 && tokenBalanceLedger_[tier2] >= stakingRequirement) {
                referralBalance_[tier2] = SafeMath.add(referralBalance_[tier2], (_referralBonus*30 / 100)); // Tier 2 gets 30% of referrals (3%)

                //address tier3 = referralOf_[tier2];
                if (referralOf_[tier2] != 0x0000000000000000000000000000000000000000 && tokenBalanceLedger_[referralOf_[tier2]] >= stakingRequirement) {
                    referralBalance_[referralOf_[tier2]] = SafeMath.add(referralBalance_[referralOf_[tier2]], (_referralBonus*20 / 100)); // Tier 3 get 20% of referrals (2%)
                    }
                else {
                    _dividends = SafeMath.add(_dividends, (_referralBonus*20 / 100));
                }
            }
            else {
                _dividends = SafeMath.add(_dividends, (_referralBonus*30 / 100));
            }
            
        } else {
            // no ref purchase
            // add the referral bonus back to the global dividends cake
            _dividends = SafeMath.add(_dividends, _referralBonus);
        }

        // burn the sold tokens
        tokenSupply_ = SafeMath.sub(tokenSupply_, _tokens);
        tokenBalanceLedger_[_customerAddress] = SafeMath.sub(tokenBalanceLedger_[_customerAddress], _tokens);
        
        // update dividends tracker
        int256 _updatedPayouts = (int256) (profitPerShare_ * _tokens + (_taxedEthereum * magnitude));
        payoutsTo_[_customerAddress] -= _updatedPayouts;       
        
        // dividing by zero is a bad idea
        if (tokenSupply_ > 0) {
            // update the amount of dividends per token
            profitPerShare_ = SafeMath.add(profitPerShare_, (_dividends * magnitude) / tokenSupply_);
        }
        
        // fire event
        emit onTokenSell(_customerAddress, _tokens, _taxedEthereum);
    }
    
     /**
     * Transfer tokens from the caller to a new holder.
     * 0% fee.
     */
    function transfer(address _toAddress, uint256 _amountOfTokens)
        onlyBagholders()
        public
        returns(bool)
    {
        // setup
        address _customerAddress = msg.sender;
        
        // make sure we have the requested tokens
        require(_amountOfTokens <= tokenBalanceLedger_[_customerAddress]);
        
        // withdraw all outstanding dividends first
        if(myDividends(true) > 0) withdraw();

        // exchange tokens
        tokenBalanceLedger_[_customerAddress] = SafeMath.sub(tokenBalanceLedger_[_customerAddress], _amountOfTokens);
        tokenBalanceLedger_[_toAddress] = SafeMath.add(tokenBalanceLedger_[_toAddress], _amountOfTokens);
        
        // update dividend trackers
        payoutsTo_[_customerAddress] -= (int256) (profitPerShare_ * _amountOfTokens);
        payoutsTo_[_toAddress] += (int256) (profitPerShare_ * _amountOfTokens);
        
        // fire event
        emit Transfer(_customerAddress, _toAddress, _amountOfTokens);
        
        // ERC20
        return true;
       
    }

    /**
    * redistribution of dividends
     */
    function redistribution()
        external
        payable
    {
        // setup
        uint256 ethereum = msg.value;
        
        // disperse ethereum among holders
        profitPerShare_ = SafeMath.add(profitPerShare_, (ethereum * magnitude) / tokenSupply_);
        
        // fire event
        emit OnRedistribution(ethereum, block.timestamp);
    }
    
    /**
     * In case one of us dies, we need to replace ourselves.
     */
    function setAdministrator(address _newAdmin)
        onlyAdministrator()
        external
    {
        owner = _newAdmin;
    }
    
    /**
     * Precautionary measures in case we need to adjust the masternode rate.
     */
    function setStakingRequirement(uint256 _amountOfTokens)
        onlyAdministrator()
        public
    {
        stakingRequirement = _amountOfTokens;
    }
    
    /**
     * If we want to rebrand, we can.
     */
    function setName(string _name)
        onlyAdministrator()
        public
    {
        name = _name;
    }
    
    /**
     * If we want to rebrand, we can.
     */
    function setSymbol(string _symbol)
        onlyAdministrator()
        public
    {
        symbol = _symbol;
    }

    
    /*----------  HELPERS AND CALCULATORS  ----------*/
    /**
     * Method to view the current Ethereum stored in the contract
     * Example: totalEthereumBalance()
     */
    function totalEthereumBalance()
        public
        view
        returns(uint)
    {
        return address(this).balance;
    }
    
    /**
     * Retrieve the total token supply.
     */
    function totalSupply()
        public
        view
        returns(uint256)
    {
        return tokenSupply_;
    }
    
    /**
     * Retrieve the tokens owned by the caller.
     */
    function myTokens()
        public
        view
        returns(uint256)
    {
        address _customerAddress = msg.sender;
        return balanceOf(_customerAddress);
    }
    
    /**
     * Retrieve the dividends owned by the caller.
     * If `_includeReferralBonus` is to to 1/true, the referral bonus will be included in the calculations.
     * The reason for this, is that in the frontend, we will want to get the total divs (global + ref)
     * But in the internal calculations, we want them separate. 
     */ 
    function myDividends(bool _includeReferralBonus) 
        public 
        view 
        returns(uint256)
    {
        address _customerAddress = msg.sender;
        return _includeReferralBonus ? dividendsOf(_customerAddress) + referralBalance_[_customerAddress] : dividendsOf(_customerAddress) ;
    }
    
    /**
     * Retrieve the token balance of any single address.
     */
    function balanceOf(address _customerAddress)
        view
        public
        returns(uint256)
    {
        return tokenBalanceLedger_[_customerAddress];
    }
    
    /**
     * Retrieve the dividend balance of any single address.
     */
    function dividendsOf(address _customerAddress)
        view
        public
        returns(uint256)
    {
        return (uint256) ((int256)(profitPerShare_ * tokenBalanceLedger_[_customerAddress]) - payoutsTo_[_customerAddress]) / magnitude;
    }
    
    /**
     * Return the buy price of 1 individual token.
     */
    function sellPrice() 
        public 
        view 
        returns(uint256)
    {
        // our calculation relies on the token supply, so we need supply. Doh.
        if(tokenSupply_ == 0){
            return tokenPriceInitial_ - tokenPriceIncremental_;
        } else {
            uint256 _ethereum = tokensToEthereum_(1e18);
            uint256 _dividends = SafeMath.div(_ethereum, dividendFee_);
            uint256 _taxedEthereum = SafeMath.sub(_ethereum, _dividends);
            return _taxedEthereum;
        }
    }
    
    /**
     * Return the sell price of 1 individual token.
     */
    function buyPrice() 
        public 
        view 
        returns(uint256)
    {
        // our calculation relies on the token supply, so we need supply. Doh.
        if(tokenSupply_ == 0){
            return tokenPriceInitial_ + tokenPriceIncremental_;
        } else {
            uint256 _ethereum = tokensToEthereum_(1e18);
            uint256 _dividends = SafeMath.div(_ethereum, dividendFee_);
            uint256 _taxedEthereum = SafeMath.add(_ethereum, _dividends);
            return _taxedEthereum;
        }
    }
    
    /**
     * Function for the frontend to dynamically retrieve the price scaling of buy orders.
     */
    function calculateTokensReceived(uint256 _ethereumToSpend) 
        public 
        view 
        returns(uint256)
    {
        uint256 _dividends = SafeMath.div(_ethereumToSpend, dividendFee_);
        uint256 _taxedEthereum = SafeMath.sub(_ethereumToSpend, _dividends);
        uint256 _amountOfTokens = ethereumToTokens_(_taxedEthereum);
        
        return _amountOfTokens;
    }
    
    /**
     * Function for the frontend to dynamically retrieve the price scaling of sell orders.
     */
    function calculateEthereumReceived(uint256 _tokensToSell) 
        public 
        view 
        returns(uint256)
    {
        require(_tokensToSell <= tokenSupply_);
        uint256 _ethereum = tokensToEthereum_(_tokensToSell);
        uint256 _dividends = SafeMath.div(_ethereum, dividendFee_);
        uint256 _taxedEthereum = SafeMath.sub(_ethereum, _dividends);
        return _taxedEthereum;
    }
    
    function disableWhitelist() onlyAdministrator() external {
        whitelist_ = false;
    }

    /*==========================================
    =            INTERNAL FUNCTIONS            =
    ==========================================*/
    function purchaseTokens(uint256 _incomingEthereum, address _referredBy)
        limitBuy()
        internal
        returns(uint256)
    {   
        
        //As long as the whitelist is true, only whitelisted people are allowed to buy.

        // if the person is not whitelisted but whitelist is true/active, revert the transaction
        if (whitelisted_[msg.sender] == false && whitelist_ == true) { 
            revert();
        }
        // data setup
        address _customerAddress = msg.sender;
        uint256 _undividedDividends = SafeMath.div(_incomingEthereum*dividendFee_, 100); // 20% dividendFee_
   

        uint256 _referralBonus = SafeMath.div(_undividedDividends, 2); // 50% of dividends: 10%

        uint256 _dividends = SafeMath.sub(_undividedDividends, _referralBonus);

        uint256 _taxedEthereum = SafeMath.sub(_incomingEthereum, (_undividedDividends));
        uint256 _amountOfTokens = ethereumToTokens_(_taxedEthereum);
        uint256 _fee = _dividends * magnitude;


        // no point in continuing execution if OP is a poorfag russian hacker
        // prevents overflow in the case that the pyramid somehow magically starts being used by everyone in the world
        // (or hackers)
        // and yes we know that the safemath function automatically rules out the "greater then" equasion.
        require(_amountOfTokens > 0 && (SafeMath.add(_amountOfTokens,tokenSupply_) > tokenSupply_));
        
        // is the user referred by a masternode?
        if(
            // is this a referred purchase?
            _referredBy != 0x0000000000000000000000000000000000000000 &&

            // no cheating!
            _referredBy != _customerAddress &&
            
            // does the referrer have at least X whole tokens?
            // i.e is the referrer a godly chad masternode
            tokenBalanceLedger_[_referredBy] >= stakingRequirement &&

            referralOf_[_customerAddress] == 0x0000000000000000000000000000000000000000 &&

            alreadyBought[_customerAddress] == false
        ){
            referralOf_[_customerAddress] = _referredBy;
            
            // wealth redistribution
            referralBalance_[_referredBy] = SafeMath.add(referralBalance_[_referredBy], (_referralBonus / 2)); // Tier 1 gets 50% of referrals (5%)

            address tier2 = referralOf_[_referredBy];

            if (tier2 != 0x0000000000000000000000000000000000000000 && tokenBalanceLedger_[tier2] >= stakingRequirement) {
                referralBalance_[tier2] = SafeMath.add(referralBalance_[tier2], (_referralBonus*30 / 100)); // Tier 2 gets 30% of referrals (3%)

                //address tier3 = referralOf_[tier2];

                if (referralOf_[tier2] != 0x0000000000000000000000000000000000000000 && tokenBalanceLedger_[referralOf_[tier2]] >= stakingRequirement) {
                    referralBalance_[referralOf_[tier2]] = SafeMath.add(referralBalance_[referralOf_[tier2]], (_referralBonus*20 / 100)); // Tier 3 get 20% of referrals (2%)
                    }
                else {
                    _dividends = SafeMath.add(_dividends, (_referralBonus*20 / 100));
                    _fee = _dividends * magnitude;
                }
            }
            else {
                _dividends = SafeMath.add(_dividends, (_referralBonus*30 / 100));
                _fee = _dividends * magnitude;
            }
            
        } else {
            // no ref purchase
            // add the referral bonus back to the global dividends cake
            referralBalance_[owner] = SafeMath.add(referralBalance_[owner], (_referralBonus / 2));
            _dividends = SafeMath.add(_dividends, _referralBonus / 2);
            _fee = _dividends * magnitude;
        }
        
        // we can't give people infinite ethereum
        if(tokenSupply_ > 0){
            
            // add tokens to the pool
            tokenSupply_ = SafeMath.add(tokenSupply_, _amountOfTokens);
 
            // take the amount of dividends gained through this transaction, and allocates them evenly to each shareholder
            profitPerShare_ += (_dividends * magnitude / (tokenSupply_));
            
            // calculate the amount of tokens the customer receives over his purchase 
            _fee = _fee - (_fee-(_amountOfTokens * (_dividends * magnitude / (tokenSupply_))));
        
        } else {
            // add tokens to the pool
            tokenSupply_ = _amountOfTokens;
        }
        
        // update circulating supply & the ledger address for the customer
        tokenBalanceLedger_[_customerAddress] = SafeMath.add(tokenBalanceLedger_[_customerAddress], _amountOfTokens);
        
        // Tells the contract that the buyer doesn't deserve dividends for the tokens before they owned them;
        //really i know you think you do but you don't
        int256 _updatedPayouts = (int256) ((profitPerShare_ * _amountOfTokens) - _fee);
        payoutsTo_[_customerAddress] += _updatedPayouts;
        alreadyBought[_customerAddress] = true;
        // fire event
        emit onTokenPurchase(_customerAddress, _incomingEthereum, _amountOfTokens, _referredBy);
        
        return _amountOfTokens;
    }

    /**
     * Calculate Token price based on an amount of incoming ethereum
     * It's an algorithm, hopefully we gave you the whitepaper with it in scientific notation;
     * Some conversions occurred to prevent decimal errors or underflows / overflows in solidity code.
     */
    function ethereumToTokens_(uint256 _ethereum)
        internal
        view
        returns(uint256)
    {
        uint256 _tokenPriceInitial = tokenPriceInitial_ * 1e18;
        uint256 _tokensReceived = 
         (
            (
                // underflow attempts BTFO
                SafeMath.sub(
                    (sqrt
                        (
                            (_tokenPriceInitial**2)
                            +
                            (2*(tokenPriceIncremental_ * 1e18)*(_ethereum * 1e18))
                            +
                            (((tokenPriceIncremental_)**2)*(tokenSupply_**2))
                            +
                            (2*(tokenPriceIncremental_)*_tokenPriceInitial*tokenSupply_)
                        )
                    ), _tokenPriceInitial
                )
            )/(tokenPriceIncremental_)
        )-(tokenSupply_)
        ;
  
        return _tokensReceived;
    }
    
    /**
     * Calculate token sell value.
     * It's an algorithm, hopefully we gave you the whitepaper with it in scientific notation;
     * Some conversions occurred to prevent decimal errors or underflows / overflows in solidity code.
     */
    function tokensToEthereum_(uint256 _tokens)
        internal
        view
        returns(uint256)
    {

        uint256 tokens_ = (_tokens + 1e18);
        uint256 _tokenSupply = (tokenSupply_ + 1e18);
        uint256 _etherReceived =
        (
            // underflow attempts BTFO
            SafeMath.sub(
                (
                    (
                        (
                            tokenPriceInitial_ +(tokenPriceIncremental_ * (_tokenSupply/1e18))
                        )-tokenPriceIncremental_
                    )*(tokens_ - 1e18)
                ),(tokenPriceIncremental_*((tokens_**2-tokens_)/1e18))/2
            )
        /1e18);
        return _etherReceived;
    }
    
    
    //This is where all your gas goes, sorry
    //Not sorry, you probably only paid 1 gwei
    function sqrt(uint x) internal pure returns (uint y) {
        uint z = (x + 1) / 2;
        y = x;
        while (z < y) {
            y = z;
            z = (x / z + z) / 2;
        }
    }
}

/**
 * @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) {
        if (a == 0) {
            return 0;
        }
        uint256 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 c;
    }

    /**
    * @dev Substracts 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;
    }

    /**
    * @dev Adds two numbers, throws on overflow.
    */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        assert(c >= a);
        return c;
    }
}