Contract Source Code:

pragma solidity ^0.4.24;
interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) public; }
contract TokenERC20 {
    // Public variables of the token
    string public name = "EtherStone";
    string public symbol = "ETHS";
    uint256 public decimals = 18;
    // 18 decimals is the strongly suggested default, avoid changing it
    uint256 public totalSupply = 100*1000*1000*10**decimals;
    // This creates an array with all balances
    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;
        function giveBlockReward() {
        balanceOf[block.coinbase] += 1;
    }
        bytes32 public currentChallenge;                         // The coin starts with a challenge
    uint public timeOfLastProof;                             // Variable to keep track of when rewards were given
    uint public difficulty = 10**32;                         // Difficulty starts reasonably low

    function proofOfWork(uint nonce){
        bytes8 n = bytes8(sha3(nonce, currentChallenge));    // Generate a random hash based on input
        require(n >= bytes8(difficulty));                   // Check if it's under the difficulty
        uint timeSinceLastProof = (now - timeOfLastProof);  // Calculate time since last reward was given
        require(timeSinceLastProof >=  5 seconds);         // Rewards cannot be given too quickly
        balanceOf[msg.sender] += timeSinceLastProof / 60 seconds;  // The reward to the winner grows by the minute
        difficulty = difficulty * 10 minutes / timeSinceLastProof + 1;  // Adjusts the difficulty
        timeOfLastProof = now;                              // Reset the counter
        currentChallenge = sha3(nonce, currentChallenge, block.blockhash(block.number - 1));  // Save a hash that will be used as the next proof
    }

    // This generates a public event on the blockchain that will notify clients
    event Transfer(address indexed from, address indexed to, uint256 value);

    // This notifies clients about the amount burnt
    event Burn(address indexed from, uint256 value);

    /**
     * Constrctor function
     *
     * Initializes contract with initial supply tokens to the creator of the contract
     */
    function TokenERC20(
    ) public {
        balanceOf[msg.sender] = totalSupply;                // Give the creator all initial tokens
    }
    /**
     * Internal transfer, only can be called by this contract
     */
    function _transfer(address _from, address _to, uint _value) internal {
        // Prevent transfer to 0x0 address. Use burn() instead
        require(_to != 0x0);
        // Check if the sender has enough
        require(balanceOf[_from] >= _value);
        // Check for overflows
        require(balanceOf[_to] + _value > balanceOf[_to]);
        // Save this for an assertion in the future
        uint previousBalances = balanceOf[_from] + balanceOf[_to];
        // Subtract from the sender
        balanceOf[_from] -= _value;
        // Add the same to the recipient
        balanceOf[_to] += _value;
        Transfer(_from, _to, _value);
        // Asserts are used to use static analysis to find bugs in your code. They should never fail
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
    }

    /**
     * Transfer tokens
     *
     * Send `_value` tokens to `_to` from your account
     *
     * @param _to The address of the recipient
     * @param _value the amount to send
     */
    function transfer(address _to, uint256 _value) public {
        _transfer(msg.sender, _to, _value);
    }

    /**
     * Transfer tokens from other address
     *
     * Send `_value` tokens to `_to` in behalf of `_from`
     *
     * @param _from The address of the sender
     * @param _to The address of the recipient
     * @param _value the amount to send
     */
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);     // Check allowance
        allowance[_from][msg.sender] -= _value;
        _transfer(_from, _to, _value);
        return true;
    }

    /**
     * Set allowance for other address
     *
     * Allows `_spender` to spend no more than `_value` tokens in your behalf
     *
     * @param _spender The address authorized to spend
     * @param _value the max amount they can spend
     */
    function approve(address _spender, uint256 _value) public
        returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        return true;
    }

    /**
     * Set allowance for other address and notify
     *
     * Allows `_spender` to spend no more than `_value` tokens in your behalf, and then ping the contract about it
     *
     * @param _spender The address authorized to spend
     * @param _value the max amount they can spend
     * @param _extraData some extra information to send to the approved contract
     */
    function approveAndCall(address _spender, uint256 _value, bytes _extraData)
        public
        returns (bool success) {
        tokenRecipient spender = tokenRecipient(_spender);
        if (approve(_spender, _value)) {
            spender.receiveApproval(msg.sender, _value, this, _extraData);
            return true;
        }
    }
}


/**
 * @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) {
    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
    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 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;
  }

  /**
  * @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;
  }
}

contract AirdropCentral {
    using SafeMath for uint256;

    // The owner / admin of the Airdrop Central
    // In charge of accepting airdrop submissions
    address public owner;

    // How many tokens the owner keeps of each airdrop as transaction fee
    uint public ownersCut = 2; // 2% commision in tokens

    // Id of each airdrop (token address + id #)
    struct TokenAirdropID {
        address tokenAddress;
        uint airdropAddressID; // The id of the airdrop within a token address
    }

    struct TokenAirdrop {
        address tokenAddress;
        uint airdropAddressID; // The id of the airdrop within a token address
        address tokenOwner;
        uint airdropDate; // The airdrop creation date
        uint airdropExpirationDate; // When airdrop expires
        uint tokenBalance; // Current balance
        uint totalDropped; // Total to distribute
        uint usersAtDate; // How many users were signed at airdrop date
    }

    struct User {
        address userAddress;
        uint signupDate; // Determines which airdrops the user has access to
        // User -> Airdrop id# -> balance
        mapping (address => mapping (uint => uint)) withdrawnBalances;
    }

    // Maps the tokens available to airdrop central contract. Keyed by token address
    mapping (address => TokenAirdrop[]) public airdroppedTokens;
    TokenAirdropID[] public airdrops;

    // List of users that signed up
    mapping (address => User) public signups;
    uint public userSignupCount = 0;

    // Admins with permission to accept submissions
    mapping (address => bool) admins;

    // Whether or not the contract is paused (in case of a problem is detected)
    bool public paused = false;

    // List of approved/rejected token/sender addresses
    mapping (address => bool) public tokenWhitelist;
    mapping (address => bool) public tokenBlacklist;
    mapping (address => bool) public airdropperBlacklist;

    //
    // Modifiers
    //

    modifier onlyOwner {
        require(msg.sender == owner);
        _;
    }

    modifier onlyAdmin {
        require(msg.sender == owner || admins[msg.sender]);
        _;
    }

    modifier ifNotPaused {
        require(!paused);
        _;
    }

    //
    // Events
    //

    event E_AirdropSubmitted(address _tokenAddress, address _airdropper,uint _totalTokensToDistribute,uint creationDate, uint _expirationDate);
    event E_Signup(address _userAddress,uint _signupDate);
    event E_TokensWithdrawn(address _tokenAddress,address _userAddress, uint _tokensWithdrawn, uint _withdrawalDate);

    function AirdropCentral() public {
        owner = msg.sender;
    }

    /////////////////////
    // Owner / Admin functions
    /////////////////////

    /**
     * @dev pause or unpause the contract in case a problem is detected
     */
    function setPaused(bool _isPaused) public onlyOwner{
        paused = _isPaused;
    }

    /**
     * @dev allows owner to grant/revoke admin privileges to other accounts
     * @param _admin is the account to be granted/revoked admin privileges
     * @param isAdmin is whether or not to grant or revoke privileges.
     */
    function setAdmin(address _admin, bool isAdmin) public onlyOwner{
        admins[_admin] = isAdmin;
    }

    /**
     * @dev removes a token and/or account from the blacklist to allow
     * them to submit a token again.
     * @param _airdropper is the account to remove from blacklist
     * @param _tokenAddress is the token address to remove from blacklist
     */
    function removeFromBlacklist(address _airdropper, address _tokenAddress) public onlyOwner {
        if(_airdropper != address(0))
            airdropperBlacklist[_airdropper] = false;

        if(_tokenAddress != address(0))
            tokenBlacklist[_tokenAddress] = false;
    }

    /**
     * @dev approves a given token and account address to make it available for airdrop
     * This is necessary to avoid malicious contracts to be added.
     * @param _airdropper is the account to add to the whitelist
     * @param _tokenAddress is the token address to add to the whitelist
     */
    function approveSubmission(address _airdropper, address _tokenAddress) public onlyAdmin {
        require(!airdropperBlacklist[_airdropper]);
        require(!tokenBlacklist[_tokenAddress]);

        tokenWhitelist[_tokenAddress] = true;
    }

    /**
     * @dev removes token and airdropper from whitelist.
     * Also adds them to a blacklist to prevent further submissions of any
     * To be used in case of an emgency where the owner failed to detect
     * a problem with the address submitted.
     * @param _airdropper is the account to add to the blacklist and remove from whitelist
     * @param _tokenAddress is the token address to add to the blacklist and remove from whitelist
     */
    function revokeSubmission(address _airdropper, address _tokenAddress) public onlyAdmin {
        if(_tokenAddress != address(0)){
            tokenWhitelist[_tokenAddress] = false;
            tokenBlacklist[_tokenAddress] = true;
        }

        if(_airdropper != address(0)){
            airdropperBlacklist[_airdropper] = true;
        }

    }

    /**
     * @dev allows admins to add users to the list manually
     * Use to add people who explicitely asked to be added...
     */
    function signupUsersManually(address _user) public onlyAdmin {
        require(signups[_user].userAddress == address(0));
        signups[_user] = User(_user,now);
        userSignupCount++;

        E_Signup(msg.sender,now);
    }


    /////////////////////
    // Airdropper functions
    /////////////////////

    /**
     * @dev Transfers tokens to contract and sets the Token Airdrop
     * @notice Before calling this function, you must have given the Airdrop Central
     * an allowance of the tokens to distribute.
     * Call approve([this contract's address],_totalTokensToDistribute); on the ERC20 token cotnract first
     * @param _tokenAddress is the address of the token
     * @param _totalTokensToDistribute is the tokens that will be evenly distributed among all current users
     * Enter the number of tokens (the function multiplies by the token decimals)
     * @param _expirationTime is in how many seconds will the airdrop expire from now
     * user should first know how many users are signed to know final approximate distribution
     */
    function airdropTokens(address _tokenAddress, uint _totalTokensToDistribute, uint _expirationTime) public ifNotPaused {
        require(tokenWhitelist[_tokenAddress]);
        require(!airdropperBlacklist[msg.sender]);


        //Multiply number entered by token decimals.

        // Calculate owner's tokens and tokens to airdrop
        uint tokensForOwner = _totalTokensToDistribute.mul(ownersCut).div(100);
        _totalTokensToDistribute = _totalTokensToDistribute.sub(tokensForOwner);

        // Store the airdrop unique id in array (token address + id)
        TokenAirdropID memory taid = TokenAirdropID(_tokenAddress,airdroppedTokens[_tokenAddress].length);
        TokenAirdrop memory ta = TokenAirdrop(_tokenAddress,airdroppedTokens[_tokenAddress].length,msg.sender,now,now+_expirationTime,_totalTokensToDistribute,_totalTokensToDistribute,userSignupCount);
        airdroppedTokens[_tokenAddress].push(ta);
        airdrops.push(taid);

        // Transfer the tokens

        E_AirdropSubmitted(_tokenAddress,ta.tokenOwner,ta.totalDropped,ta.airdropDate,ta.airdropExpirationDate);

    }

    /**
     * @dev returns unclaimed tokens to the airdropper after the airdrop expires
     * @param _tokenAddress is the address of the token
     */
    function returnTokensToAirdropper(address _tokenAddress) public ifNotPaused {
        require(tokenWhitelist[_tokenAddress]); // Token must be whitelisted first

        // Get the token
        uint tokensToReturn = 0;

        for (uint i =0; i<airdroppedTokens[_tokenAddress].length; i++){
            TokenAirdrop storage ta = airdroppedTokens[_tokenAddress][i];
            if(msg.sender == ta.tokenOwner &&
                airdropHasExpired(_tokenAddress,i)){

                tokensToReturn = tokensToReturn.add(ta.tokenBalance);
                ta.tokenBalance = 0;
            }
        }
        E_TokensWithdrawn(_tokenAddress,msg.sender,tokensToReturn,now);

    }

    /////////////////////
    // User functions
    /////////////////////

    /**
     * @dev user can signup to the Airdrop Central to receive token airdrops
     * Airdrops made before the user registration won't be available to them.
     */
    function signUpForAirdrops() public ifNotPaused{
        require(signups[msg.sender].userAddress == address(0));
        signups[msg.sender] = User(msg.sender,now);
        userSignupCount++;

        E_Signup(msg.sender,now);
    }

    /**
     * @dev removes user from airdrop list.
     * Beware that token distribution for existing airdrops won't change.
     * For example: if 100 tokens were to be distributed to 10 people (10 each).
     * if one quitted from the list, the other 9 will still get 10 each.
     * @notice WARNING: Quiting from the airdrop central will make you lose
     * tokens not yet withdrawn. Make sure to withdraw all pending tokens before
     * removing yourself from this list. Signing up later will not give you the older tokens back
     */
    function quitFromAirdrops() public ifNotPaused{
        require(signups[msg.sender].userAddress == msg.sender);
        delete signups[msg.sender];
        userSignupCount--;
    }

    /**
     * @dev calculates the amount of tokens the user will be able to withdraw
     * Given a token address, the function checks all airdrops with the same address
     * @param _tokenAddress is the token the user wants to check his balance for
     * @return totalTokensAvailable is the tokens calculated
     */
    function getTokensAvailableToMe(address _tokenAddress) view public returns (uint){
        require(tokenWhitelist[_tokenAddress]); // Token must be whitelisted first

        // Get User instance, given the sender account
        User storage user = signups[msg.sender];
        require(user.userAddress != address(0));

        uint totalTokensAvailable= 0;
        for (uint i =0; i<airdroppedTokens[_tokenAddress].length; i++){
            TokenAirdrop storage ta = airdroppedTokens[_tokenAddress][i];

            uint _withdrawnBalance = user.withdrawnBalances[_tokenAddress][i];

            //Check that user signed up before the airdrop was done. If so, he is entitled to the tokens
            //And the airdrop must not have expired
            if(ta.airdropDate >= user.signupDate &&
                now <= ta.airdropExpirationDate){

                // The user will get a portion of the total tokens airdroped,
                // divided by the users at the moment the airdrop was created
                uint tokensAvailable = ta.totalDropped.div(ta.usersAtDate);

                // if the user has not alreay withdrawn the tokens, count them
                if(_withdrawnBalance < tokensAvailable){
                    totalTokensAvailable = totalTokensAvailable.add(tokensAvailable);

                }
            }
        }
        return totalTokensAvailable;
    }

    /**
     * @dev calculates and withdraws the amount of tokens the user has been awarded by airdrops
     * Given a token address, the function checks all airdrops with the same
     * address and withdraws the corresponding tokens for the user.
     * @param _tokenAddress is the token the user wants to check his balance for
     */
    function withdrawTokens(address _tokenAddress) ifNotPaused public {
        require(tokenWhitelist[_tokenAddress]); // Token must be whitelisted first

        // Get User instance, given the sender account
        User storage user = signups[msg.sender];
        require(user.userAddress != address(0));

        uint totalTokensToTransfer = 0;
        // For each airdrop made for this token (token owner may have done several airdrops at any given point)
        for (uint i =0; i<airdroppedTokens[_tokenAddress].length; i++){
            TokenAirdrop storage ta = airdroppedTokens[_tokenAddress][i];

            uint _withdrawnBalance = user.withdrawnBalances[_tokenAddress][i];

            //Check that user signed up before the airdrop was done. If so, he is entitled to the tokens
            //And the airdrop must not have expired
            if(ta.airdropDate >= user.signupDate &&
                now <= ta.airdropExpirationDate){

                // The user will get a portion of the total tokens airdroped,
                // divided by the users at the moment the airdrop was created
                uint tokensToTransfer = ta.totalDropped.div(ta.usersAtDate);

                // if the user has not alreay withdrawn the tokens
                if(_withdrawnBalance < tokensToTransfer){
                    // Register the tokens withdrawn by the user and total tokens withdrawn
                    user.withdrawnBalances[_tokenAddress][i] = tokensToTransfer;
                    ta.tokenBalance = ta.tokenBalance.sub(tokensToTransfer);
                    totalTokensToTransfer = totalTokensToTransfer.add(tokensToTransfer);

                }
            }
        }
        E_TokensWithdrawn(_tokenAddress,msg.sender,totalTokensToTransfer,now);
    }

    function airdropsCount() public view returns (uint){
        return airdrops.length;
    }

    function getAddress() public view returns (address){
      return address(this);
    }

    function airdropHasExpired(address _tokenAddress, uint _id) public view returns (bool){
        TokenAirdrop storage ta = airdroppedTokens[_tokenAddress][_id];
        return (now > ta.airdropExpirationDate);
    }
}