contract HashPayToken is ERC20Interface, Owned {

    using SafeMath for uint;
    using ExtendedMath for uint;

    string public symbol;
    string public  name;
    uint8 public decimals;
    uint public _totalSupply;
    uint public latestDifficultyPeriodStarted;
    uint public epochCount;//number of 'blocks' mined
    uint public _BLOCKS_PER_READJUSTMENT = 1024;

    //a little number
    uint public  _MINIMUM_TARGET = 2**16;

      //a big number is easier ; just find a solution that is smaller
    //uint public  _MAXIMUM_TARGET = 2**224;  bitcoin uses 224
    uint public  _MAXIMUM_TARGET = 2**234;

    uint public miningTarget;

    bytes32 public challengeNumber;   //generate a new one when a new reward is minted

    uint public rewardEra;
    uint public maxSupplyForEra;

    address public lastRewardTo;
    uint public lastRewardAmount;
    uint public lastRewardEthBlockNumber;

    bool locked = false;

    mapping(bytes32 => bytes32) solutionForChallenge;

    uint public tokensMinted; 
    address public parentAddress; //address of 0xbtc
    uint public miningReward; //initial reward

    mapping(address => uint) balances;
    
    mapping(address => uint) merge_mint_ious;
    mapping(address => uint) merge_mint_payout_threshold;

    mapping(address => mapping(address => uint)) allowed;

    uint public burnPercent;

    event Mint(address indexed from, uint reward_amount, uint epochCount, bytes32 newChallengeNumber);

    // ------------------------------------------------------------------------
    // Constructor
    // ------------------------------------------------------------------------

    constructor() public onlyOwner {

        symbol = "0xHASH";
        name = "Hash Pay Token";
        decimals = 4; 
        _totalSupply = 1000000 * 10 ** uint(decimals);
        if(locked) revert();
        locked = true;
        tokensMinted = 0;
        rewardEra = 0;
        miningReward = 5;
        maxSupplyForEra = _totalSupply.div(2);

        miningTarget = _MAXIMUM_TARGET;

        latestDifficultyPeriodStarted = block.number;

        burnPercent = 20; //it's divided by 1000, then 20/1000 = 0.02 = 2%

        _startNewMiningEpoch();

        parentAddress = 0xB6eD7644C69416d67B522e20bC294A9a9B405B31;

        balances[owner] = balances[owner].add(tokensMinted);
        Transfer(address(this), owner, tokensMinted); 


    }
    
    // ------------------------------------------------------------------------
    //The owner gets nothing! You must mine this ERC20 token
    //balances[owner] = _totalSupply;
    //Transfer(address(0), owner, _totalSupply);
    // ------------------------------------------------------------------------
    

    function ParentCoinAddress(address parent) public onlyOwner{
        parentAddress = parent;
    }


    function mint(uint256 nonce, bytes32 challenge_digest) public returns (bool success) {


            //the PoW must contain work that includes a recent ethereum block hash (challenge number) and the msg.sender's address to prevent MITM attacks
            bytes32 digest =  keccak256(challengeNumber, msg.sender, nonce );

            //the challenge digest must match the expected
            if (digest != challenge_digest) revert();

            //the digest must be smaller than the target
            if(uint256(digest) > miningTarget) revert();


            //only allow one reward for each challenge
            bytes32 solution = solutionForChallenge[challengeNumber];
            solutionForChallenge[challengeNumber] = digest;
            if(solution != 0x0) revert();  //prevent the same answer from awarding twice


            uint reward_amount = getMiningReward();

            balances[msg.sender] = balances[msg.sender].add(reward_amount);

            tokensMinted = tokensMinted.add(reward_amount);


            //Cannot mint more tokens than there are
            assert(tokensMinted <= maxSupplyForEra);

            //set readonly diagnostics data
            lastRewardTo = msg.sender;
            lastRewardAmount = reward_amount;
            lastRewardEthBlockNumber = block.number;
            
            _startNewMiningEpoch();

            Mint(msg.sender, reward_amount, epochCount, challengeNumber );

           return true;

    }


    function merge() public returns (bool success) {

            // Function for the Merge mining (0xbitcoin as a parent coin)
            // original idea by 0xbitcoin developers
            // the idea is that the miner uses https://github.com/0xbitcoin/mint-helper/blob/master/contracts/MintHelper.sol 
            // to call mint() and then mergeMint() in the same transaction
            // hard code a reference to the "Parent" ERC918 Contract ( in this case 0xBitcoin)
            // Verify that the Parent contract was minted in this block, by the same person calling this contract
            // then followthrough with the resulting mint logic
            // don't call revert, but return true or false based on success
            // this method shouldn't revert because it will be calleed in the same transaction as a "Parent" mint attempt
            //ensure that mergeMint() can only be called once per Parent::mint()
            //do this by ensuring that the "new" challenge number from Parent::challenge post mint can be called once
            //and that this block time is the same as this mint, and the caller is msg.sender
            //only allow one reward for each challenge
            // do this by calculating what the new challenge will be in _startNewMiningEpoch, and verify that it is not that value
            // this checks happen in the local contract, not in the parent

            bytes32 future_challengeNumber = block.blockhash(block.number - 1);

            if(challengeNumber == future_challengeNumber){
                return false; // ( this is likely the second time that mergeMint() has been called in a transaction, so return false (don't revert))
            }

            if(ERC918Interface(parentAddress).lastRewardTo() != msg.sender){
                return false; // a different address called mint last so return false ( don't revert)
            }
            

            if(ERC918Interface(parentAddress).lastRewardEthBlockNumber() != block.number){
                return false; // parent::mint() was called in a different block number so return false ( don't revert)
            }

            //we have verified that _startNewMiningEpoch has not been run more than once this block by verifying that
            // the challenge is not the challenge that will be set by _startNewMiningEpoch
            //we have verified that this is the same block as a call to Parent::mint() and that the sender
            // is the sender that has called mint
            
            //0xHASH will have the same challenge numbers as 0xBitcoin, this means that mining for one is literally the same process as mining for the other
            // we want to make sure that one can't use a combination of merge and mint to get two blocks of 0xHASH for each valid nonce, since the same solution 
            //    applies to each coin
            // for this reason, we update the solutionForChallenge hashmap with the value of parent::challengeNumber when a solution is merge minted.
            // when a miner finds a valid solution, if they call this::mint(), without the next few lines of code they can then subsequently use the mint helper and in one transaction
            //   call parent::mint() this::merge(). the following code will ensure that this::merge() does not give a block reward, because the challenge number will already be set in the 
            //   solutionForChallenge map
            //only allow one reward for each challenge based on parent::challengeNumber
            
            bytes32 parentChallengeNumber = ERC918Interface(parentAddress).challengeNumber();
            bytes32 solution = solutionForChallenge[parentChallengeNumber];
            if(solution != 0x0) return false;  //prevent the same answer from awarding twice

            //now that we've checked that the next challenge wasn't reused, apply the current 0xHASH challenge 
            //this will prevent the 'previous' challenge from being reused
            
            bytes32 digest = 'merge';
            solutionForChallenge[challengeNumber] = digest;

            //so now we may safely run the relevant logic to give an award to the sender, and update the contract

            uint reward_amount = getMiningReward();

            balances[msg.sender] = balances[msg.sender].add(reward_amount);

            tokensMinted = tokensMinted.add(reward_amount);


            //Cannot mint more tokens than there are
            assert(tokensMinted <= maxSupplyForEra);

            //set readonly diagnostics data
            lastRewardTo = msg.sender;
            lastRewardAmount = reward_amount;
            lastRewardEthBlockNumber = block.number;


            _startNewMiningEpoch();

            Mint(msg.sender, reward_amount, epochCount, 0 ); // use 0 to indicate a merge mine

            return true;

    }

    //a new 'block' to be mined
    
    function _startNewMiningEpoch() internal {

      //if max supply for the era will be exceeded next reward round then enter the new era before that happens
      //40 is the final reward era, almost all tokens minted
      //once the final era is reached, more tokens will not be given out because the assert function
      if (tokensMinted.add(getMiningReward()) > maxSupplyForEra && rewardEra < 39)
      {
        rewardEra = rewardEra + 1;
      }
      //set the next minted supply at which the era will change
      // total supply is 10000000000  because of 4 decimal places
      maxSupplyForEra = _totalSupply - _totalSupply.div(2**(rewardEra + 1));

      epochCount = epochCount.add(1);

      //every so often, readjust difficulty. Dont readjust when deploying
      if(epochCount % _BLOCKS_PER_READJUSTMENT == 0)
      {
        _reAdjustDifficulty();
      }
      //make the latest ethereum block hash a part of the next challenge for PoW to prevent pre-mining future blocks
      //do this last since this is a protection mechanism in the mint() function
      challengeNumber = block.blockhash(block.number - 1);

    }


    //https://en.bitcoin.it/wiki/Difficulty#What_is_the_formula_for_difficulty.3F
    //as of 2017 the bitcoin difficulty was up to 17 zeroes, it was only 8 in the early days

    //readjust the target by 5 percent
    
    function _reAdjustDifficulty() internal {


        uint ethBlocksSinceLastDifficultyPeriod = block.number - latestDifficultyPeriodStarted;

        uint epochsMined = _BLOCKS_PER_READJUSTMENT; //256

        uint targetEthBlocksPerDiffPeriod = epochsMined * 60; //should be 60 times slower than ethereum

        //if there were less eth blocks passed in time than expected
        if (ethBlocksSinceLastDifficultyPeriod < targetEthBlocksPerDiffPeriod )
        {
            uint excess_block_pct = (targetEthBlocksPerDiffPeriod.mul(100)).div( ethBlocksSinceLastDifficultyPeriod );

            uint excess_block_pct_extra = excess_block_pct.sub(100).limitLessThan(1000);
            // If there were 5% more blocks mined than expected then this is 5.  If there were 100% more blocks mined than expected then this is 100.

            //make it harder
            miningTarget = miningTarget.sub(miningTarget.div(2000).mul(excess_block_pct_extra));   //by up to 50 %
        }else{
            uint shortage_block_pct = (ethBlocksSinceLastDifficultyPeriod.mul(100)).div( targetEthBlocksPerDiffPeriod );

            uint shortage_block_pct_extra = shortage_block_pct.sub(100).limitLessThan(1000); //always between 0 and 1000

            //make it easier
            miningTarget = miningTarget.add(miningTarget.div(2000).mul(shortage_block_pct_extra));   //by up to 50 %
        }


        latestDifficultyPeriodStarted = block.number;

        if(miningTarget < _MINIMUM_TARGET) //very difficult
        {
          miningTarget = _MINIMUM_TARGET;
        }

        if(miningTarget > _MAXIMUM_TARGET) //very easy
        {
          miningTarget = _MAXIMUM_TARGET;
        }
    }


    //this is a recent ethereum block hash, used to prevent pre-mining future blocks
    function getChallengeNumber() public constant returns (bytes32) {
        return challengeNumber;
    }

    //the number of zeroes the digest of the PoW solution requires.  Auto adjusts
     function getMiningDifficulty() public constant returns (uint) {
        return _MAXIMUM_TARGET.div(miningTarget);
    }

    function getMiningTarget() public constant returns (uint) {
       return miningTarget;
   }


    //1m coins total
    //reward begins at 5 and is cut in half every reward era (as tokens are mined)
    function getMiningReward() public constant returns (uint) {
        //once we get half way thru the coins, only get 2.5 per block

         //every reward era, the reward amount halves.

         return (5 * 10 ** uint(decimals)).div(2 ** rewardEra);

    }

    //help debug mining software
    function getMintDigest(uint256 nonce, bytes32 challenge_digest, bytes32 challenge_number) public view returns (bytes32 digesttest) {

        bytes32 digest = keccak256(challenge_number,msg.sender,nonce);

        return digest;

    }

        //help debug mining software
    function checkMintSolution(uint256 nonce, bytes32 challenge_digest, bytes32 challenge_number, uint testTarget) public view returns (bool success) {

          bytes32 digest = keccak256(challenge_number,msg.sender,nonce);

          if(uint256(digest) > testTarget) revert();

          return (digest == challenge_digest);

    }

    // ------------------------------------------------------------------------
    // Total supply
    // ------------------------------------------------------------------------

    function totalSupply() public constant returns (uint) {

        return _totalSupply  - balances[address(0)];

    }

    // ------------------------------------------------------------------------
    // Get the token balance for account `tokenOwner`
    // ------------------------------------------------------------------------

    function balanceOf(address tokenOwner) public constant returns (uint balance) {

        return balances[tokenOwner];

    }

    // ------------------------------------------------------------------------
    // Transfer the balance from token owner's account to `to` account
    // - Owner's account must have sufficient balance to transfer
    // - 0 value transfers are allowed
    // ------------------------------------------------------------------------

    function transfer(address to, uint tokens) public returns (bool success) {

        uint toBurn = tokens.mul(burnPercent).div(1000);
        uint toSend = tokens.sub(toBurn);

        balances[msg.sender] = balances[msg.sender].sub(tokens);

        balances[to] = balances[to].add(toSend);
        Transfer(msg.sender, to, toSend);

        balances[address(0)] = balances[address(0)].add(toBurn);
        Transfer(msg.sender, address(0), toBurn);

        return true;

    }

    // ------------------------------------------------------------------------
    // Token owner can approve for `spender` to transferFrom(...) `tokens`
    // from the token owner's account
    //
    // https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md
    // recommends that there are no checks for the approval double-spend attack
    // as this should be implemented in user interfaces
    // ------------------------------------------------------------------------

    function approve(address spender, uint tokens) public returns (bool success) {

        allowed[msg.sender][spender] = tokens;

        Approval(msg.sender, spender, tokens);

        return true;

    }

    // ------------------------------------------------------------------------
    // Transfer `tokens` from the `from` account to the `to` account
    //
    // The calling account must already have sufficient tokens approve(...)-d
    // for spending from the `from` account and
    // - From account must have sufficient balance to transfer
    // - Spender must have sufficient allowance to transfer
    // - 0 value transfers are allowed
    // ------------------------------------------------------------------------

    function transferFrom(address from, address to, uint tokens) public returns (bool success) {

        uint toBurn = tokens.mul(burnPercent).div(1000);
        uint toSend = tokens.sub(toBurn);
        balances[msg.sender] = balances[msg.sender].sub(tokens);
        balances[to] = balances[to].add(toSend);
        Transfer(msg.sender, to, toSend);
        balances[address(0)] = balances[address(0)].add(toBurn);
        Transfer(msg.sender, address(0), toBurn);
        return true;

    }

    // ------------------------------------------------------------------------
    // Returns the amount of tokens approved by the owner that can be
    // transferred to the spender's account
    // ------------------------------------------------------------------------

    function allowance(address tokenOwner, address spender) public constant returns (uint remaining) {

        return allowed[tokenOwner][spender];

    }


    // ------------------------------------------------------------------------
    // Token owner can approve for `spender` to transferFrom(...) `tokens`
    // from the token owner's account. The `spender` contract function
    // `receiveApproval(...)` is then executed
    // ------------------------------------------------------------------------

    function approveAndCall(address spender, uint tokens, bytes data) public returns (bool success) {

        allowed[msg.sender][spender] = tokens;

        Approval(msg.sender, spender, tokens);

        ApproveAndCallFallBack(spender).receiveApproval(msg.sender, tokens, this, data);

        return true;

    }

    // ------------------------------------------------------------------------
    // Don't accept ETH
    // ------------------------------------------------------------------------

    function () public payable {

        revert();

    }


    // ------------------------------------------------------------------------
    // Owner can transfer out any accidentally sent ERC20 tokens
    // ------------------------------------------------------------------------

    function transferAnyERC20Token(address tokenAddress, uint tokens) public onlyOwner returns (bool success) {

        return ERC20Interface(tokenAddress).transfer(owner, tokens);

    }

}
// ----------------------------------------------------------------------------
// ERC20 Token, with the addition of symbol, name and decimals and an
// initial fixed supply
// ----------------------------------------------------------------------------
function getChallengeNumber() public constant returns (bytes32) {
    return challengeNumber;
}

//this is a recent ethereum block hash, used to prevent pre-mining future blocks