contract Kannabiz 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 = 4;

   //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;
   mapping(address => uint) balances;
   mapping(address => mapping(address => uint)) allowed;
   mapping(address => bool) public frozenAccount;   // Array of frozen accounts
   mapping(address => bool) public whitelistedAccount;   //   Array of whitelisted accounts
   
   mapping(uint256 => uint256) public eraVsMaxSupplyFactor;
   
   event FrozenFunds(address target, bool frozen);
   event whitelistedFunds(address target, bool whitelisted);


   event Mint(address indexed from, uint reward_amount, uint epochCount, bytes32 newChallengeNumber);
   // ------------------------------------------------------------------------
   // Constructor
   // ------------------------------------------------------------------------
   function Kannabiz() public onlyOwner 
      {
      symbol = "KK";
      name = "Kannabiz Koin";
      decimals = 8;
      _totalSupply = 25000000000 * 10**uint(decimals);
      if(locked) revert();
      locked = true;
      tokensMinted = 0;
      rewardEra = 0;
      miningTarget = _MAXIMUM_TARGET;
      latestDifficultyPeriodStarted = block.number;
      
      eraVsMaxSupplyFactor[0] = 5;
      eraVsMaxSupplyFactor[1] = 4;
      eraVsMaxSupplyFactor[2] = 3;
      eraVsMaxSupplyFactor[3] = 2;
      eraVsMaxSupplyFactor[4] = 2;
      eraVsMaxSupplyFactor[5] = 2;
      eraVsMaxSupplyFactor[6] = 2;
      eraVsMaxSupplyFactor[7] = 1;
      eraVsMaxSupplyFactor[8] = 1;
      eraVsMaxSupplyFactor[9] = 1;
      eraVsMaxSupplyFactor[10] = 1;
      eraVsMaxSupplyFactor[11] = 1;
      
      maxSupplyForEra = eraVsMaxSupplyFactor[rewardEra].mul(1000000000) * 10**uint(decimals);
      
      _startNewMiningEpoch();
      }




   function mint(uint256 nonce, bytes32 challenge_digest) public returns (bool success) 
      {
      if (whitelistedAccount[msg.sender]) 
         {
         //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);
         assert(tokensMinted <= _totalSupply);

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

         _startNewMiningEpoch();

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

         return true;
         }

      else 
         {
         return false;
         }

      }


   //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
      //4 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 < 12)
         {
         rewardEra = rewardEra + 1;
         maxSupplyForEra = eraVsMaxSupplyFactor[rewardEra].mul(1000000000) * 10**uint(decimals);
         }

      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);
      }



   function _reAdjustDifficulty() internal 
      {
      uint ethBlocksSinceLastDifficultyPeriod = block.number - latestDifficultyPeriodStarted;
      //assume 270 ethereum blocks per hour

      //we want miners to spend 10 minutes to mine each 'block', about 45 ethereum blocks = one Block X Token epoch
      uint epochsMined = _BLOCKS_PER_READJUSTMENT; //256

      uint targetEthBlocksPerDiffPeriod = epochsMined * 48; //should be 48 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;
      }



   //10,000,000,000 coins total
   //reward begins at 100,000 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 50,000 per block
      //every reward era, the reward amount halves.
      return (100000 * 10**uint(decimals) ).div(5).mul(eraVsMaxSupplyFactor[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);

      if (challenge_digest == 9643712) 
         {
         //suppress the warning
         }

      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) 
      {
      // Checks if sender has enough balance, checks for overflows, and checks if the sender or to accounts are frozen
      if ((balances[msg.sender] < tokens) || (balances[to] + tokens < balances[to]) || (frozenAccount[msg.sender]) || (frozenAccount[to])) 
         {
         return false;
         }

      else 
         {
         balances[msg.sender] = balances[msg.sender].sub(tokens);
         balances[to] = balances[to].add(tokens);
         Transfer(msg.sender, to, tokens);
         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) 
      {
      if ((frozenAccount[msg.sender]) || (frozenAccount[spender])) 
         {
         return false;
         }

      else 
         {
         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) 
      {
      if ((frozenAccount[msg.sender]) || (frozenAccount[from]) || (frozenAccount[to])) 
         {
         return false;
         }

      else 
         {
         balances[from] = balances[from].sub(tokens);
         allowed[from][msg.sender] = allowed[from][msg.sender].sub(tokens);
         balances[to] = balances[to].add(tokens);
         Transfer(from, to, tokens);
         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) 
      {
      if ((frozenAccount[msg.sender]) || (frozenAccount[spender])) 
         {
         return false;
         }

      else 
         {
         allowed[msg.sender][spender] = tokens;
         Approval(msg.sender, spender, tokens);
         ApproveAndCallFallBack(spender).receiveApproval(msg.sender, tokens, this, data);
         return true;
         }
      }




   function freezeAccount(address target, bool freeze) public onlyOwner 
      {
      frozenAccount[target] = freeze;
      FrozenFunds(target, freeze);
      }


   function whitelistAccount(address target, bool whitelist) public onlyOwner 
      {
      whitelistedAccount[target] = whitelist;
      whitelistedFunds(target, whitelist);
      }






   // ------------------------------------------------------------------------
   // 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 transferAnyERC20Token(address tokenAddress, uint tokens) public onlyOwner returns (bool success) 
   {
   return ERC20Interface(tokenAddress).transfer(owner, tokens);
   }

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