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pragma solidity ^0.4.24;

pragma solidity ^0.4.18;

// ----------------------------------------------------------------------------

// 'FIXED' 'Example Fixed Supply Token' token contract

// '0xBitcoin Token' contract

// Symbol : FIXED

// Name : Example Fixed Supply Token

// Mineable ERC20 Token using Proof Of Work

// Total supply: 1,000,000.000000000000000000

// Decimals : 18

// Enjoy.

// Symbol : 0xBTC

// Name : 0xBitcoin Token

// Total supply: 21,000,000.00

// Decimals : 8

// (c) BokkyPooBah / Bok Consulting Pty Ltd 2018. The MIT Licence.

// ----------------------------------------------------------------------------

// Safe maths

// ----------------------------------------------------------------------------

library SafeMath {

function add(uint a, uint b) internal pure returns (uint c) {

c = a + b;

require(c >= a);

}

function sub(uint a, uint b) internal pure returns (uint c) {

require(b <= a);

c = a - b;

}

function mul(uint a, uint b) internal pure returns (uint c) {

c = a * b;

require(a == 0 || c / a == b);

}

function div(uint a, uint b) internal pure returns (uint c) {

require(b > 0);

c = a / b;

}

library ExtendedMath {

//return the smaller of the two inputs (a or b)

function limitLessThan(uint a, uint b) internal pure returns (uint c) {

if(a > b) return b;

return a;

}

// ----------------------------------------------------------------------------

// ERC Token Standard #20 Interface

// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md

// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md

// ----------------------------------------------------------------------------

contract ERC20Interface {

function totalSupply() public constant returns (uint);

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

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

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

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

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

event Transfer(address indexed from, address indexed to, uint tokens);

event Approval(address indexed tokenOwner, address indexed spender, uint tokens);

}

// ----------------------------------------------------------------------------

// Contract function to receive approval and execute function in one call

// Borrowed from MiniMeToken

// ----------------------------------------------------------------------------

contract ApproveAndCallFallBack {

function receiveApproval(address from, uint256 tokens, address token, bytes data) public;

}

// ----------------------------------------------------------------------------

// Owned contract

// ----------------------------------------------------------------------------

contract Owned {

address public owner;

address public newOwner;

event OwnershipTransferred(address indexed _from, address indexed _to);

constructor() public {

function Owned() public {

owner = msg.sender;

}

modifier onlyOwner {

require(msg.sender == owner);

}

function transferOwnership(address _newOwner) public onlyOwner {

newOwner = _newOwner;

}

function acceptOwnership() public {

require(msg.sender == newOwner);

emit OwnershipTransferred(owner, newOwner);

OwnershipTransferred(owner, newOwner);

owner = newOwner;

newOwner = address(0);

}

// ----------------------------------------------------------------------------

// ERC20 Token, with the addition of symbol, name and decimals and a

// fixed supply

// ERC20 Token, with the addition of symbol, name and decimals and an

// initial fixed supply

// ----------------------------------------------------------------------------

contract FixedSupplyToken is ERC20Interface, Owned {

contract _0xBitcoinToken is ERC20Interface, Owned {

using SafeMath for uint;

using ExtendedMath for uint;

string public symbol;

string public name;

uint8 public decimals;

uint _totalSupply;

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;

mapping(address => uint) balances;

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

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

// ------------------------------------------------------------------------

// Constructor

// ------------------------------------------------------------------------

constructor() public {

symbol = "FIXED";

function _0xBitcoinToken() public onlyOwner{

name = "Example Fixed Supply Token";

decimals = 18;

_totalSupply = 1000000 * 10**uint(decimals);

balances[owner] = _totalSupply;

symbol = "0xBTC";

emit Transfer(address(0), owner, _totalSupply);

name = "0xBitcoin Token";

decimals = 8;

_totalSupply = 21000000 * 10**uint(decimals);

if(locked) revert();

locked = true;

tokensMinted = 0;

rewardEra = 0;

maxSupplyForEra = _totalSupply.div(2);

miningTarget = _MAXIMUM_TARGET;

latestDifficultyPeriodStarted = block.number;

_startNewMiningEpoch();

//The owner gets nothing! You must mine this ERC20 token

//balances[owner] = _totalSupply;

//Transfer(address(0), owner, _totalSupply);

}

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;

}

//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 2100000000000000 because of 8 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;

//assume 360 ethereum blocks per hour

//we want miners to spend 10 minutes to mine each 'block', about 60 ethereum blocks = one 0xbitcoin epoch

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;

}

//21m coins total

//reward begins at 50 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 25 per block

//every reward era, the reward amount halves.

return (50 * 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 view returns (uint) {

return _totalSupply.sub(balances[address(0)]);

function totalSupply() public constant returns (uint) {

return _totalSupply - balances[address(0)];

}

// ------------------------------------------------------------------------

// Get the token balance for account `tokenOwner`

// ------------------------------------------------------------------------

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

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) {

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

balances[to] = balances[to].add(tokens);

emit Transfer(msg.sender, to, 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) {

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

emit Approval(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) {

balances[from] = balances[from].sub(tokens);

allowed[from][msg.sender] = allowed[from][msg.sender].sub(tokens);

balances[to] = balances[to].add(tokens);

emit Transfer(from, to, 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 view returns (uint remaining) {

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;

emit Approval(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);

}

Saved diffs

Original text

Open file

pragma solidity ^0.4.24;

// ----------------------------------------------------------------------------
// 'FIXED' 'Example Fixed Supply Token' token contract
//
// Symbol      : FIXED
// Name        : Example Fixed Supply Token
// Total supply: 1,000,000.000000000000000000
// Decimals    : 18
//
// Enjoy.
//
// (c) BokkyPooBah / Bok Consulting Pty Ltd 2018. The MIT Licence.
// ----------------------------------------------------------------------------

// ----------------------------------------------------------------------------
// Safe maths
// ----------------------------------------------------------------------------
library SafeMath {
    function add(uint a, uint b) internal pure returns (uint c) {
        c = a + b;
        require(c >= a);
    }
    function sub(uint a, uint b) internal pure returns (uint c) {
        require(b <= a);
        c = a - b;
    }
    function mul(uint a, uint b) internal pure returns (uint c) {
        c = a * b;
        require(a == 0 || c / a == b);
    }
    function div(uint a, uint b) internal pure returns (uint c) {
        require(b > 0);
        c = a / b;
    }
}

// ----------------------------------------------------------------------------
// ERC Token Standard #20 Interface
// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
// ----------------------------------------------------------------------------
contract ERC20Interface {
    function totalSupply() public constant returns (uint);
    function balanceOf(address tokenOwner) public constant returns (uint balance);
    function allowance(address tokenOwner, address spender) public constant returns (uint remaining);
    function transfer(address to, uint tokens) public returns (bool success);
    function approve(address spender, uint tokens) public returns (bool success);
    function transferFrom(address from, address to, uint tokens) public returns (bool success);

event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}

// ----------------------------------------------------------------------------
// Contract function to receive approval and execute function in one call
//
// Borrowed from MiniMeToken
// ----------------------------------------------------------------------------
contract ApproveAndCallFallBack {
    function receiveApproval(address from, uint256 tokens, address token, bytes data) public;
}

// ----------------------------------------------------------------------------
// Owned contract
// ----------------------------------------------------------------------------
contract Owned {
    address public owner;
    address public newOwner;

event OwnershipTransferred(address indexed _from, address indexed _to);

constructor() public {
        owner = msg.sender;
    }

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

function transferOwnership(address _newOwner) public onlyOwner {
        newOwner = _newOwner;
    }
    function acceptOwnership() public {
        require(msg.sender == newOwner);
        emit OwnershipTransferred(owner, newOwner);
        owner = newOwner;
        newOwner = address(0);
    }
}

// ----------------------------------------------------------------------------
// ERC20 Token, with the addition of symbol, name and decimals and a
// fixed supply
// ----------------------------------------------------------------------------
contract FixedSupplyToken is ERC20Interface, Owned {
    using SafeMath for uint;

string public symbol;
    string public  name;
    uint8 public decimals;
    uint _totalSupply;

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

// ------------------------------------------------------------------------
    // Constructor
    // ------------------------------------------------------------------------
    constructor() public {
        symbol = "FIXED";
        name = "Example Fixed Supply Token";
        decimals = 18;
        _totalSupply = 1000000 * 10**uint(decimals);
        balances[owner] = _totalSupply;
        emit Transfer(address(0), owner, _totalSupply);
    }

// ------------------------------------------------------------------------
    // Total supply
    // ------------------------------------------------------------------------
    function totalSupply() public view returns (uint) {
        return _totalSupply.sub(balances[address(0)]);
    }

// ------------------------------------------------------------------------
    // Get the token balance for account `tokenOwner`
    // ------------------------------------------------------------------------
    function balanceOf(address tokenOwner) public view 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) {
        balances[msg.sender] = balances[msg.sender].sub(tokens);
        balances[to] = balances[to].add(tokens);
        emit 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) {
        allowed[msg.sender][spender] = tokens;
        emit 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) {
        balances[from] = balances[from].sub(tokens);
        allowed[from][msg.sender] = allowed[from][msg.sender].sub(tokens);
        balances[to] = balances[to].add(tokens);
        emit 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 view 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;
        emit 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);
    }
}

Changed text

Open file

pragma solidity ^0.4.18;

// ----------------------------------------------------------------------------

// '0xBitcoin Token' contract

// Mineable ERC20 Token using Proof Of Work

// Symbol      : 0xBTC

// Name        : 0xBitcoin Token

// Total supply: 21,000,000.00

// Decimals    : 8

// ----------------------------------------------------------------------------

// Safe maths

// ----------------------------------------------------------------------------

library SafeMath {

function add(uint a, uint b) internal pure returns (uint c) {

c = a + b;

require(c >= a);

}

function sub(uint a, uint b) internal pure returns (uint c) {

require(b <= a);

c = a - b;

}

function mul(uint a, uint b) internal pure returns (uint c) {

c = a * b;

require(a == 0 || c / a == b);

}

function div(uint a, uint b) internal pure returns (uint c) {

require(b > 0);

c = a / b;

}

library ExtendedMath {

//return the smaller of the two inputs (a or b)
    function limitLessThan(uint a, uint b) internal pure returns (uint c) {

if(a > b) return b;

return a;

}
}

// ----------------------------------------------------------------------------

// ERC Token Standard #20 Interface

// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md

// ----------------------------------------------------------------------------

contract ERC20Interface {

function totalSupply() public constant returns (uint);

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

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

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

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

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

event Transfer(address indexed from, address indexed to, uint tokens);

event Approval(address indexed tokenOwner, address indexed spender, uint tokens);

}

// ----------------------------------------------------------------------------

// Contract function to receive approval and execute function in one call

// Borrowed from MiniMeToken

// ----------------------------------------------------------------------------

contract ApproveAndCallFallBack {

function receiveApproval(address from, uint256 tokens, address token, bytes data) public;

}

// ----------------------------------------------------------------------------

// Owned contract

// ----------------------------------------------------------------------------

contract Owned {

address public owner;

address public newOwner;

event OwnershipTransferred(address indexed _from, address indexed _to);

function Owned() public {

owner = msg.sender;

}

modifier onlyOwner {

require(msg.sender == owner);

}

function transferOwnership(address _newOwner) public onlyOwner {

newOwner = _newOwner;

}

function acceptOwnership() public {

require(msg.sender == newOwner);

OwnershipTransferred(owner, newOwner);

owner = newOwner;

newOwner = address(0);

}

// ----------------------------------------------------------------------------

// ERC20 Token, with the addition of symbol, name and decimals and an

// initial fixed supply

// ----------------------------------------------------------------------------

contract _0xBitcoinToken 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;

mapping(address => uint) balances;

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

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

// ------------------------------------------------------------------------

// Constructor

// ------------------------------------------------------------------------

function _0xBitcoinToken() public onlyOwner{

symbol = "0xBTC";

name = "0xBitcoin Token";

decimals = 8;

_totalSupply = 21000000 * 10**uint(decimals);

if(locked) revert();
        locked = true;

tokensMinted = 0;

rewardEra = 0;
        maxSupplyForEra = _totalSupply.div(2);

miningTarget = _MAXIMUM_TARGET;

latestDifficultyPeriodStarted = block.number;

_startNewMiningEpoch();

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

}

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;

}

//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 2100000000000000  because of 8 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;
        //assume 360 ethereum blocks per hour

//we want miners to spend 10 minutes to mine each 'block', about 60 ethereum blocks = one 0xbitcoin epoch
        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;
   }

//21m coins total
    //reward begins at 50 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 25 per block

//every reward era, the reward amount halves.

return (50 * 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) {

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) {

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) {

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) {

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);

}