pragma solidity ^0.4.24;

/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/179
 */
contract ERC20Basic {
	uint256 public totalSupply;
	function balanceOf(address who) public constant returns (uint256);
	function transfer(address to, uint256 value) public returns (bool);
	event Transfer(address indexed from, address indexed to, uint256 value);
}

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20 is ERC20Basic {
	function allowance(address owner, address spender) public constant returns (uint256);
	function transferFrom(address from, address to, uint256 value) public returns (bool);
	function approve(address spender, uint256 value) public returns (bool);
	event Approval(address indexed owner, address indexed spender, uint256 value);
}

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {
	function mul(uint256 a, uint256 b) internal pure returns (uint256) {
		uint256 c = a * b;
		assert(a == 0 || c / a == b);
		return c;
	}

	function div(uint256 a, uint256 b) internal pure returns (uint256) {
		// assert(b > 0); // Solidity automatically throws when dividing by 0
		uint256 c = a / b;
		// assert(a == b * c + a % b); // There is no case in which this doesn't hold
		return c;
	}

	function sub(uint256 a, uint256 b) internal pure returns (uint256) {
		assert(b <= a); 
		return a - b; 
	} 
	
	function add(uint256 a, uint256 b) internal pure returns (uint256) { 
		uint256 c = a + b; assert(c >= a);
		return c;
	}

}

/**
 * @title Basic token
 * @dev Basic version of StandardToken, with no allowances.
 */
contract BasicToken is ERC20Basic {
	using SafeMath for uint256;

	mapping(address => uint256) balances;

	/**
	* @dev transfer token for a specified address
	* @param _to The address to transfer to.
	* @param _value The amount to be transferred.
	*/
	function transfer(address _to, uint256 _value) public returns (bool) {
		require(_to != address(0));
		require(_value <= balances[msg.sender]); 
		balances[msg.sender] = balances[msg.sender].sub(_value); 
		balances[_to] = balances[_to].add(_value); 
		emit Transfer(msg.sender, _to, _value); 
		return true; 
	} 

	/** 
	 * @dev Gets the balance of the specified address. 
	 * @param _owner The address to query the the balance of. 
	 * @return An uint256 representing the amount owned by the passed address. 
	 */ 
	function balanceOf(address _owner) public constant returns (uint256 balance) { 
		return balances[_owner]; 
	} 
} 

/** 
 * @title Standard ERC20 token 
 * 
 * @dev Implementation of the basic standard token. 
 * @dev https://github.com/ethereum/EIPs/issues/20 
 * @dev Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol 
 */ 
contract StandardToken is ERC20, BasicToken {

	mapping (address => mapping (address => uint256)) internal allowed;

	/**
	 * @dev Transfer tokens from one address to another
	 * @param _from address The address which you want to send tokens from
	 * @param _to address The address which you want to transfer to
	 * @param _value uint256 the amount of tokens to be transferred
	 */
	function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
		require(_to != address(0));
		require(_value <= balances[_from]);
		require(_value <= allowed[_from][msg.sender]); 
		balances[_from] = balances[_from].sub(_value); 
		balances[_to] = balances[_to].add(_value); 
		allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value); 
		emit Transfer(_from, _to, _value); 
		return true; 
	} 

 /** 
	* @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender. 
	* 
	* Beware that changing an allowance with this method brings the risk that someone may use both the old 
	* and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this 
	* race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards: 
	* https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729 
	* @param _spender The address which will spend the funds. 
	* @param _value The amount of tokens to be spent. 
	*/ 
	function approve(address _spender, uint256 _value) public returns (bool) { 
		allowed[msg.sender][_spender] = _value; 
		emit Approval(msg.sender, _spender, _value); 
		return true; 
	}

 /** 
	* @dev Function to check the amount of tokens that an owner allowed to a spender. 
	* @param _owner address The address which owns the funds. 
	* @param _spender address The address which will spend the funds. 
	* @return A uint256 specifying the amount of tokens still available for the spender. 
	*/ 
	function allowance(address _owner, address _spender) public constant returns (uint256 remaining) { 
		return allowed[_owner][_spender]; 
	} 

 /** 
	* approve should be called when allowed[_spender] == 0. To increment 
	* allowed value is better to use this function to avoid 2 calls (and wait until 
	* the first transaction is mined) * From MonolithDAO Token.sol 
	*/ 
	function increaseApproval (address _spender, uint _addedValue) public returns (bool success) {
		allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_addedValue);
		emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]); 
		return true; 
	}

	function decreaseApproval (address _spender, uint _subtractedValue) public returns (bool success) {
		uint oldValue = allowed[msg.sender][_spender]; 
		if (_subtractedValue > oldValue) {
			allowed[msg.sender][_spender] = 0;
		} else {
			allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
		}
		emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
		return true;
	}

	function () public payable {
		revert();
	}

}

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
	address public owner;

	event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

	/**
	 * @dev The Ownable constructor sets the original `owner` of the contract to the sender
	 * account.
	 */
	constructor() public {
		owner = msg.sender;
	}

	/**
	 * @dev Throws if called by any account other than the owner.
	 */
	modifier onlyOwner() {
		require(msg.sender == owner);
		_;
	}

	/**
	 * @dev Allows the current owner to transfer control of the contract to a newOwner.
	 * @param newOwner The address to transfer ownership to.
	 */
	function transferOwnership(address newOwner) onlyOwner public {
		require(newOwner != address(0));
		emit OwnershipTransferred(owner, newOwner);
		owner = newOwner;
	}

}

/**
 * @title Mintable token
 * @dev Simple ERC20 Token example, with mintable token creation
 * @dev Issue: * https://github.com/OpenZeppelin/zeppelin-solidity/issues/120
 * Based on code by TokenMarketNet: https://github.com/TokenMarketNet/ico/blob/master/contracts/MintableToken.sol
 */
contract MintableToken is StandardToken, Ownable {
		
	event Mint(address indexed to, uint256 amount);
    
    uint public totalMined;

	function mint(address _to, uint256 _amount) onlyOwner public returns (bool) {
		require(totalMined.sub(totalSupply) >= _amount);
		totalSupply = totalSupply.add(_amount);
		balances[_to] = balances[_to].add(_amount);
		emit Mint(_to, _amount);
		return true;
	}
}

contract vBitcoin is MintableToken {
	string public constant name = "Virtual Bitcoin";
	string public constant symbol = "vBTC";
	uint32 public constant decimals = 18;
	
    uint public start = 1529934560;
    uint public startBlockProfit = 50;
    uint public blockBeforeChange = 210000;
    uint public blockTime = 15 minutes;
    
    function defrosting() onlyOwner public {
        
        uint _totalMined = 0;
        uint timePassed = now.sub(start);
        uint blockPassed = timePassed.div(blockTime);
        uint blockProfit = startBlockProfit;
        
        while(blockPassed > 0) {
            if(blockPassed > blockBeforeChange) {
                _totalMined = _totalMined.add(blockBeforeChange.mul(blockProfit));
                blockProfit = blockProfit.div(2);
                blockPassed = blockPassed.sub(blockBeforeChange);
            } else {
                _totalMined = _totalMined.add(blockPassed.mul(blockProfit));
                blockPassed = 0;
            }
        }
        
        totalMined = _totalMined;
    }
}