pragma solidity ^0.6.0;


/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
abstract contract ERC20 {
    function totalSupply() public view virtual returns (uint256);
    function balanceOf(address _who) public view virtual returns (uint256);
    function allowance(address _owner, address _spender) public view virtual returns (uint256);
    function transfer(address _to, uint256 _value) public virtual returns (bool);
    function approve(address _spender, uint256 _value) public virtual returns (bool);
    function transferFrom(address _from, address _to, uint256 _value) public virtual returns (bool);

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}


/**
 * @title SafeMath
 * @dev Math operations with safety checks that revert on error
 */
library SafeMath {
    /**
    * @dev Multiplies two numbers, reverts on overflow.
    */
    function mul(uint256 _a, uint256 _b) internal pure returns (uint256) {
        uint256 c = _a * _b;
        require(_a == 0 || c / _a == _b);

        return c;
    }

    /**
    * @dev Integer division of two numbers truncating the quotient, reverts on division by zero.
    */
    function div(uint256 _a, uint256 _b) internal pure returns (uint256) {
        uint256 c = _a / _b;
        return c;
    }

    /**
    * @dev Subtracts two numbers, reverts on overflow (i.e. if subtrahend is greater than minuend).
    */
    function sub(uint256 _a, uint256 _b) internal pure returns (uint256) {
        require(_b <= _a);
        uint256 c = _a - _b;

        return c;
    }

    /**
    * @dev Adds two numbers, reverts on overflow.
    */
    function add(uint256 _a, uint256 _b) internal pure returns (uint256) {
        uint256 c = _a + _b;
        require(c >= _a);

        return c;
    }
}



/**
 * @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 OwnershipRenounced(address indexed previousOwner);
    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 relinquish control of the contract.
    * @notice Renouncing to ownership will leave the contract without an owner.
    * It will not be possible to call the functions with the `onlyOwner`
    * modifier anymore.
    */
    function renounceOwnership() public onlyOwner {
        emit OwnershipRenounced(owner);
        owner = address(0);
    }

    /**
    * @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) public onlyOwner {
		require(_newOwner != address(0));
		emit OwnershipTransferred(owner, _newOwner);
        owner = _newOwner;
    }
}


/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * https://github.com/ethereum/EIPs/issues/20
 * Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 */
contract StandardToken is ERC20 {
    using SafeMath for uint256;

    mapping(address => uint256) balances;

    mapping (address => mapping (address => uint256)) internal allowed;

    uint256 _totalSupply;

    /**
    * @dev Total number of tokens in existence
    */
    function totalSupply() public override view returns (uint256) {
        return _totalSupply;
    }

    /**
    * @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 override view returns (uint256) {
        return balances[_owner];
    }

    /**
    * @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 override view returns (uint256) {
        return allowed[_owner][_spender];
    }

    /**
    * @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 override returns (bool) {
        require(_to != address(0));

        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(_value);

        emit Transfer(msg.sender, _to, _value);
        return true;
    }

    /**
    * @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 override returns (bool) {
        require(_to != address(0));

        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.
    * @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 override returns (bool) {
		require(_value == 0 || allowed[msg.sender][_spender] == 0 );
        allowed[msg.sender][_spender] = _value;

        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    /**
    * @dev Increase the amount of tokens that an owner allowed to a 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
    * @param _spender The address which will spend the funds.
    * @param _addedValue The amount of tokens to increase the allowance by.
    */
    function increaseApproval(address _spender, uint256 _addedValue) public returns (bool) {
        allowed[msg.sender][_spender] = (allowed[msg.sender][_spender].add(_addedValue));

        emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
        return true;
    }

    /**
    * @dev Decrease the amount of tokens that an owner allowed to a spender.
    * approve should be called when allowed[_spender] == 0. To decrement
    * allowed value is better to use this function to avoid 2 calls (and wait until
    * the first transaction is mined)
    * From MonolithDAO Token.sol
    * @param _spender The address which will spend the funds.
    * @param _subtractedValue The amount of tokens to decrease the allowance by.
    */
    function decreaseApproval(address _spender, uint256 _subtractedValue) public returns (bool) {
        uint256 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;
    }
}


/**
* @title Burnable Token
* @dev Token that can be irreversibly burned (destroyed).
*/
contract BurnableToken is StandardToken, Ownable {
    event Burn(address indexed burner, uint256 value);

    function _burn(address _who, uint256 _value) internal {
        require(_value <= balances[_who]);

        balances[_who] = balances[_who].sub(_value);
        _totalSupply = _totalSupply.sub(_value);

        emit Burn(_who, _value);
        emit Transfer(_who, address(0), _value);
    }

    /**
    * @dev Burns a specific amount of tokens.
    * @param _value The amount of token to be burned.
    */
    function burn(uint256 _value) public {
        _burn(msg.sender, _value);
    }

    /**
    * @dev Burns a specific amount of tokens from the target address and decrements allowance
    * @param _from address The address which you want to send tokens from
    * @param _value uint256 The amount of token to be burned
    */
    function burnFrom(address _from, uint256 _value) public {
        require(_value <= allowed[_from][msg.sender]);

        allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
        _burn(_from, _value);
    }
}

/**
* @title Mintable Token
*/
contract MintableToken is StandardToken, Ownable {
	event Mint(address indexed to, uint256 amount);
	event MintFinished();

	bool public mintingFinished = false;
	
	modifier canMint() {
		require(!mintingFinished);
		_;
	}

	/**
	* @dev Internal function that mints an amount of the token and assigns it to
	* an account. This encapsulates the modification of balances such that the
    * proper events are emitted.
    * @param _to The account that will receive the created tokens.
    * @param _value The amount that will be created.
    */
	function _mint(address _to, uint256 _value) internal {
		require(_to != address(0));

		_totalSupply = _totalSupply.add(_value);
		balances[_to] = balances[_to].add(_value);
     
		emit Mint(_to, _value);
		emit Transfer(address(0), _to, _value);
	}

	/**
	* @dev Function to mint tokens
	* @param _to The address that will receive the minted tokens.
	* @param _value The amount of tokens to mint.
	* @return A boolean that indicates if the operation was successful.
	*/
	function mint(address _to, uint256 _value) onlyOwner canMint public returns (bool) {
		_mint(_to, _value);
	
		return true;
	}

	/**
	* @dev Function to stop minting new tokens.
	* @return True if the operation was successful.
	*/
	function finishMinting() onlyOwner canMint public returns (bool) {
		mintingFinished = true;
		MintFinished();
		return true;
	}
}

/**
* @title USD BTC Standard Token
* @dev	main contract
*/
contract USDSToken is BurnableToken, MintableToken {
    string public name;
    string public symbol;
    uint8 public decimals;
    
    constructor(string memory _name, string memory _symbol, uint8 _decimals, uint256 _initalSupply, address _owner) public {
		require(_owner != address(0) && _decimals != 0);
		
		name = _name;
		symbol = _symbol;
		decimals = _decimals;
		owner = _owner;

        _mint(_owner, _initalSupply * (10 ** uint256(decimals)));
    }
}