pragma solidity ^0.4.24;

library SafeMath {
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }
        uint256 c = a * b;
        assert(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;
    }
}

contract Ownable {
    address public owner;
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    constructor () public {
        owner = msg.sender;
    }
    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }
    function transferOwnership(address newOwner) public onlyOwner {
        require(newOwner != address(0));
        emit OwnershipTransferred(owner, newOwner);
        owner = newOwner;
    }
}

contract ERC20Basic {
    function totalSupply() public view returns (uint256);
    function balanceOf(address who) public view returns (uint256);
    function transfer(address to, uint256 value) public returns (bool);

    event Transfer(address indexed from, address indexed to, uint256 value);
}

contract ERC20 is ERC20Basic {
    function allowance(address owner, address spender) public view 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);
}

contract BasicToken is ERC20Basic{
    using SafeMath for uint256;
    mapping(address => uint256) balances;
    uint256 internal totalSupply_;
    /**
     * @dev total number of tokens in existence
     */
    function totalSupply() public view returns (uint256) {
        return totalSupply_;
    }

    function _transfer(address _from, address _to, uint256 _value) internal returns (bool) {
        require(_to != address(0));
        require(_from != address(0));
        require(_value != 0);
        require(_value <= balances[_from]);
        // SafeMath.sub will throw if there is not enough balance.
        balances[_from] = balances[_from].sub(_value);
        balances[_to] = balances[_to].add(_value);
        emit Transfer(_from, _to, _value);
        return true;
    }

    /**
    * @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) {
        return _transfer(msg.sender, _to, _value);
    }
    /**
     * @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 view returns (uint256 balance) {
        return balances[_owner];
    }
}
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(_value <= allowed[_from][msg.sender]);
        _transfer(_from, _to, _value);
        allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_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 spenders 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) {
        require(_spender != address(0));
        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 view returns (uint256) {
        return allowed[_owner][_spender];
    }
    /**
     * @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, uint _addedValue) public returns (bool) {
        require(_spender != address(0));
        require(_addedValue != 0);
        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, uint _subtractedValue) public returns (bool) {
        require(_spender != address(0));
        require(_subtractedValue != 0);
        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;
    }
}

contract BurnableToken is StandardToken {
    event Burn(address indexed burner, uint256 value);
    /**
    * @dev Burns a specific amount of tokens.
    * @param _value The amount of token to be burned.
    */
    function burn(uint256 _value) public {
        require(_value <= balances[msg.sender]);
        require(_value != 0);
        // no need to require value <= totalSupply, since that would imply the
        // sender's balance is greater than the totalSupply, which *should* be an assertion failure
        balances[msg.sender] = balances[msg.sender].sub(_value);
        totalSupply_ = totalSupply_.sub(_value);
        emit Transfer(msg.sender, address(0), _value);
        emit Burn(msg.sender, _value);
    }
}

contract ISTCoin is BurnableToken, Ownable {
    string public constant name = "ISTCoin";
    string public constant symbol = "IST";
    uint8 public constant decimals = 8;

    bool public unFreeze = false;
    mapping (address => bool) public frozenAccount;

    modifier notFrozen(address sender) {
        require(!frozenAccount[sender] || unFreeze);
        _;
    }

    event FrozenFunds(address target, bool frozen);
    event FreezeAll(bool flag);
    event AccountFrozenError();

    uint256 public constant INITIAL_SUPPLY = 200000000 * (10 ** uint256(decimals));
    constructor () public {
        totalSupply_ = INITIAL_SUPPLY;
        balances[msg.sender] = INITIAL_SUPPLY;
        emit Transfer(0x0, msg.sender, INITIAL_SUPPLY);
    }

    function freezeAccount(address target, bool freeze) public onlyOwner {
        frozenAccount[target] = freeze;
        emit FrozenFunds(target, freeze);
    }

    // unfreeze all accounts
    function unFreezeAll(bool flag) public onlyOwner {
        unFreeze = flag;
        emit FreezeAll(flag);
    }

    function transfer(address _to, uint256 _value) public notFrozen(msg.sender) returns (bool) {
        return super.transfer(_to, _value);
    }

    function transferFrom(address _from, address _to, uint256 _value) public notFrozen(_from) returns (bool) {
        return super.transferFrom(_from, _to, _value);
    }

    function approve(address _spender, uint256 _value) public notFrozen(msg.sender) returns (bool) {
        return super.approve(_spender, _value);
    }
}