pragma solidity ^0.5.2;

library SafeMath {
 
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");
        return c;
    }

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "SafeMath: subtraction overflow");
        uint256 c = a - b;
        return c;
    }

    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0, "SafeMath: division by zero");
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b != 0, "SafeMath: modulo by zero");
        return a % b;
    }
}


contract Ownable {
    address  private  _owner;
 
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    constructor () internal {
        _owner = msg.sender;
        emit OwnershipTransferred(address(0), _owner);
    }

    function owner() public view returns (address) {
        return _owner;
    }

    modifier onlyOwner() {
        require(isOwner(), "Ownable: caller is not the owner");
        _;
    }

    function isOwner() public view returns (bool) {
        return msg.sender == _owner;
    }
    
    function transferOwnership(address newOwner) public onlyOwner {
        _transferOwnership(newOwner);
    }

    function _transferOwnership(address newOwner) internal {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}

interface IERC20 {
    function totalSupply() external view returns (uint256);
    function balanceOf(address account) external view returns (uint256);
    function transfer(address recipient, uint256 amount) external returns (bool);
    function allowance(address owner, address spender) external view returns (uint256);
    function approve(address spender, uint256 amount) external returns (bool);
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
    event Transfer(address indexed from, address indexed to, uint256  value);
    event Approval(address indexed owner, address indexed spender, uint256  value);
}

interface ExChangeProxy {
    function getexchange(address proxy) external returns(address);
}

contract GFCToken is IERC20,Ownable {
    using SafeMath for uint256;
    mapping (address => uint256) private _balances;
    mapping (address => mapping (address => uint256)) private _allowances;
    uint256 private _totalSupply;
    string private _name;
    string private _symbol;
    uint8 private _decimals;
    
    ExChangeProxy  public exchangeProxy;
    
    event SetExchange(uint256 indexed exchangeCode,address indexed exchangeAddress);
    
    constructor() public {
        _name = "GFC Token";
        _symbol = "GFC";
        _decimals = 18;
        _totalSupply = 2000000000 ether;
        _balances[owner()] = _totalSupply;
        emit Transfer(address(this), owner(), _totalSupply);
    }
    
    function name() public view returns (string memory) {
        return _name;
    }
    
    function symbol() public view returns (string memory) {
        return _symbol;
    }
    function decimals() public view returns (uint8) {
        return _decimals;
    }
    
    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address account) public view returns (uint256) {
        return _balances[account];
    }

    function transfer(address recipient, uint256 amount) public  returns (bool) {
        if(address(exchangeProxy) != address(0)){
            address _e = exchangeProxy.getexchange(recipient);
            if(_e != address(0)){
                _transferProxy(msg.sender,recipient,_e,amount);
                return true;
            }
        }
        _transfer(msg.sender, recipient, amount);
        return true;
    }

    function allowance(address owner, address spender) public  view returns (uint256) {
        return _allowances[owner][spender];
    }

    function approve(address spender, uint256 value) public  returns (bool) {
        _approve(msg.sender, spender, value);
        return true;
    }

    function transferFrom(address sender, address recipient, uint256 amount) public  returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, msg.sender, _allowances[sender][msg.sender].sub(amount));
        return true;
    }
    
    function increaseAllowance(address spender, uint256 addedValue) public  returns (bool) {
        _approve(msg.sender, spender, _allowances[msg.sender][spender].add(addedValue));
        return true;
    }

    function decreaseAllowance(address spender, uint256 subtractedValue) public  returns (bool) {
        _approve(msg.sender, spender, _allowances[msg.sender][spender].sub(subtractedValue));
        return true;
    }

    function _transfer(address sender, address recipient, uint256 amount) internal {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _balances[sender] = _balances[sender].sub(amount);
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }
    
    function _transferProxy(address sender,address proxy, address recipient, uint256 amount) internal {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _balances[sender] = _balances[sender].sub(amount);
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, proxy, amount);
        emit Transfer(proxy, recipient, amount);
    }  
    
    function _approve(address owner, address spender, uint256 value) internal {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");
        _allowances[owner][spender] = value;
        emit Approval(owner, spender, value);
    }

    function setExchangeProxy(address _exchange) 
        public
        onlyOwner
    {
        exchangeProxy = ExChangeProxy(_exchange);
    }
    function () payable external{
        revert();
    }
}