pragma solidity ^0.5.0;

contract Ownable {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    constructor() internal {
        _owner = msg.sender;
        emit OwnershipTransferred(address(0), _owner);
    }

    modifier onlyOwner() {
        require(isOwner(), "Ownable: caller is not the owner");
        _;
    }

    function isOwner() public view returns (bool) {
        return msg.sender == _owner;
    }    

    function owner() public view returns (address) {
        return _owner;
    }

    function transferOwnership(address newOwner) public onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}


library SafeMath {
    function mul(uint a, uint b) internal pure returns (uint) {
        uint c = a * b;
        assert(a == 0 || c / a == b);
        return c;
    }

    function div(uint a, uint b) internal pure returns (uint) {
        assert(b > 0);
        uint c = a / b;
        assert(a == b * c + a % b);
        return c;
    }

    function sub(uint a, uint b) internal pure returns (uint) {
        assert(b <= a);
        return a - b;
    }

    function add(uint a, uint b) internal pure returns (uint) {
        uint c = a + b;
        assert(c >= a);
        return c;
    }
}

contract Pausable is Ownable {
    bool private _paused;

    event Paused(address account);
    event Unpaused(address account);

    constructor () internal {
        _paused = false;
    }

    function paused() public view returns (bool) {
        return _paused;
    }

    modifier whenNotPaused() {
        require(!_paused, "Pausable: paused");
        _;
    }

    modifier whenPaused() {
        require(_paused, "Pausable: not paused");
        _;
    }

    function pause() public onlyOwner whenNotPaused {
        _paused = true;
        emit Paused(msg.sender);
    }

    function unpause() public onlyOwner whenPaused {
        _paused = false;
        emit Unpaused(msg.sender);
    }    
}

contract EIP20Interface {
    function transfer(address to, uint256 value) public returns (bool);
    function approve(address spender, uint256 value) public returns (bool);
    function transferFrom(address from, address to, uint256 value) public returns (bool);
    function totalSupply() public view returns (uint256);
    function balanceOf(address who) public view returns (uint256);
    function allowance(address owner, address spender) public view returns (uint256);
    
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}


contract ERC20 is Ownable, Pausable, EIP20Interface {
    using SafeMath for uint256;
    
    uint256 private _totalSupply;
    string private _name;
    string private _symbol;
    uint8 private _decimals;
    
    
    mapping (address => uint256) private _balances;
    mapping (address => mapping (address => uint256)) private _allowed;

    constructor() public {
        _name = "Qobit.com Token";
        _symbol = "QOB";
        _decimals = 8;
        _totalSupply = 1500000000 * 10 ** uint256(_decimals);
        _balances[msg.sender] = _totalSupply;
    }
    
    // erc20
    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address owner) public view returns (uint256) {
        return _balances[owner];
    }

    function allowance(address owner, address spender) public view returns (uint256) {
        return _allowed[owner][spender];
    }    

    function transfer(address to, uint256 value) public whenNotPaused returns (bool) {
        _transfer(msg.sender, to, value);
        return true;
    }

    function approve(address spender, uint256 value) public whenNotPaused returns (bool) {
        require(value > 0);
        _approve(msg.sender, spender, value);
        return true;
    }    

    function transferFrom(address from, address to, uint256 value) public whenNotPaused returns (bool) {
        _transfer(from, to, value);
        _approve(from, msg.sender, _allowed[from][msg.sender].sub(value));
        return true;
    }

    function increaseAllowance(address spender, uint256 addedValue) public whenNotPaused returns (bool) {
        _approve(msg.sender, spender, _allowed[msg.sender][spender].add(addedValue));
        return true;
    }

    function decreaseAllowance(address spender, uint256 subtractedValue) public whenNotPaused returns (bool) {
        _approve(msg.sender, spender, _allowed[msg.sender][spender].sub(subtractedValue));
        return true;
    }    
    
    function _transfer(address from, address to, uint256 value) internal {
        require(to != address(0), "ERC20: transfer to the zero address");
        
        _balances[from] = _balances[from].sub(value);
        _balances[to] = _balances[to].add(value);
        emit Transfer(from, to, value);
    }

    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");

        _allowed[owner][spender] = value;
        emit Approval(owner, spender, value);
    }    
    
    // details
    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;
    }
}