pragma solidity >=0.5.0 <0.6.0;

import "openzeppelin-solidity/contracts/ownership/Ownable.sol";
import "openzeppelin-solidity/contracts/math/SafeMath.sol";


/**
 * @title Token_V0
 * @dev Version 0 of a token to show upgradeability.
 */
contract Token_V0 is Ownable {
    using SafeMath for uint256;

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

    bool internal _initialized;
    uint256 internal _totalSupply;
    mapping (address => uint256) internal _balances;
    mapping (address => mapping (address => uint256)) internal _allowances;

    function initialize(address owner) public {
        require(!_initialized);
        _transferOwnership(owner);
        _initialized = true;
    }

    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 _allowances[owner][spender];
    }

    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;
    }

    function transferFrom(address from, address to, uint256 value) public returns (bool) {
        require(to != address(0));
        require(value <= _balances[from]);
        require(value <= _allowances[from][msg.sender]);

        _balances[from] = _balances[from].sub(value);
        _balances[to] = _balances[to].add(value);
        _allowances[from][msg.sender] = _allowances[from][msg.sender].sub(value);
        emit Transfer(from, to, value);
        return true;
    }

    function approve(address spender, uint256 value) public {
        _allowances[msg.sender][spender] = value;
        emit Approval(msg.sender, spender, value);
    }

    function increaseApproval(address spender, uint256 addedValue) public {
        _allowances[msg.sender][spender] = _allowances[msg.sender][spender].add(addedValue);
        emit Approval(msg.sender, spender, _allowances[msg.sender][spender]);
    }

    function decreaseApproval(address spender, uint256 subtractedValue) public {
        uint oldValue = _allowances[msg.sender][spender];
        if (subtractedValue > oldValue) {
            _allowances[msg.sender][spender] = 0;
        } else {
            _allowances[msg.sender][spender] = oldValue.sub(subtractedValue);
        }
        emit Approval(msg.sender, spender, _allowances[msg.sender][spender]);
    }

    function mint(address to, uint256 value) public onlyOwner {
        _balances[to] = _balances[to].add(value);
        _totalSupply = _totalSupply.add(value);
        emit Transfer(address(0), to, value);
    }
}