Ethereum Solidity Dapp Game Contract

pragma solidity >=0.4.17 <0.6.0;

import "./SafeMath.sol";
import "./Strings.sol";

contract Game {
    using Strings for *;
    using SafeMath for *;
    address public manager;
    // address[] public players;
    
    struct GameInstructor {
        string number;
        uint256 totalSupply;
    }

    uint256 _totalSupply = 0;
    uint _GamePeriod = 1;
    uint _players = 0;

    mapping(uint => GameInstructor) GameInstructors;

    event log(string, uint); //变动时调用事件

    function Game() public {
        manager = msg.sender;
    }

    function enter(uint256 _value) public onlyManagerCanCall {
        // require(msg.value >= .01 ether);
        _totalSupply = _totalSupply.add(_value);
        _players++;
    }

    function fetchSeed() public view returns (uint) {
        // 
        uint value = uint(sha3(block.difficulty, now, _players));

        log("seed number", value);

        return value;
    }

    function convert() view public returns (string){
        uint _i = fetchSeed();
        uint j = _i;
        uint len;
        while (j != 0) {
            len++;
            j /= 10;
        }
        bytes memory bstr = new bytes(len);
        uint k = len - 1;
        while (_i != 0) {
            bstr[k--] = byte(uint8(48 + _i % 10));
            _i /= 10;
        }

        return string(bstr);
    }

    function random() public view returns (string) {
        string memory x = convert();
        return x._substring(4, 4);
    }

    // function modifier, reduce code we need to write
    modifier onlyManagerCanCall() {
        // only manage can pick winner
        require(msg.sender == manager);
        _;
        // all code in your function will replace the "_"
    }

    function pickWinner() public onlyManagerCanCall {
        string memory GameNumber = random();
        // this.balance has all the ether in current smart contract
        // players[index].transfer(address(this).balance); 
        // inital dynamic array whose length is 0;
        // players = new address[](0);
        addGameInstructor(_GamePeriod, _totalSupply, GameNumber);
 
        _totalSupply = 0;
        _players = 0;
        _GamePeriod++;
    }

    function addGameInstructor(uint GamePeriodKey, uint256 periodTotal, string GameNumber) public onlyManagerCanCall {
        var GameInstructor = GameInstructors[GamePeriodKey];
        GameInstructor.number = GameNumber;
        GameInstructor.totalSupply = periodTotal;
    }

    function getGame(uint GamePeriodKey) view public returns (string, uint256){
        return (GameInstructors[GamePeriodKey].number, GameInstructors[GamePeriodKey].totalSupply);
    }

    function getCurrentPlayers() public view returns (uint) {
        return _players;
    }

    function getCurrentTotalSupply() public view returns (uint) {
        return _totalSupply;
    }

}