A game like Ethereum fund in Solidity

fund.sol

pragma solidity ^0.5.11;

contract Fund {
    address payable manager = msg.sender;
    uint reserve = 0;
    uint redemptionFee = 100;
    
    struct Account{
        uint balance;
        bool isValid;
    }
    
    uint perAccountExtra;
    uint accountCount;
    
    mapping(address => Account) account;

    modifier onlyManager() {
        require(manager == msg.sender);
        _;
    }
    
    function Invest() public payable {
        if(account[msg.sender].isValid == false){
            account[msg.sender].isValid = true;
            accountCount = accountCount + 1;
        }
        account[msg.sender].balance = account[msg.sender].balance + msg.value;
    }
    
    function Redeem() public {
        require(account[msg.sender].balance  > redemptionFee);
        
        uint redemption = account[msg.sender].balance - redemptionFee + perAccountExtra;
        reserve = reserve + redemptionFee;
        
        msg.sender.transfer(redemption);
        
        account[msg.sender].balance = 0;
        account[msg.sender].isValid = false;
        accountCount = accountCount - 1;
    }
    
    function getBalance()  public view returns (uint) {
        return account[msg.sender].balance;
    }
    
    function ChangeRedemptionFee(uint _redemptionFee) public onlyManager {
        redemptionFee = _redemptionFee;
    }
    
    function RedeemReserve() public onlyManager {
        manager.transfer(reserve);
    }
    
    function DistributeReserve() public onlyManager {
        require(reserve/accountCount > 0);
        
        uint _perAccount =  reserve/accountCount;
        
        perAccountExtra = perAccountExtra + _perAccount;
        reserve = reserve - _perAccount * accountCount;
    }
}