Smart contract function throwing error in remix

Question

I am writing a PreICO Smart contract. I got a code snippet which I am trying to understand. I am stuck in function buyXYZTokens function which is throwing error. Here is the code snippet:

Contract XYZ is StandardToken, Ownable{
string public name = "XYZ";
string public symbol = "X";
string public standard = "X v1.0";
uint256 public decimals = 18;

modifier inActivePeriod() {
    require((preSaleStart < now && now <= preSaleEnd));
    _;
}
 function XYZ( uint256 _initialsupply, address _escrow, uint _preSaleStart, uint _preSaleEnd) {
    require(_escrow != 0x0);
    require(_preSaleStart < _preSaleEnd);       
    escrow = _escrow;
    totalSupply_=_initialsupply;
    balances[escrow] = _initialsupply;
}
    function() inActivePeriod payable{
    buyXYZTokens(msg.sender);
}
function buyXYZTokens(address _sender) inActivePeriod payable{
    require(_sender!=0x0);
    require(msg.value>=minTransactionAmount);
    uint256 weiCount = msg.value;
    amountRaised = amountRaised.add(weiCount);
    uint256 tokens = weiCount.mul(rate);
    tokens += getBonus(tokens);
    tokenReserve(_sender, tokens);
    forwardFunds();
} 
  function getBonus(uint256 _tokens) constant returns (uint256 bonus) {
        require(_tokens != 0);

        if (preSaleStart <= now && now < preSaleStart + 1 days) {
              return _tokens.div(2);
        } else if (preSaleStart + 1 days <= now && now < preSaleStart + 2 days ) {
              return _tokens.div(4);
        } else if (preSaleStart + 2 days <= now && now < preSaleStart + 3 days ) {
              return _tokens.div(10);
        }

        return 0;
    }

    function tokenReserve(address _to, uint256 _value) internal returns (bool) {
        balances[_to] = balances[_to].add(_value);
        Transfer(msg.sender, _to, _value);
        return true;
    }
    function forwardFunds() internal {
        escrow.transfer(msg.value);
    }

I got this error: I am not getting what this fallback function used for and why buyXYZTokens function throwing me error?

Answer 1

As your function call is reverting, it has to be caused by one of the require statements somewhere in the contracts.

If you are really deploying your contract with the parameters shown in the picture, the problem occurs (at least) in your inActivePeriod modifier which makes sure that the current block number is between the values _preSaleStart and _preSaleEnd.

After you fix that problem, you will probably face other similar problems if you do not fully understand what the code is doing (or what you are doing). One way to try to figure out this kind of reversion problems is by commenting out require statements until you find the one which causes the current problem - you can for example start by commenting them all out and enabling them one by one to see where you get issues. It's not a very fast method but it works.