Safe Remote Purchase

contract Purchase {
    uint public value;
    address public seller;
    address public buyer;
    
    enum State { Created, Locked, Inactive }
    State public state;
    
    function Purchase() {
        seller = msg.sender;
        value = msg.value/2;
        if (2*value != msg.value) throw;
    }
    
    modifier require(bool _condition) {
        if (!_condition) throw;
        _
    }
    
    modifier onlyBuyer() {
        if (msg.sender != buyer) throw;
        _
    }
    
    modifier onlySeller() {
        if (msg.sender != seller) throw;
        _
    }
    
    modifier inState(State _state) {
        if (state != _state) throw;
        _
    }
    
    event aborted();
    event purchaseConfirmed();
    event itemReceived();
    
    // 구매를 취소하고 ether 를 refund 한다
    // Seller 만 취소할 수 있으며 contract 가 lock 되기 전에만 가능
    function abort() 
        onlySeller
        inState(State.Created) {
        aborted();
        seller.send(this.balance);
        state = State.Locked;
    }
    
    function confirmReceived()
        onlyBuyer
        inState(State.Locked) {
        itemReceived();
        buyer.send(value);
        seller.send(this.balance);
        state = State.Inactive;
    }
    
    function() {throw;}
}