Purchase.sol

pragma solidity ^0.4.22;
contract Purchase{
    uint public value;
    address public seller;
    address public buyer;
    enum State { Created, Locked, Inactive }
    State public state;
    
    constructor() public payable{
        seller = msg.sender;
        value = msg.value / 2;
        require((2 * value) == msg.value, "Value has to be even.");
    }
    
    modifier condition(bool _condition) {
        require(_condition);
        _;
    }
    
    modifier onlyBuyer() {
        require(msg.sender == buyer,"Only buyer can call this.");
        _;
    }
    
    modifier onlySeller() {
        require(msg.sender == seller,"Only seller can call this.");
        _;
    }
    
    modifier inState(State _state) {
        require(state == _state, "Invalid state.");
        _;
    }
    
    event Aborted();
    event PurchaseConfirmed();
    event ItemReceived();
    
    function abort() public onlySeller inState(State.Created){
        emit Aborted();
        state = State.Inactive;
        seller.transfer(address(this).balance);
    }
    
    function comfirmPurchase() public inState(State.Created) condition(msg.value == (2 * value)) payable{
        emit PurchaseConfirmed();
        buyer = msg.sender;
        state = State.Locked;
    }
    
    function confirmReceived() public onlyBuyer inState(State.Locked){
        emit ItemReceived();
        state = State.Inactive;
        buyer.transfer(value);
        seller.transfer(address(this).balance);
    }
}