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contracts...LemonadeStand.sol

// SPDX-License-Identifier: MIT
pragma solidity 0.8.11;

contract LemonadeStand {

    address owner; 
    uint skuCount;
    
    enum State { ForSale , Sold, Shipped }
    
    struct Item {
        string name;
        uint sku;
        uint price;
        State state;
        address seller;
        address buyer;
    }

    constructor() {
        owner = msg.sender;
        skuCount = 0;
    }

    //items that maps the SKU to a number on an item
    mapping (uint => Item) items;

    //Events
    event ForSale(uint skuCount);
    event Sold (uint sku);
    event Shipped(uint sku);

    //modifiers 
    modifier onlyOwner(){
        require(msg.sender == owner);
        _;
    }

    modifier verifyCaller(address _address){
        require(msg.sender == _address);
        _;
    }

    modifier paidEnough(uint _price){
        require(msg.value >= _price);
        _;
    }

    modifier forSale(uint _sku){
        require(items[_sku].state == State.ForSale);
        _;
    }

    modifier sold(uint _sku){
        require(items[_sku].state == State.Sold);
        _;
    }

    //Adding a new item
    function addItem(string calldata _name, uint _price) onlyOwner public{
        skuCount += 1;
        items[skuCount] = Item({name: _name, sku: skuCount, price: _price, state: State.ForSale, seller: msg.sender, buyer: address(0)});
        emit ForSale(skuCount);
    }

    //buying an item
    function buyItem(uint _sku) forSale(_sku) paidEnough(items[_sku].price) public payable {
        //get price from item
        uint price = items[_sku].price;
        //set buyer to sender address
        items[_sku].buyer = msg.sender;
        //set item state
        items[_sku].state = State.Sold;
        //transfer money to seller
        payable(items[_sku].seller).transfer(price);
        returnBalance(_sku);
        emit Sold(_sku);
    }

    function returnBalance(uint sku) internal {
          uint balance = msg.value - items[sku].price;
          payable(items[sku].buyer).transfer(balance);     
    }

    function shipItem(uint _sku) verifyCaller(items[_sku].seller) sold(_sku) public {
        items[_sku].state = State.Shipped;
        emit Shipped(_sku);
    }

    function fetchItem(uint _sku) public view returns(string memory name, uint sku, uint price, string memory stateIs, address  buyer, address seller){
        uint state;
        name = items[_sku].name;
        sku = items[_sku].sku;
        price = items[_sku].price;
        state = uint(items[_sku].state);
        if(state == 0){ stateIs = "ForSale"; }
        if(state == 1) { stateIs = "Sold"; }
        if(state == 2) { stateIs = "Shipped"; }
        seller = items[_sku].seller;
        buyer = items[_sku].buyer;
    }

}