Smart_Contract

contract NFTSimple {
    string public name = "KlayLion";
    string public symbol = "KL";

    mapping (uint256 => address) public tokenOwner;
    mapping (uint256 => string) public tokenURIs;

    mapping (address => uint256[]) private _ownedTokens;
    
    bytes4 private constant _KIP17_RECEIVED = 0x6745782b;

    using Counters for Counters.Counter;
    mapping(address => Counters.Counter) private _ownedTokensCount;

    // mint(tokenId, uri, owner)
    // transferFrom(from, to, tokenId)

    function mintWithTokenURI(address to, uint256 tokenId, string memory tokenURI) public returns (bool) {
        // to에게 tokenId(일련번호)를 발행하겠다.
        // 적힐 글자는 tokenURI
        tokenOwner[tokenId] = to;
        tokenURIs[tokenId] = tokenURI;
        _ownedTokens[to].push(tokenId);
        _ownedTokensCount[to].increment();
        return true;
    }

    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public {
        require(from == msg.sender, "from != msg.sender");
        require(from == tokenOwner[tokenId], "you are not the owner of the token");
        // 
        _removeTokenFromList(from, tokenId);
        _ownedTokens[to].push(tokenId);
        //
        tokenOwner[tokenId] = to;
        _ownedTokensCount[from].decrement();
        _ownedTokensCount[to].increment();

        // 만약에 받는쪽이 실행할 코드가 있는 스마트 컨트랙트이면 코드를 실행할 것
        require(
            _checkOnKIP17Received(from, to, tokenId, _data), "KIP17: transfer to non KIP17Receiver implementer"
        );

    }

    function _checkOnKIP17Received(address from, address to, uint256 tokenId, bytes memory _data) internal returns (bool) {
        bool success;
        bytes memory returndata;

        if (!isContract(to)) {
            return true;
        }

        (success, returndata) = to.call(
            abi.encodeWithSelector(_KIP17_RECEIVED, msg.sender, from, tokenId, _data)
        );

        if (returndata.length !=0 && abi.decode(returndata, (bytes4)) == _KIP17_RECEIVED) {
            return true;
        }

        return false;
    }

    function balanceOf(address owner) public view returns (uint256) {
        require(
            owner != address(0),
            "KIP17: balance query for the zero address"
        );

        return _ownedTokensCount[owner].current();
    }

    function isContract(address account) internal view returns (bool) {
        uint256 size;
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    function _removeTokenFromList(address from, uint256 tokenId) private {
        uint256 lastTokenIndex = _ownedTokens[from].length-1;
        for(uint256 i=0; i<_ownedTokens[from].length; i++) {
            if (tokenId == _ownedTokens[from][i]) {
                // Swap last token with deleting token;
                _ownedTokens[from][i] = _ownedTokens[from][lastTokenIndex];
                _ownedTokens[from][lastTokenIndex] = tokenId;
                break;
            }
        }
        _ownedTokens[from].length--;
    }
    function ownedTokens(address owner) public view returns (uint256[] memory) {
        return _ownedTokens[owner];
    }
    function setTokenUri(uint256 id, string memory uri) public {
        tokenURIs[id] = uri;
    }
}