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contracts...Practice2.sol

//Klaytn IDE uses solidity 0.4.24 0.5.6 versions.
pragma solidity >=0.4.24 <=0.5.6;

contract Practice2 {
    string public name = "KlayLion";
    string public symbol = "KL"; // 단위 

    mapping(uint256 => address) public tokenOwner; // token owner mapping
    mapping(uint256 => string) public tokenURIs; // key - value type declare

    // 소유한 토큰 리스트
    mapping(address => uint256[]) private _ownedTokens;

    // mint(tokenId, uri, owner) : 발행
    // transferFrom(from, to, tokenId) : 전송 -> owner가 바뀌는 것 (from -> to)

    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;

        // add token to the list
        _ownedTokens[to].push(tokenId);

        return true;
    }

    function safeTransferFrom(address from, address to, uint256 tokenId) public{
        require(from == msg.sender, "from !- msg.sender"); // 보내는 사람이 현재 이 함수를 실행한 주체 
        require(from == tokenOwner[tokenId], "you are not the owner of the token"); // 보내는 사람이 보내려고 하는 토큰의 주인
        //
        _removeTokenFromList(from, tokenId);
        _ownedTokens[to].push(tokenId);
        //
        tokenOwner[tokenId] = to;

    }

    function _removeTokenFromList(address from, uint256 tokenId) private{
        // [10, 15, 19, 20] -> 19번을 삭제하고 싶어요.
        // [10, 15, 20, 19]
        // [10, 15, 20]
        uint256 lastTokenIdx = _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][lastTokenIdx];
               _ownedTokens[from][lastTokenIdx] = 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 { // memory는 좀 긴 타입의 데이터를 표시할 떄 쓴다. 
        tokenURIs[id] = uri; // mapping
    }
}