pragma solidity ^0.6.1;

import "@openzeppelin/contracts-ethereum-package/contracts/introspection/IERC165.sol";
import "./IERC721.sol";
import "@openzeppelin/contracts-ethereum-package/contracts/token/ERC721/IERC721Receiver.sol";

contract TestNFT is IERC165, IERC721 {
    bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;

    mapping (uint256 => address) private _tokenOwner;
    mapping (uint256 => address) private _tokenApprovals;
    mapping (address => uint256) private _ownedTokensCount;
    mapping (address => mapping (address => bool)) private _operatorApprovals;

    bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;

    function balanceOf(address owner) public override(IERC721) view returns (uint256) {
        require(owner != address(0), "ERC721: balance query for the zero address");
        return _ownedTokensCount[owner];
    }

    function ownerOf(uint256 tokenId) public override(IERC721) view returns (address) {
        address owner = _tokenOwner[tokenId];
        require(owner != address(0), "ERC721: owner query for nonexistent token");

        return owner;
    }

    function approve(address to, uint256 tokenId) public override(IERC721) {
        address owner = ownerOf(tokenId);
        require(to != owner, "ERC721: approval to current owner");

        require(msg.sender == owner || isApprovedForAll(owner, msg.sender),
            "ERC721: approve caller is not owner nor approved for all"
        );

        _tokenApprovals[tokenId] = to;
        emit Approval(owner, to, tokenId);
    }

    function getApproved(uint256 tokenId) public override(IERC721) view returns (address) {
        require(_exists(tokenId), "ERC721: approved query for nonexistent token");

        return _tokenApprovals[tokenId];
    }

    function setApprovalForAll(address to, bool approved) public override(IERC721) {
        require(to != msg.sender, "ERC721: approve to caller");

        _operatorApprovals[msg.sender][to] = approved;
        emit ApprovalForAll(msg.sender, to, approved);
    }

    function isApprovedForAll(address owner, address operator) public override(IERC721) view returns (bool) {
        return _operatorApprovals[owner][operator];
    }

    function transferFrom(address from, address to, uint256 tokenId) public override(IERC721) {
        require(_isApprovedOrOwner(msg.sender, tokenId), "ERC721: transfer caller is not owner nor approved");

        _transferFrom(from, to, tokenId);
    }

    function safeTransferFrom(address from, address to, uint256 tokenId) public override(IERC721) {
        safeTransferFrom(from, to, tokenId, "");
    }

    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public override(IERC721) {
        transferFrom(from, to, tokenId);
        require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    function _exists(uint256 tokenId) internal view returns (bool) {
        address owner = _tokenOwner[tokenId];
        return owner != address(0);
    }

    function _isApprovedOrOwner(address spender, uint256 tokenId) internal view returns (bool) {
        require(_exists(tokenId), "ERC721: operator query for nonexistent token");
        address owner = ownerOf(tokenId);
        return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
    }

    function mint(address to, uint256 tokenId) public returns (bool) {
        return mint(to, tokenId, "");
    }

    function mint(address to, uint256 tokenId, bytes memory _data) public returns (bool) {
        require(to != address(0), "ERC721: mint to the zero address");
        require(!_exists(tokenId), "ERC721: token already minted");

        _tokenOwner[tokenId] = to;
        _ownedTokensCount[to]++;

        emit Transfer(address(0), to, tokenId);
        require(_checkOnERC721Received(msg.sender, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    function _transferFrom(address from, address to, uint256 tokenId) internal {
        require(ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
        require(to != address(0), "ERC721: transfer to the zero address");

        _clearApproval(tokenId);

        _ownedTokensCount[from]--;
        _ownedTokensCount[to]++;

        _tokenOwner[tokenId] = to;

        emit Transfer(from, to, tokenId);
    }

    function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data) internal returns (bool) {
        if (!isContract(to)) {
            return true;
        }

        bytes4 retval = IERC721Receiver(to).onERC721Received(msg.sender, from, tokenId, _data);
        return (retval == _ERC721_RECEIVED);
    }

    function _clearApproval(uint256 tokenId) private {
        if (_tokenApprovals[tokenId] != address(0)) {
            _tokenApprovals[tokenId] = address(0);
        }
    }

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

    function supportsInterface(bytes4 interfaceID) external override(IERC165) view returns (bool) {
        return interfaceID == _INTERFACE_ID_ERC721;
    }
}