# PNC-721

// SPDX-License-Identifier: MIT // // ERC-721 Non-Fungible Token implementation based on the OpenZeppelin Lib //

pragma solidity ^0.6.2;

### Abstract contract Context

{% code overflow="wrap" %}

```
function _msgData() 
internal view virtual returns (bytes memory) 
{
    this; // silence state mutability warning without generating bytecode - see                https://github.com/ethereum/solidity/issues/2691
    return msg.data;
}
```

{% endcode %}

### Library SafeMath

```
library SafeMath { function add(uint256 a, uint256 b) 
internal pure returns (uint256) 
{ 
 uint256 c = a + b; require(c >= a, "SafeMath: addition overflow"); 
 return c;
}
 function sub(uint256 a, uint256 b) internal pure returns (uint256) 
{
 return sub(a, b, "SafeMath: subtraction overflow");
}
 function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns    (uint256) 
 {
    require(b <= a, errorMessage);
    uint256 c = a - b;
    return c;
}
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
    if (a == 0) {
        return 0;
    }
    uint256 c = a * b;
    require(c / a == b, "SafeMath: multiplication overflow");
    return c;
}
function div(uint256 a, uint256 b) internal pure returns (uint256) {
    return div(a, b, "SafeMath: division by zero");
}
function div(uint256 a, uint256 b, string memory errorMessage) 
internal pure returns (uint256) 
{
    require(b > 0, errorMessage);
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
}
function mod(uint256 a, uint256 b) internal pure returns (uint256) {
    return mod(a, b, "SafeMath: modulo by zero");
}
function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
    require(b != 0, errorMessage);
    return a % b;
}
}
```

### Library Address

```
library Address 
{
function isContract(address account) internal view returns (bool) 
   {
    // This method relies on extcodesize, which returns 0 for contracts in
    // construction, since the code is only stored at the end of the
    // constructor execution.
    uint256 size;
    // solhint-disable-next-line no-inline-assembly
    assembly { size := extcodesize(account) }
    return size > 0;
    }
    function sendValue(address payable recipient, uint256 amount) internal {
    require(address(this).balance >= amount, "Address: insufficient balance");

    // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
    (bool success, ) = recipient.call{ value: amount }("");
    require(success, "Address: unable to send value, recipient may have reverted");
}
function functionCall(address target, bytes memory data) internal returns (bytes memory) {
  return functionCall(target, data, "Address: low-level call failed");
}

function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
    return _functionCallWithValue(target, data, 0, errorMessage);
}

function functionCallWithValue(address target, bytes memory data, uint256 value) internal returns (bytes memory) {
    return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
}

function functionCallWithValue(address target, bytes memory data, uint256 value, string memory errorMessage) internal returns (bytes memory) {
    require(address(this).balance >= value, "Address: insufficient balance for call");
    return _functionCallWithValue(target, data, value, errorMessage);
}

function _functionCallWithValue(address target, bytes memory data, uint256 weiValue, string memory errorMessage) private returns (bytes memory) {
    require(isContract(target), "Address: call to non-contract");
    // solhint-disable-next-line avoid-low-level-calls
    (bool success, bytes memory returndata) = target.call{ value: weiValue }(data);
    if (success) {
        return returndata;
    } else {
        // Look for revert reason and bubble it up if present
        if (returndata.length > 0) {
            // The easiest way to bubble the revert reason is using memory via assembly

            // solhint-disable-next-line no-inline-assembly
            assembly {
                let returndata_size := mload(returndata)
                revert(add(32, returndata), returndata_size)
            }
        } else {
            revert(errorMessage);
        }
    }
}
}
```

### Library EnumerableSet

```
library EnumerableSet 
{
struct Set {
    // Storage of set values
    bytes32[] _values;
    // Position of the value in the `values` array, plus 1 because index 0
    // means a value is not in the set.
    mapping (bytes32 => uint256) _indexes;
}
function _add(Set storage set, bytes32 value) private returns (bool) {
    if (!_contains(set, value)) {
        set._values.push(value);
        // The value is stored at length-1, but we add 1 to all indexes
        // and use 0 as a sentinel value
        set._indexes[value] = set._values.length;
        return true;
    } else {
        return false;
    }
}
function _remove(Set storage set, bytes32 value) private returns (bool) {
    // We read and store the value's index to prevent multiple reads from the same storage slot
    uint256 valueIndex = set._indexes[value];
    if (valueIndex != 0) { // Equivalent to contains(set, value)
        // To delete an element from the _values array in O(1), we swap the element to delete with the last one in
        // the array, and then remove the last element (sometimes called as 'swap and pop').
        // This modifies the order of the array, as noted in {at}.
        uint256 toDeleteIndex = valueIndex - 1;
        uint256 lastIndex = set._values.length - 1;
        // When the value to delete is the last one, the swap operation is unnecessary. However, since this occurs
        // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.
        bytes32 lastvalue = set._values[lastIndex];
        // Move the last value to the index where the value to delete is
        set._values[toDeleteIndex] = lastvalue;
        // Update the index for the moved value
        set._indexes[lastvalue] = toDeleteIndex + 1; // All indexes are 1-based
        // Delete the slot where the moved value was stored
        set._values.pop();
        // Delete the index for the deleted slot
        delete set._indexes[value];
        return true;
    } else {
        return false;
    }
}
function _contains(Set storage set, bytes32 value) private view returns (bool) {
    return set._indexes[value] != 0;
}
function _length(Set storage set) private view returns (uint256) {
    return set._values.length;
}
function _at(Set storage set, uint256 index) private view returns (bytes32) {
    require(set._values.length > index, "EnumerableSet: index out of bounds");
    return set._values[index];
}
struct AddressSet {
    Set _inner;
}
function add(AddressSet storage set, address value) internal returns (bool) {
    return _add(set._inner, bytes32(uint256(value)));
}
function remove(AddressSet storage set, address value) internal returns (bool) {
    return _remove(set._inner, bytes32(uint256(value)));
}
function contains(AddressSet storage set, address value) internal view returns (bool) {
    return _contains(set._inner, bytes32(uint256(value)));
}
function length(AddressSet storage set) internal view returns (uint256) {
    return _length(set._inner);
}
function at(AddressSet storage set, uint256 index) internal view returns (address) {
    return address(uint256(_at(set._inner, index)));
}
struct UintSet {
    Set _inner;
}
function add(UintSet storage set, uint256 value) internal returns (bool) {
    return _add(set._inner, bytes32(value));
}
function remove(UintSet storage set, uint256 value) internal returns (bool) {
    return _remove(set._inner, bytes32(value));
}
function contains(UintSet storage set, uint256 value) internal view returns (bool) {
    return _contains(set._inner, bytes32(value));
}
function length(UintSet storage set) internal view returns (uint256) {
    return _length(set._inner);
}
function at(UintSet storage set, uint256 index) internal view returns (uint256) {
    return uint256(_at(set._inner, index));
}
}
```

### Library EnumerableMap

```
library EnumerableMap 
{
struct MapEntry {
    bytes32 _key;
    bytes32 _value;
}
struct Map {
    // Storage of map keys and values
    MapEntry[] _entries;
    // Position of the entry defined by a key in the `entries` array, plus 1
    // because index 0 means a key is not in the map.
    mapping (bytes32 => uint256) _indexes;
}
function _set(Map storage map, bytes32 key, bytes32 value) private returns (bool) {
    // We read and store the key's index to prevent multiple reads from the same storage slot
    uint256 keyIndex = map._indexes[key];
    if (keyIndex == 0) { // Equivalent to !contains(map, key)
        map._entries.push(MapEntry({ _key: key, _value: value }));
        // The entry is stored at length-1, but we add 1 to all indexes
        // and use 0 as a sentinel value
        map._indexes[key] = map._entries.length;
        return true;
    } else {
        map._entries[keyIndex - 1]._value = value;
        return false;
    }
}
function _remove(Map storage map, bytes32 key) private returns (bool) {
    // We read and store the key's index to prevent multiple reads from the same storage slot
    uint256 keyIndex = map._indexes[key];
    if (keyIndex != 0) { // Equivalent to contains(map, key)
        // To delete a key-value pair from the _entries array in O(1), we swap the entry to delete with the last one
        // in the array, and then remove the last entry (sometimes called as 'swap and pop').
        // This modifies the order of the array, as noted in {at}.
        uint256 toDeleteIndex = keyIndex - 1;
        uint256 lastIndex = map._entries.length - 1;
        // When the entry to delete is the last one, the swap operation is unnecessary. However, since this occurs
        // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.
        MapEntry storage lastEntry = map._entries[lastIndex];
        // Move the last entry to the index where the entry to delete is
        map._entries[toDeleteIndex] = lastEntry;
        // Update the index for the moved entry
        map._indexes[lastEntry._key] = toDeleteIndex + 1; // All indexes are 1-based
        // Delete the slot where the moved entry was stored
        map._entries.pop();
        // Delete the index for the deleted slot
        delete map._indexes[key];
        return true;
    } else {
        return false;
    }
}
function _contains(Map storage map, bytes32 key) private view returns (bool) {
    return map._indexes[key] != 0;
}
function _length(Map storage map) private view returns (uint256) {
    return map._entries.length;
}
function _at(Map storage map, uint256 index) private view returns (bytes32, bytes32) {
    require(map._entries.length > index, "EnumerableMap: index out of bounds");
    MapEntry storage entry = map._entries[index];
    return (entry._key, entry._value);
}
function _get(Map storage map, bytes32 key) private view returns (bytes32) {
    return _get(map, key, "EnumerableMap: nonexistent key");
}
function _get(Map storage map, bytes32 key, string memory errorMessage) private view returns (bytes32) {
    uint256 keyIndex = map._indexes[key];
    require(keyIndex != 0, errorMessage); // Equivalent to contains(map, key)
    return map._entries[keyIndex - 1]._value; // All indexes are 1-based
}
struct UintToAddressMap {
    Map _inner;
}
function set(UintToAddressMap storage map, uint256 key, address value) internal returns (bool) {
    return _set(map._inner, bytes32(key), bytes32(uint256(value)));
}
function remove(UintToAddressMap storage map, uint256 key) internal returns (bool) {
    return _remove(map._inner, bytes32(key));
}
function contains(UintToAddressMap storage map, uint256 key) internal view returns (bool) {
    return _contains(map._inner, bytes32(key));
}
function length(UintToAddressMap storage map) internal view returns (uint256) {
    return _length(map._inner);
}
function at(UintToAddressMap storage map, uint256 index) internal view returns (uint256, address) {
    (bytes32 key, bytes32 value) = _at(map._inner, index);
    return (uint256(key), address(uint256(value)));
}
function get(UintToAddressMap storage map, uint256 key) internal view returns (address) {
    return address(uint256(_get(map._inner, bytes32(key))));
}
function get(UintToAddressMap storage map, uint256 key, string memory errorMessage) internal view returns (address) {
    return address(uint256(_get(map._inner, bytes32(key), errorMessage)));
}
}
```

### Library Strings

```
library Strings 
{
function toString(uint256 value) internal pure returns (string memory) {
    // Inspired by OraclizeAPI's implementation - MIT licence
    // https://github.com/oraclize/ethereum-api/blob/b42146b063c7d6ee1358846c198246239e9360e8/oraclizeAPI_0.4.25.sol
    if (value == 0) {
        return "0";
    }
    uint256 temp = value;
    uint256 digits;
    while (temp != 0) {
        digits++;
        temp /= 10;
    }
    bytes memory buffer = new bytes(digits);
    uint256 index = digits - 1;
    temp = value;
    while (temp != 0) {
        buffer[index--] = byte(uint8(48 + temp % 10));
        temp /= 10;
    }
    return string(buffer);
}
}
```

### Interface IERC165

{% code overflow="wrap" %}

```
Interface IERC165 
{ 
function supportsInterface(bytes4 interfaceId) external view returns (bool);
}
Interface IERC721 is IERC165 
{ 
event Transfer(address indexed from, address indexed to, uint256 indexed tokenId); 
event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);
event ApprovalForAll(address indexed owner, address indexed operator, bool approved); 
function balanceOf(address owner) external view returns (uint256 balance); 
function ownerOf(uint256 tokenId) external view returns (address owner); 
function safeTransferFrom(address from, address to, uint256 tokenId) external; 
function transferFrom(address from, address to, uint256 tokenId) external; 
function approve(address to, uint256 tokenId) external; function getApproved(uint256 tokenId) external view returns (address operator); 
function setApprovalForAll(address operator, bool _approved) external; 
function isApprovedForAll(address owner, address operator) external view returns (bool); 
function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external; 
}
interface IERC721Metadata is IERC721 { function name() external view returns (string memory); 
function symbol() external view returns (string memory); 
function tokenURI(uint256 tokenId) external view returns (string memory); 
}
interface IERC721Enumerable is IERC721 
{ 
function totalSupply() external view returns (uint256); 
function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256 tokenId); 
function tokenByIndex(uint256 index) external view returns (uint256); 
}
interface IERC721Receiver { function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data) external returns (bytes4); 
}
```

{% endcode %}

### Contract ERC165

```
Contract ERC165 is IERC165 
{
bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;

mapping(bytes4 => bool) private _supportedInterfaces;

constructor () internal {
    _registerInterface(_INTERFACE_ID_ERC165);
}

function supportsInterface(bytes4 interfaceId) public view override returns (bool) {
    return _supportedInterfaces[interfaceId];
}

function _registerInterface(bytes4 interfaceId) internal virtual {
    require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
    _supportedInterfaces[interfaceId] = true;
}
}
```

### Panda Network Token contract ERC721

<pre data-overflow="wrap"><code>// ERC stands for "Pando Network Token", ERC-721 is the non-fungible token // standard on the Pando network, similar to the ERC-721 standard on Pando Network Token contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable 
{ 
using SafeMath for uint256; 
using Address for address; 
using EnumerableSet for EnumerableSet.UintSet; 
using EnumerableMap for EnumerableMap.UintToAddressMap; 
using Strings for uint256; 
bytes4 private constant _ERC721_RECEIVED = 0x150b7a02; 
mapping (address => EnumerableSet.UintSet) private _holderTokens; 
EnumerableMap.UintToAddressMap private _tokenOwners; mapping (uint256 => address) private _tokenApprovals; 
mapping (address => mapping (address => bool)) private _operatorApprovals; 
string private _name; 
string private _symbol; 
mapping (uint256 => string) private _tokenURIs; 
string private _baseURI; bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;
bytes4 private constant _INTERFACE_ID_ERC721_METADATA = 0x5b5e139f; 
bytes4 private constant _INTERFACE_ID_ERC721_ENUMERABLE = 0x780e9d63;
constructor (string memory name, string memory symbol) public 
{ 
    _name = name;
    _symbol = symbol;
    _registerInterface(_INTERFACE_ID_ERC721);
    _registerInterface(_INTERFACE_ID_ERC721_METADATA);
    _registerInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
}
    function balanceOf(address owner) public view override returns (uint256) {
    require(owner != address(0), "ERC721: balance query for the zero address");
    return _holderTokens[owner].length();
}
    function ownerOf(uint256 tokenId) public view override returns (address) {
    return _tokenOwners.get(tokenId, "ERC721: owner query for nonexistent token");
}
    function name() public view override returns (string memory) {
    return _name;
}
    function symbol() public view override returns (string memory) {
    return _symbol;
}
    function tokenURI(uint256 tokenId) public view override returns (string memory) {
    require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");
    string memory _tokenURI = _tokenURIs[tokenId];
    // If there is no base URI, return the token URI.
    if (bytes(_baseURI).length == 0) 
    {
        return _tokenURI;
    }
    // If both are set, concatenate the baseURI and tokenURI (via abi.encodePacked).
    if (bytes(_tokenURI).length > 0) 
    {
        return string(abi.encodePacked(_baseURI, _tokenURI));
    }
    // If there is a baseURI but no tokenURI, concatenate the tokenID to the baseURI.
    return string(abi.encodePacked(_baseURI, tokenId.toString()));
}
    function baseURI() public view returns (string memory) {
    return _baseURI;
}
   function tokenOfOwnerByIndex(address owner, uint256 index) public view override returns (uint256) 
{
    return _holderTokens[owner].at(index);
}
   function totalSupply() public view override returns (uint256) {
    // _tokenOwners are indexed by tokenIds, so .length() returns the number of tokenIds
    return _tokenOwners.length();
}
    function tokenByIndex(uint256 index) public view override returns (uint256) {
    (uint256 tokenId, ) = _tokenOwners.at(index);
    return tokenId;
}

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

    require(_msgSender() == owner || isApprovedForAll(owner, _msgSender()),
    "ERC721: approve caller is not owner nor approved for all");
    _approve(to, tokenId);
}
    function getApproved(uint256 tokenId) public view override returns (address) {
    require(_exists(tokenId), "ERC721: approved query for nonexistent token");
    return _tokenApprovals[tokenId];
}
   function setApprovalForAll(address operator, bool approved) public virtual override 
 {
    require(operator != _msgSender(), "ERC721: approve to caller");
    _operatorApprovals[_msgSender()][operator] = approved;
    emit ApprovalForAll(_msgSender(), operator, approved);
}
    function isApprovedForAll(address owner, address operator) public view override returns (bool) 
    {
    return _operatorApprovals[owner][operator];
<strong>}
</strong>
    function transferFrom(address from, address to, uint256 tokenId) public virtual override 
{
    //solhint-disable-next-line max-line-length
    require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");
    _transfer(from, to, tokenId);
}
     function safeTransferFrom(address from, address to, uint256 tokenId) public virtual override 
     {
    safeTransferFrom(from, to, tokenId, "");
}
function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public virtual override {
    require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");
    _safeTransfer(from, to, tokenId, _data);
}
function _safeTransfer(address from, address to, uint256 tokenId, bytes memory _data) internal virtual {
    _transfer(from, to, tokenId);
    require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
}
function _exists(uint256 tokenId) internal view returns (bool) {
    return _tokenOwners.contains(tokenId);
}
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 _safeMint(address to, uint256 tokenId) internal virtual {
    _safeMint(to, tokenId, "");
}
function _safeMint(address to, uint256 tokenId, bytes memory _data) internal virtual {
    _mint(to, tokenId);
    require(_checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
}
function _mint(address to, uint256 tokenId) internal virtual {
    require(to != address(0), "ERC721: mint to the zero address");
    require(!_exists(tokenId), "ERC721: token already minted");
    _beforeTokenTransfer(address(0), to, tokenId);
    _holderTokens[to].add(tokenId);
    _tokenOwners.set(tokenId, to);
    emit Transfer(address(0), to, tokenId);
}
function _burn(uint256 tokenId) internal virtual {
    address owner = ownerOf(tokenId);
    _beforeTokenTransfer(owner, address(0), tokenId);
    // Clear approvals
    _approve(address(0), tokenId);
    // Clear metadata (if any)
    if (bytes(_tokenURIs[tokenId]).length != 0) {
    delete _tokenURIs[tokenId];
    }
    _holderTokens[owner].remove(tokenId);
    _tokenOwners.remove(tokenId);
    emit Transfer(owner, address(0), tokenId);
}
function _transfer(address from, address to, uint256 tokenId) internal virtual {
    require(ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
    require(to != address(0), "ERC721: transfer to the zero address");
    _beforeTokenTransfer(from, to, tokenId);
    // Clear approvals from the previous owner
    _approve(address(0), tokenId);
    _holderTokens[from].remove(tokenId);
    _holderTokens[to].add(tokenId);
    _tokenOwners.set(tokenId, to);
    emit Transfer(from, to, tokenId);
}
function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
    require(_exists(tokenId), "ERC721Metadata: URI set of nonexistent token");
    _tokenURIs[tokenId] = _tokenURI;
}
function _setBaseURI(string memory baseURI_) internal virtual {
    _baseURI = baseURI_;
}
function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data)
    private returns (bool)
{
    if (!to.isContract()) {
        return true;
    }
    bytes memory returndata = to.functionCall(abi.encodeWithSelector(
        IERC721Receiver(to).onERC721Received.selector,
        _msgSender(),
        from,
        tokenId,
        _data
    ), "ERC721: transfer to non ERC721Receiver implementer");
    bytes4 retval = abi.decode(returndata, (bytes4));
    return (retval == _ERC721_RECEIVED);
}
function _approve(address to, uint256 tokenId) private {
    _tokenApprovals[tokenId] = to;
    emit Approval(ownerOf(tokenId), to, tokenId);
}
function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal virtual { }
</code></pre>

}

contract CoolNFT is ERC721 { constructor (string memory name, string memory symbol, string memory uri) public ERC721(name, symbol) { uint mintIndex = totalSupply(); \_safeMint(msg.sender, mintIndex); \_setTokenURI(mintIndex, uri); } }


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