pragma solidity ^0.4.24;

interface ERC721TokenReceiver
{

    function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4);

}

contract Astroglyphs {

    event Generated(uint indexed index, address indexed a, string value);

    /// @dev This emits when ownership of any NFT changes by any mechanism.
    ///  This event emits when NFTs are created (`from` == 0) and destroyed
    ///  (`to` == 0). Exception: during contract creation, any number of NFTs
    ///  may be created and assigned without emitting Transfer. At the time of
    ///  any transfer, the approved address for that NFT (if any) is reset to none.
    event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);

    /// @dev This emits when the approved address for an NFT is changed or
    ///  reaffirmed. The zero address indicates there is no approved address.
    ///  When a Transfer event emits, this also indicates that the approved
    ///  address for that NFT (if any) is reset to none.
    event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

    /// @dev This emits when an operator is enabled or disabled for an owner.
    ///  The operator can manage all NFTs of the owner.
    event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved);

    bytes4 internal constant MAGIC_ON_ERC721_RECEIVED = 0x150b7a02;

    uint public constant TOKEN_LIMIT = 12; // 8 for testing, 256 or 512 for prod;
    uint public constant ARTIST_PRINTS = 12; // 2 for testing, 64 for prod;

    uint public constant PRICE = 200 finney;

    // The beneficiary is 350.org
    address public constant BENEFICIARY = 0x28A9dE2183817164B91B0b693788E84Da3eAc59e;

    mapping (uint => address) private idToCreator;
    mapping (uint => uint8) private idToSymbolScheme;

    // ERC 165
    mapping(bytes4 => bool) internal supportedInterfaces;

    /**
     * @dev A mapping from NFT ID to the address that owns it.
     */
    mapping (uint256 => address) internal idToOwner;

    /**
     * @dev A mapping from NFT ID to the seed used to make it.
     */
    mapping (uint256 => uint256) internal idToSeed;
    mapping (uint256 => uint256) internal seedToId;

    /**
     * @dev Mapping from NFT ID to approved address.
     */
    mapping (uint256 => address) internal idToApproval;

    /**
     * @dev Mapping from owner address to mapping of operator addresses.
     */
    mapping (address => mapping (address => bool)) internal ownerToOperators;

    /**
     * @dev Mapping from owner to list of owned NFT IDs.
     */
    mapping(address => uint256[]) internal ownerToIds;

    /**
     * @dev Mapping from NFT ID to its index in the owner tokens list.
     */
    mapping(uint256 => uint256) internal idToOwnerIndex;

    /**
     * @dev Total number of tokens.
     */
    uint internal numTokens = 0;

    /**
     * @dev Guarantees that the msg.sender is an owner or operator of the given NFT.
     * @param _tokenId ID of the NFT to validate.
     */
    modifier canOperate(uint256 _tokenId) {
        address tokenOwner = idToOwner[_tokenId];
        require(tokenOwner == msg.sender || ownerToOperators[tokenOwner][msg.sender]);
        _;
    }

    /**
     * @dev Guarantees that the msg.sender is allowed to transfer NFT.
     * @param _tokenId ID of the NFT to transfer.
     */
    modifier canTransfer(uint256 _tokenId) {
        address tokenOwner = idToOwner[_tokenId];
        require(
            tokenOwner == msg.sender
            || idToApproval[_tokenId] == msg.sender
            || ownerToOperators[tokenOwner][msg.sender]
        );
        _;
    }

    /**
     * @dev Guarantees that _tokenId is a valid Token.
     * @param _tokenId ID of the NFT to validate.
     */
    modifier validNFToken(uint256 _tokenId) {
        require(idToOwner[_tokenId] != address(0));
        _;
    }

    /**
     * @dev Contract constructor.
     */
    constructor() public {
        supportedInterfaces[0x01ffc9a7] = true; // ERC165
        supportedInterfaces[0x80ac58cd] = true; // ERC721
        supportedInterfaces[0x780e9d63] = true; // ERC721 Enumerable
        supportedInterfaces[0x5b5e139f] = true; // ERC721 Metadata
    }

    ///////////////////
    //// GENERATOR ////
    ///////////////////

    int constant ONE = int(0x100000000);
    uint constant USIZE = 64;
    int constant SIZE = int(USIZE);
    int constant HALF_SIZE = SIZE / int(2);

    int constant SCALE = int(0x1b81a81ab1a81a823);
    int constant HALF_SCALE = SCALE / int(2);    

    string internal nftName = "Astroglyphs";
    string internal nftSymbol = "Ψ";

    // 0x2E = .
    // 0x4F = O
    // 0x2B = +
    // 0x58 = X
    // 0x7C = |
    // 0x2D = -
    // 0x5C = \
    // 0x2F = /
    // 0x23 = #

    function abs(int n) internal pure returns (int) {
        if (n >= 0) return n;
        return -n;
    }
    

    /* * ** *** ***** ******** ************* ******** ***** *** ** * */

    // The following code generates art.

    function draw(uint id) public view returns (string) {        
        //bytes memory output = new bytes(USIZE * (USIZE + 3) + 30);
        bytes memory output;
        if (id == 0) {
            revert();
        } else if (id == 1) {
        output="data:text/plain;charset=utf-8,....................................................%0A....................................................%0A.....>/X0####XX/+..................|/X0####0X/+.....%0A....|0#0XXXXX0##0X...............+/###0XXXXX##0/....%0A..+X##/.......+X0#0/...........+X##X/.......+X##/...%0A..X##X...........X##X>.........###X........../##0+..%0A..X##/............0##X........X##X+..........|##0|..%0A../##X............+X##|......X#0/............/##X...%0A.../0#/............+X#0|....X##/............+X##+...%0A..../#0/............|0#X.../0##.............X##X....%0A.....................|##/.+X##X.....................%0A......................##0+X##X+.....................%0A......................X##/X##/......................%0A....................../##0##X.......................%0A......................+X####/.......................%0A.......................X###0>.......................%0A......................./###X........................%0A.......................>0##/........................%0A.......................+X##/........................%0A........................X##/........................%0A........................X##/........................%0A........................X##/........................%0A....................................................%0A....................................................";
        
        }
        else if (id == 2) {
        output="data:text/plain;charset=utf-8,....................................................%0A....................................................%0A.....###0X/+...........................+/X0###/.....%0A.....+/X0##0X.........................>0##0X>+......%0A.........X0##0X...................../0##0/..........%0A..........+X###0|..................0###/............%0A............/###0/...............+X###X.............%0A.............|X###X/+..........X0##0X+..............%0A................|X####0XXXX00####0/.................%0A.................>##############0X+.................%0A..............+X0##0X/++...++|/0###/................%0A.............0###0..............+X0##X+.............%0A............|###X+................/###X.............%0A...........|0##/...................X###.............%0A.........../##X+.................../###>............%0A.........../##0+.................../###+............%0A...........+0##X...................0###.............%0A............+###X|...............+X##0/.............%0A.............X###0+............+/0##0/..............%0A.............../0###0X/>++>|/XX###0>................%0A................../X0#########0X/+..................%0A.....................+|/////|++.....................%0A....................................................%0A....................................................";
        
        }
        else if (id == 3) {
        output="data:text/plain;charset=utf-8,....................................................%0A....................................................%0A.....#0XX//|>++.....................+++|//XXX0/.....%0A.....##########0XXXXXXXXXXXXXXXXXXX00#########/.....%0A........++|/XX0###000##########00###0XX/|+++........%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A............../###/............./###/...............%0A.......++/XXXX0#####################0XXXX/|>+.......%0A.....########XXXXXXX////////////XXXXXX0#######/.....%0A.....XXX/>+++..........................++>|/XX/.....%0A....................................................%0A....................................................";
        
        }
        else if (id == 4) {
        output="data:text/plain;charset=utf-8,....................................................%0A....................................................%0A......................+.............................%0A............+/XX0#####################0XXXXX|>++....%0A.........+/X0#########0/+++++++++++>||/XXX0####0....%0A......../##0X++...+|X0##X>...................++|....%0A....../0#X|...........X0##+.........................%0A.....+X##|.............X##/.........................%0A...../###............../##X.........................%0A.....+X##/.............X##/.........................%0A......|0#0/...........X0##..........................%0A......../##0X>++.++/X0#0X+.....>/XX####0X/|+........%0A........../X0#######0X+....../0#0XX|>>|XX0##X+......%0A.............+|///|++......+X##X>........+X0#0/.....%0A........................../0##X.............###X....%0A..........................X###/.............X##0....%0A..........................X###/.............X##0....%0A..........................|0##0............+##0/....%0A......+.....................X0#0X>......+/0#0X+.....%0A...../0XX/>++................+X###0XXXX00##0/.......%0A......|/XXX0#####0XXXXXXXXXXXX0########XX|+.........%0A..................++++++++++>>>>>+++................%0A....................................................%0A....................................................";
        
        }
        else if (id == 5) {
        output="data:text/plain;charset=utf-8,....................................................%0A..........................++>||>++..................%0A.....................+X00XX////XX000X>..............%0A..................+X0#/............/0#0+............%0A..................X##X..............+X#X+...........%0A.................+##X................./#0>..........%0A.................|##/..................##/..........%0A.................+##/..................##/..........%0A..................X#X+................/#0|..........%0A..................+X#X...............+X#/...........%0A...........+>///|+./##...............X#X............%0A......../0#0XXXXX0####X............+X#0.............%0A......X0#/........+X0##/...........0#X+.............%0A...../##X...........X##X........../#X+..............%0A.....X##/............X#X........>0#/................%0A.....X##X.........../00|......./0#X.................%0A.....+X##+........./0#/........0#0>.................%0A.......+X#0XX///XX##X/........|#0|..................%0A..........+|/XXXX/+...........X#X...................%0A..............................X#/...................%0A............................../#X+.......+/+........%0A...............................X0#0XXXX0#0X/........%0A................................+/X000XX/+..........%0A....................................................";
        
        }
        else if (id == 6) {
        output="data:text/plain;charset=utf-8,....................................................%0A....................................................%0A.......+/+.......+XXX/.......+/XX+..................%0A......../#0/.../00/X0#/...+X0XX0##X.................%0A.........##0/+X0X+../00|..0X+..X##0.................%0A.........X##00X......X#X/X+..../###.................%0A........./###/.......X###X...../###...|0#0X+........%0A........./###........X##X+...../###.+X#00##X........%0A........./###........X#0+....../###X00/../0#+.......%0A........./###........X#X+....../###0/.....X#/.......%0A........./###........X#X+....../###X....../#/.......%0A........./###........X#X+....../###......./#/.......%0A........./###........X#X+....../###.......X#/.......%0A........./###........X#X+....../###......|0#+.......%0A........./###........X#X+....../###.....+0#0........%0A........./###........X#X+....../###..../0#X+........%0A........./###........X#X+....../###...+0#X+.........%0A........./###........X#X+....../###.+X0X+...........%0A.........+XXX........|X/.......|0##00X|.............%0A.............................../0##X>...............%0A.........................>/XX0#XX0#X................%0A........................+X|++....+X#X+..............%0A..................................+00X/.............%0A....................................................";
        
        }
        else if (id == 7) {
        output="data:text/plain;charset=utf-8,....................................................%0A....................................................%0A....................................................%0A.....................+/XXXXX/|+.....................%0A................../X0##########X/+..................%0A...............+X0##0X/+++++|/X###0+................%0A.............+X###X+...........+X0##X+..............%0A.............X###0.............../0##X..............%0A.............####/................/###/.............%0A.............####/................>###/.............%0A.............0###/................/###/.............%0A............./0###+.............+X0#0/..............%0A...../////////X####0+..........0####X///////////....%0A....+###############/..........#################....%0A.....|||||||||||||||+..........|||||||||||||||||....%0A....................................................%0A....................................................%0A....................................................%0A....+###########################################....%0A....+###########################################....%0A....................................................%0A....................................................%0A....................................................%0A....................................................";
        
        }
        else if (id == 8) {
        output="data:text/plain;charset=utf-8,....................................................%0A....+++.........+|X/+.........+///..................%0A.....0#0X....+X0#00##0.....|X0#####.................%0A.....>0##X..|#X/...X##/...00/..|0##/................%0A....../##0//0/...../##X++XX+....X##/................%0A.......X####X......>0##000....../##X................%0A......./###X.......+X###X+....../##X................%0A......./##0/.......+X##0/......./##X................%0A......./##X........+X##/......../##X................%0A......./##X........+X##/......../##X................%0A......./##X........+X##/......../##X................%0A......./##X........+X##/......../##X................%0A......./##X........+X##/......../##X................%0A......./##X........+X##/........|##X................%0A......./##X........+X##/........+X#X................%0A......./##X........+X##/........+X#X................%0A......./##X........+X##/.........X#X................%0A......./##X........+X##/.........X#X+...............%0A......./##X........+X##/.........|0#X+......+.......%0A......./##X........+X##/........../##X/.....XX/+....%0A........++...........++............+XX0#########/...%0A............................................X0X|....%0A............................................++......%0A....................................................";
        
        }
        else if (id == 9) {
        output="data:text/plain;charset=utf-8,....................................................%0A....................................................%0A..........................XXXXXXXXXXXXXXXXXXXXX/....%0A..........................XXXXXXXXXXXXXX0######X....%0A.......................................X0######X....%0A...................................../X##0XX###X....%0A...................................X###X|...###X....%0A................................./X##0X.....###X....%0A...............................X###X|.......###X....%0A.........+//................/X##0X/.........###X....%0A.........0##0+............/X###X|...........###X....%0A.........+X0##0/+......./X##0X|.............###X....%0A............+0###0+...X###X/........................%0A.............+X0##0//X##0X..........................%0A................+X####0/............................%0A.............../X######0/+..........................%0A............./X###X/|X###0+.........................%0A.........../X##0X|....+X0##0/+......................%0A.........X###X|..........+0###0.....................%0A......./X##0X.............+X0#0.....................%0A...../0##X|.........................................%0A.......++...........................................%0A....................................................%0A....................................................";
        
        }
        else if (id == 10) {
        output="data:text/plain;charset=utf-8,....................................................%0A.......................++...........................%0A.....0XX/............/####+.........................%0A.....+|X00X+......./0#####X+........................%0A........|##X+.....X0##>+X##/........................%0A........./0#0....+##0/...X#X+.......................%0A........../##|..>0#0>....+##/.......................%0A..........+X#X..X##/......##/.......................%0A...........|00/X##/.......##X.......................%0A............X#####........##0|......................%0A............/#####........X##/.........+++..........%0A............+####X........X##X....+X0#######X+......%0A.............####/......../##0|./0#X/+.....|0#0|....%0A.............##00/........+0##XX0#0........./0#0....%0A.........................../0####X...........X##....%0A............................/###0/..........+X##....%0A............................X####0X.........X##0....%0A...........................X###+/0##XX/|//X0#0X+....%0A..........................X###/....|/XXXXXXX+.......%0A.........................+###X......................%0A......................./X0#0/.......................%0A................./#######0|.........................%0A.................+XXXX/|++..........................%0A....................................................";
        
        }
        else if (id == 11) {
        output="data:text/plain;charset=utf-8,....................................................%0A....................................................%0A....................................................%0A....................................................%0A....................................................%0A............................+++.....................%0A............+/X0#0+.....+/0####+....../X00X/........%0A.........+/X0#####X/++/X0######0/++/X0######+.......%0A......+/X#0X|...>X#####0X/...>X######0X>|/X0#X+.....%0A....+0#0X.........XXX/.........0##X|........0#X+....%0A.....X/+........................++........../XX+....%0A....................................................%0A..............+|//......../X0#0........|/|+.........%0A...........+/0####/....+/X#####/....+/X###0X........%0A........+0##0///X0#######0X/XX0########X00##0>......%0A...../0##/+......+###0/+......+####X/+...../##X.....%0A....+#0X|.........|/|+........./XX/.........X#0/....%0A....................................................%0A....................................................%0A....................................................%0A....................................................%0A....................................................%0A....................................................%0A....................................................";
        
        }
        else if (id == 12) {
        output="data:text/plain;charset=utf-8,....................................................%0A.........++++..........................+++++........%0A........../0#0/....................../0#0X+.........%0A............/##0/................../0##/............%0A.............X###X................+0##X.............%0A..............|0##/............../0#0/..............%0A................X##X+..........+X##X+...............%0A................+X##X..........X###+................%0A..................###X.........###X.................%0A..................0###........|###/.................%0A..................X###+......+X##0/.................%0A.........X################################X.........%0A........./XXXXXXXX0###XXXXXXXX0##0XXXXXXXX/.........%0A..................X###......../##X+.................%0A..................###X........+###/.................%0A................./##0|.........0##0.................%0A................+X##/........../0##>................%0A.............../0#0/............|0#0/...............%0A.............+X##0................X##X|.............%0A.............0##0/.................0##0.............%0A.........../0#0/..................../0#0/...........%0A........./X00|........................>000X+........%0A....................................................%0A....................................................";
        
        }else{
            revert();
        }
        string memory result = string(output);
        return result;
        
    }

    /* * ** *** ***** ******** ************* ******** ***** *** ** * */

    function creator(uint _id) external view returns (address) {
        return idToCreator[_id];
    }
    

    function createGlyph(uint seed) external payable returns (string) {
        return _mint(msg.sender, seed);
    }

    //////////////////////////
    //// ERC 721 and 165  ////
    //////////////////////////

    /**
     * @dev Returns whether the target address is a contract.
     * @param _addr Address to check.
     * @return True if _addr is a contract, false if not.
     */
    function isContract(address _addr) internal view returns (bool addressCheck) {
        uint256 size;
        assembly { size := extcodesize(_addr) } // solhint-disable-line
        addressCheck = size > 0;
    }

    /**
     * @dev Function to check which interfaces are suported by this contract.
     * @param _interfaceID Id of the interface.
     * @return True if _interfaceID is supported, false otherwise.
     */
    function supportsInterface(bytes4 _interfaceID) external view returns (bool) {
        return supportedInterfaces[_interfaceID];
    }

    /**
     * @dev Transfers the ownership of an NFT from one address to another address. This function can
     * be changed to payable.
     * @notice Throws unless `msg.sender` is the current owner, an authorized operator, or the
     * approved address for this NFT. Throws if `_from` is not the current owner. Throws if `_to` is
     * the zero address. Throws if `_tokenId` is not a valid NFT. When transfer is complete, this
     * function checks if `_to` is a smart contract (code size > 0). If so, it calls
     * `onERC721Received` on `_to` and throws if the return value is not
     * `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`.
     * @param _from The current owner of the NFT.
     * @param _to The new owner.
     * @param _tokenId The NFT to transfer.
     * @param _data Additional data with no specified format, sent in call to `_to`.
     */
    function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes _data) external {
        _safeTransferFrom(_from, _to, _tokenId, _data);
    }

    /**
     * @dev Transfers the ownership of an NFT from one address to another address. This function can
     * be changed to payable.
     * @notice This works identically to the other function with an extra data parameter, except this
     * function just sets data to ""
     * @param _from The current owner of the NFT.
     * @param _to The new owner.
     * @param _tokenId The NFT to transfer.
     */
    function safeTransferFrom(address _from, address _to, uint256 _tokenId) external {
        _safeTransferFrom(_from, _to, _tokenId, "");
    }

    /**
     * @dev Throws unless `msg.sender` is the current owner, an authorized operator, or the approved
     * address for this NFT. Throws if `_from` is not the current owner. Throws if `_to` is the zero
     * address. Throws if `_tokenId` is not a valid NFT. This function can be changed to payable.
     * @notice The caller is responsible to confirm that `_to` is capable of receiving NFTs or else
     * they maybe be permanently lost.
     * @param _from The current owner of the NFT.
     * @param _to The new owner.
     * @param _tokenId The NFT to transfer.
     */
    function transferFrom(address _from, address _to, uint256 _tokenId) external canTransfer(_tokenId) validNFToken(_tokenId) {
        address tokenOwner = idToOwner[_tokenId];
        require(tokenOwner == _from);
        require(_to != address(0));
        _transfer(_to, _tokenId);
    }

    /**
     * @dev Set or reaffirm the approved address for an NFT. This function can be changed to payable.
     * @notice The zero address indicates there is no approved address. Throws unless `msg.sender` is
     * the current NFT owner, or an authorized operator of the current owner.
     * @param _approved Address to be approved for the given NFT ID.
     * @param _tokenId ID of the token to be approved.
     */
    function approve(address _approved, uint256 _tokenId) external canOperate(_tokenId) validNFToken(_tokenId) {
        address tokenOwner = idToOwner[_tokenId];
        require(_approved != tokenOwner);
        idToApproval[_tokenId] = _approved;
        emit Approval(tokenOwner, _approved, _tokenId);
    }

    /**
     * @dev Enables or disables approval for a third party ("operator") to manage all of
     * `msg.sender`'s assets. It also emits the ApprovalForAll event.
     * @notice This works even if sender doesn't own any tokens at the time.
     * @param _operator Address to add to the set of authorized operators.
     * @param _approved True if the operators is approved, false to revoke approval.
     */
    function setApprovalForAll(address _operator, bool _approved) external {
        ownerToOperators[msg.sender][_operator] = _approved;
        emit ApprovalForAll(msg.sender, _operator, _approved);
    }

    /**
     * @dev Returns the number of NFTs owned by `_owner`. NFTs assigned to the zero address are
     * considered invalid, and this function throws for queries about the zero address.
     * @param _owner Address for whom to query the balance.
     * @return Balance of _owner.
     */
    function balanceOf(address _owner) external view returns (uint256) {
        require(_owner != address(0));
        return _getOwnerNFTCount(_owner);
    }

    /**
     * @dev Returns the address of the owner of the NFT. NFTs assigned to zero address are considered
     * invalid, and queries about them do throw.
     * @param _tokenId The identifier for an NFT.
     * @return Address of _tokenId owner.
     */
    function ownerOf(uint256 _tokenId) external view returns (address _owner) {
        _owner = idToOwner[_tokenId];
        require(_owner != address(0));
    }

    /**
     * @dev Get the approved address for a single NFT.
     * @notice Throws if `_tokenId` is not a valid NFT.
     * @param _tokenId ID of the NFT to query the approval of.
     * @return Address that _tokenId is approved for.
     */
    function getApproved(uint256 _tokenId) external view validNFToken(_tokenId) returns (address) {
        return idToApproval[_tokenId];
    }

    /**
     * @dev Checks if `_operator` is an approved operator for `_owner`.
     * @param _owner The address that owns the NFTs.
     * @param _operator The address that acts on behalf of the owner.
     * @return True if approved for all, false otherwise.
     */
    function isApprovedForAll(address _owner, address _operator) external view returns (bool) {
        return ownerToOperators[_owner][_operator];
    }

    /**
     * @dev Actually preforms the transfer.
     * @notice Does NO checks.
     * @param _to Address of a new owner.
     * @param _tokenId The NFT that is being transferred.
     */
    function _transfer(address _to, uint256 _tokenId) internal {
        address from = idToOwner[_tokenId];
        _clearApproval(_tokenId);

        _removeNFToken(from, _tokenId);
        _addNFToken(_to, _tokenId);

        emit Transfer(from, _to, _tokenId);
}

    /**
     * @dev Mints a new NFT.
     * @notice This is an internal function which should be called from user-implemented external
     * mint function. Its purpose is to show and properly initialize data structures when using this
     * implementation.
     * @param _to The address that will own the minted NFT.
     */
    function _mint(address _to, uint seed) internal returns (string) {
        require(_to != address(0));
        require(numTokens < TOKEN_LIMIT);
        uint amount = 0;
        if (numTokens >= ARTIST_PRINTS) {
            amount = PRICE;
            require(msg.value >= amount);
        }
        require(seedToId[seed] == 0);
        uint id = numTokens + 1;

        idToCreator[id] = _to;
        idToSeed[id] = seed;
        seedToId[seed] = id;        
        string memory uri = draw(id);
        emit Generated(id, _to, uri);

        numTokens = numTokens + 1;
        _addNFToken(_to, id);

        if (msg.value > amount) {
            msg.sender.transfer(msg.value - amount);
        }
        if (amount > 0) {
            BENEFICIARY.transfer(amount);
        }

        emit Transfer(address(0), _to, id);
        return uri;
    }

    /**
     * @dev Assigns a new NFT to an address.
     * @notice Use and override this function with caution. Wrong usage can have serious consequences.
     * @param _to Address to which we want to add the NFT.
     * @param _tokenId Which NFT we want to add.
     */
    function _addNFToken(address _to, uint256 _tokenId) internal {
        require(idToOwner[_tokenId] == address(0));
        idToOwner[_tokenId] = _to;

        uint256 length = ownerToIds[_to].push(_tokenId);
        idToOwnerIndex[_tokenId] = length - 1;
    }

    /**
     * @dev Removes a NFT from an address.
     * @notice Use and override this function with caution. Wrong usage can have serious consequences.
     * @param _from Address from wich we want to remove the NFT.
     * @param _tokenId Which NFT we want to remove.
     */
    function _removeNFToken(address _from, uint256 _tokenId) internal {
        require(idToOwner[_tokenId] == _from);
        delete idToOwner[_tokenId];

        uint256 tokenToRemoveIndex = idToOwnerIndex[_tokenId];
        uint256 lastTokenIndex = ownerToIds[_from].length - 1;

        if (lastTokenIndex != tokenToRemoveIndex) {
            uint256 lastToken = ownerToIds[_from][lastTokenIndex];
            ownerToIds[_from][tokenToRemoveIndex] = lastToken;
            idToOwnerIndex[lastToken] = tokenToRemoveIndex;
        }

        ownerToIds[_from].length--;
    }

    /**
     * @dev Helper function that gets NFT count of owner. This is needed for overriding in enumerable
     * extension to remove double storage (gas optimization) of owner nft count.
     * @param _owner Address for whom to query the count.
     * @return Number of _owner NFTs.
     */
    function _getOwnerNFTCount(address _owner) internal view returns (uint256) {
        return ownerToIds[_owner].length;
    }

    /**
     * @dev Actually perform the safeTransferFrom.
     * @param _from The current owner of the NFT.
     * @param _to The new owner.
     * @param _tokenId The NFT to transfer.
     * @param _data Additional data with no specified format, sent in call to `_to`.
     */
    function _safeTransferFrom(address _from,  address _to,  uint256 _tokenId,  bytes memory _data) private canTransfer(_tokenId) validNFToken(_tokenId) {
        address tokenOwner = idToOwner[_tokenId];
        require(tokenOwner == _from);
        require(_to != address(0));

        _transfer(_to, _tokenId);

        if (isContract(_to)) {
            bytes4 retval = ERC721TokenReceiver(_to).onERC721Received(msg.sender, _from, _tokenId, _data);
            require(retval == MAGIC_ON_ERC721_RECEIVED);
        }
    }

    /**
     * @dev Clears the current approval of a given NFT ID.
     * @param _tokenId ID of the NFT to be transferred.
     */
    function _clearApproval(uint256 _tokenId) private {
        if (idToApproval[_tokenId] != address(0)) {
            delete idToApproval[_tokenId];
        }
    }

    //// Enumerable

    function totalSupply() public view returns (uint256) {
        return numTokens;
    }

    function tokenByIndex(uint256 index) public view returns (uint256) {
        require(index < numTokens);
        return index;
    }

    /**
     * @dev returns the n-th NFT ID from a list of owner's tokens.
     * @param _owner Token owner's address.
     * @param _index Index number representing n-th token in owner's list of tokens.
     * @return Token id.
     */
    function tokenOfOwnerByIndex(address _owner, uint256 _index) external view returns (uint256) {
        require(_index < ownerToIds[_owner].length);
        return ownerToIds[_owner][_index];
    }

    //// Metadata

    /**
      * @dev Returns a descriptive name for a collection of NFTokens.
      * @return Representing name.
      */
    function name() external view returns (string memory _name) {
        _name = nftName;
    }

    /**
     * @dev Returns an abbreviated name for NFTokens.
     * @return Representing symbol.
     */
    function symbol() external view returns (string memory _symbol) {
        _symbol = nftSymbol;
    }

    /**
     * @dev A distinct URI (RFC 3986) for a given NFT.
     * @param _tokenId Id for which we want uri.
     * @return URI of _tokenId.
     */
    function tokenURI(uint256 _tokenId) external view validNFToken(_tokenId) returns (string memory) {
        return draw(_tokenId);
    }

}