// Contract: NFTich
// Target function: function tokenURI(uint256 tokenId) public view virtual override returns (string memory dataURI)
 uint256 length = ERC721.balanceOf(to);
        _ownedTokens[to][length] = tokenId;
        _ownedTokensIndex[tokenId] = length;
    }

    /**
     * @dev Private function to add a token to this extension's token tracking data structures.
     * @param tokenId uint256 ID of the token to be added to the tokens list
     */
    function _addTokenToAllTokensEnumeration(uint256 tokenId) private {
        _allTokensIndex[tokenId] = _allTokens.length;
        _allTokens.push(tokenId);
    }

    /**
     * @dev Private function to remove a token from this extension's ownership-tracking data structures. Note that
     * while the token is not assigned a new owner, the `_ownedTokensIndex` mapping is _not_ updated: this allows for
     * gas optimizations e.g. when performing a transfer operation (avoiding double writes).
     * This has O(1) time complexity, but alters the order of the _ownedTokens array.
     * @param from address representing the previous owner of the given token ID
     * @param tokenId uint256 ID of the token to be removed from the tokens list of the given address
     */
    function _removeTokenFromOwnerEnumeration(address from, uint256 tokenId) private {
        // To prevent a gap in from's tokens array, we store the last token in the index of the token to delete, and
        // then delete the last slot (swap and pop).

        uint256 lastTokenIndex = ERC721.balanceOf(from) - 1;
        uint256 tokenIndex = _ownedTokensIndex[tokenId];

        // When the token to delete is the last token, the swap operation is unnecessary
        if (tokenIndex != lastTokenIndex) {
            uint256 lastTokenId = _ownedTokens[from][lastTokenIndex];

            _ownedTokens[from][tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
            _ownedTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
        }

        // This also deletes the contents at the last position of the array
        delete _ownedTokensIndex[tokenId];
        delete _ownedTokens[from][lastTokenIndex];
    }

    /**
     * @dev Private function to remove a token from this extension's token tracking data structures.
     * This has O(1) time complexity, but alters the order of the _allTokens array.
     * @param tokenId uint256 ID of the token to be removed from the tokens list
     */
    function _removeTokenFromAllTokensEnumeration(uint256 tokenId) private {
        // To prevent a gap in the tokens array, we store the last token in the index of the token to delete, and
        // then delete the last slot (swap and pop).

        uint256 lastTokenIndex = _allTokens.length - 1;
        uint256 tokenIndex = _allTokensIndex[tokenId];

        // When the token to delete is the last token, the swap operation is unnecessary. However, since this occurs so
        // rarely (when the last minted token is burnt) that we still do the swap here to avoid the gas cost of adding
        // an 'if' statement (like in _removeTokenFromOwnerEnumeration)
        uint256 lastTokenId = _allTokens[lastTokenIndex];

        _allTokens[tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
        _allTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index

        // This also deletes the contents at the last position of the array
        delete _allTokensIndex[tokenId];
        _allTokens.pop();
    }
}

pragma solidity ^0.8.0;

/**
 * @dev Provides a set of functions to operate with Base64 strings.
 *
 * _Available since v4.5._
 */
library Base64 {
    /**
     * @dev Base64 Encoding/Decoding Table
     */
    string internal constant _TABLE = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    /**
     * @dev Converts a `bytes` to its Bytes64 `string` representation.
     */
    function encode(bytes memory data) internal pure returns (string memory) {
        /**
         * Inspired by Brecht Devos (Brechtpd) implementation - MIT licence
         * https://github.com/Brechtpd/base64/blob/e78d9fd951e7b0977ddca77d92dc85183770daf4/base64.sol
         */
        if (data.length == 0) return "";

        // Loads the table into memory
        string memory table = _TABLE;

        // Encoding takes 3 bytes chunks of binary data from `bytes` data parameter
        // and split into 4 numbers of 6 bits.
        // The final Base64 length should be `bytes` data length multiplied by 4/3 rounded up
        // - `data.length + 2`  -> Round up
        // - `/ 3`              -> Number of 3-bytes chunks
        // - `4 *`              -> 4 characters for each chunk
        string memory result = new string(4 * ((data.length + 2) / 3));

        /// @solidity memory-safe-assembly
        assembly {
            // Prepare the lookup table (skip the first "length" byte)
            let tablePtr := add(table, 1)

            // Prepare result pointer, jump over length
            let resultPtr := add(result, 32)

            // Run over the input, 3 bytes at a time
            for {
                let dataPtr := data
                let endPtr := add(data, mload(data))
            } lt(dataPtr, endPtr) {

            } {
                // Advance 3 bytes
                dataPtr := add(dataPtr, 3)
                let input := mload(dataPtr)

                // To write each character, shift the 3 bytes (18 bits) chunk
                // 4 times in blocks of 6 bits for each character (18, 12, 6, 0)
                // and apply logical AND with 0x3F which is the number of
                // the previous character in the ASCII table prior to the Base64 Table
                // The result is then added to the table to get the character to write,
                // and finally write it in the result pointer but with a left shift
                // of 256 (1 byte) - 8 (1 ASCII char) = 248 bits

                mstore8(resultPtr, mload(add(tablePtr, and(shr(18, input), 0x3F))))
                resultPtr := add(resultPtr, 1) // Advance

                mstore8(resultPtr, mload(add(tablePtr, and(shr(12, input), 0x3F))))
                resultPtr := add(resultPtr, 1) // Advance

                mstore8(resultPtr, mload(add(tablePtr, and(shr(6, input), 0x3F))))
                resultPtr := add(resultPtr, 1) // Advance

                mstore8(resultPtr, mload(add(tablePtr, and(input, 0x3F))))
                resultPtr := add(resultPtr, 1) // Advance
            }

            // When data `bytes` is not exactly 3 bytes long
            // it is padded with `=` characters at the end
            switch mod(mload(data), 3)
            case 1 {
                mstore8(sub(resultPtr, 1), 0x3d)
                mstore8(sub(resultPtr, 2), 0x3d)
            }
            case 2 {
                mstore8(sub(resultPtr, 1), 0x3d)
            }
        }

        return result;
    }
}

pragma solidity ^0.8.0;

library DateTime {
    /*
     *  Date and Time utilities for ethereum contracts
     *
     */
    struct _DateTime {
        uint16 year;
        uint8 month;
        uint8 day;
        uint8 hour;
        uint8 minute;
        uint8 second;
        uint8 weekday;
    }

    uint256 constant DAY_IN_SECONDS = 86400;
    uint256 constant YEAR_IN_SECONDS = 31536000;
    uint256 constant LEAP_YEAR_IN_SECONDS = 31622400;

    uint256 constant HOUR_IN_SECONDS = 3600;
    uint256 constant MINUTE_IN_SECONDS = 60;

    uint16 constant ORIGIN_YEAR = 1970;

    function isLeapYear(uint16 year) public pure returns (bool) {
        if (year % 4 != 0) {
            return false;
        }
        if (year % 100 != 0) {
            return true;
        }
        if (year % 400 != 0) {
            return false;
        }
        return true;
    }

    function leapYearsBefore(uint256 year) public pure returns (uint256) {
        year -= 1;
        return year / 4 - year / 100 + year / 400;
    }

    function getDaysInMonth(uint8 month, uint16 year)
        public
        pure
        returns (uint8)
    {
        if (
            month == 1 ||
            month == 3 ||
            month == 5 ||
            month == 7 ||
            month == 8 ||
            month == 10 ||
            month == 12
        ) {
            return 31;
        } else if (month == 4 || month == 6 || month == 9 || month == 11) {
            return 30;
        } else if (isLeapYear(year)) {
            return 29;
        } else {
            return 28;
        }
    }

    function parseTimestamp(uint256 timestamp)
        internal
        pure
        returns (_DateTime memory dt)
    {
        uint256 secondsAccountedFor = 0;
        uint256 buf;
        uint8 i;

        // Year
        dt.year = getYear(timestamp);
        buf = leapYearsBefore(dt.year) - leapYearsBefore(ORIGIN_YEAR);

        secondsAccountedFor += LEAP_YEAR_IN_SECONDS * buf;
        secondsAccountedFor += YEAR_IN_SECONDS * (dt.year - ORIGIN_YEAR - buf);

        // Month
        uint256 secondsInMonth;
        for (i = 1; i <= 12; i++) {
            secondsInMonth = DAY_IN_SECONDS * getDaysInMonth(i, dt.year);
            if (secondsInMonth + secondsAccountedFor > timestamp) {
                dt.month = i;
                break;
            }
            secondsAccountedFor += secondsInMonth;
        }

        // Day
        for (i = 1; i <= getDaysInMonth(dt.month, dt.year); i++) {
            if (DAY_IN_SECONDS + secondsAccountedFor > timestamp) {
                dt.day = i;
                break;
            }
            secondsAccountedFor += DAY_IN_SECONDS;
        }

        // Hour
        dt.hour = getHour(timestamp);

        // Minute
        dt.minute = getMinute(timestamp);

        // Second
        dt.second = getSecond(timestamp);

        // Day of week.
        dt.weekday = getWeekday(timestamp);
    }

    function getYear(uint256 timestamp) public pure returns (uint16) {
        uint256 secondsAccountedFor = 0;
        uint16 year;
        uint256 numLeapYears;

        // Year
        year = uint16(ORIGIN_YEAR + timestamp / YEAR_IN_SECONDS);
        numLeapYears = leapYearsBefore(year) - leapYearsBefore(ORIGIN_YEAR);

        secondsAccountedFor += LEAP_YEAR_IN_SECONDS * numLeapYears;
        secondsAccountedFor +=
            YEAR_IN_SECONDS *
            (year - ORIGIN_YEAR - numLeapYears);

        while (secondsAccountedFor > timestamp) {
            if (isLeapYear(uint16(year - 1))) {
                secondsAccountedFor -= LEAP_YEAR_IN_SECONDS;
            } else {
                secondsAccountedFor -= YEAR_IN_SECONDS;
            }
            year -= 1;
        }
        return year;
    }

    function getMonth(uint256 timestamp) public pure returns (uint8) {
        return parseTimestamp(timestamp).month;
    }

    function getDay(uint256 timestamp) public pure returns (uint8) {
        return parseTimestamp(timestamp).day;
    }

    function getHour(uint256 timestamp) public pure returns (uint8) {
        return uint8((timestamp / 60 / 60) % 24);
    }

    function getMinute(uint256 timestamp) public pure returns (uint8) {
        return uint8((timestamp / 60) % 60);
    }

    function getSecond(uint256 timestamp) public pure returns (uint8) {
        return uint8(timestamp % 60);
    }

    function getWeekday(uint256 timestamp) public pure returns (uint8) {
        return uint8((timestamp / DAY_IN_SECONDS + 4) % 7);
    }

    function toTimestamp(
        uint16 year,
        uint8 month,
        uint8 day
    ) public pure returns (uint256 timestamp) {
        return toTimestamp(year, month, day, 0, 0, 0);
    }

    function toTimestamp(
        uint16 year,
        uint8 month,
        uint8 day,
        uint8 hour
    ) public pure returns (uint256 timestamp) {
        return toTimestamp(year, month, day, hour, 0, 0);
    }

    function toTimestamp(
        uint16 year,
        uint8 month,
        uint8 day,
        uint8 hour,
        uint8 minute
    ) public pure returns (uint256 timestamp) {
        return toTimestamp(year, month, day, hour, minute, 0);
    }

    function toTimestamp(
        uint16 year,
        uint8 month,
        uint8 day,
        uint8 hour,
        uint8 minute,
        uint8 second
    ) public pure returns (uint256 timestamp) {
        uint16 i;

        // Year
        for (i = ORIGIN_YEAR; i < year; i++) {
            if (isLeapYear(i)) {
                timestamp += LEAP_YEAR_IN_SECONDS;
            } else {
                timestamp += YEAR_IN_SECONDS;
            }
        }

        // Month
        uint8[12] memory monthDayCounts;
        monthDayCounts[0] = 31;
        if (isLeapYear(year)) {
            monthDayCounts[1] = 29;
        } else {
            monthDayCounts[1] = 28;
        }
        monthDayCounts[2] = 31;
        monthDayCounts[3] = 30;
        monthDayCounts[4] = 31;
        monthDayCounts[5] = 30;
        monthDayCounts[6] = 31;
        monthDayCounts[7] = 31;
        monthDayCounts[8] = 30;
        monthDayCounts[9] = 31;
        monthDayCounts[10] = 30;
        monthDayCounts[11] = 31;

        for (i = 1; i < month; i++) {
            timestamp += DAY_IN_SECONDS * monthDayCounts[i - 1];
        }

        // Day
        timestamp += DAY_IN_SECONDS * (day - 1);

        // Hour
        timestamp += HOUR_IN_SECONDS * (hour);

        // Minute
        timestamp += MINUTE_IN_SECONDS * (minute);

        // Second
        timestamp += second;

        return timestamp;
    }
}

pragma solidity ^0.8.0;

contract NFTich is ERC721Enumerable, Ownable {

    uint256 public fee = 0.01 * (10 ** 18);

    using DateTime for uint256;
    using NFTichSVG for uint256;

    event NFTExists(uint256 totalSupply, uint256 tokenId, address owner);

    constructor() ERC721("I Ching Hexagram NFT", "NFTich") Ownable() {}

    receive() external payable {

        require(msg.value >= fee, "NFTich: Insufficient fees, at least 0.01 ETH required.");

        uint256 amount = _divination(uint256(uint160(_msgSender())), msg.value, block.timestamp);
        payable(_msgSender()).transfer(amount);
        payable(owner()).transfer(address(this).balance);

        if (_exists(amount)) {
            emit NFTExists(totalSupply(), amount, ownerOf(amount));
        } else {
            _safeMint(_msgSender(), amount);
        }
    }

    function _divination(uint256 sender, uint256 value, uint timestamp) private pure returns (uint256) {
        
        uint8[24] memory gs = [8, 8, 8, 8, 8, 7, 8, 7, 8, 8, 7, 7, 7, 8, 8, 7, 8, 7, 7, 7, 8, 7, 7, 7];
        uint8[6] memory result;

        DateTime._DateTime memory date = timestamp.parseTimestamp();

        uint up = ( ( date.year + date.month + date.day ) % 8 ) * 3;
        uint down = ( ( date.year + date.month + date.day + date.hour ) % 8 ) * 3;
        uint c = sender + value + date.minute + date.second;

        for (uint8 i = 0; i < 3; i ++) {
            result[i] = gs[up + i];
            result[i+3] = gs[down + i];
        }

        if (c % 6 == 1) {
            result[0] = result[0] == 7 ? 9 : 6;
        }

        for (uint8 i = 2; i <= 6; i ++) {
            if (c % i == 0) {
                result[i-1] = result[i-1] == 7 ? 9 : 6;
            }
        }

        uint amount = 0;
        for (uint8 i = 0; i < 6; i ++) {
            amount += result[5-i] * (10 ** i);
        }

        require(amount >= 666666 && amount <= 999999, "NFTich: Unknown error.");
        return amount;
    }