// Vulnerability label: clean
pragma solidity ^0.4.23;

// File: contracts/zeppelin-solidity/contracts/ownership/Ownable.sol

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
  address public owner;


  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);


  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   */
  function Ownable() public {
    owner = msg.sender;
  }

  /**
   * @dev Throws if called by any account other than the owner.
   */
  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }

  /**
   * @dev Allows the current owner to transfer control of the contract to a newOwner.
   * @param newOwner The address to transfer ownership to.
   */
  function transferOwnership(address newOwner) public onlyOwner {
    require(newOwner != address(0));
    OwnershipTransferred(owner, newOwner);
    owner = newOwner;
  }

}

// File: contracts/Acceptable.sol

// @title Acceptable
// @author Takayuki Jimba
// @dev Provide basic access control.
contract Acceptable is Ownable {
    address public sender;

    // @dev Throws if called by any address other than the sender.
    modifier onlyAcceptable {
        require(msg.sender == sender);
        _;
    }

    // @dev Change acceptable address
    // @param _sender The address to new sender
    function setAcceptable(address _sender) public onlyOwner {
        sender = _sender;
    }
}

// File: contracts/zeppelin-solidity/contracts/token/ERC721/ERC721Basic.sol

/**
 * @title ERC721 Non-Fungible Token Standard basic interface
 * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721Basic {
  event Transfer(address indexed _from, address indexed _to, uint256 _tokenId);
  event Approval(address indexed _owner, address indexed _approved, uint256 _tokenId);
  event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved);  

  function balanceOf(address _owner) public view returns (uint256 _balance);
  function ownerOf(uint256 _tokenId) public view returns (address _owner);
  function exists(uint256 _tokenId) public view returns (bool _exists);
  
  function approve(address _to, uint256 _tokenId) public;
  function getApproved(uint256 _tokenId) public view returns (address _operator);
  
  function setApprovalForAll(address _operator, bool _approved) public;
  function isApprovedForAll(address _owner, address _operator) public view returns (bool);

  function transferFrom(address _from, address _to, uint256 _tokenId) public;
  function safeTransferFrom(address _from, address _to, uint256 _tokenId) public;  
  function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes _data) public;
}

// File: contracts/zeppelin-solidity/contracts/token/ERC721/ERC721.sol

/**
 * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
 * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721Enumerable is ERC721Basic {
  function totalSupply() public view returns (uint256);
  function tokenOfOwnerByIndex(address _owner, uint256 _index) public view returns (uint256 _tokenId);
  function tokenByIndex(uint256 _index) public view returns (uint256);
}

/**
 * @title ERC-721 Non-Fungible Token Standard, optional metadata extension
 * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721Metadata is ERC721Basic {
  function name() public view returns (string _name);
  function symbol() public view returns (string _symbol);
  function tokenURI(uint256 _tokenId) public view returns (string);
}

/**
 * @title ERC-721 Non-Fungible Token Standard, full implementation interface
 * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721 is ERC721Basic, ERC721Enumerable, ERC721Metadata {
}

// File: contracts/zeppelin-solidity/contracts/token/ERC721/DeprecatedERC721.sol

/**
 * @title ERC-721 methods shipped in OpenZeppelin v1.7.0, removed in the latest version of the standard
 * @dev Only use this interface for compatibility with previously deployed contracts
 * @dev Use ERC721 for interacting with new contracts which are standard-compliant
 */
contract DeprecatedERC721 is ERC721 {
  function takeOwnership(uint256 _tokenId) public;
  function transfer(address _to, uint256 _tokenId) public;
  function tokensOf(address _owner) public view returns (uint256[]);
}

// File: contracts/zeppelin-solidity/contracts/AddressUtils.sol

/**
 * Utility library of inline functions on addresses
 */
library AddressUtils {

  /**
   * Returns whether there is code in the target address
   * @dev This function will return false if invoked during the constructor of a contract,
   *  as the code is not actually created until after the constructor finishes.
   * @param addr address address to check
   * @return whether there is code in the target address
   */
  function isContract(address addr) internal view returns (bool) {
    uint256 size;
    assembly { size := extcodesize(addr) }
    return size > 0;
  }

}

// File: contracts/zeppelin-solidity/contracts/math/SafeMath.sol

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {

  /**
  * @dev Multiplies two numbers, throws on overflow.
  */
  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0) {
      return 0;
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c;
  }

  /**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }

  /**
  * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }

  /**
  * @dev Adds two numbers, throws on overflow.
  */
  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}

// File: contracts/zeppelin-solidity/contracts/token/ERC721/ERC721Receiver.sol

/**
 * @title ERC721 token receiver interface
 * @dev Interface for any contract that wants to support safeTransfers
 *  from ERC721 asset contracts.
 */
contract ERC721Receiver {
  /**
   * @dev Magic value to be returned upon successful reception of an NFT
   *  Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`,
   *  which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
   */
  bytes4 constant ERC721_RECEIVED = 0xf0b9e5ba; 

  /**
   * @notice Handle the receipt of an NFT
   * @dev The ERC721 smart contract calls this function on the recipient
   *  after a `safetransfer`. This function MAY throw to revert and reject the
   *  transfer. This function MUST use 50,000 gas or less. Return of other
   *  than the magic value MUST result in the transaction being reverted.
   *  Note: the contract address is always the message sender.
   * @param _from The sending address 
   * @param _tokenId The NFT identifier which is being transfered
   * @param _data Additional data with no specified format
   * @return `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
   */
  function onERC721Received(address _from, uint256 _tokenId, bytes _data) public returns(bytes4);
}

// File: contracts/zeppelin-solidity/contracts/token/ERC721/ERC721BasicToken.sol

/**
 * @title ERC721 Non-Fungible Token Standard basic implementation
 * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721BasicToken is ERC721Basic {
  using SafeMath for uint256;
  using AddressUtils for address;
  
  // Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
  // which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
  bytes4 constant ERC721_RECEIVED = 0xf0b9e5ba; 

  // Mapping from token ID to owner
  mapping (uint256 => address) internal tokenOwner;

  // Mapping from token ID to approved address
  mapping (uint256 => address) internal tokenApprovals;

  // Mapping from owner to number of owned token
  mapping (address => uint256) internal ownedTokensCount;

  // Mapping from owner to operator approvals
  mapping (address => mapping (address => bool)) internal operatorApprovals;

  /**
  * @dev Guarantees msg.sender is owner of the given token
  * @param _tokenId uint256 ID of the token to validate its ownership belongs to msg.sender
  */
  modifier onlyOwnerOf(uint256 _tokenId) {
    require(ownerOf(_tokenId) == msg.sender);
    _;
  }

  /**
  * @dev Checks msg.sender can transfer a token, by being owner, approved, or operator
  * @param _tokenId uint256 ID of the token to validate
  */
  modifier canTransfer(uint256 _tokenId) {
    require(isApprovedOrOwner(msg.sender, _tokenId));
    _;
  }

  /**
  * @dev Gets the balance of the specified address
  * @param _owner address to query the balance of
  * @return uint256 representing the amount owned by the passed address
  */
  function balanceOf(address _owner) public view returns (uint256) {
    require(_owner != address(0));
    return ownedTokensCount[_owner];
  }

  /**
  * @dev Gets the owner of the specified token ID
  * @param _tokenId uint256 ID of the token to query the owner of
  * @return owner address currently marked as the owner of the given token ID
  */
  function ownerOf(uint256 _tokenId) public view returns (address) {
    address owner = tokenOwner[_tokenId];
    require(owner != address(0));
    return owner;
  }

  /**
  * @dev Returns whether the specified token exists
  * @param _tokenId uint256 ID of the token to query the existance of
  * @return whether the token exists
  */
  function exists(uint256 _tokenId) public view returns (bool) {
    address owner = tokenOwner[_tokenId];
    return owner != address(0);
  }

  /**
  * @dev Approves another address to transfer the given token ID
  * @dev The zero address indicates there is no approved address.
  * @dev There can only be one approved address per token at a given time.
  * @dev Can only be called by the token owner or an approved operator.
  * @param _to address to be approved for the given token ID
  * @param _tokenId uint256 ID of the token to be approved
  */
  function approve(address _to, uint256 _tokenId) public {
    address owner = ownerOf(_tokenId);
    require(_to != owner);
    require(msg.sender == owner || isApprovedForAll(owner, msg.sender));

    if (getApproved(_tokenId) != address(0) || _to != address(0)) {
      tokenApprovals[_tokenId] = _to;
      Approval(owner, _to, _tokenId);
    }
  }

  /**
   * @dev Gets the approved address for a token ID, or zero if no address set
   * @param _tokenId uint256 ID of the token to query the approval of
   * @return address currently approved for a the given token ID
   */
  function getApproved(uint256 _tokenId) public view returns (address) {
    return tokenApprovals[_tokenId];
  }


  /**
  * @dev Sets or unsets the approval of a given operator
  * @dev An operator is allowed to transfer all tokens of the sender on their behalf
  * @param _to operator address to set the approval
  * @param _approved representing the status of the approval to be set
  */
  function setApprovalForAll(address _to, bool _approved) public {
    require(_to != msg.sender);
    operatorApprovals[msg.sender][_to] = _approved;
    ApprovalForAll(msg.sender, _to, _approved);
  }

  /**
   * @dev Tells whether an operator is approved by a given owner
   * @param _owner owner address which you want to query the approval of
   * @param _operator operator address which you want to query the approval of
   * @return bool whether the given operator is approved by the given owner
   */
  function isApprovedForAll(address _owner, address _operator) public view returns (bool) {
    return operatorApprovals[_owner][_operator];
  }

  /**
  * @dev Transfers the ownership of a given token ID to another address
  * @dev Usage of this method is discouraged, use `safeTransferFrom` whenever possible
  * @dev Requires the msg sender to be the owner, approved, or operator
  * @param _from current owner of the token
  * @param _to address to receive the ownership of the given token ID
  * @param _tokenId uint256 ID of the token to be transferred
  */
  function transferFrom(address _from, address _to, uint256 _tokenId) public canTransfer(_tokenId) {
    require(_from != address(0));
    require(_to != address(0));

    clearApproval(_from, _tokenId);
    removeTokenFrom(_from, _tokenId);
    addTokenTo(_to, _tokenId);
    
    Transfer(_from, _to, _tokenId);
  }

  /**
  * @dev Safely transfers the ownership of a given token ID to another address
  * @dev If the target address is a contract, it must implement `onERC721Received`,
  *  which is called upon a safe transfer, and return the magic value
  *  `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`; otherwise,
  *  the transfer is reverted.
  * @dev Requires the msg sender to be the owner, approved, or operator
  * @param _from current owner of the token
  * @param _to address to receive the ownership of the given token ID
  * @param _tokenId uint256 ID of the token to be transferred
  */
  function safeTransferFrom(address _from, address _to, uint256 _tokenId) public canTransfer(_tokenId) {
    safeTransferFrom(_from, _to, _tokenId, "");
  }

  /**
  * @dev Safely transfers the ownership of a given token ID to another address
  * @dev If the target address is a contract, it must implement `onERC721Received`,
  *  which is called upon a safe transfer, and return the magic value
  *  `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`; otherwise,
  *  the transfer is reverted.
  * @dev Requires the msg sender to be the owner, approved, or operator
  * @param _from current owner of the token
  * @param _to address to receive the ownership of the given token ID
  * @param _tokenId uint256 ID of the token to be transferred
  * @param _data bytes data to send along with a safe transfer check
  */
  function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes _data) public canTransfer(_tokenId) {
    transferFrom(_from, _to, _tokenId);
    require(checkAndCallSafeTransfer(_from, _to, _tokenId, _data));
  }

  /**
   * @dev Returns whether the given spender can transfer a given token ID
   * @param _spender address of the spender to query
   * @param _tokenId uint256 ID of the token to be transferred
   * @return bool whether the msg.sender is approved for the given token ID,
   *  is an operator of the owner, or is the owner of the token
   */
  function isApprovedOrOwner(address _spender, uint256 _tokenId) internal view returns (bool) {
    address owner = ownerOf(_tokenId);
    return _spender == owner || getApproved(_tokenId) == _spender || isApprovedForAll(owner, _spender);
  }

  /**
  * @dev Internal function to mint a new token
  * @dev Reverts if the given token ID already exists
  * @param _to The address that will own the minted token
  * @param _tokenId uint256 ID of the token to be minted by the msg.sender
  */
  function _mint(address _to, uint256 _tokenId) internal {
    require(_to != address(0));
    addTokenTo(_to, _tokenId);
    Transfer(address(0), _to, _tokenId);
  }

  /**
  * @dev Internal function to burn a specific token
  * @dev Reverts if the token does not exist
  * @param _tokenId uint256 ID of the token being burned by the msg.sender
  */
  function _burn(address _owner, uint256 _tokenId) internal {
    clearApproval(_owner, _tokenId);
    removeTokenFrom(_owner, _tokenId);
    Transfer(_owner, address(0), _tokenId);
  }

  /**
  * @dev Internal function to clear current approval of a given token ID
  * @dev Reverts if the given address is not indeed the owner of the token
  * @param _owner owner of the token
  * @param _tokenId uint256 ID of the token to be transferred
  */
  function clearApproval(address _owner, uint256 _tokenId) internal {
    require(ownerOf(_tokenId) == _owner);
    if (tokenApprovals[_tokenId] != address(0)) {
      tokenApprovals[_tokenId] = address(0);
      Approval(_owner, address(0), _tokenId);
    }
  }

  /**
  * @dev Internal function to add a token ID to the list of a given address
  * @param _to address representing the new owner of the given token ID
  * @param _tokenId uint256 ID of the token to be added to the tokens list of the given address
  */
  function addTokenTo(address _to, uint256 _tokenId) internal {
    require(tokenOwner[_tokenId] == address(0));
    tokenOwner[_tokenId] = _to;
    ownedTokensCount[_to] = ownedTokensCount[_to].add(1);
  }

  /**
  * @dev Internal function to remove a token ID from the list of a given address
  * @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 removeTokenFrom(address _from, uint256 _tokenId) internal {
    require(ownerOf(_tokenId) == _from);
    ownedTokensCount[_from] = ownedTokensCount[_from].sub(1);
    tokenOwner[_tokenId] = address(0);
  }

  /**
  * @dev Internal function to invoke `onERC721Received` on a target address
  * @dev The call is not executed if the target address is not a contract
  * @param _from address representing the previous owner of the given token ID
  * @param _to target address that will receive the tokens
  * @param _tokenId uint256 ID of the token to be transferred
  * @param _data bytes optional data to send along with the call
  * @return whether the call correctly returned the expected magic value
  */
  function checkAndCallSafeTransfer(address _from, address _to, uint256 _tokenId, bytes _data) internal returns (bool) {
    if (!_to.isContract()) {
      return true;
    }
    bytes4 retval = ERC721Receiver(_to).onERC721Received(_from, _tokenId, _data);
    return (retval == ERC721_RECEIVED);
  }
}

// File: contracts/zeppelin-solidity/contracts/token/ERC721/ERC721Token.sol

/**
 * @title Full ERC721 Token
 * This implementation includes all the required and some optional functionality of the ERC721 standard
 * Moreover, it includes approve all functionality using operator terminology
 * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721Token is ERC721, ERC721BasicToken {
  // Token name
  string internal name_;

  // Token symbol
  string internal symbol_;

  // Mapping from owner to list of owned token IDs
  mapping (address => uint256[]) internal ownedTokens;

  // Mapping from token ID to index of the owner tokens list
  mapping(uint256 => uint256) internal ownedTokensIndex;

  // Array with all token ids, used for enumeration
  uint256[] internal allTokens;

  // Mapping from token id to position in the allTokens array
  mapping(uint256 => uint256) internal allTokensIndex;

  // Optional mapping for token URIs 
  mapping(uint256 => string) internal tokenURIs;

  /**
  * @dev Constructor function
  */
  function ERC721Token(string _name, string _symbol) public {
    name_ = _name;
    symbol_ = _symbol;
  }

  /**
  * @dev Gets the token name
  * @return string representing the token name
  */
  function name() public view returns (string) {
    return name_;
  }

  /**
  * @dev Gets the token symbol
  * @return string representing the token symbol
  */
  function symbol() public view returns (string) {
    return symbol_;
  }

  /**
  * @dev Returns an URI for a given token ID
  * @dev Throws if the token ID does not exist. May return an empty string.
  * @param _tokenId uint256 ID of the token to query
  */
  function tokenURI(uint256 _tokenId) public view returns (string) {
    require(exists(_tokenId));
    return tokenURIs[_tokenId];
  }

  /**
  * @dev Internal function to set the token URI for a given token
  * @dev Reverts if the token ID does not exist
  * @param _tokenId uint256 ID of the token to set its URI
  * @param _uri string URI to assign
  */
  function _setTokenURI(uint256 _tokenId, string _uri) internal {
    require(exists(_tokenId));
    tokenURIs[_tokenId] = _uri;
  }

  /**
  * @dev Gets the token ID at a given index of the tokens list of the requested owner
  * @param _owner address owning the tokens list to be accessed
  * @param _index uint256 representing the index to be accessed of the requested tokens list
  * @return uint256 token ID at the given index of the tokens list owned by the requested address
  */
  function tokenOfOwnerByIndex(address _owner, uint256 _index) public view returns (uint256) {
    require(_index < balanceOf(_owner));
    return ownedTokens[_owner][_index];
  }

  /**
  * @dev Gets the total amount of tokens stored by the contract
  * @return uint256 representing the total amount of tokens
  */
  function totalSupply() public view returns (uint256) {
    return allTokens.length;
  }

  /**
  * @dev Gets the token ID at a given index of all the tokens in this contract
  * @dev Reverts if the index is greater or equal to the total number of tokens
  * @param _index uint256 representing the index to be accessed of the tokens list
  * @return uint256 token ID at the given index of the tokens list
  */
  function tokenByIndex(uint256 _index) public view returns (uint256) {
    require(_index < totalSupply());
    return allTokens[_index];
  }

  /**
  * @dev Internal function to add a token ID to the list of a given address
  * @param _to address representing the new owner of the given token ID
  * @param _tokenId uint256 ID of the token to be added to the tokens list of the given address
  */
  function addTokenTo(address _to, uint256 _tokenId) internal {
    super.addTokenTo(_to, _tokenId);
    uint256 length = ownedTokens[_to].length;
    ownedTokens[_to].push(_tokenId);
    ownedTokensIndex[_tokenId] = length;
  }

  /**
  * @dev Internal function to remove a token ID from the list of a given address
  * @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 removeTokenFrom(address _from, uint256 _tokenId) internal {
    super.removeTokenFrom(_from, _tokenId);

    uint256 tokenIndex = ownedTokensIndex[_tokenId];
    uint256 lastTokenIndex = ownedTokens[_from].length.sub(1);
    uint256 lastToken = ownedTokens[_from][lastTokenIndex];

    ownedTokens[_from][tokenIndex] = lastToken;
    ownedTokens[_from][lastTokenIndex] = 0;
    // Note that this will handle single-element arrays. In that case, both tokenIndex and lastTokenIndex are going to
    // be zero. Then we can make sure that we will remove _tokenId from the ownedTokens list since we are first swapping
    // the lastToken to the first position, and then dropping the element placed in the last position of the list

    ownedTokens[_from].length--;
    ownedTokensIndex[_tokenId] = 0;
    ownedTokensIndex[lastToken] = tokenIndex;
  }

  /**
  * @dev Internal function to mint a new token
  * @dev Reverts if the given token ID already exists
  * @param _to address the beneficiary that will own the minted token
  * @param _tokenId uint256 ID of the token to be minted by the msg.sender
  */
  function _mint(address _to, uint256 _tokenId) internal {
    super._mint(_to, _tokenId);
    
    allTokensIndex[_tokenId] = allTokens.length;
    allTokens.push(_tokenId);
  }

  /**
  * @dev Internal function to burn a specific token
  * @dev Reverts if the token does not exist
  * @param _owner owner of the token to burn
  * @param _tokenId uint256 ID of the token being burned by the msg.sender
  */
  function _burn(address _owner, uint256 _tokenId) internal {
    super._burn(_owner, _tokenId);

    // Clear metadata (if any)
    if (bytes(tokenURIs[_tokenId]).length != 0) {
      delete tokenURIs[_tokenId];
    }

    // Reorg all tokens array
    uint256 tokenIndex = allTokensIndex[_tokenId];
    uint256 lastTokenIndex = allTokens.length.sub(1);
    uint256 lastToken = allTokens[lastTokenIndex];

    allTokens[tokenIndex] = lastToken;
    allTokens[lastTokenIndex] = 0;

    allTokens.length--;
    allTokensIndex[_tokenId] = 0;
    allTokensIndex[lastToken] = tokenIndex;
  }

}

// File: contracts/CrystalBase.sol

// @title CrystalBase
// @author Takayuki Jimba
// @dev ERC721 token.
//      mint, burn and transferFrom are supposed to be called from CryptoCrystal contract only.
contract CrystalBase is Acceptable, ERC721Token {

    // Each of Crystal occupies 3 slots.
    struct Crystal {
        /* slot1 */

        uint256 tokenId;

        /* slot2 */
        uint256 gene;

        /* slot3 */

        // kind is between 0 and 100.
        // 2^8 = 256 is adequate.
        uint8 kind;

        // totalWeight of cryptocrystals is 21 * 10**13.
        // 2^128 = 3.4028237e+38 is adequate.
        uint128 weight;

        // unix timestamp
        uint64 mintedAt;
    }

    mapping(uint256 => Crystal) internal tokenIdToCrystal;
    event CrystalBurned(address indexed owner, uint256 tokenId);
    event CrystalMinted(address indexed owner, uint256 tokenId, uint256 gene, uint256 kind, uint256 weight);

    uint256 currentTokenId = 1;

    constructor() ERC721Token("CryptoCrystal", "CC") public {

    }

    function mint(
        address _owner,
        uint256 _gene,
        uint256 _kind,
        uint256 _weight
    ) public onlyAcceptable returns(uint256) {
        require(_gene > 0);
        require(_weight > 0);

        uint256 _tokenId = currentTokenId;
        currentTokenId++;
        super._mint(_owner, _tokenId);
        Crystal memory _crystal = Crystal({
            tokenId: _tokenId,
            gene: _gene,
            kind: uint8(_kind),
            weight: uint128(_weight),
            mintedAt: uint64(now)
            });
        tokenIdToCrystal[_tokenId] = _crystal;
        emit CrystalMinted(_owner, _tokenId, _gene, _kind, _weight);
        return _tokenId;
    }

    function burn(address _owner, uint256 _tokenId) public onlyAcceptable {
        require(ownerOf(_tokenId) == _owner);

        delete tokenIdToCrystal[_tokenId];
        super._burn(_owner, _tokenId);
        emit CrystalBurned(_owner, _tokenId);
    }

    // @dev Transfers the ownership of a given token ID to another address.
    //      _transferFrom is almost the same to openzeppelin-solidity's implementation.
    //      ref. https://github.com/OpenZeppelin/openzeppelin-solidity/blob/ad12381549c4c0711c2f3310e9fb1f65d51c299c/contracts/token/ERC721/ERC721BasicToken.sol#L140
    //      We use onlyAcceptable modifier instead of canTransfer.
    //      _transferFrom is intended to be called only from cryptocrystal contract.
    // @param _from owner of the token
    // @param _to address to receive the ownership of the given token ID
    // @param _tokenId uint256 ID of the token to be transferred
    function _transferFrom(address _from, address _to, uint256 _tokenId) public onlyAcceptable {
        require(ownerOf(_tokenId) == _from);
        require(_to != address(0));

        clearApproval(_from, _tokenId);
        removeTokenFrom(_from, _tokenId);
        addTokenTo(_to, _tokenId);

        emit Transfer(_from, _to, _tokenId);
    }

    function getCrystalKindWeight(uint256 _tokenId) public onlyAcceptable view returns(
        uint256 kind,
        uint256 weight
    ) {
        require(exists(_tokenId));

        Crystal memory _crystal = tokenIdToCrystal[_tokenId];
        kind = _crystal.kind;
        weight = _crystal.weight;
    }

    function getCrystalGeneKindWeight(uint256 _tokenId) public onlyAcceptable view returns(
        uint256 gene,
        uint256 kind,
        uint256 weight
    ) {
        require(exists(_tokenId));

        Crystal memory _crystal = tokenIdToCrystal[_tokenId];
        gene = _crystal.gene;
        kind = _crystal.kind;
        weight = _crystal.weight;
    }

    function getCrystal(uint256 _tokenId) external view returns(
        address owner,
        uint256 gene,
        uint256 kind,
        uint256 weight,
        uint256 mintedAt
    ) {
        require(exists(_tokenId));

        Crystal memory _crystal = tokenIdToCrystal[_tokenId];

        owner = ownerOf(_tokenId);
        gene = _crystal.gene;
        kind = _crystal.kind;
        weight = _crystal.weight;
        mintedAt = _crystal.mintedAt;
    }

    function getCrystalsSummary(address _owner) external view returns(
        uint256[] amounts,
        uint256[] weights
    ) {
        amounts = new uint256[](100);
        weights = new uint256[](100);
        uint256 _tokenCount = ownedTokensCount[_owner];
        for (uint256 i = 0; i < _tokenCount; i++) {
            uint256 _tokenId = ownedTokens[_owner][i];
            Crystal memory _crystal = tokenIdToCrystal[_tokenId];
            amounts[_crystal.kind] = amounts[_crystal.kind].add(1);
            weights[_crystal.kind] = weights[_crystal.kind].add(_crystal.weight);
        }
    }

    function getCrystals(address _owner) external view returns(
        uint256[] tokenIds,
        uint256[] genes,
        uint256[] kinds,
        uint256[] weights,
        uint256[] mintedAts
    ) {
        uint256 _tokenCount = ownedTokensCount[_owner];
        tokenIds = new uint256[](_tokenCount);
        genes = new uint256[](_tokenCount);
        kinds = new uint256[](_tokenCount);
        weights = new uint256[](_tokenCount);
        mintedAts = new uint256[](_tokenCount);
        for (uint256 i = 0; i < _tokenCount; i++) {
            uint256 _tokenId = ownedTokens[_owner][i];
            Crystal memory _crystal = tokenIdToCrystal[_tokenId];
            tokenIds[i] = _tokenId;
            genes[i] = _crystal.gene;
            kinds[i] = _crystal.kind;
            weights[i] = _crystal.weight;
            mintedAts[i] = _crystal.mintedAt;
        }
    }

    function getCrystalsByKind(address _owner, uint256 _kind) external view returns(
        uint256[] tokenIds,
        uint256[] genes,
        uint256[] weights,
        uint256[] mintedAts
    ) {
        require(_kind < 100);

        uint256 _tokenCount = ownedTokensCount[_owner];
        tokenIds = new uint256[](_tokenCount);
        genes = new uint256[](_tokenCount);
        weights = new uint256[](_tokenCount);
        mintedAts = new uint256[](_tokenCount);
        uint256 index;
        for (uint256 i = 0; i < _tokenCount; i++) {
            uint256 _tokenId = ownedTokens[_owner][i];
            Crystal memory _crystal = tokenIdToCrystal[_tokenId];
            if (_crystal.kind == _kind) {
                tokenIds[index] = _tokenId;
                genes[index] = _crystal.gene;
                weights[index] = _crystal.weight;
                mintedAts[i] = _crystal.mintedAt;
                index = index.add(1);
            }
        }
    }
}