pragma solidity ^0.4.24;

// File: contracts/openzeppelin-solidity/introspection/ERC165.sol

/**
 * @title ERC165
 * @dev https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md
 */
interface ERC165 {

  /**
   * @notice Query if a contract implements an interface
   * @param _interfaceId The interface identifier, as specified in ERC-165
   * @dev Interface identification is specified in ERC-165. This function
   * uses less than 30,000 gas.
   */
  function supportsInterface(bytes4 _interfaceId)
    external
    view
    returns (bool);
}

// File: contracts/openzeppelin-solidity/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 is ERC165 {
  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) 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/openzeppelin-solidity/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() external view returns (string _name);
  function symbol() external 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/openzeppelin-solidity/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,address,uint256,bytes)"))`,
   *  which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
   */
  bytes4 internal constant ERC721_RECEIVED = 0x150b7a02;

  /**
   * @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. Return of other than the magic value MUST result in the 
   * transaction being reverted.
   * Note: the contract address is always the message sender.
   * @param _operator The address which called `safeTransferFrom` function
   * @param _from The address which previously owned the token
   * @param _tokenId The NFT identifier which is being transfered
   * @param _data Additional data with no specified format
   * @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
   */
  function onERC721Received(
    address _operator,
    address _from,
    uint256 _tokenId,
    bytes _data
  )
    public
    returns(bytes4);
}

// File: contracts/openzeppelin-solidity/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 c) {
    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
    if (a == 0) {
      return 0;
    }

    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 a / b;
  }

  /**
  * @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 c) {
    c = a + b;
    assert(c >= a);
    return c;
  }
}

// File: contracts/openzeppelin-solidity/AddressUtils.sol

/**
 * Utility library of inline functions on addresses
 */
library AddressUtils {

  /**
   * Returns whether the target address is a contract
   * @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 to check
   * @return whether the target address is a contract
   */
  function isContract(address addr) internal view returns (bool) {
    uint256 size;
    // XXX Currently there is no better way to check if there is a contract in an address
    // than to check the size of the code at that address.
    // See https://ethereum.stackexchange.com/a/14016/36603
    // for more details about how this works.
    // TODO Check this again before the Serenity release, because all addresses will be
    // contracts then.
    // solium-disable-next-line security/no-inline-assembly
    assembly { size := extcodesize(addr) }
    return size > 0;
  }

}

// File: contracts/openzeppelin-solidity/introspection/SupportsInterfaceWithLookup.sol

/**
 * @title SupportsInterfaceWithLookup
 * @author Matt Condon (@shrugs)
 * @dev Implements ERC165 using a lookup table.
 */
contract SupportsInterfaceWithLookup is ERC165 {
  bytes4 public constant InterfaceId_ERC165 = 0x01ffc9a7;
  /**
   * 0x01ffc9a7 ===
   *   bytes4(keccak256('supportsInterface(bytes4)'))
   */

  /**
   * @dev a mapping of interface id to whether or not it's supported
   */
  mapping(bytes4 => bool) internal supportedInterfaces;

  /**
   * @dev A contract implementing SupportsInterfaceWithLookup
   * implement ERC165 itself
   */
  constructor()
    public
  {
    _registerInterface(InterfaceId_ERC165);
  }

  /**
   * @dev implement supportsInterface(bytes4) using a lookup table
   */
  function supportsInterface(bytes4 _interfaceId)
    external
    view
    returns (bool)
  {
    return supportedInterfaces[_interfaceId];
  }

  /**
   * @dev private method for registering an interface
   */
  function _registerInterface(bytes4 _interfaceId)
    internal
  {
    require(_interfaceId != 0xffffffff);
    supportedInterfaces[_interfaceId] = true;
  }
}

// File: contracts/openzeppelin-solidity/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 SupportsInterfaceWithLookup, ERC721Basic {

  bytes4 private constant InterfaceId_ERC721 = 0x80ac58cd;
  /*
   * 0x80ac58cd ===
   *   bytes4(keccak256('balanceOf(address)')) ^
   *   bytes4(keccak256('ownerOf(uint256)')) ^
   *   bytes4(keccak256('approve(address,uint256)')) ^
   *   bytes4(keccak256('getApproved(uint256)')) ^
   *   bytes4(keccak256('setApprovalForAll(address,bool)')) ^
   *   bytes4(keccak256('isApprovedForAll(address,address)')) ^
   *   bytes4(keccak256('transferFrom(address,address,uint256)')) ^
   *   bytes4(keccak256('safeTransferFrom(address,address,uint256)')) ^
   *   bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)'))
   */

  bytes4 private constant InterfaceId_ERC721Exists = 0x4f558e79;
  /*
   * 0x4f558e79 ===
   *   bytes4(keccak256('exists(uint256)'))
   */

  using SafeMath for uint256;
  using AddressUtils for address;

  // Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
  // which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
  bytes4 private constant ERC721_RECEIVED = 0x150b7a02;

  // 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));
    _;
  }

  constructor()
    public
  {
    // register the supported interfaces to conform to ERC721 via ERC165
    _registerInterface(InterfaceId_ERC721);
    _registerInterface(InterfaceId_ERC721Exists);
  }

  /**
   * @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 existence 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
   * The zero address indicates there is no approved address.
   * There can only be one approved address per token at a given time.
   * 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));

    tokenApprovals[_tokenId] = _to;
    emit 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 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
   * 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;
    emit 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
   * Usage of this method is discouraged, use `safeTransferFrom` whenever possible
   * 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);

    emit Transfer(_from, _to, _tokenId);
  }

  /**
   * @dev Safely transfers the ownership of a given token ID to another address
   * 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,address,uint256,bytes)"))`; otherwise,
   * the transfer is reverted.
   *
   * 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)
  {
    // solium-disable-next-line arg-overflow
    safeTransferFrom(_from, _to, _tokenId, "");
  }

  /**
   * @dev Safely transfers the ownership of a given token ID to another address
   * 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,address,uint256,bytes)"))`; otherwise,
   * the transfer is reverted.
   * 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);
    // solium-disable-next-line arg-overflow
    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);
    // Disable solium check because of
    // https://github.com/duaraghav8/Solium/issues/175
    // solium-disable-next-line operator-whitespace
    return (
      _spender == owner ||
      getApproved(_tokenId) == _spender ||
      isApprovedForAll(owner, _spender)
    );
  }

  /**
   * @dev Internal function to mint a new token
   * 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);
    emit Transfer(address(0), _to, _tokenId);
  }

  /**
   * @dev Internal function to burn a specific token
   * 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);
    emit Transfer(_owner, address(0), _tokenId);
  }

  /**
   * @dev Internal function to clear current approval of a given token ID
   * 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);
    }
  }

  /**
   * @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
   * 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(
      msg.sender, _from, _tokenId, _data);
    return (retval == ERC721_RECEIVED);
  }
}

// File: contracts/openzeppelin-solidity/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 SupportsInterfaceWithLookup, ERC721BasicToken, ERC721 {

  bytes4 private constant InterfaceId_ERC721Enumerable = 0x780e9d63;
  /**
   * 0x780e9d63 ===
   *   bytes4(keccak256('totalSupply()')) ^
   *   bytes4(keccak256('tokenOfOwnerByIndex(address,uint256)')) ^
   *   bytes4(keccak256('tokenByIndex(uint256)'))
   */

  bytes4 private constant InterfaceId_ERC721Metadata = 0x5b5e139f;
  /**
   * 0x5b5e139f ===
   *   bytes4(keccak256('name()')) ^
   *   bytes4(keccak256('symbol()')) ^
   *   bytes4(keccak256('tokenURI(uint256)'))
   */

  // 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
   */
  constructor(string _name, string _symbol) public {
    name_ = _name;
    symbol_ = _symbol;

    // register the supported interfaces to conform to ERC721 via ERC165
    _registerInterface(InterfaceId_ERC721Enumerable);
    _registerInterface(InterfaceId_ERC721Metadata);
  }

  /**
   * @dev Gets the token name
   * @return string representing the token name
   */
  function name() external view returns (string) {
    return name_;
  }

  /**
   * @dev Gets the token symbol
   * @return string representing the token symbol
   */
  function symbol() external view returns (string) {
    return symbol_;
  }

  /**
   * @dev Returns an URI for a given token ID
   * 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 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
   * 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 set the token URI for a given token
   * 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 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
   * 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
   * 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/openzeppelin-solidity/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 OwnershipRenounced(address indexed previousOwner);
  event OwnershipTransferred(
    address indexed previousOwner,
    address indexed newOwner
  );


  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   */
  constructor() 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 relinquish control of the contract.
   * @notice Renouncing to ownership will leave the contract without an owner.
   * It will not be possible to call the functions with the `onlyOwner`
   * modifier anymore.
   */
  function renounceOwnership() public onlyOwner {
    emit OwnershipRenounced(owner);
    owner = address(0);
  }

  /**
   * @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 {
    _transferOwnership(_newOwner);
  }

  /**
   * @dev Transfers control of the contract to a newOwner.
   * @param _newOwner The address to transfer ownership to.
   */
  function _transferOwnership(address _newOwner) internal {
    require(_newOwner != address(0));
    emit OwnershipTransferred(owner, _newOwner);
    owner = _newOwner;
  }
}

// File: contracts/ERC721TokenWithData.sol

// import "./ERC721SlimTokenArray.sol";



// an ERC721 token with additional data storage,
contract ERC721TokenWithData is ERC721Token("CryptoAssaultUnit", "CAU"), Ownable {

  /**
   * @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);
		// Disable solium check because of
		// https://github.com/duaraghav8/Solium/issues/175
		// solium-disable-next-line operator-whitespace
		return (
			_spender == owner ||
			approvedContractAddresses[_spender] ||
			getApproved(_tokenId) == _spender ||
			isApprovedForAll(owner, _spender)
		);
	}

	mapping (address => bool) internal approvedContractAddresses;
	bool approvedContractsFinalized = false;

	/**
	* @notice Approve a contract address for minting tokens and transferring tokens, when approved by the owner
	* @param contractAddress The address that will be approved
	*/
	function addApprovedContractAddress(address contractAddress) public onlyOwner
	{
		require(!approvedContractsFinalized);
		approvedContractAddresses[contractAddress] = true;
	}

	/**
	* @notice Unapprove a contract address for minting tokens and transferring tokens
	* @param contractAddress The address that will be unapproved
	*/
	function removeApprovedContractAddress(address contractAddress) public onlyOwner
	{
		require(!approvedContractsFinalized);
		approvedContractAddresses[contractAddress] = false;
	}

	/**
	* @notice Finalize the contract so it will be forever impossible to change the approved contracts list
	*/
	function finalizeApprovedContracts() public onlyOwner {
		approvedContractsFinalized = true;
	}

	mapping(uint256 => mapping(uint256 => uint256)) data;

	function getData(uint256 _tokenId, uint256 _index) public view returns (uint256) {
		return data[_index][_tokenId];
	}

	function getData3(uint256 _tokenId1, uint256 _tokenId2, uint256 _tokenId3, uint256 _index) public view returns (uint256, uint256, uint256) {
		return (
			data[_index][_tokenId1],
			data[_index][_tokenId2],
			data[_index][_tokenId3]
		);
	}
	
	function getDataAndOwner3(uint256 _tokenId1, uint256 _tokenId2, uint256 _tokenId3, uint256 _index) public view returns (uint256, uint256, uint256, address, address, address) {
		return (
			data[_index][_tokenId1],
			data[_index][_tokenId2],
			data[_index][_tokenId3],
			ownerOf(_tokenId1),
			ownerOf(_tokenId2),
			ownerOf(_tokenId3)
		);
	}
	
	function _setData(uint256 _tokenId, uint256 _index, uint256 _data) internal {
		
		data[_index][_tokenId] = _data;
	}

	function setData(uint256 _tokenId, uint256 _index, uint256 _data) public {
		
		require(approvedContractAddresses[msg.sender], "not an approved sender");
		data[_index][_tokenId] = _data;
	}

	/**
	* @notice Gets the list of tokens owned by a given address
	* @param _owner address to query the tokens of
	* @return uint256[] representing the list of tokens owned by the passed address
	*/
	function tokensOfWithData(address _owner, uint256 _index) public view returns (uint256[], uint256[]) {
		uint256[] memory tokensList = ownedTokens[_owner];
		uint256[] memory dataList = new uint256[](tokensList.length);
		for (uint i=0; i<tokensList.length; i++) {
			dataList[i] = data[_index][tokensList[i]];
		}
		return (tokensList, dataList);
	}

	// The tokenId of the next minted token. It auto-increments.
	uint256 nextTokenId = 1;

	function getNextTokenId() public view returns (uint256) {
		return nextTokenId;
	}

	/**
	* @notice Mint token function
	* @param _to The address that will own the minted token
	*/
	function mintAndSetData(address _to, uint256 _data) public returns (uint256) {

		require(approvedContractAddresses[msg.sender], "not an approved sender");

		uint256 tokenId = nextTokenId;
		nextTokenId++;
		_mint(_to, tokenId);
		_setData(tokenId, 0, _data);

		return tokenId;
	}

	function burn(uint256 _tokenId) public {
		require(
			approvedContractAddresses[msg.sender] ||
			msg.sender == owner, "burner not approved"
		);

		_burn(ownerOf(_tokenId), _tokenId);
	}
	
	function burn3(uint256 _tokenId1, uint256 _tokenId2, uint256 _tokenId3) public {
		require(
			approvedContractAddresses[msg.sender] ||
			msg.sender == owner, "burner not approved"
		);

		_burn(ownerOf(_tokenId1), _tokenId1);
		_burn(ownerOf(_tokenId2), _tokenId2);
		_burn(ownerOf(_tokenId3), _tokenId3);
	}
}

// File: contracts/strings/Strings.sol

library Strings {
  // via https://github.com/oraclize/ethereum-api/blob/master/oraclizeAPI_0.5.sol
  function strConcat(string _a, string _b, string _c, string _d, string _e) internal pure returns (string) {
      bytes memory _ba = bytes(_a);
      bytes memory _bb = bytes(_b);
      bytes memory _bc = bytes(_c);
      bytes memory _bd = bytes(_d);
      bytes memory _be = bytes(_e);
      string memory abcde = new string(_ba.length + _bb.length + _bc.length + _bd.length + _be.length);
      bytes memory babcde = bytes(abcde);
      uint k = 0;
      for (uint i = 0; i < _ba.length; i++) babcde[k++] = _ba[i];
      for (i = 0; i < _bb.length; i++) babcde[k++] = _bb[i];
      for (i = 0; i < _bc.length; i++) babcde[k++] = _bc[i];
      for (i = 0; i < _bd.length; i++) babcde[k++] = _bd[i];
      for (i = 0; i < _be.length; i++) babcde[k++] = _be[i];
      return string(babcde);
    }

    function strConcat(string _a, string _b, string _c, string _d) internal pure returns (string) {
        return strConcat(_a, _b, _c, _d, "");
    }

    function strConcat(string _a, string _b, string _c) internal pure returns (string) {
        return strConcat(_a, _b, _c, "", "");
    }

    function strConcat(string _a, string _b) internal pure returns (string) {
        return strConcat(_a, _b, "", "", "");
    }

    function uint2str(uint i) internal pure returns (string) {
        if (i == 0) return "0";
        uint j = i;
        uint len;
        while (j != 0){
            len++;
            j /= 10;
        }
        bytes memory bstr = new bytes(len);
        uint k = len - 1;
        while (i != 0){
            bstr[k--] = byte(48 + i % 10);
            i /= 10;
        }
        return string(bstr);
    }
}

// File: contracts/Token.sol

contract Token is ERC721TokenWithData {

	string metadataUrlPrefix = "https://metadata.cryptoassault.io/unit/";

	/**
	* @dev Returns an URI for a given token ID
	* 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 Strings.strConcat(metadataUrlPrefix, Strings.uint2str(_tokenId));
	}

	function setMetadataUrlPrefix(string _metadataUrlPrefix) public onlyOwner
	{
		metadataUrlPrefix = _metadataUrlPrefix;
	}
}

// File: contracts/openzeppelin-solidity/lifecycle/Pausable.sol

/**
 * @title Pausable
 * @dev Base contract which allows children to implement an emergency stop mechanism.
 */
contract Pausable is Ownable {
  event Pause();
  event Unpause();

  bool public paused = false;


  /**
   * @dev Modifier to make a function callable only when the contract is not paused.
   */
  modifier whenNotPaused() {
    require(!paused);
    _;
  }

  /**
   * @dev Modifier to make a function callable only when the contract is paused.
   */
  modifier whenPaused() {
    require(paused);
    _;
  }

  /**
   * @dev called by the owner to pause, triggers stopped state
   */
  function pause() onlyOwner whenNotPaused public {
    paused = true;
    emit Pause();
  }

  /**
   * @dev called by the owner to unpause, returns to normal state
   */
  function unpause() onlyOwner whenPaused public {
    paused = false;
    emit Unpause();
  }
}

// File: contracts/Fusion.sol

contract Fusion is Pausable {

	event Fused(uint32 unit1, uint32 unit2, uint32 unit3, uint256 price);
	event FinishedFusing(uint32 unit1, uint32 unit2, uint32 unit3, uint32 newUnit);

	Token token;

	function setTokenContractAddress(address newAddress) onlyOwner public {
		token = Token(newAddress);
	}

	struct WaitingToFuse {
		address owner;
		uint32 unit1;
		uint32 unit2;
		uint32 unit3;
		uint48 fusedOnBlockNumber;
		//TODO: maybe fit into 256 bits
	}
	mapping (uint256 => WaitingToFuse) waitingToFuse; // This is a LIFO stack.

	uint64 waitingToFuseNum = 0;
	uint64 waitingToFuseFirstIndex = 0;
	uint64 fuseNonce = 1;

	uint256 fusePrice = 0.005 ether;

	function withdrawBalance() onlyOwner public {
		owner.transfer(address(this).balance);
	}

	function setFusePrice(uint256 price) public onlyOwner {
		fusePrice = price;
	}

	function pushFuse(uint32 unit1, uint32 unit2, uint32 unit3) private {

		waitingToFuse[waitingToFuseFirstIndex + waitingToFuseNum] = WaitingToFuse(msg.sender, unit1, unit2, unit3, uint48(block.number));
		waitingToFuseNum = waitingToFuseNum + 1;
	}

	function popFuse() private {

		require(waitingToFuseNum > 0, "trying to popFuse() an empty stack");
		waitingToFuseNum = waitingToFuseNum - 1;
		if (waitingToFuseNum == 0) {
			waitingToFuseFirstIndex = 0;
		} else {
			waitingToFuseFirstIndex++;
		}
	}

	function peekFuse() private view returns (WaitingToFuse) {

		return waitingToFuse[waitingToFuseFirstIndex];
	}

	function fuse(uint32 unit1, uint32 unit2, uint32 unit3) external payable whenNotPaused {

		require(msg.value == fusePrice, "Price doesnt match the amount payed");

		address owner1;
		address owner2;
		address owner3;
		uint256 data1;
		uint256 data2;
		uint256 data3;
		(data1, data2, data3, owner1, owner2, owner3) = token.getDataAndOwner3(unit1, unit2, unit3, 0);

		require(msg.sender == owner1, "not the owner");
		require(msg.sender == owner2, "not the owner");
		require(msg.sender == owner3, "not the owner");

		uint256 category1 = ((data1 >> 248) & 0xff) / 6;
		uint256 category2 = ((data2 >> 248) & 0xff) / 6;
		uint256 category3 = ((data3 >> 248) & 0xff) / 6;
		require(
			category1 == category2 &&
			category1 == category3,
			"categories don't match"
		);

		uint256 tier1 = (data1 >> 244) & 0x0f;
		// uint256 tier2 = (data2 >> 244) & 0x0f;
		// uint256 tier3 = (data3 >> 244) & 0x0f;
		require(
			(tier1 == (data2 >> 244) & 0x0f) &&
			(tier1 == (data3 >> 244) & 0x0f),
			"tiers don't match"
		);
		require (tier1 <= 2, "4 is the maximum tier");

		// burn the tokens.
		// their data will still be used though.
		token.burn3(unit1, unit2, unit3);

		pushFuse(unit1, unit2, unit3);

		emit Fused(unit1, unit2, unit3, fusePrice);
	}

	function getProjectedBlockHash(uint256 blockNumber) internal view returns (uint256) {

		uint256 blockToHash = blockNumber;
		uint256 blocksAgo = block.number - blockToHash;
		blockToHash += ((blocksAgo-1) / 256) * 256;
		return uint256(blockhash(blockToHash));
	}

	function fusionsNeeded() external view returns (uint256) {

		return waitingToFuseNum;
	}

	function getRandomRarity(uint256 data1, uint256 data2, uint256 data3, uint16 rarityRand) internal pure returns (uint256, uint256) {

		uint256 rarityPattern = 0;
		rarityPattern += 1 << (((data1 >> 216) & 0x0f) * 4);
		rarityPattern += 1 << (((data2 >> 216) & 0x0f) * 4);
		rarityPattern += 1 << (((data3 >> 216) & 0x0f) * 4);

		int256 rarity;
		int256 lowestParentRarity;

		if (rarityPattern == 0x0003) {
			rarity = 0;
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x0030) {
			rarity = 1;
			lowestParentRarity = 1;
		}
		else if (rarityPattern == 0x0300) {
			rarity = 2;
			lowestParentRarity = 2;
		}
		else if (rarityPattern == 0x3000) {
			rarity = 3;
			lowestParentRarity = 3;
		}
		else if (rarityPattern == 0x0111) {
			rarity = (rarityRand < 21845) ? 0 : ((rarityRand < 43691) ? 1 : 2);
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x1110) {
			rarity = (rarityRand < 21845) ? 1 : ((rarityRand < 43691) ? 2 : 3);
			lowestParentRarity = 1;
		}
		else if (rarityPattern == 0x1011) {
			rarity = (rarityRand < 10923) ? 0 : ((rarityRand < 36409) ? 1 : ((rarityRand < 54613) ? 2 : 3));
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x1101) {
			rarity = (rarityRand < 10923) ? 0 : ((rarityRand < 29127) ? 1 : ((rarityRand < 54613) ? 2 : 3));
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x2001) {
			rarity = (rarityRand < 10923) ? 0 : ((rarityRand < 25486) ? 1 : ((rarityRand < 43691) ? 2 : 3));
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x1002) {
			rarity = (rarityRand < 21845) ? 0 : ((rarityRand < 40050) ? 1 : ((rarityRand < 54613) ? 2 : 3));
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x2010) {
			rarity = (rarityRand < 14564) ? 1 : ((rarityRand < 36409) ? 2 : 3);
			lowestParentRarity = 1;
		}
		else if (rarityPattern == 0x0201) {
			rarity = (rarityRand < 14564) ? 0 : ((rarityRand < 36409) ? 1 : 2);
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x0102) {
			rarity = (rarityRand < 29127) ? 0 : ((rarityRand < 50972) ? 1 : 2);
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x1020) {
			rarity = (rarityRand < 29127) ? 1 : ((rarityRand < 50972) ? 2 : 3);
			lowestParentRarity = 1;
		}
		else if (rarityPattern == 0x0012) {
			rarity = (rarityRand < 43691) ? 0 : 1;
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x0021) {
			rarity = (rarityRand < 43691) ? 1 : 0;
			lowestParentRarity = 0;
		}
		else if (rarityPattern == 0x0120) {
			rarity = (rarityRand < 43691) ? 1 : 2;
			lowestParentRarity = 1;
		}
		else if (rarityPattern == 0x0210) {
			rarity = (rarityRand < 43691) ? 2 : 1;
			lowestParentRarity = 1;
		}
		else if (rarityPattern == 0x1200) {
			rarity = (rarityRand < 43691) ? 2 : 3;
			lowestParentRarity = 2;
		}
		else if (rarityPattern == 0x2100) {
			rarity = (rarityRand < 43691) ? 3 : 2;
			lowestParentRarity = 2;
		}
		else {
			require(false, "invalid rarity pattern");//TODO: remove this
			rarity = 0;
		}

		// Apply the penalty for when the child rarity is higher than the lowest parent rarity:
		// child is 3 rarities higher: 0.85
		// child is 2 rarities higher: 0.89
		// child is 1 rarity higher: 0.95
		int256 rarityDifference = rarity - lowestParentRarity;
		uint256 penalty;
		if (rarityDifference == 3) {
			penalty = 55705;
		} 
		else if (rarityDifference == 2) {
			penalty = 58327;
		} 
		else if (rarityDifference == 1) {
			penalty = 62259;
		} 
		else {
			penalty = 65536;
		} 

		return (uint256(rarity), penalty);
	}

	function getOldestBirthTimestamp(uint256 data1, uint256 data2, uint256 data3) internal pure returns (uint256)
	{
		uint256 oldestBirthTimestamp = ((data1 >> 220) & 0xffffff);
		uint256 birthTimestamp2 = ((data2 >> 220) & 0xffffff);
		uint256 birthTimestamp3 = ((data3 >> 220) & 0xffffff);
		if (birthTimestamp2 < oldestBirthTimestamp) oldestBirthTimestamp = birthTimestamp2;
		if (birthTimestamp3 < oldestBirthTimestamp) oldestBirthTimestamp = birthTimestamp3;
		return oldestBirthTimestamp;
	}

	function finishFusion() external whenNotPaused {

		require(waitingToFuseNum > 0, "nothing to fuse");

		WaitingToFuse memory w = peekFuse();
		
		// can't fuse on the same block. its block hash would be unknown.
		require(w.fusedOnBlockNumber < block.number, "Can't fuse on the same block.");

		uint256 rand = uint256(keccak256(abi.encodePacked(getProjectedBlockHash(w.fusedOnBlockNumber))));

		uint256 data1;
		uint256 data2;
		uint256 data3;
		(data1, data2, data3) = token.getData3(w.unit1, w.unit2, w.unit3, 0);

		uint256 data = 0;
		data |= ((data1 >> 248) & 0xff) << 248; // type
		data |= (((data1 >> 244) & 0x0f) + 1) << 244; // tier


		// uint256 oldestBirthTimestamp = getOldestBirthTimestamp(data1, data2, data3);

		// Get the oldest birthday
		// uint256 oldestBirthTimestamp = ((data1 >> 220) & 0xffffff);
		// if (((data2 >> 220) & 0xffffff) < oldestBirthTimestamp) oldestBirthTimestamp = ((data2 >> 220) & 0xffffff);
		// if (((data3 >> 220) & 0xffffff) < oldestBirthTimestamp) oldestBirthTimestamp = ((data3 >> 220) & 0xffffff);
		data |= getOldestBirthTimestamp(data1, data2, data3) << 220;

		(uint256 rarity, uint256 penalty) = getRandomRarity(data1, data2, data3, uint16(rand));
		rand >>= 16;

		data |= rarity << 216;

		data |= ((data1 >> 208) & 0xff) << 208; // sku

		// Apply the penalty for fusing non-matching types:
		// 1 matching: 0.93
		// 0 matching: 0.88
		uint256 numMatchingTypes = 0;
		if ((((data1 >> 248) & 0xff) << 248) == (((data2 >> 248) & 0xff) << 248)) numMatchingTypes++;
		if ((((data1 >> 248) & 0xff) << 248) == (((data3 >> 248) & 0xff) << 248)) numMatchingTypes++;
		if (numMatchingTypes == 1)
		{
			penalty = (penalty * 60948) / 65536; // *= 0.93
		}
		else if (numMatchingTypes == 0)
		{
			penalty = (penalty * 57671) / 65536; // *= 0.88
		}

		// generate child stats
		for (uint256 i=0; i<18; i++) {
			data |= (((
					((data1 >> (200-i*8)) & 0xff) +
					((data2 >> (200-i*8)) & 0xff) +
					((data3 >> (200-i*8)) & 0xff)
				) * penalty // the penalty from mismatched types/rarities
				   * (63488 + (rand&0x3ff)) // a random penalty from 97% to 100%
			) / 0x300000000) << (200-i*8);
			rand >>= 10;
		}


		// TODO: maybe re-use the unit1 token as the new fused unit, to save gas
		uint32 newUnit = uint32(token.mintAndSetData(w.owner, data));

		popFuse();

		emit FinishedFusing(w.unit1, w.unit2, w.unit3, newUnit);
	}

}