pragma solidity ^0.4.24;

// File: openzeppelin-solidity/contracts/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: openzeppelin-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 is ERC165 {

  bytes4 internal 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 internal constant InterfaceId_ERC721Exists = 0x4f558e79;
  /*
   * 0x4f558e79 ===
   *   bytes4(keccak256('exists(uint256)'))
   */

  bytes4 internal constant InterfaceId_ERC721Enumerable = 0x780e9d63;
  /**
   * 0x780e9d63 ===
   *   bytes4(keccak256('totalSupply()')) ^
   *   bytes4(keccak256('tokenOfOwnerByIndex(address,uint256)')) ^
   *   bytes4(keccak256('tokenByIndex(uint256)'))
   */

  bytes4 internal constant InterfaceId_ERC721Metadata = 0x5b5e139f;
  /**
   * 0x5b5e139f ===
   *   bytes4(keccak256('name()')) ^
   *   bytes4(keccak256('symbol()')) ^
   *   bytes4(keccak256('tokenURI(uint256)'))
   */

  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: openzeppelin-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() 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: openzeppelin-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,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 transferred
   * @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: openzeppelin-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 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: openzeppelin-solidity/contracts/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: openzeppelin-solidity/contracts/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: openzeppelin-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 SupportsInterfaceWithLookup, ERC721Basic {

  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;

  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
  {
    require(isApprovedOrOwner(msg.sender, _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
  {
    // 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
  {
    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: openzeppelin-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 SupportsInterfaceWithLookup, ERC721BasicToken, ERC721 {

  // 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);

    // To prevent a gap in the array, we store the last token in the index of the token to delete, and
    // then delete the last slot.
    uint256 tokenIndex = ownedTokensIndex[_tokenId];
    uint256 lastTokenIndex = ownedTokens[_from].length.sub(1);
    uint256 lastToken = ownedTokens[_from][lastTokenIndex];

    ownedTokens[_from][tokenIndex] = lastToken;
    // This also deletes the contents at the last position of the array
    ownedTokens[_from].length--;

    // 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

    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: openzeppelin-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 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: openzeppelin-solidity/contracts/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() public onlyOwner whenNotPaused {
    paused = true;
    emit Pause();
  }

  /**
   * @dev called by the owner to unpause, returns to normal state
   */
  function unpause() public onlyOwner whenPaused {
    paused = false;
    emit Unpause();
  }
}

// File: contracts/MEHAccessControl.sol

contract MarketInerface {
    function buyBlocks(address, uint16[]) external returns (uint) {}
    function sellBlocks(address, uint, uint16[]) external returns (uint) {}
    function isMarket() public view returns (bool) {}
    function isOnSale(uint16) public view returns (bool) {}
    function areaPrice(uint16[]) public view returns (uint) {}
    function importOldMEBlock(uint8, uint8) external returns (uint, address) {}
}

contract RentalsInterface {
    function rentOutBlocks(address, uint, uint16[]) external returns (uint) {}
    function rentBlocks(address, uint, uint16[]) external returns (uint) {}
    function blocksRentPrice(uint, uint16[]) external view returns (uint) {}
    function isRentals() public view returns (bool) {}
    function isRented(uint16) public view returns (bool) {}
    function renterOf(uint16) public view returns (address) {}
}

contract AdsInterface {
    function advertiseOnBlocks(address, uint16[], string, string, string) external returns (uint) {}
    function canAdvertiseOnBlocks(address, uint16[]) public view returns (bool) {}
    function isAds() public view returns (bool) {}
}

/// @title MEHAccessControl: Part of MEH contract responsible for communication with external modules:
///  Market, Rentals, Ads contracts. Provides authorization and upgradability methods.
contract MEHAccessControl is Pausable {

    // Allows a module being plugged in to verify it is MEH contract. 
    bool public isMEH = true;

    // Modules
    MarketInerface public market;
    RentalsInterface public rentals;
    AdsInterface public ads;

    // Emitted when a module is plugged.
    event LogModuleUpgrade(address newAddress, string moduleName);
    
// GUARDS
    
    /// @dev Functions allowed to market module only. 
    modifier onlyMarket() {
        require(msg.sender == address(market));
        _;
    }

    /// @dev Functions allowed to balance operators only (market and rentals contracts are the 
    ///  only balance operators)
    modifier onlyBalanceOperators() {
        require(msg.sender == address(market) || msg.sender == address(rentals));
        _;
    }

// ** Admin set Access ** //
    /// @dev Allows admin to plug a new Market contract in.
    // credits to cryptokittes! - https://github.com/Lunyr/crowdsale-contracts/blob/cfadd15986c30521d8ba7d5b6f57b4fefcc7ac38/contracts/LunyrToken.sol#L117
    // NOTE: verify that a contract is what we expect
    function adminSetMarket(address _address) external onlyOwner {
        MarketInerface candidateContract = MarketInerface(_address);
        require(candidateContract.isMarket());
        market = candidateContract;
        emit LogModuleUpgrade(_address, "Market");
    }

    /// @dev Allows admin to plug a new Rentals contract in.
    function adminSetRentals(address _address) external onlyOwner {
        RentalsInterface candidateContract = RentalsInterface(_address);
        require(candidateContract.isRentals());
        rentals = candidateContract;
        emit LogModuleUpgrade(_address, "Rentals");
    }

    /// @dev Allows admin to plug a new Ads contract in.
    function adminSetAds(address _address) external onlyOwner {
        AdsInterface candidateContract = AdsInterface(_address);
        require(candidateContract.isAds());
        ads = candidateContract;
        emit LogModuleUpgrade(_address, "Ads");
    }
}

// File: contracts/MehERC721.sol

// ERC721 



/// @title MehERC721: Part of MEH contract responsible for ERC721 token management. Openzeppelin's
///  ERC721 implementation modified for the Million Ether Homepage. 
contract MehERC721 is ERC721Token("MillionEtherHomePage","MEH"), MEHAccessControl {

    /// @dev Checks rights to transfer block ownership. Locks tokens on sale.
    ///  Overrides OpenZEppelin's isApprovedOrOwner function - so that tokens marked for sale can 
    ///  be transferred by Market contract only.
    function isApprovedOrOwner(
        address _spender,
        uint256 _tokenId
    )
        internal
        view
        returns (bool)
    {   
        bool onSale = market.isOnSale(uint16(_tokenId));

        address owner = ownerOf(_tokenId);
        bool spenderIsApprovedOrOwner =
            _spender == owner ||
            getApproved(_tokenId) == _spender ||
            isApprovedForAll(owner, _spender);

        return (
            (onSale && _spender == address(market)) ||
            (!(onSale) && spenderIsApprovedOrOwner)
        );
    }

    /// @dev mints a new block.
    ///  overrides _mint function to add pause/unpause functionality, onlyMarket access,
    ///  restricts totalSupply of blocks to 10000 (as there is only a 100x100 blocks field).
    function _mintCrowdsaleBlock(address _to, uint16 _blockId) external onlyMarket whenNotPaused {
        if (totalSupply() <= 9999) {
        _mint(_to, uint256(_blockId));
        }
    }

    /// @dev overrides approve function to add pause/unpause functionality
    function approve(address _to, uint256 _tokenId) public whenNotPaused {
        super.approve(_to, _tokenId);
    }
 
    /// @dev overrides setApprovalForAll function to add pause/unpause functionality
    function setApprovalForAll(address _to, bool _approved) public whenNotPaused {
        super.setApprovalForAll(_to, _approved);
    }    

    /// @dev overrides transferFrom function to add pause/unpause functionality
    ///  affects safeTransferFrom functions as well
    function transferFrom(
        address _from,
        address _to,
        uint256 _tokenId
    )
        public
        whenNotPaused
    {
        super.transferFrom(_from, _to, _tokenId);
    }
}

// File: contracts/Accounting.sol

// import "../installed_contracts/math.sol";



// @title Accounting: Part of MEH contract responsible for eth accounting.
contract Accounting is MEHAccessControl {
    using SafeMath for uint256;

    // Balances of users, admin, charity
    mapping(address => uint256) public balances;

    // Emitted when a user deposits or withdraws funds from the contract
    event LogContractBalance(address payerOrPayee, int balanceChange);

// ** PAYMENT PROCESSING ** //
    
    /// @dev Withdraws users available balance.
    function withdraw() external whenNotPaused {
        address payee = msg.sender;
        uint256 payment = balances[payee];

        require(payment != 0);
        assert(address(this).balance >= payment);

        balances[payee] = 0;

        // reentrancy safe
        payee.transfer(payment);
        emit LogContractBalance(payee, int256(-payment));
    }

    /// @dev Lets external authorized contract (operators) to transfer balances within MEH contract.
    ///  MEH contract doesn't transfer funds on its own. Instead Market and Rentals contracts
    ///  are granted operator access.
    function operatorTransferFunds(
        address _payer, 
        address _recipient, 
        uint _amount) 
    external 
    onlyBalanceOperators
    whenNotPaused
    {
        require(balances[_payer] >= _amount);
        _deductFrom(_payer, _amount);
        _depositTo(_recipient, _amount);
    }

    /// @dev Deposits eth to msg.sender balance.
    function depositFunds() internal whenNotPaused {
        _depositTo(msg.sender, msg.value);
        emit LogContractBalance(msg.sender, int256(msg.value));
    }

    /// @dev Increases recipients internal balance.
    function _depositTo(address _recipient, uint _amount) internal {
        balances[_recipient] = balances[_recipient].add(_amount);
    }

    /// @dev Increases payers internal balance.
    function _deductFrom(address _payer, uint _amount) internal {
        balances[_payer] = balances[_payer].sub(_amount);
    }

// ** ADMIN ** //

    /// @notice Allows admin to withdraw contract balance in emergency. And distribute manualy
    ///  aftrewards.
    /// @dev As the contract is not designed to keep users funds (users can withdraw
    ///  at anytime) it should be relatively easy to manualy transfer unclaimed funds to 
    ///  their owners. This is an alternatinve to selfdestruct allowing blocks ledger (ERC721 tokens)
    ///  to be immutable.
    function adminRescueFunds() external onlyOwner whenPaused {
        address payee = owner;
        uint256 payment = address(this).balance;
        payee.transfer(payment);
    }

    /// @dev Checks if a msg.sender has enough balance to pay the price _needed.
    function canPay(uint _needed) internal view returns (bool) {
        return (msg.value.add(balances[msg.sender]) >= _needed);
    }
}

// File: contracts/MEH.sol

/*
MillionEther smart contract - decentralized advertising platform.

This program is free software: you can redistribute it and/or modifromY
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

/*
* A 1000x1000 pixel field is displayed at TheMillionEtherHomepage.com. 
* This smart contract lets anyone buy 10x10 pixel blocks and place ads there.
* It also allows to sell blocks and rent them out to other advertisers. 
*
* 10x10 pixels blocks are addressed by xy coordinates. So 1000x1000 pixel field is 100 by 100 blocks.
* Making up 10 000 blocks in total. Each block is an ERC721 (non fungible token) token. 
*
* At the initial sale the price for each block is $1 (price is feeded by an oracle). After
* every 1000 blocks sold (every 10%) the price doubles. Owners can sell and rent out blocks at any
* price they want. Owners and renters can place and replace ads to their blocks as many times they 
* want.
*
* All heavy logic is delegated to external upgradable contracts. There are 4 main modules (contracts):
*     - MEH: Million Ether Homepage (MEH) contract. Provides user interface and accounting 
*         functionality. It is immutable and it keeps Non fungible ERC721 tokens (10x10 pixel blocks) 
*         ledger and eth balances. 
*     - Market: Plugable. Provides methods for buy-sell functionality, keeps buy-sell ledger, 
*         querries oracle for a ETH-USD price, 
*     - Rentals: Plugable. Provides methods for rentout-rent functionality, keeps rentout-rent ledger.
*     - Ads: Plugable. Provides methods for image placement functionality.
* 
*/

/// @title MEH: Million Ether Homepage. Buy, sell, rent out pixels and place ads.
/// @author Peter Porobov (https://keybase.io/peterporobov)
/// @notice The main contract, accounting and user interface. Immutable.
contract MEH is MehERC721, Accounting {

    /// @notice emited when an area blocks is bought
    event LogBuys(
        uint ID,
        uint8 fromX,
        uint8 fromY,
        uint8 toX,
        uint8 toY,
        address newLandlord
    );

    /// @notice emited when an area blocks is marked for sale
    event LogSells(
        uint ID,
        uint8 fromX,
        uint8 fromY,
        uint8 toX,
        uint8 toY,
        uint sellPrice
    );

    /// @notice emited when an area blocks is marked for rent
    event LogRentsOut(
        uint ID,
        uint8 fromX,
        uint8 fromY,
        uint8 toX,
        uint8 toY,
        uint rentPricePerPeriodWei
    );

    /// @notice emited when an area blocks is rented
    event LogRents(
        uint ID,
        uint8 fromX,
        uint8 fromY,
        uint8 toX,
        uint8 toY,
        uint numberOfPeriods,
        uint rentedFrom
    );

    /// @notice emited when an ad is placed to an area
    event LogAds(uint ID, 
        uint8 fromX,
        uint8 fromY,
        uint8 toX,
        uint8 toY,
        string imageSourceUrl,
        string adUrl,
        string adText,
        address indexed advertiser);

// ** BUY AND SELL BLOCKS ** //
    
    /// @notice lets a message sender to buy blocks within area
    /// @dev if using a contract to buy an area make sure to implement ERC721 functionality 
    ///  as tokens are transfered using "transferFrom" function and not "safeTransferFrom"
    ///  in order to avoid external calls.
    function buyArea(uint8 fromX, uint8 fromY, uint8 toX, uint8 toY) 
        external
        whenNotPaused
        payable
    {   
        // check input parameters and eth deposited
        require(isLegalCoordinates(fromX, fromY, toX, toY));
        require(canPay(areaPrice(fromX, fromY, toX, toY)));
        depositFunds();

        // try to buy blocks through market contract
        // will get an id of buy-sell operation if succeeds (if all blocks available)
        uint id = market.buyBlocks(msg.sender, blocksList(fromX, fromY, toX, toY));
        emit LogBuys(id, fromX, fromY, toX, toY, msg.sender);
    }

    /// @notice lets a message sender to mark blocks for sale at price set for each block in wei
    /// @dev (priceForEachBlockCents = 0 - not for sale)
    function sellArea(uint8 fromX, uint8 fromY, uint8 toX, uint8 toY, uint priceForEachBlockWei)
        external 
        whenNotPaused
    {   
        // check input parameters
        require(isLegalCoordinates(fromX, fromY, toX, toY));

        // try to mark blocks for sale through market contract
        // will get an id of buy-sell operation if succeeds (if owns all blocks)
        uint id = market.sellBlocks(
            msg.sender, 
            priceForEachBlockWei, 
            blocksList(fromX, fromY, toX, toY)
        );
        emit LogSells(id, fromX, fromY, toX, toY, priceForEachBlockWei);
    }

    /// @notice get area price in wei
    function areaPrice(uint8 fromX, uint8 fromY, uint8 toX, uint8 toY) 
        public 
        view 
        returns (uint) 
    {   
        // check input
        require(isLegalCoordinates(fromX, fromY, toX, toY));

        // querry areaPrice in wei at market contract
        return market.areaPrice(blocksList(fromX, fromY, toX, toY));
    }

// ** RENT OUT AND RENT BLOCKS ** //
        
    /// @notice Rent out an area of blocks at coordinates [fromX, fromY, toX, toY] at a price for 
    ///  each block in wei
    /// @dev if rentPricePerPeriodWei = 0 then makes area not available for rent
    function rentOutArea(uint8 fromX, uint8 fromY, uint8 toX, uint8 toY, uint rentPricePerPeriodWei)
        external
        whenNotPaused
    {   
        // check input
        require(isLegalCoordinates(fromX, fromY, toX, toY));

        // try to mark blocks as rented out through rentals contract
        // will get an id of rent-rentout operation if succeeds (if message sender owns blocks)
        uint id = rentals.rentOutBlocks(
            msg.sender, 
            rentPricePerPeriodWei, 
            blocksList(fromX, fromY, toX, toY)
        );
        emit LogRentsOut(id, fromX, fromY, toX, toY, rentPricePerPeriodWei);
    }
    
    /// @notice Rent an area of blocks at coordinates [fromX, fromY, toX, toY] for a number of 
    ///  periods specified
    ///  (period length is specified in rentals contract)
    function rentArea(uint8 fromX, uint8 fromY, uint8 toX, uint8 toY, uint numberOfPeriods)
        external
        payable
        whenNotPaused
    {   
        // check input parameters and eth deposited
        // checks number of periods > 0 in rentals contract
        require(isLegalCoordinates(fromX, fromY, toX, toY));
        require(canPay(areaRentPrice(fromX, fromY, toX, toY, numberOfPeriods)));
        depositFunds();

        // try to rent blocks through rentals contract
        // will get an id of rent-rentout operation if succeeds (if all blocks available for rent)
        uint id = rentals.rentBlocks(
            msg.sender, 
            numberOfPeriods, 
            blocksList(fromX, fromY, toX, toY)
        );
        emit LogRents(id, fromX, fromY, toX, toY, numberOfPeriods, 0);
    }

    /// @notice get area rent price in wei for number of periods specified 
    ///  (period length is specified in rentals contract) 
    function areaRentPrice(uint8 fromX, uint8 fromY, uint8 toX, uint8 toY, uint numberOfPeriods)
        public 
        view 
        returns (uint) 
    {   
        // check input 
        require(isLegalCoordinates(fromX, fromY, toX, toY));

        // querry areaPrice in wei at rentals contract
        return rentals.blocksRentPrice (numberOfPeriods, blocksList(fromX, fromY, toX, toY));
    }

// ** PLACE ADS ** //
    
    /// @notice places ads (image, caption and link to a website) into desired coordinates
    /// @dev nothing is stored in any of the contracts except an image id. All other data is 
    ///  only emitted in event. Basicaly this function just verifies if an event is allowed 
    ///  to be emitted.
    function placeAds( 
        uint8 fromX, 
        uint8 fromY, 
        uint8 toX, 
        uint8 toY, 
        string imageSource, 
        string link, 
        string text
    ) 
        external
        whenNotPaused
    {   
        // check input
        require(isLegalCoordinates(fromX, fromY, toX, toY));

        // try to place ads through ads contract
        // will get an image id if succeeds (if advertiser owns or rents all blocks within area)
        uint AdsId = ads.advertiseOnBlocks(
            msg.sender, 
            blocksList(fromX, fromY, toX, toY), 
            imageSource, 
            link, 
            text
        );
        emit LogAds(AdsId, fromX, fromY, toX, toY, imageSource, link, text, msg.sender);
    }

    /// @notice check if an advertiser is allowed to put ads within area (i.e. owns or rents all 
    ///  blocks)
    function canAdvertise(
        address advertiser,
        uint8 fromX, 
        uint8 fromY, 
        uint8 toX, 
        uint8 toY
    ) 
        external
        view
        returns (bool)
    {   
        // check user input
        require(isLegalCoordinates(fromX, fromY, toX, toY));

        // querry permission at ads contract
        return ads.canAdvertiseOnBlocks(advertiser, blocksList(fromX, fromY, toX, toY));
    }

// ** IMPORT BLOCKS ** //

    /// @notice import blocks from previous version Million Ether Homepage
    function adminImportOldMEBlock(uint8 x, uint8 y) external onlyOwner {
        (uint id, address newLandlord) = market.importOldMEBlock(x, y);
        emit LogBuys(id, x, y, x, y, newLandlord);
    }

// ** INFO GETTERS ** //
    
    /// @notice get an owner(address) of block at a specified coordinates
    function getBlockOwner(uint8 x, uint8 y) external view returns (address) {
        return ownerOf(blockID(x, y));
    }

// ** UTILS ** //
    
    /// @notice get ERC721 token id corresponding to xy coordinates
    function blockID(uint8 x, uint8 y) public pure returns (uint16) {
        return (uint16(y) - 1) * 100 + uint16(x);
    }

    /// @notice get a number of blocks within area
    function countBlocks(
        uint8 fromX, 
        uint8 fromY, 
        uint8 toX, 
        uint8 toY
    ) 
        internal 
        pure 
        returns (uint16)
    {
        return (toX - fromX + 1) * (toY - fromY + 1);
    }

    /// @notice get an array of all block ids (i.e. ERC721 token ids) within area
    function blocksList(
        uint8 fromX, 
        uint8 fromY, 
        uint8 toX, 
        uint8 toY
    ) 
        internal 
        pure 
        returns (uint16[] memory r) 
    {
        uint i = 0;
        r = new uint16[](countBlocks(fromX, fromY, toX, toY));
        for (uint8 ix=fromX; ix<=toX; ix++) {
            for (uint8 iy=fromY; iy<=toY; iy++) {
                r[i] = blockID(ix, iy);
                i++;
            }
        }
    }
    
    /// @notice insures that area coordinates are within 100x100 field and 
    ///  from-coordinates >= to-coordinates
    /// @dev function is used instead of modifier as modifier 
    ///  required too much stack for placeImage and rentBlocks
    function isLegalCoordinates(
        uint8 _fromX, 
        uint8 _fromY, 
        uint8 _toX, 
        uint8 _toY
    )    
        private 
        pure 
        returns (bool) 
    {
        return ((_fromX >= 1) && (_fromY >=1)  && (_toX <= 100) && (_toY <= 100) 
            && (_fromX <= _toX) && (_fromY <= _toY));
    }
}