pragma solidity ^0.4.24;

/**
 * @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;
  }

}

/**
 * @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 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;
  }

}



/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/179
 */
contract ERC20Basic
{
  function totalSupply() public view returns (uint256);
  function balanceOf(address who) public view returns (uint256);
  function transfer(address to, uint256 value) public;
  event Transfer(address indexed from, address indexed to, uint256 value);
}

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20 is ERC20Basic
{
  function allowance(address owner, address spender) public view returns (uint256);

  function transferFrom(address from, address to, uint256 value) public returns (bool);

  function approve(address spender, uint256 value) public returns (bool);

  event Approval(
    address indexed owner,
    address indexed spender,
    uint256 value
  );

}




/**
 * @title SafeERC20
 * @dev Wrappers around ERC20 operations that throw on failure.
 * To use this library you can add a `using SafeERC20 for ERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
  function safeTransfer(
    ERC20 _token,
    address _to,
    uint256 _value
  )
    internal
  {
    _token.transfer(_to, _value);
  }

  function safeTransferFrom(
    ERC20 _token,
    address _from,
    address _to,
    uint256 _value
  )
    internal
  {
    require(_token.transferFrom(_from, _to, _value));
  }

  function safeApprove(
    ERC20 _token,
    address _spender,
    uint256 _value
  )
    internal
  {
    require(_token.approve(_spender, _value));
  }
}


/**
 * @title TokenVesting
 * @dev A token holder contract that can release its token balance gradually like a
 * typical vesting scheme, with a cliff and vesting period. Optionally revocable by the
 * owner.
 */
contract TokenVesting is Ownable {
  using SafeMath for uint256;
  using SafeERC20 for ERC20;

  event Released(uint256 amount);
  event Revoked();

  // beneficiary of tokens after they are released
  address public beneficiary;
  address public admin;
  address public backup;
  address private __proposedNewBeneficiary = address(0);

  uint256 public cliff;
  uint256 public start;
  uint256 public duration;

  bool public revocable;

  mapping (address => uint256) public released;
  mapping (address => bool) public revoked;

  /**
   * @dev Creates a vesting contract that vests its balance of any ERC20 token to the
   * _beneficiary, gradually in a linear fashion until _start + _duration. By then all
   * of the balance will have vested.
   * @param _beneficiary address of the beneficiary to whom vested tokens are transferred
   * @param _cliff duration in seconds of the cliff in which tokens will begin to vest
   * @param _start the time (as Unix time) at which point vesting starts
   * @param _duration duration in seconds of the period in which the tokens will vest
   * @param _revocable whether the vesting is revocable or not
   */
  constructor(
    address _beneficiary,
    address _admin,
    address _backup,
    uint256 _start,
    uint256 _cliff,
    uint256 _duration,
    bool _revocable
  )
    public
  {
    require(_beneficiary != address(0));
    require(_admin != address(0));
    require(_beneficiary != _backup);
    require(_cliff <= _duration);

    beneficiary = _beneficiary;
    backup = _backup;
    admin = _admin;
    revocable = _revocable;
    duration = _duration;
    cliff = _start.add(_cliff);
    start = _start;
  }

  /**
   * @notice Transfers vested tokens to beneficiary.
   * @param _token ERC20 token which is being vested
   */
  function release(ERC20 _token) public {
    uint256 unreleased = releasableAmount(_token);

    require(unreleased > 0);

    released[_token] = unreleased.add(released[_token]);

    _token.safeTransfer(beneficiary, unreleased);

    emit Released(unreleased);
  }

  /**
   * @notice Allows the owner to revoke the vesting. Tokens already vested
   * remain in the contract, the rest are returned to the owner.
   * @param _token ERC20 token which is being vested
   */
  function revoke(ERC20 _token) public onlyOwner {
    require(revocable);
    require(!revoked[_token]);

    uint256 balance = _token.balanceOf(address(this));

    uint256 unreleased = releasableAmount(_token);
    uint256 refund = balance.sub(unreleased);

    revoked[_token] = true;

    _token.safeTransfer(owner, refund);

    emit Revoked();
  }

  /**
   * @dev Calculates the amount that has already vested but hasn't been released yet.
   * @param _token ERC20 token which is being vested
   */
  function releasableAmount(ERC20 _token) public view returns (uint256) {
    return vestedAmount(_token).sub(released[_token]);
  }

  /**
   * @dev Calculates the amount that has already vested.
   * @param _token ERC20 token which is being vested
   */
  function vestedAmount(ERC20 _token) public view returns (uint256) {
    uint256 currentBalance = _token.balanceOf(this);
    uint256 totalBalance = currentBalance.add(released[_token]);

    if (block.timestamp < cliff) {
      return 0;
    } else if (block.timestamp >= start.add(duration) || revoked[_token]) {
      return totalBalance;
    } else {
      return totalBalance.mul(block.timestamp.sub(start)).div(duration);
    }
  }
  
  
  /**
   * @dev Reset backup address for current beneficiary only by current beneficiary
   * @param _backup new backup address for current beneficiary
   */
  function resetBackup(address _backup) external {
    require(msg.sender == beneficiary);
    require(_backup != address(0));
    require(_backup != backup);
    require(_backup != beneficiary);
      
    backup = _backup;
  }
  
  
  /**
   * @dev Request an update to current beneficiary only by backup address
   * @param _newBeneficiary new beneficiary
   */
  function updateBeneficiary(address _newBeneficiary) external {
    require(msg.sender == backup);
    require(_newBeneficiary != address(0));
    require(_newBeneficiary != beneficiary);
    require(_newBeneficiary != backup);
      
    __proposedNewBeneficiary = _newBeneficiary;
  }
  
  
    /**
   * @dev Calculates the amount that has already vested.
   * @param _newBeneficiary new beneficiary
   */
  function approveBeneficiaryUpdate(address _newBeneficiary) external {
    require(msg.sender == admin);
    require(__proposedNewBeneficiary != address(0));
    require(__proposedNewBeneficiary == _newBeneficiary);
      
    beneficiary = __proposedNewBeneficiary;
    __proposedNewBeneficiary = address(0);
  }
}