openzeppelin-contracts/contracts/crowdsale/emission/AllowanceCrowdsale.sol

pragma solidity ^0.4.24;

import "../Crowdsale.sol";
import "../../token/ERC20/IERC20.sol";
import "../../token/ERC20/SafeERC20.sol";
import "../../math/SafeMath.sol";
import "../../math/Math.sol";

/**
 * @title AllowanceCrowdsale
 * @dev Extension of Crowdsale where tokens are held by a wallet, which approves an allowance to the crowdsale.
 */
contract AllowanceCrowdsale is Crowdsale {
  using SafeMath for uint256;
  using SafeERC20 for IERC20;

  address private _tokenWallet;

  /**
   * @dev Constructor, takes token wallet address.
   * @param tokenWallet Address holding the tokens, which has approved allowance to the crowdsale
   */
  constructor(address tokenWallet) internal {
    require(tokenWallet != address(0));
    _tokenWallet = tokenWallet;
  }

  /**
   * @return the address of the wallet that will hold the tokens.
   */
  function tokenWallet() public view returns(address) {
    return _tokenWallet;
  }

  /**
   * @dev Checks the amount of tokens left in the allowance.
   * @return Amount of tokens left in the allowance
   */
  function remainingTokens() public view returns (uint256) {
    return Math.min(
      token().balanceOf(_tokenWallet),
      token().allowance(_tokenWallet, this)
    );
  }

  /**
   * @dev Overrides parent behavior by transferring tokens from wallet.
   * @param beneficiary Token purchaser
   * @param tokenAmount Amount of tokens purchased
   */
  function _deliverTokens(
    address beneficiary,
    uint256 tokenAmount
  )
    internal
  {
    token().safeTransferFrom(_tokenWallet, beneficiary, tokenAmount);
  }
}