// SPDX-License-Identifier: MIT
pragma solidity ^0.7.0;

abstract contract LinkTokenReceiver {

  bytes4 constant private ORACLE_REQUEST_SELECTOR = 0x40429946;
  uint256 constant private SELECTOR_LENGTH = 4;
  uint256 constant private EXPECTED_REQUEST_WORDS = 2;
  uint256 constant private MINIMUM_REQUEST_LENGTH = SELECTOR_LENGTH + (32 * EXPECTED_REQUEST_WORDS);
  /**
   * @notice Called when LINK is sent to the contract via `transferAndCall`
   * @dev The data payload's first 2 words will be overwritten by the `sender` and `amount`
   * values to ensure correctness. Calls oracleRequest.
   * @param sender Address of the sender
   * @param amount Amount of LINK sent (specified in wei)
   * @param data Payload of the transaction
   */
  function onTokenTransfer(
    address sender,
    uint256 amount,
    bytes memory data
  )
    public
    onlyLINK
    validRequestLength(data)
    permittedFunctionsForLINK(data)
  {
    assembly {
      // solhint-disable-next-line avoid-low-level-calls
      mstore(add(data, 36), sender) // ensure correct sender is passed
      // solhint-disable-next-line avoid-low-level-calls
      mstore(add(data, 68), amount)    // ensure correct amount is passed
    }
    // solhint-disable-next-line avoid-low-level-calls
    (bool success, ) = address(this).delegatecall(data); // calls oracleRequest
    require(success, "Unable to create request");
  }

  function getChainlinkToken() public view virtual returns (address);

  /**
   * @dev Reverts if not sent from the LINK token
   */
  modifier onlyLINK() {
    require(msg.sender == getChainlinkToken(), "Must use LINK token");
    _;
  }

  /**
   * @dev Reverts if the given data does not begin with the `oracleRequest` function selector
   * @param data The data payload of the request
   */
  modifier permittedFunctionsForLINK(bytes memory data) {
    bytes4 funcSelector;
    assembly {
      // solhint-disable-next-line avoid-low-level-calls
      funcSelector := mload(add(data, 32))
    }
    require(funcSelector == ORACLE_REQUEST_SELECTOR, "Must use whitelisted functions");
    _;
  }

  /**
   * @dev Reverts if the given payload is less than needed to create a request
   * @param data The request payload
   */
  modifier validRequestLength(bytes memory data) {
    require(data.length >= MINIMUM_REQUEST_LENGTH, "Invalid request length");
    _;
  }
}