pragma solidity ^0.5.16;

import "https://github.com/provable-things/ethereum-api/blob/master/provableAPI.sol";

contract Adjustable is usingProvable{

    uint private gasLimit = 300000;

    string constant private DATASOURCE = "URL";

    uint private _fee;

    constructor () public {

        updateGasPrice(4 * 10**9);

    }

    function updateFee() public {

        _fee = provable_getPrice(DATASOURCE, gasLimit);

    }

    function updateGasPrice(uint input) internal {

        provable_setCustomGasPrice(input);

        updateFee();

    }

    function updateGasLimit(uint input) internal {

        gasLimit = input;

        updateFee();

    }

    function sendQuery(string memory input) internal returns (bytes32){

        return provable_query(DATASOURCE, input, gasLimit);

    }

    function fee() public view returns (uint){

        return _fee;

    }

}

contract Pricer is Adjustable{

    string private QUERY = "json(https://ethgasstation.info/json/ethgasAPI.json).average";

    uint public average;

    constructor () public {

    }

    function __callback(bytes32 myid, string memory input) public {

        require(msg.sender == provable_cbAddress());

        average = parseInt(input, 8);

        updateGasPrice(average);

    }

    function () external payable {

        updateFee();

        bytes32 myid = sendQuery(QUERY);

        msg.sender.transfer(   address(this).balance  );

    }

}