contract Keep3rV2Oracle {

    constructor(address _pair) {
        _factory = msg.sender;
        pair = _pair;
        (,,uint32 timestamp) = IUniswapV2Pair(_pair).getReserves();
        uint112 _price0CumulativeLast = uint112(IUniswapV2Pair(_pair).price0CumulativeLast() * e10 / Q112);
        uint112 _price1CumulativeLast = uint112(IUniswapV2Pair(_pair).price1CumulativeLast() * e10 / Q112);
        observations[length++] = Observation(timestamp, _price0CumulativeLast, _price1CumulativeLast);
    }

    struct Observation {
        uint32 timestamp;
        uint112 price0Cumulative;
        uint112 price1Cumulative;
    }

    modifier factory() {
        require(msg.sender == _factory, "!F");
        _;
    }

    Observation[65535] public observations;
    uint16 public length;

    address immutable _factory;
    address immutable public pair;
    // this is redundant with granularity and windowSize, but stored for gas savings & informational purposes.
    uint constant periodSize = 1800;
    uint Q112 = 2**112;
    uint e10 = 10**18;

    // Pre-cache slots for cheaper oracle writes
    function cache(uint size) external {
        uint _length = length+size;
        for (uint i = length; i < _length; i++) observations[i].timestamp = 1;
    }

    // update the current feed for free
    function update() external factory returns (bool) {
        return _update();
    }

    function updateable() external view returns (bool) {
        Observation memory _point = observations[length-1];
        (,, uint timestamp) = IUniswapV2Pair(pair).getReserves();
        uint timeElapsed = timestamp - _point.timestamp;
        return timeElapsed > periodSize;
    }

    function _update() internal returns (bool) {
        Observation memory _point = observations[length-1];
        (,, uint32 timestamp) = IUniswapV2Pair(pair).getReserves();
        uint32 timeElapsed = timestamp - _point.timestamp;
        if (timeElapsed > periodSize) {
            uint112 _price0CumulativeLast = uint112(IUniswapV2Pair(pair).price0CumulativeLast() * e10 / Q112);
            uint112 _price1CumulativeLast = uint112(IUniswapV2Pair(pair).price1CumulativeLast() * e10 / Q112);
            observations[length++] = Observation(timestamp, _price0CumulativeLast, _price1CumulativeLast);
            return true;
        }
        return false;
    }
}
// sliding oracle that uses observations collected to provide moving price averages in the past
function cache(uint size) external {
    uint _length = length+size;
    for (uint i = length; i < _length; i++) observations[i].timestamp = 1;
}

// Pre-cache slots for cheaper oracle writes