Summary

In LinearDistributionIntervalDecrease::_calculateFullPeriodReward() total interval passed in a period is calculated. However the calculation is bit complex which expenses more gas. We can oversimplify it.

Vulnerability Details

We can structure the time passed thing like this:

Here interval passed over a period was calculated here:

If we look more into intervalPassedBefore we can see how it was calculated, an private function, _divideCeil(uint256 a_, uint256 b_), was called, where a = timePassedBefore_ and b = interval_.
timePassedBefore is the timestamp from startTime to payoutStart, as we can see in the starting of the _calculateFullPeriodReward() :

As uint256 intervalsPassedBefore_ = _divideCeil(timePassedBefore_, interval_); we can say timePassedBefore_ = intervalsPassedBefore_ * interval_.
So, while calculating the total interval period i.e ip_ we can use timePassedBefore_ instead of intervalsPassedBefore_ * interval_.
So if we simplify the calculation of ip_ it will look like :

As we can see we can get the interval passed over a period just by substracting the startTime from endTime.

Impact

Here we are unnecessarily calling a function and made the process complicated.

Tools Used

Manual analysis.

Recommendations

Calculate the ip_ like this: (endTime_ - startTime_) / interval_