Gas Optimizations

[code423n4]

1. Save gas by keeping track of nextEpochDate instead of reading from storage.

Details

In AuraLocker._checkpointEpoch(), there is a while loop to fill epoch gaps until the next epoch date. Each iter, it reads from storage 4 times (2 in the while condition, 2 when calculate nextEpochDate).
In fact, nextEpochDate can be kept track simply by adding rewardsDuration each epoch.

Proof of concept

https://github.com/code-423n4/2022-05-aura/blob/4989a2077546a5394e3650bf3c224669a0f7e690/contracts/AuraLocker.sol#L334-L335

Recommended Mitigation Steps

Use variable nextEpochDate to cache like this

uint256 nextEpochDate = uint256(epochs[epochs.length - 1].date);
while (nextEpockDate != currentEpoch) {
  uint256 nextEpochDate = nextEpockDate.add(rewardsDuration);
  epochs.push(Epoch({ supply: 0, date: uint32(nextEpochDate) }));
}