Summary

Same pieces of data are calculated multiple times throughout calculate*Reward functions. Passing this data pre-computed would optimize gas cost and provide better readability

Vulnerability Details

After validating startTime and endTime, the variable payoutStart_ is repeatedly used to compute a time difference: either startTime_ - payoutStart_ (which was already saved as a variable at line 48), or endTime_ - payoutStart_ (which can be saved in a variable named timePassedTotal_). These could be pre-calculated in the getPeriodReward function and passed to the calculate*Reward functions. The following changes are needed:

function _calculatePartPeriodReward:

• line 102: Change signature to replace first two parameters with the timePassed_ parameter, which is equal to the difference between them (either startTime or endTime minus payoutStart). This would also solve a naming inconsistency: function signature currently contains "startTime" while line 75 provides "endTime" as argument, which is confusing. Function would change as following:
  before: function calculatePartPeriodReward(uint128 payoutStart, uint128 startTime_, uint128 interval_, uint256 initialAmount_, uint256 decreaseAmount_, bool toEnd_ ) private pure returns (uint256)
  after: function calculatePartPeriodReward(uint128 timePassed, uint128 interval_, uint256 initialAmount_, uint256 decreaseAmount_, bool toEnd_ ) private pure returns (uint256)

• line 110: before: uint256 intervalsPassed_ = (startTime_ - payoutStart_) / interval_;
  after: uint256 intervalsPassed_ = timePassed_ / interval_;

• line 119: before: intervalPart_ = interval_ * (intervalsPassed_ + 1) + payoutStart_ - startTime_;
  after: intervalPart_ = interval_ * (intervalsPassed_ + 1) - timePassed_;

• line 121: before: intervalPart_ = startTime_ - interval_ * intervalsPassed_ - payoutStart_;
  after: intervalPart_ = timePassed_ - interval_ * intervalsPassed_;

function _calculateFullPeriodReward:

• line 131: Change signature to replace first three parameters with timePassedBefore_ (defined on line 48) and timePassedTotal_ (endTime_ - payoutStart_):
  before: function calculateFullPeriodReward(uint128 payoutStart, uint128 startTime_, uint128 endTime_, uint128 interval_, uint256 initialAmount_, uint256 decreaseAmount_ ) private pure returns (uint256)
  after: function calculateFullPeriodReward(uint128 timePassedBefore, uint128 timePassedTotal_, uint128 interval_, uint256 initialAmount_, uint256 decreaseAmount_ ) private pure returns (uint256)

• line 140: not needed anymore

• line 153: before: uint256 ip_ = ((endTime_ - payoutStart_ - intervalsPassedBefore_ * interval_) / interval_);
  after: uint256 ip_ = ((timePassedTotal_ - intervalsPassedBefore_ * interval_) / interval_);
  If taking into account a defect that I submitted in parallel, this can be further reduced to:
  after 2: uint256 ip_ = timePassedTotal_ / interval_ - intervalsPassedBefore_;

Impact

Gas used inefficiently, cluttered code

Tools Used

Manual inspection

Recommendations

Apply changes mentioned above for an optimized gas consumption and better readability