QA Report

[code423n4]

Low risk findings:

1. tokenURI pass in the wrong parameter for getPremium

It should accept a vaultId, but here the code pass in premiumId

getPremium(vault.premiumIndex),

This will make the NFT showing the wrong trait.
P.S. I suggested changing the input parameter of getPremium from vaultId to premiumIndex, as described below (gas optimization #2)

2. wrong revert message in createVault

This revert message:

require(dutchAuctionReserveStrike < strikeOptions[dutchAuctionStartingStrikeIndex], "Reserve strike too small");

should instead be reserve strike too high.

Gas optimizations

1. pack variables feeRate and protocolUnclaimedFees together to save gas on exercise.

Because these 2 variables are used together in exercise function, (reading feeRate and updating protocolUnclaimedFees), packing them into the same storage slot will save you one more SLOAD if feeRate > 0. on average this saves around 2000 gas. (49k -> 47K)

2. change getPremium input to premiumIndex.

As it currently is, getPremium function receive vaultId read the vault from storage again and than get the premiumIndex. This will cause an extra SLOAD to load the vault from storage.
This can be avoided if we just pass in premiumIndex, as all 2 functions that call this already have their vault instance in memory.
This makes average cost of buyOption drops from 75.7K -> 74.8.

3. pack currentStrike and dutchAuctionReserveStrike into a single storage slot.

Since packing variables is highly used in the vault struct, i suggest also pack dutchAuctionReserveStrike and currentStrike to uint128 to save 1 SLOAD and SSTORE from read and write.
The gas saving from running the benchmark is 74.8K -> 73.5K