Gas: shadow pools are only required for burn types

[code423n4]

Handle

cmichel

Vulnerability details

The PoolCommiter.shadowPools track commitments for all four Long/Short Mint/Burn types and uses these to reconstruct the initial total supply to correctly compute the token amounts for the sequence of commitments (as short/long tokens already get burned in the commitment phase and reduced the total supply).
However, the two burn types LongBurn and ShortBurn are all that's needed for the reconstruction which can be seen from the fact that shadowPools[.] is only accessed with them.

Recommendation

Only store shadowPools for LongBurn and ShortBurn types, and remove the shadowPools[_commitType] = shadowPools[_commitType] - _commit.amount; statement in _uncommit which is unnecessary for the mints as it just pays out what's already tracked in the commitments (_commit).

[GalloDaSballo]

Agree with the finding, avoid interacting with storage when unnecessary for sizeable gas savings