# Use a flag to increase or decrease the index of the pool instead of a heuristic Medium · Three Sigma · NFT perpetuals · 29th January, 2024 Finding 3S-NFTPerp-L03 of the Nftperp Matching Engine security review. - Report: /reports/nftperp-ii - Source: https://cdn.sanity.io/files/qoqld077/production/87f617e82d5468500e950a669f30607376b37c32.pdf --- ### Description [AmmRouter:_shitPool()](https://github.com/nftperp/NFTPerp-V2-Contracts/blob/8ea05fc0ecf299ca8374b571486846652c8725f8/src/AMMRouter.sol#L757) increases the index if the price is within 100 of the upper bound of the current pool price. This is error prone as a fill could for example increase the price of a pool, but not be close enough to the upper bound, making the index decrease, entering in a wrong state. ### Recommendation Use the direction flag to increase or decrease the index, as in [AmmRouter:_getQuoteValue()](https://github.com/nftperp/NFTPerp-V2-Contracts/blob/8ea05fc0ecf299ca8374b571486846652c8725f8/src/AMMRouter.sol#L809). ### Status Addressed in [#68c722c](https://github.com/nftperp/NFTPerp-V2-Contracts/commit/68c722ccb3e71532ac8642e97be45c072dba4e05) --- Related findings: - [Adapters could store the index instead of true in the `VaultV2` to remove the O(n) search](https://0xsimao.com/findings/morpho-vault-v2-adapters-store-index-search): Morpho Vault V2 - [safeApprove(...) reverts if approval is different than 0, use safeIncreaseAllowance(...) instead](https://0xsimao.com/findings/fuji-finance-approve-reverts-approval-allowance): Fuji Finance