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Vec could be mapping

Description​

When using a Vec to store key-value pairs, it is possible to use a Mapping instead. This will reduce the gas usage of the contract, as the Vec will have to iterate over all elements to find the desired key-value pair.

Exploit Scenario​

Consider the following ink! contract, where you have a balances vec of tuples of (AccountId, Balance). If you want to find the Balance from a specific AccountId, you will have to iterate over all elements of the balances vec to find the desired AccountId.

    pub struct Contract {
        balances: Vec<(AccountId, Balance)>,
    }

    pub enum Error {
        NotFound,
    }

    impl Contract {
        /* --- snip --- */
        #[ink(message)]
        pub fn get_balance(&mut self, acc: AccountId) -> Result<Balance, Error> {
            self.balances
                .iter()
                .find(|(a, _)| *a == acc)
                .map(|(_, b)| *b)
                .ok_or(Error::NotFound)
        }
        /* --- snip --- */
    }

Using .find(...) over an iterator of tuples consumes more gas than using a Mapping to store the key-value pairs.

Remediation​

    pub struct VecCouldBeMapping {
        balances: Mapping<AccountId, Balance>,
    }

    pub enum Error {
        NotFound,
    }

    impl Contract {
        /* --- snip --- */
        #[ink(message)]
        pub fn get_balance(&mut self, acc: AccountId) -> Result<Balance, Error> {
            self.balances.get(&acc).ok_or(Error::NotFound)
        }
        /* --- snip --- */
    }