Function diff_balances_iter 

pub fn diff_balances_iter<I1, I2>(base: I1, target: I2) -> BalancesDiff
where
    I1: ExactSizeIterator<Item = u64>,
    I2: ExactSizeIterator<Item = u64>,

Computes a compact balance delta between two balance iterators.

This is the generic counterpart to diff_balances. It is intended for consensus clients whose balance storage is not represented as a contiguous &[u64].

The iterators must implement ExactSizeIterator, allowing the function to determine the size of the common portion and distinguish existing balances from balances appended to the target.

The iterators are consumed during encoding and do not need to implement Clone.

The common portion is first collected into a compact list of changed balances so that the most frequently occurring balance difference can be selected as the encoding mode. Remaining items in the target iterator are stored as appended balances.

Complexity

O(n) time and O(k + a) additional space, where:

• n is the number of balances in the common portion;
• k is the number of changed balances; and
• a is the number of balances appended to the target.