Function compact 

pub fn compact<T, F>(arr: &mut [T], is_dummy: F) -> usize
where
    F: Fn(&T) -> bool,
    T: Cmov + Copy,

Stably compacts an array arr of length n using oblivious compaction. The payload array payload is the prefix sum of valid elements. Uses https://eprint.iacr.org/2022/1333.pdf

Requires

Behavior

• returns ret - the number of non-dummy elements (new real length of the array).
• first ret elements of arr are the non-dummy elements in the same order as they were in the original array.
• the rest of the elements in arr are the dummy elements in no particular order.

Oblivious

• Fully data-independent memory access pattern.
• Leaks: arr.len() - the full length of the original array

Returns the number of non-dummy elements in the array after compaction.