Trailing zeros for poseidon channel cannot return values between 3 and 7

[ajgara]

stwo/crates/prover/src/core/channel/poseidon252.rs

Line 59 in a51f630

fn trailing_zeros(&self) -> u32 {

This is because Felts have 252 bits and it's mixing big endian with little endian. Is this the expected behaviour? Using little endian or reversing the bytes are alternative options.

For example, when running this test:

#[test]
    pub fn test_trailing_zeros() {
        let mut counter = vec![0; 128];
        let mut channel = Poseidon252Channel::default();

        for i in 0..100000 {
            channel.mix_nonce(i);
            let index: usize = channel.trailing_zeros().try_into().unwrap();
            counter[index] = counter[index] + 1;
        }

        println!("{:?}", counter);
    }

It outputs the following distribution of trailing_zeros():