Removes window_num_bits < 8 in decompose_word

[Janmajayamall]

No description provided.

[therealyingtong]

If we're returning Vec<u8>, I think we would have to at least check that window_num_bits <= (1 << 8). However, in future I would like this function to return

Suggested change

	) -> Vec<u8> {
	) -> Vec<[bool; K]> {

[Janmajayamall]

I am not sure how to do this since array lengths don't support generics.

Shall we add the following assert statement for now
assert(window_num_bits <= (1 << 8))
Or should we try making it generic?

[therealyingtong]

We could add a const generic to

decompose_word<F: PrimeFieldBits, const K: usize>(

[Janmajayamall]

So something like the following? -

pub fn decompose_word<F: PrimeFieldBits, const K: usize>(
    word: &F,
    word_num_bits: usize,
) -> Vec<[bool; K]> {
    // Pad bits to multiple of window_num_bits
    let padding = (K - (word_num_bits % K)) % K;
    let bits: Vec<bool> = word
        .to_le_bits()
        .into_iter()
        .take(word_num_bits)
        .chain(std::iter::repeat(false).take(padding))
        .collect();
    assert_eq!(bits.len(), word_num_bits + padding);

    bits.chunks_exact(K)
        .map(|x| {
            let chunks = [false; K];
            chunks.copy_from_slice(x);
            chunks
        })
        .collect()
}