Function decompose

saver/src/utils.rs:17–45

Given an element `F`, break it into chunks where each chunk is of `chunk_bit_size` bits. This is essentially an n-ary representation where n is `chunk_bit_size`. Returns big-endian representation.

(message: &F, chunk_bit_size: u8)

/// Given an element `F`, break it into chunks where each chunk is of `chunk_bit_size` bits. This is
/// essentially an n-ary representation where n is `chunk_bit_size`. Returns big-endian representation.
pub fn decompose<F: PrimeField>(message: &F, chunk_bit_size: u8) -> crate::Result<Vec<CHUNK_TYPE>> {
    let bytes = message.into_bigint().to_bytes_be();
    let mut decomposition = Vec::<CHUNK_TYPE>::new();
    match chunk_bit_size {
        4 => {
            for b in bytes {
                decomposition.push((b >> 4) as CHUNK_TYPE);
                decomposition.push((b & 15) as CHUNK_TYPE);
            }
        }
        8 => {
            for b in bytes {
                decomposition.push(b as CHUNK_TYPE);
            }
        }
        16 => {
            // Process 2 bytes at a time
            for bytes_2 in bytes.chunks(2) {
                let mut b = (bytes_2[0] as CHUNK_TYPE) << (8 as CHUNK_TYPE);
                if bytes_2.len() > 1 {
                    b += bytes_2[1] as CHUNK_TYPE;
                }
                decomposition.push(b);
            }
        }
        b => return Err(SaverError::UnexpectedBase(b)),
    }
    Ok(decomposition)
}

/// Recreate a field element back from output of `decompose`. Assumes big-endian representation in `decomposed`
pub fn compose<F: PrimeField>(decomposed: &[CHUNK_TYPE], chunk_bit_size: u8) -> crate::Result<F> {