Function unpack_arithmetic

src/pack.rs:436–482

Opposite of `pack_arithmetic`. `out` will be overwritten with the unpacked bytes.

(
    input: &mut &[u8],
    unpacked_len: usize,
    out: &mut Vec<u8>,
)

/// Opposite of `pack_arithmetic`. `out` will be overwritten with the unpacked bytes.
fn unpack_arithmetic<const FACTOR: usize>(
    input: &mut &[u8],
    unpacked_len: usize,
    out: &mut Vec<u8>,
) -> Result<()> {
    let divisor = factor_to_divisor::<FACTOR>();

    // TODO STRICT: check that packed.all(|&b| b < FACTOR.powi(divisor)).
    let floor = unpacked_len / divisor;
    let ceil = crate::nightly::div_ceil_usize(unpacked_len, divisor);
    let packed = consume_bytes(input, ceil)?;

    debug_assert!(out.is_empty());
    out.reserve(unpacked_len);
    let unpacked = &mut out.spare_capacity_mut()[..unpacked_len];

    for i in 0..floor {
        unsafe {
            let mut packed = *packed.get_unchecked(i);
            if FACTOR == 2 && BMI2 {
                #[cfg(not(all(target_arch = "x86_64", target_feature = "bmi2")))]
                unreachable!();
                #[cfg(all(target_arch = "x86_64", target_feature = "bmi2"))]
                {
                    // Could use on any pow2 FACTOR, but only 2 is faster (target-cpu=native).
                    let chunk = std::arch::x86_64::_pdep_u64(packed as u64, 0x0101010101010101);
                    *(unpacked.as_mut_ptr() as *mut [u8; 8]).add(i) = chunk.to_le_bytes();
                }
            } else {
                for byte in unpacked.get_unchecked_mut(i * divisor..i * divisor + divisor) {
                    byte.write(packed % FACTOR as u8);
                    packed /= FACTOR as u8;
                }
            }
        }
    }
    if floor < ceil {
        let mut packed = packed[floor];
        for byte in unpacked[floor * divisor..].iter_mut() {
            byte.write(packed % FACTOR as u8);
            packed /= FACTOR as u8;
        }
    }
    // Safety: `unpacked_len` elements were initialized by the loops above.
    unsafe { out.set_len(unpacked_len) };
    Ok(())
}

#[cfg(test)]
mod tests {