Function decode_helper 

pub(crate) fn decode_helper(
    input: &[u8],
    estimate: &GeneralPurposeEstimate,
    output: &mut [u8],
    decode_table: &[u8; 256],
    decode_allow_trailing_bits: bool,
    padding: Symbol,
    padding_mode: DecodePaddingMode,
    simd_prefix: impl FnOnce(&[u8], usize, &mut [u8]) -> (usize, usize),
) -> Result<DecodeMetadata, DecodeSliceError>

Helper to avoid duplicating num_chunks calculation, which is costly on short inputs. Returns the decode metadata, or an error.