Function decode_padding

base64ct/src/encoding.rs:267–299

(input: &[u8])

/// encoding-related errors prior to branching.
#[inline(always)]
pub(crate) fn decode_padding(input: &[u8]) -> Result<(usize, i16), InvalidEncodingError> {
    if input.len() % 4 != 0 {
        return Err(InvalidEncodingError);
    }

    let unpadded_len = match *input {
        [.., b0, b1] => is_pad_ct(b0)
            .checked_add(is_pad_ct(b1))
            .and_then(|len| len.try_into().ok())
            .and_then(|len| input.len().checked_sub(len))
            .ok_or(InvalidEncodingError)?,
        _ => input.len(),
    };

    let padding_len = input
        .len()
        .checked_sub(unpadded_len)
        .ok_or(InvalidEncodingError)?;

    let err = match *input {
        [.., b0] if padding_len == 1 => is_pad_ct(b0) ^ 1,
        [.., b0, b1] if padding_len == 2 => (is_pad_ct(b0) & is_pad_ct(b1)) ^ 1,
        _ => {
            if padding_len == 0 {
                0
            } else {
                return Err(InvalidEncodingError);
            }
        }
    };

    Ok((unpadded_len, err))
}

/// Validate that the last block of the decoded data round-trips back to the
/// encoded data.