In-place base64 decoder

base64.rs

fn main() {
  const ENCODED : &'static [u8] = b"aGVsbG8gd29ybGQ=";
//const ENCODED : &'static [u8] = b"aGVlbGxvIHdvcmxk";
  let mut encoded = [0u8; 16];
  assert_eq!(ENCODED.len(), encoded.len());
  for i in 0..ENCODED.len() {
      encoded[i] = ENCODED[i];
  }
  
  let decoded = base64_decode(encoded.as_mut()).unwrap();
  println!("{}", std::str::from_utf8(decoded).unwrap());
}

fn base64_decode<'a>(src: &'a mut [u8]) -> Option<&'a mut [u8]> {
  let mut previous_6bit = 0u8;
  let mut first_padding_idx : Option<usize> = None;
  for i in 0..src.len() {
    let current = src[i];
    let decoded_6bit = match current {
      65...90 => Some(current - 65),// A-Z,
      97...122 => Some(current - 97 + 26),// a-z,
      48...57 => Some(current + 4),  //0-9 (after 2x26 comes 52 which is 48 + 4)
      43 => Some(62), // +,
      47 => Some(63), // /,
      61 => {
        first_padding_idx = first_padding_idx.or(Some(i));
        Some(0)
      }, // = (padding)
      _ => None //invalid character
    }?;
    let decoded_8bit = match i % 4 {
      0 => None,
      1 => Some(previous_6bit << 2 | (decoded_6bit & 0b11_0000) >> 4),
      2 => Some(previous_6bit << 4 | (decoded_6bit & 0b11_1100) >> 2),
      3 => Some(previous_6bit << 6 | (decoded_6bit & 0b11_1111)),
      _ => panic!("wtf")
    };
    if let Some(d) = decoded_8bit {
      let index = i - (i / 4) - 1;
      src[index] = d;
    }
    previous_6bit = decoded_6bit;
  }
  let old_len = first_padding_idx.unwrap_or(src.len());
  let new_len = old_len - (old_len / 4);
  Some(&mut src[0 .. new_len])
}