+const CHARS: &[u8; 64] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

+const PADDING: u8 = b'=';

+

+fn decode_char(val: u8) -> Option<usize>

+{

+ match val

+ {

+ b'A'..=b'Z' => Some((val - b'A') as usize),

+ b'a'..=b'z' => Some(26 + (val - b'a') as usize),

+ b'0'..=b'9' => Some(52 + (val - b'0') as usize),

+ b'+' => Some(0x3E),

+ b'/' => Some(0x3F),

+ _ => None

+ }

+}

+

+pub fn encode(input: &[u8], output: &mut [u8]) -> Result<usize, EncodeError>

+{

+ let use_pad = input.len() % 3 != 0;

+ let expect_len = if use_pad {4 * (input.len() / 3 + 1)} else {4 * (input.len() / 3)};

+ if output.len() < expect_len

+ {

+ return Err(EncodeError::Overflow{need: expect_len, have: output.len()});

+ }

+ let mut in_pos = 0usize;

+ let mut out_pos = 0usize;

+ while input.len() - in_pos >= 3

+ {

+ let buff = ((input[in_pos] as usize) << 16) | ((input[in_pos + 1] as usize) << 8) | (input[in_pos + 2] as usize);

+ output[out_pos] = CHARS[buff >> 18];

+ output[out_pos + 1] = CHARS[(buff >> 12) & 0x3F];

+ output[out_pos + 2] = CHARS[(buff >> 6) & 0x3F];

+ output[out_pos + 3] = CHARS[buff & 0x3F];

+ out_pos += 4;

+ in_pos += 3;

+ }

+ let remain = input.len() - in_pos;

+ if remain > 0

+ {

+ let mut buff = (input[in_pos] as usize) << 16;

+ if remain > 1

+ {

+ buff |= (input[in_pos + 1] as usize) << 8;

+ }

+ in_pos += if remain > 1 {2} else {1};

+ output[out_pos] = CHARS[buff >> 18];

+ output[out_pos + 1] = CHARS[(buff >> 12) & 0x3F];

+ if remain > 1

+ {

+ output[out_pos + 2] = CHARS[(buff >> 6) & 0x3F];

+ }

+ else {output[out_pos + 2] = PADDING;}

+ output[out_pos + 3] = PADDING;

+ out_pos += 4;

+ }

+ assert_eq!(in_pos, input.len(), "missed input ({in_pos}, expected {})", input.len());

+ assert_eq!(out_pos, expect_len, "missed output ({out_pos}, expected {expect_len})");

+ Ok(out_pos)

+}

+

+macro_rules!do_decode

+{

+ ($input:ident, $in_pos:expr) =>

+ {

+ match decode_char($input[$in_pos])

+ {

+ None => return Err(DecodeError::Malformed{at: $in_pos, value: $input[$in_pos]}),

+ Some(v) => v,

+ }

+ };

+}

+

+pub fn decode(input: &[u8], output: &mut [u8]) -> Result<usize, DecodeError>

+{

+ if input.len() % 4 != 0

+ {

+ // can't decode, but check for malformed data first

+ let mut in_pad = false;

+ for (i, &c) in input.iter().enumerate()

+ {

+ if c == PADDING

+ {

+ if i % 4 < 2

+ {

+ return Err(DecodeError::Malformed{at: i, value: c});

+ }

+ in_pad = true;

+ }

+ else if in_pad && i % 4 == 0

+ {

+ return Err(DecodeError::TrailingData{at: i});

+ }

+ else if (in_pad && i % 4 == 3) || decode_char(c).is_none()

+ {

+ return Err(DecodeError::Malformed{at: i, value: c});

+ }

+ }

+ return Err(DecodeError::Truncated);

+ }

+ let pad_len = if input.len() > 0

+ {

+ if input[input.len() - 1] != PADDING {0}

+ else if input[input.len() - 2] != PADDING {1}

+ else {2}

+ }

+ else {0};

+ let expect_len = input.len() / 4 * 3 - pad_len;

+ if output.len() < expect_len

+ {

+ return Err(DecodeError::Overflow{need: expect_len, have: output.len()});

+ }

+ if !input.is_empty()

+ {

+ let mut in_pos = 0usize;

+ let mut out_pos = 0usize;

+ while in_pos < input.len() - 4

+ {

+ let c0 = do_decode!(input, in_pos);

+ let c1 = do_decode!(input, in_pos + 1);

+ let c2 = do_decode!(input, in_pos + 2);

+ let c3 = do_decode!(input, in_pos + 3);

+ let buff = (c0 << 18) | (c1 << 12) | (c2 << 6) | c3;

+ output[out_pos] = (buff >> 16) as u8;

+ output[out_pos + 1] = (buff >> 8) as u8;

+ output[out_pos + 2] = buff as u8;

+ in_pos += 4;

+ out_pos += 3;

+ }

+

+ let c0 = do_decode!(input, in_pos);

+ let c1 = do_decode!(input, in_pos + 1);

+ let mut buff = (c0 << 18) | (c1 << 12);

+ output[out_pos] = (buff >> 16) as u8;

+ out_pos += 1;

+ if input[in_pos + 2] == PADDING

+ {

+ if input[in_pos + 3] != PADDING

+ {

+ return Err(DecodeError::Malformed{at: in_pos + 3, value: input[in_pos + 3]});

+ }

+ }

+ else

+ {

+ buff |= do_decode!(input, in_pos + 2) << 6;

+ output[out_pos] = (buff >> 8) as u8;

+ out_pos += 1;

+ if input[in_pos + 3] != PADDING

+ {

+ buff |= do_decode!(input, in_pos + 3);

+ output[out_pos] = buff as u8;

+ out_pos += 1;

+ }

+ }

+ in_pos += 4;

+

+ assert_eq!(in_pos, input.len(), "missed input ({in_pos}, expected {})", input.len());

+ assert_eq!(out_pos, expect_len, "missed output ({out_pos}, expected {expect_len})");

+ Ok(out_pos)

+ }

+ else {Ok(0)}

+}

+

+#[derive(Clone, Copy, Debug, Eq, PartialEq)]

+pub enum DecodeError

+{

+ Malformed{at: usize, value: u8},

+ Overflow{need: usize, have: usize},

+ Truncated,

+ TrailingData{at: usize},

+}

+

+#[derive(Clone, Copy, Debug, Eq, PartialEq)]

+pub enum EncodeError

+{

+ Overflow{need: usize, have: usize}

+}

+

+#[cfg(test)]

+mod test

+{

+ use super::*;

+

+ #[test]

+ fn validate_chars()

+ {

+ for (i, &c0) in CHARS.iter().enumerate()

+ {

+ assert_ne!(c0, PADDING, "padding character in data charset at {i}");

+ if i > 0

+ {

+ for (j, &c1) in CHARS[..i].iter().enumerate()

+ {

+ assert_ne!(c1, c0, "duplicate data character at {j} and {i}")

+ }

+ }

+ }

+ }

+

+ #[test]

+ fn decode_matches_chars()

+ {

+ for (i, &c0) in CHARS.iter().enumerate()

+ {

+ assert_eq!(decode_char(c0), Some(i), "data character {c0} (at {i}) isn't decoded properly");

+ }

+ }

+

+ macro_rules!test_codec

+ {

+ ($func:ident, $input:expr => $expect:expr) =>

+ {

+ {

+ const EMPTY: u8 = 0xC9u8; // arbitrary

+ let mut output = [EMPTY; 64];

+ let input = $input;

+ let expect = $expect;

+ assert_eq!($func(input, &mut output), Ok(expect.len()));

+ assert_eq!(&output[..expect.len()], expect);

+ assert!(output[expect.len()..].iter().all(|&x| x == EMPTY), "output buffer overflow");

+ }

+ };

+ }

+

+ #[test]

+ fn encoder_success()

+ {

+ test_codec!(encode, b"Hello Wor" => b"SGVsbG8gV29y");

+ test_codec!(encode, b"Hello Worl" => b"SGVsbG8gV29ybA==");

+ test_codec!(encode, b"Hello World" => b"SGVsbG8gV29ybGQ=");

+ test_codec!(encode, b"Hello World!" => b"SGVsbG8gV29ybGQh");

+ }

+

+ #[test]

+ fn decoder_success()

+ {

+ test_codec!(decode, b"SGVsbG8gV29y" => b"Hello Wor");

+ test_codec!(decode, b"SGVsbG8gV29ybA==" => b"Hello Worl");

+ test_codec!(decode, b"SGVsbG8gV29ybGQ=" => b"Hello World");

+ test_codec!(decode, b"SGVsbG8gV29ybGQh" => b"Hello World!");

+ }

+

+ #[test]

+ fn encoder_fail()

+ {

+ let mut output = [0u8; 64];

+ assert_eq!(encode(b"Hello Worl", &mut output[..15]), Err(EncodeError::Overflow{need: 16, have: 15}));

+ assert_eq!(encode(b"Hello World!", &mut output[..0]), Err(EncodeError::Overflow{need: 16, have: 0}));

+ }

+

+ #[test]

+ fn decoder_fail()

+ {

+ let mut output = [0u8; 64];

+ assert_eq!(decode(b"SGVsbG8gV29ybA==", &mut output[..9]), Err(DecodeError::Overflow{need: 10, have: 9}));

+ assert_eq!(decode(b"SGVsbG8gV29ybGQh", &mut output[..11]), Err(DecodeError::Overflow{need: 12, have: 11}));

+ assert_eq!(decode(b"SGVsbG8gV29ybA", &mut output), Err(DecodeError::Truncated));

+ assert_eq!(decode(b"SGVsbG8gV29yb", &mut output), Err(DecodeError::Truncated));

+ assert_eq!(decode(b"SGVsbG8gV29y\n", &mut output), Err(DecodeError::Malformed{at: 12, value: b'\n'}));

+ assert_eq!(decode(b"SGVs_bG8gV29y\n", &mut output), Err(DecodeError::Malformed{at: 4, value: b'_'}));

+ assert_eq!(decode(b"SGVsbG8gV29ybA==*", &mut output), Err(DecodeError::TrailingData{at: 16}));

+ assert_eq!(decode(b"SGVsbG8gV29ybA=*", &mut output), Err(DecodeError::Malformed{at: 15, value: b'*'}));

+ assert_eq!(decode(b"SGVsbG8gV29ybA=A", &mut output), Err(DecodeError::Malformed{at: 15, value: b'A'}));

+ }

+}