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#![feature(iter_array_chunks)]
pub fn encode(bytes: &[u8]) -> String {
let remainder = bytes.len() % 3;
let bytes = match remainder {
0 => bytes.iter().chain(&[0u8;0]),
1 => bytes.iter().chain(&[0u8;2]),
2 => bytes.iter().chain(&[0u8;1]),
_ => unreachable!(),
};
let mut string = String::new();
let mut triples = bytes.array_chunks::<3>();
let mut n = 0u32;
let mut hextets = [0u32;4];
while let Some(triple) = triples.next() {
for i in 0..3 {
n <<= 8;
n += *triple[i] as u32;
}
for i in (0..4).rev() {
hextets[i] = n & 0b111111;
n >>= 6;
}
for i in 0..4 {
let symbol = hextets[i];
let code = match symbol {
0..=25 => symbol + 65, // A..Z
26..=51 => symbol + 71, // a..z
52..=61 => symbol - 4, // 0..9
62 => 43, // '+'
63 => 47, // '/'
_ => unreachable!(),
};
string.push(char::from_u32(code).unwrap());
}
}
match remainder {
0 => { string},
1 => {string.pop(); string.pop(); string.push_str("=="); string},
2 => { string.pop(); string.push('='); string},
_ => unreachable!(),
}
}
pub fn decode(string: &str) -> Result<Vec<u8>, String> {
if string.len() % 4 != 0 {
return Err("Input string length is not divisible by 4".into())
}
let mut bytes: Vec<u8> = Vec::new();
let mut n = 0u32;
let mut padding = 0;
for (i, c) in string.chars().enumerate() {
let code: u32 = c.into();
if padding > 0 && code != 61 {
return Err(format!("Character at position {} is a non-terminating padding character", i-1));
}
let hextet = match code {
65..=90 => code - 65, // A..Z
97..=122 => code - 71, // a..z
48..=57 => code + 4, // 0..9
43 => 62, // '+'
47 => 63, // '/'
61 => {padding += 1; 0}, // '='
_ => return Err(format!("Character '{c}' at position {i} is not a member of the base64 alphabet")),
};
n <<= 6;
n += hextet;
if i & 0b11 == 3 {
bytes.extend(&n.to_be_bytes()[1..4]);
n = 0;
}
}
if padding > 2 {
return Err(format!("Input string contains more than 2 terminating padding characters"))
}
for _ in 0..padding {
bytes.pop();
}
Ok(bytes)
}
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