struct B64 {
    w: usize,
    table: Vec<char>,
}
impl B64 {
    fn new() -> B64 {B64::with_w(6)}
    fn with_w(w: usize) -> B64 {
        B64{w: w, table: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/".chars().collect()}
    }
    fn encode(&self, s: &str) -> String {
        s.as_bytes().iter()
            .flat_map(|b| (0..8).map(move |i| b >> (7 - i) & 1u8 != 0u8))
            .collect::<Vec<_>>().chunks(self.w)
            .map(|bw| bw.iter().enumerate().fold(0, |acc, (i, b)| acc + ((*b as u8) << (self.w - 1 - i))))
            .map(|u| self.table[u as usize])
            .chain(std::iter::repeat('=').take(s.as_bytes().len() * 8 % self.w % 4 % 4))
            .collect()
    }
    fn decode(&self, s: &str) -> String {
        let us: Vec<u8> = s.chars().take_while(|d| *d != '=')
            .flat_map(|c| self.table.iter().position(|e| *e == c))
            .flat_map(|b| (0..self.w).map(move |i| b as u8 >> (self.w - 1 - i) & 1u8 != 0u8))
            .collect::<Vec<_>>().chunks(8).filter(|b8| b8.len() == 8)
            .map(|b8| b8.iter().enumerate().fold(0, |acc, (i, b)| acc + ((*b as u8) << (7 - i))))
            .collect();
        String::from_utf8(us).unwrap_or(String::new())
    }
}
fn main() {
    let p = |b: &B64, s| {
        let e = b.encode(s);
        let d = b.decode(e.as_str());
        println!("{}\n{}\n{}\n", s, e, d);
    };
    let f = |b| for s in &["ABCDEFG", "Hello, World!", "0123456789\"#$%&'()`=@", "💖"] {p(&b, s)};
    f(B64::new());
    f(B64::with_w(5));
    f(B64::with_w(3));
}
 

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