Encoder and Decoder for John Hammond's Snyk Rust challenge. https://www.youtube.com/watch?v=4ZFypGpGfAo

probe.rs

use std::fs;

const CHARSET: &[u8] = b"QWlKoxp3mT9EeRb4YzgG6rNj1OLvZ5SDfMBaXtP8JyIFVH07uh2wicdnUAC#@q";

fn encode(input: String) -> String {
    let input_bytes = input.as_bytes();
    let mut output = Vec::new();

    let mut temp = 0u32;
    let mut temp_len = 0u8;

    for &byte in input_bytes {
        temp = (temp << 8) | byte as u32;
        temp_len += 8;

        while temp_len >= 6 {
            temp_len -= 6;
            output.push(CHARSET[((temp >> temp_len) & 0x3F) as usize]);
        }
    }
    if temp_len > 0 {
        output.push(CHARSET[((temp << (6 - temp_len)) & 0x3F) as usize]);
    }

    while output.len() % 4 != 0 {
        output.push(b'=');
    }

    String::from_utf8(output).unwrap()
}

fn decode(input: String) -> String {
    let mut output = Vec::new();

    let mut temp = 0u32;
    let mut temp_len = 0u8;

    for char in input.chars() {
        if char == '=' { break; }
        let index = CHARSET.iter().position(|&r| r == char as u8).unwrap();
        temp = (temp << 6) | index as u32;
        temp_len += 6;

        while temp_len >= 8 {
            output.push(((temp >> temp_len) & 0xFF) as u8);
            temp_len -= 8;
        }
    }
    if temp_len > 0 {
        output.push(((temp << (8 - temp_len)) & 0xFF) as u8);
    }

    String::from_utf8(output).unwrap()
}

fn main() {
    let content = fs::read_to_string("flag.txt").expect("Unable to read flag.txt");
    let out = encode(content);

    println!("{}", out);

    let reversed = decode(out);

    println!("{}", reversed);
}