// This only works if your offset is bigger than half the length of your data…
fn run_big_fft(data: &[char], iterations: usize, offset: usize) -> String {
    let mut curr: Vec<i32> = data
        .iter()
        .skip(offset)
        .map(|&i| i.to_digit(10).unwrap() as i32)
        .collect();
    for _ in 0..iterations {
        let mut next: Vec<i32> = vec![];
        let mut sum: i32 = curr.iter().sum();
        for elem in curr {
            next.push((sum % 10).abs());
            sum -= elem;
        }
        curr = next;
    }
    curr.into_iter().map(|x| x.to_string()).take(8).collect()
}

fn process_data_b(data: &str) -> String {
    let offset: usize = data[0..7].parse().unwrap();
    run_big_fft(
        &data.chars().collect::<Vec<_>>().repeat(10_000),
        100,
        offset,
    )
}