Add two big nums.

add_big_num.rs

#![allow(unused)]

pub fn add_big_num(num_1: &str, num_2: &str) -> String {
    let to_u8_arr = |num: &str| {
        num.split("")
            .filter(|&n| !n.is_empty())
            .map(|n| n.parse::<u8>().unwrap())
            .collect::<Vec<_>>()
    };

    let mut num_1_arr = to_u8_arr(num_1);
    let mut num_2_arr = to_u8_arr(num_2);

    match num_1.len().cmp(&num_2.len()) {
        std::cmp::Ordering::Greater => {
            num_2_arr.splice(0..0, vec![0; num_1.len() - num_2.len()]);
        }
        _ => {
            num_1_arr.splice(0..0, vec![0; num_2.len() - num_1.len()]);
        }
    }

    let mut padding = 0u8;

    for idx in (0..num_1_arr.len()).rev() {
        let current = num_2_arr[idx] + num_1_arr[idx] + padding;

        num_1_arr[idx] = current % 10;
        padding = current / 10;
    }

    if padding != 0 {
        num_1_arr.insert(0, padding);
    }

    num_1_arr
        .iter()
        .map(|x| x.to_string())
        .collect::<Vec<_>>()
        .join("")
}