Multiply big numbers by base converting algorithm (https://play.rust-lang.org/?gist=50e4da80a076623ba5de3b04410c1739&version=stable)

big-number-multiply.rs

fn base_conv(a: &str, n: usize) -> Vec<u64> {
    let mut result = vec![];
    let mut s = a.to_owned();
    let mut len = s.len();
    while len > 0 {
        let mut pos = 0;
        if n <= len {
            pos = len - n;
        }
        let n = s[pos..].to_string().parse::<u64>().unwrap();
        result.push(n);
        s = s[..pos].to_string();
        len = s.len();
    }
    return result;
}

fn multiply_base(a: Vec<u64>, b: Vec<u64>, n: u32) -> Vec<u64> {
    let mut result = vec![];
    result.resize(a.len() + b.len(), 0u64);
    let mut bot = 0; let mut up = 0;
    let mut flag = 0u64; let base = 10u64.pow(n);
    for i in 0..b.len() {
        for j in 0..a.len() {
            let mut t = (a[j] * b[i]) as u64 + flag + result[up];
            flag = 0;
            if t >= base {
                flag = t / base;
                t = t % base;
            }
            result[up] = t;
            if j >= a.len()-1 {
                result[up+1] = flag;
                flag = 0;
            }
            up += 1;
        }
        bot += 1;
        up = bot;
    }
    result
}

fn convert_back(num: Vec<u64>, base: usize) -> String {
    let mut result = String::new();
    let filtered: Vec<u64> = num.into_iter().filter(|x| *x > 0u64).collect();
    for i in 0..filtered.len() {
        let n = filtered[i];
        if n != 0 {
            if i >= filtered.len()-1 {
                result = format!("{}{}", n, result);
            } else {
                result = format!("{}{}", format!("{num:>0width$}", num=n, width=base), result);
            }
        }
    }
    result
}

fn main() {
    let base = 9;
    println!("{:?}", convert_back(multiply_base(
        base_conv("1234", base),
        base_conv("3214", base),
        base as u32
    ), base));
    println!("{:?}", convert_back(multiply_base(
        base_conv("12", base),
        base_conv("13", base),
        base as u32
    ), base));
    println!("{:?}", convert_back(multiply_base(
        base_conv("2", base),
        base_conv("8", base),
        base as u32
    ), base));
    println!("{:?}", convert_back(multiply_base(
        base_conv("2", base),
        base_conv("2", base),
        base as u32
    ), base));
    println!("{:?}", convert_back(multiply_base(
        base_conv("29123841234812351239412", base),
        base_conv("3496123842341123491234123", base),
        base as u32
    ), base));
}

Run it directly here: https://play.rust-lang.org/?gist=50e4da80a076623ba5de3b04410c1739&version=stable