fn factorial(n: usize) -> usize {
    match n {
        0 => 1,
        _ => n * factorial(n - 1),
    }
}

fn factorial_to_base10(val: usize) -> usize {
    let mut result = 0;
    let mut i = 0;
    let mut val = val;
    while val > 0 {
        result += (val % 10) * factorial(i);
        i += 1;
        val /= 10;
    }
    result
}

fn base10_to_factorial(val: usize) -> usize {
    let mut result = 0;
    let mut val = val;
    let mut digits = Vec::new();
    let mut i = 1;
    while val > 0 {
        digits.push(val % i);
        val /= i;
        i += 1;
    }
    for &d in digits.iter().rev() {
        result *= 10;
        result += d;
    }
    result
}

fn main() {
    assert_eq!(463, factorial_to_base10(341010));
    assert_eq!(719, factorial_to_base10(543210));

    assert_eq!(341010, base10_to_factorial(463));

    for i in 0..factorial(8) {
        let f = base10_to_factorial(i);
        assert_eq!(factorial_to_base10(f), i);
    }
}