aoc 2019 day 16 part 2

part2.rs

pub fn part2(input: &str) -> i64 {

    // parse input
    let onums = input
        .trim()
        .bytes()
        .map(|x| (x - b'0') as i64)
        .collect::<Vec<_>>();

    let offset = input[..7].parse::<usize>().unwrap();

    // calculate new input
    let mut nums = std::iter::repeat(&onums)
        .take(10000)
        .flatten()
        .copied()
        .skip(offset as usize)
        .collect::<Vec<_>>();

    // 100 phases
    for n in 0..100 {

        // reverse prefix sum
        for i in (0..nums.len()-1).rev() {
            nums[i] += nums[i+1];
        }
        nums.iter_mut().for_each(|x| *x %= 10);
    }

    // final answer
    nums[..8].iter().fold(0i64, |a, &n| a*10 + n)
}