aoc 2018 day8

solution.rs

use std::collections::*;
pub fn part1(input: &str) -> usize {
    let mut input = input.split_whitespace().peekable();
    let mut entries = Vec::new();
    parse_node(input.by_ref(), &mut entries);
    entries.into_iter().sum()
}

fn parse_node<'a>(input: &mut impl Iterator<Item = &'a str>, entries: &mut Vec<usize>) {
    let child_node_count = input.next().unwrap().parse::<usize>().unwrap();
    let metadata_entries_count = input.next().unwrap().parse::<usize>().unwrap();
    for _ in 0..child_node_count {
        parse_node(input, entries);
    }
    for _ in 0..metadata_entries_count {
        entries.push(input.next().unwrap().parse().unwrap());
    }
}

fn build_tree<'a, I: Iterator<Item = &'a str>>(input: &mut I, entries: &mut Vec<(Vec<usize>, Vec<usize>)>) {
    let child_node_count = input.next().unwrap().parse::<usize>().unwrap(); // ^ children, metadata
    let metadata_entries_count = input.next().unwrap().parse::<usize>().unwrap();
    let mut children = Vec::new();
    let mut met = Vec::new();
    let this_index = entries.len();
    entries.push((children, met));
    let mut children = Vec::new();
    let mut met = Vec::new();
    for _ in 0..child_node_count {
        children.push(entries.len());
        build_tree(input, entries);
    }
    for _ in 0..metadata_entries_count {
        met.push(input.next().unwrap().parse().unwrap());
    }
    entries[this_index] = (children, met);
}

fn value_of(tree: &[(Vec<usize>, Vec<usize>)], node: usize) -> usize {
    if tree[node].0.len() == 0 {
        tree[node].1.iter().cloned().sum()
    } else {
        let mut sum = 0;
        for ent in &tree[node].1 {
            tree[node].0.get(ent-1)
                .map(|&i| sum += value_of(tree, i));
        }
        sum
    }
}

pub fn part2(input: &str) -> impl std::fmt::Display {
    let mut input = input.split_whitespace().peekable();
    let mut entries = Vec::new();
    build_tree(input.by_ref(), &mut entries);
    value_of(&entries, 0)
}