98 lines
2.6 KiB
Rust
98 lines
2.6 KiB
Rust
use aoc_runner_derive::aoc;
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use std::collections::HashSet;
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struct Preamble {
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entries: [usize; 25],
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entries_start_idx: usize,
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set: HashSet<usize>,
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}
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impl Preamble {
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// Take the first 25 items from the iterator.
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fn new<I>(iter: &mut I) -> Option<Self>
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where
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I: Iterator<Item = usize>,
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{
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let mut entries = [0; 25];
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let mut set = HashSet::new();
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let mut count = 0;
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for (idx, entry) in iter.enumerate().take(25) {
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count += 1;
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entries[idx] = entry;
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set.insert(entry);
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}
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if count != 25 {
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return None;
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}
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Some(Preamble {
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entries,
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entries_start_idx: 0,
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set,
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})
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}
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// Insert a new entry into the preamble letting the oldest one roll off.
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fn insert(&mut self, entry: usize) {
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self.set.remove(&self.entries[self.entries_start_idx]);
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self.set.insert(entry);
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self.entries[self.entries_start_idx] = entry;
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self.entries_start_idx = (self.entries_start_idx + 1) % 25;
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}
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fn find_sum_pair(&self, total: usize) -> Option<(usize, usize)> {
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for idx in 0..25 {
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let entry = self.entries[(self.entries_start_idx + idx) % 25];
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let needed = total - entry;
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if self.set.contains(&needed) {
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return Some((entry, needed));
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}
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}
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None
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}
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}
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#[aoc(day9, part1)]
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pub fn solve_d9_p1(input: &str) -> usize {
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let mut iter = input.split('\n').map(|x| -> usize { x.parse().unwrap() });
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let mut preamble = Preamble::new(&mut iter).unwrap();
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for entry in iter {
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if preamble.find_sum_pair(entry).is_none() {
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return entry;
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}
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preamble.insert(entry);
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}
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panic!("not found");
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}
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#[aoc(day9, part2)]
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pub fn solve_d9_p2(input: &str) -> usize {
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const EXPECTED_SUM: usize = 36845998;
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let entries: Vec<usize> = input
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.split('\n')
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.map(|x| -> usize { x.parse().unwrap() })
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.collect();
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let mut sum = 0;
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let mut start_idx = 0;
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let mut end_idx = 0;
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loop {
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if sum < EXPECTED_SUM {
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sum += entries[end_idx];
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end_idx += 1;
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} else if sum > EXPECTED_SUM {
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sum -= entries[start_idx];
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start_idx += 1;
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} else {
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let (min, max) = &entries[start_idx..end_idx]
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.iter()
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.copied()
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.fold((usize::MAX, 0), |(min, max), entry| {
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(std::cmp::min(min, entry), std::cmp::max(max, entry))
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});
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return min + max;
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}
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}
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}
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