day 4 part 2

src/bin/day4.rs

@@ -2,16 +2,15 @@
 
 use aoc23::read;
 use chumsky::{prelude::*, text::int};
-use std::collections::HashSet;
+use std::collections::{BTreeMap, HashSet};
 
 struct Card {
-	id: usize,
 	winning_nums: HashSet<usize>,
 	nums: HashSet<usize>
 }
 
 impl Card {
-	fn parser() -> impl Parser<char, Self, Error = Simple<char>> {
+	fn parser() -> impl Parser<char, (usize, Self), Error = Simple<char>> {
 		let nums = just(" ")
 			.repeated()
 			.ignore_then(int(10))
@@ -29,36 +28,62 @@ impl Card {
 			.then(nums)
 			.then_ignore(just(" | "))
 			.then(nums)
-			.map(|((id, winning_nums), nums): ((String, _), _)| Self {
-				id: id.parse().unwrap(),
-				winning_nums,
-				nums
+			.map(|((id, winning_nums), nums): ((String, _), _)| {
+				(id.parse().unwrap(), Self { winning_nums, nums })
 			})
 	}
 }
 
-fn parser() -> impl Parser<char, Vec<Card>, Error = Simple<char>> {
+fn parser() -> impl Parser<char, BTreeMap<usize, Card>, Error = Simple<char>> {
 	Card::parser()
 		.then_ignore(just("\n"))
 		.repeated()
+		.map(|cards| cards.into_iter().collect())
 		.then_ignore(end())
 }
 
 fn main() -> anyhow::Result<()> {
 	let cards = read("inputs/day4.txt", parser())?;
 
-	let mut points = 0;
-	for card in &cards {
-		let matches = card
-			.nums
+	let matches = cards
+		.iter()
+		.map(|(id, card)| {
+			(
+				*id,
+				card.nums
 					.iter()
 					.filter(|num| card.winning_nums.contains(num))
-			.count();
-		if matches > 0 {
-			points += 1 << (matches - 1);
+					.count()
+			)
+		})
+		.collect::<BTreeMap<usize, usize>>();
+
+	// part 1
+	println!(
+		"{}",
+		matches
+			.values()
+			.map(|count| match count {
+				0 => 0,
+				_ => 1 << (count - 1)
+			})
+			.sum::<usize>()
+	);
+
+	// part 2
+	let mut copies = cards
+		.keys()
+		.map(|key| (*key, 1))
+		.collect::<BTreeMap<usize, usize>>();
+	for id in cards.keys() {
+		let num_copies = copies[id];
+		for next in id + 1 ..= id + matches[id] {
+			if let Some(next_copies) = copies.get_mut(&next) {
+				*next_copies += num_copies;
 			}
 		}
-	println!("{points}");
+	}
+	println!("{}", copies.values().sum::<usize>());
 
 	Ok(())
 }