day 4 part 2

src/day4.rs

@@ -1,12 +1,12 @@
 use chumsky::{
     error::Simple,
-    primitive::{choice, just},
+    primitive::just,
     text::{self, TextParser},
     Parser,
 };
 use eyre::{eyre, Context};
 use std::{
-    collections::HashSet,
+    collections::{HashMap, HashSet},
     io::{BufRead, BufReader, Read},
     str::FromStr,
 };
@@ -24,6 +24,33 @@ pub fn part1(input: impl Read) -> eyre::Result<usize> {
     Ok(sum)
 }
 
+pub fn part2(input: impl Read) -> eyre::Result<usize> {
+    let buf_reader = BufReader::new(input);
+    let mut counts: HashMap<_, usize> = HashMap::new();
+    for (num, line) in buf_reader.lines().enumerate() {
+        let line = line.wrap_err_with(|| format!("could not read line {num}"))?;
+        let card: Card = line
+            .parse()
+            .wrap_err_with(|| format!("could not parse card at line {num}"))?;
+        // the count of copies won by past cards
+        let count = counts.entry(card.id).or_default();
+        *count += 1; // count the original card
+        let count = *count; // drop the borrow
+        let matches = card.matches();
+        for id in card.id + 1..=card.id + matches {
+            *counts.entry(id).or_default() += count;
+        }
+        // println!(
+        //     "card: {} has {}, matched {} -- set is {:?}",
+        //     card.id,
+        //     count,
+        //     card.matches(),
+        //     counts
+        // );
+    }
+    Ok(counts.into_values().sum())
+}
+
 #[derive(Default, Debug)]
 struct Card {
     id: usize,
@@ -33,13 +60,16 @@ struct Card {
 
 impl Card {
     fn points(&self) -> usize {
-        let matches = self.winning.intersection(&self.scratched).count();
+        let matches = self.matches();
         if matches == 0 {
             0
         } else {
             2_usize.pow((matches - 1) as u32)
         }
     }
+    fn matches(&self) -> usize {
+        self.winning.intersection(&self.scratched).count()
+    }
 }
 
 impl FromStr for Card {

tests/day4.rs

@@ -11,6 +11,7 @@ const SAMPLE: &str = indoc! { r#"
 	Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
 	Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 1
 "# };
+const INPUT_PATH: &str = "day4/input.txt";
 
 #[test]
 fn part1_sample() {
@@ -20,7 +21,20 @@ fn part1_sample() {
 
 #[test]
 fn part1_input() {
-    let input = common::read("day4/input.txt").unwrap();
+    let input = common::read(INPUT_PATH).unwrap();
     let result = day4::part1(input).unwrap();
     assert_eq!(result, 25004);
 }
+
+#[test]
+fn part2_sample() {
+    let result = day4::part2(SAMPLE.as_bytes()).unwrap();
+    assert_eq!(result, 30);
+}
+
+#[test]
+fn part2_input() {
+    let input = common::read(INPUT_PATH).unwrap();
+    let result = day4::part2(input).unwrap();
+    assert_eq!(result, 14427616);
+}