2023: Day 4, part 2

2023/day4/src/main.rs

@@ -10,6 +10,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     let mut points = Vec::new();
 
+    let card_count = input.trim().lines().count().try_into()?;
+    let mut copies = vec![0; card_count];
     for line in input.lines() {
         if line.is_empty() {
             continue;
@@ -17,6 +19,12 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         let (fst, mine) = line.split_once(" | ").ok_or("expected |")?;
         let (fst, winning) = fst.split_once(": ").ok_or("expected :")?;
         let (_, card) = fst.split_once(' ').ok_or("expected ' '")?;
+        let card = card
+            .trim()
+            .parse()
+            .ok()
+            .and_then(|n: usize| n.checked_sub(1))
+            .ok_or("invalid card number")?;
         let mine = mine
             .split(' ')
             .filter(|s| !s.is_empty())
@@ -28,14 +36,30 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             .map(|n| n.parse())
             .collect::<Result<HashSet<u32>, _>>()?;
 
-        let count: u32 = winning.intersection(&mine).count().try_into().unwrap();
+        let count = winning.intersection(&mine).count();
         if count > 0 {
-            points.push(2_u64.pow(count - 1));
+            points.push(2_u64.pow((count - 1).try_into()?));
         }
+
+        // part 2 calculations
+        // add the current card to the count
+        copies[card] += 1;
+        let instances = copies[card];
+
+        // add 'count' copies of subsequent cards
+        let next_card = card + 1;
+        (next_card..next_card + count).for_each(|n| {
+            if let Some(entry) = copies.get_mut(n) {
+                *entry += instances;
+            }
+        });
     }
 
     let total_points: u64 = points.iter().sum();
+    let total_cards: u64 = copies.iter().sum();
+
     println!("Part 1: {}", total_points);
+    println!("Part 2: {}", total_cards);
 
     Ok(())
 }