day 4 part 2 (in Rust)

src/main.rs

@@ -1,9 +1,15 @@
 use anyhow::Result;
 use std::{collections::HashSet, io::stdin};
 
+struct Row {
+    cards: usize,
+    matches: usize,
+}
+
 fn main() -> Result<()> {
     let mut sum = 0;
 
+    let mut rows = vec![];
     for line in stdin().lines().map_while(Result::ok) {
         let p: Vec<_> = line.split(':').collect();
         let nums: Vec<_> = p[1].split('|').collect();
@@ -22,8 +28,21 @@ fn main() -> Result<()> {
 
         let both = winning.intersection(&mine);
         let c = both.count();
-        sum += if c > 0 { 2i32.pow(c as u32 - 1) } else { 0 };
+        rows.push(Row {
+            cards: 1,
+            matches: c,
+        });
     }
+    for i in 0..rows.len() {
+        // let row = &rows[i];
+        let (front, back) = rows.split_at_mut(i + 1);
+        let row = front.last().unwrap();
+        for r in back.iter_mut().take(row.matches) {
+            r.cards += row.cards;
+        }
+        sum += row.cards;
+    }
+
     println!("{sum}");
     Ok(())
 }