Year 2023, Day 15, part 2

year2023/src/day15.rs

@@ -8,9 +8,51 @@ impl Day for Day15 {
         input.split(',').map(hash).sum::<usize>().to_string()
     }
 
-    fn star2(&self, _input: &str) -> String {
-        String::from("not implemented")
+    fn star2(&self, input: &str) -> String {
+        let mut boxes: [_; 256] = std::array::from_fn(|_| vec![]);
+
+        let input = input.replace('\n', "");
+        for s in input.split(',') {
+            boxes = insert(boxes, s);
+        }
+
+        focusing_power(&boxes).to_string()
+    }
+}
+
+type Boxes<'a> = [Vec<(&'a str, usize)>; 256];
+
+fn insert<'a>(mut boxes: Boxes<'a>, s: &'a str) -> Boxes<'a> {
+    if s.ends_with('-') {
+        let label = &s[0..s.len() - 1];
+        let target_box = &mut boxes[hash(label)];
+        if let Some(p) = target_box.iter().position(|(l, _)| l == &label) {
+            target_box.remove(p);
+        }
+    } else {
+        let (label, focal_length) = s.split_once('=').unwrap();
+        let focal_length = focal_length.parse().unwrap();
+        let target_box = &mut boxes[hash(label)];
+        if let Some(p) = target_box.iter().position(|(l, _)| l == &label) {
+            target_box[p].1 = focal_length;
+        } else {
+            target_box.push((label, focal_length));
+        }
     }
+    boxes
+}
+
+fn focusing_power(boxes: &Boxes<'_>) -> usize {
+    boxes
+        .iter()
+        .enumerate()
+        .map(|(i, b)| {
+            b.iter()
+                .enumerate()
+                .map(|(j, (_, focal_length))| (i + 1) * (j + 1) * focal_length)
+                .sum::<usize>()
+        })
+        .sum::<usize>()
 }
 
 fn hash(s: &str) -> usize {
@@ -35,6 +77,12 @@ mod tests {
         assert_eq!(d.star1(INPUT), "1320");
     }
 
+    #[test]
+    fn ex2() {
+        let d = Day15 {};
+        assert_eq!(d.star2(INPUT), "145");
+    }
+
     #[test]
     fn test_hash() {
         assert_eq!(hash("HASH"), 52);