Day 9

src/day9.rs

@@ -0,0 +1,49 @@
+const ALLOW_SEPARATION: bool = false;
+
+pub fn day9() {
+    let mut regions: Vec<(Option<usize>, usize)> = crate::input(9)
+        .chars()
+        .filter_map(|c| c.to_string().parse::<usize>().ok())
+        .enumerate()
+        .flat_map(|(i, size)| {
+            let file_id = (i % 2 == 0).then_some(i / 2);
+            if ALLOW_SEPARATION {
+                vec![(file_id, 1); size]
+            } else {
+                vec![(file_id, size)]
+            }
+        })
+        .collect();
+
+    for i in (0..regions.len()).rev() {
+        let (Some(_), file_size) = regions[i] else {
+            continue;
+        };
+        let Some(slot_index) = regions[..i]
+            .iter()
+            .position(|&(slot, slot_size)| slot.is_none() && slot_size >= file_size)
+        else {
+            continue;
+        };
+        let slot_size = regions[slot_index].1;
+
+        regions[slot_index] = regions[i];
+        regions[i] = (None, file_size);
+        if slot_size > file_size {
+            regions.insert(slot_index + 1, (None, slot_size - file_size));
+        }
+    }
+
+    let disk: Vec<Option<usize>> = regions
+        .into_iter()
+        .flat_map(|(file_id, size)| vec![file_id; size])
+        .collect();
+
+    let checksum: usize = disk
+        .into_iter()
+        .enumerate()
+        .filter_map(|(i, f)| Some(i * f? as usize))
+        .sum();
+
+    println!("Checksum: {}", checksum);
+}

src/main.rs

@@ -12,10 +12,12 @@ mod day5;
 mod day6;
 #[allow(dead_code)]
 mod day7;
+#[allow(dead_code)]
 mod day8;
+mod day9;
 
 fn main() {
-    day8::day8();
+    day9::day9();
 }
 
 pub fn input(day: u8) -> String {