diff --git a/bin/day2409.ml b/bin/day2409.ml
new file mode 100644
index 0000000..438bf44
--- /dev/null
+++ b/bin/day2409.ml
@@ -0,0 +1,71 @@
+let load_file fname =
+  match In_channel.with_open_text fname In_channel.input_line with
+  | Some x -> x
+  | None -> failwith "load_file"
+
+(*
+  let disk_print disk =
+    let prnt c = if c = -1 then print_char '.' else print_int c in
+    Array.iter prnt disk;
+    print_newline();
+*)
+let disk_size disk_str =
+  let rec impl acc disk_str =
+    let len = String.length disk_str in
+    match len with
+    | 0 -> acc
+    | len ->
+        let h = int_of_string (String.sub disk_str 0 1) in
+        flush_all ();
+        let t = String.sub disk_str 1 (len - 1) in
+        flush_all ();
+        impl (acc + h) t
+  in
+  impl 0 disk_str
+
+let disk_init disk_str =
+  let size = disk_size disk_str in
+  let disk = Array.make size (-1) in
+  let rec add_id offset id = function
+    | 0 -> offset
+    | x ->
+        disk.(offset) <- id;
+        add_id (offset + 1) id (x - 1)
+  in
+  let rec impl offset id disk_str =
+    match String.length disk_str with
+    | 0 -> ()
+    | 1 ->
+        let _ = add_id offset id (int_of_string (String.sub disk_str 0 1)) in
+        ()
+    | str_len ->
+        let len = int_of_string (String.sub disk_str 0 1) in
+        impl
+          (add_id offset id len + int_of_string (String.sub disk_str 1 1))
+          (id + 1)
+          (String.sub disk_str 2 (str_len - 2))
+  in
+  impl 0 0 disk_str;
+  disk
+
+let disk_defrag disk =
+  let rec impl front back =
+    if front >= back then ()
+    else if disk.(front) <> -1 then impl (front + 1) back
+    else if disk.(back) = -1 then impl front (back - 1)
+    else (
+      disk.(front) <- disk.(back);
+      disk.(back) <- -1;
+      impl (front + 1) (back - 1))
+  in
+  impl 0 (Array.length disk - 1);
+  disk
+
+let calc_checksum disk =
+  let rec impl acc idx =
+    if disk.(idx) = -1 then acc else impl (acc + (disk.(idx) * idx)) (idx + 1)
+  in
+  impl 0 0
+
+let part1 str = disk_init str |> disk_defrag |> calc_checksum
+let _ = Aoc.main load_file [ (string_of_int, part1) ]
diff --git a/bin/dune b/bin/dune
index 8536833..6b6c429 100644
--- a/bin/dune
+++ b/bin/dune
@@ -7,6 +7,16 @@
   day2405
   day2406
   day2407
-  day2408)
- (names day2401 day2402 day2403 day2404 day2405 day2406 day2407 day2408)
+  day2408
+  day2409)
+ (names
+  day2401
+  day2402
+  day2403
+  day2404
+  day2405
+  day2406
+  day2407
+  day2408
+  day2409)
  (libraries str aoc))