diff --git a/2024/days/Day09.ml b/2024/days/Day09.ml
index 4be40dc..d0b8885 100644
--- a/2024/days/Day09.ml
+++ b/2024/days/Day09.ml
@@ -1 +1,70 @@
-let day09 _input = ("TODO", "TODO")
+open Utils
+
+let parse input =
+  let open Iter in
+  of_str input |> filter is_digit
+  |> map (fun c -> (int_of_char c - int_of_char '0', 0))
+  |> map_by_2 (fun (a, _) (b, _) -> (a, b))
+  |> mapi (fun i (a, b) -> (i, a, b))
+  |> to_list
+
+let sum_range a l = (a * l) + (l * (l - 1) / 2)
+
+let checksum files =
+  let rec aux fwd bwd pos max_pos =
+    match (fwd, bwd) with
+    | _ when pos >= max_pos -> 0
+    | (_, 0, 0) :: fwd, _ -> aux fwd bwd pos max_pos
+    | (_, 0, f) :: fwd, (i, b, p) :: bwd when b > f ->
+        aux ((i, f, 0) :: fwd) ((i, b - f, p) :: bwd) pos max_pos
+    | (_, 0, f) :: fwd, (i, b, _) :: bwd when b <= f ->
+        aux ((i, b, f - b) :: fwd) bwd pos max_pos
+    | (i, b, f) :: fwd, _ ->
+        let a = min (max_pos - pos) b in
+        (i * sum_range pos a) + aux ((i, 0, f) :: fwd) bwd (pos + a) max_pos
+    | _ -> failwith "Unreachable"
+  in
+  let max_pos = Iter.(of_list files |> map (fun (_, a, _) -> a) |> sum) in
+  aux files (List.rev files) 0 max_pos
+
+let id_eq i (j, _, _) = i = j
+
+let reorder files =
+  let rec aux fwd arr buf n =
+    let open Iter in
+    match fwd with
+    | [] -> List.rev buf
+    | (i, b, f) :: fwd -> (
+        let e =
+          if f = 0 then None
+          else
+            n --^ 0
+            |> map (fun j -> arr.(j))
+            |> keep_some
+            |> find (fun e ->
+                   let j, b, _ = e in
+                   if b <= f && i != j then Some e else None)
+        in
+        match e with
+        | Some (j, p, _) -> aux ((i, b, 0) :: (j, p, f - p) :: fwd) arr buf n
+        | None ->
+            if Option.is_some arr.(i) then (
+              arr.(i) <- None;
+              aux fwd arr ((i, b, f) :: buf) n)
+            else
+              let pi, pb, pf = List.hd buf in
+              aux fwd arr ((pi, pb, pf + b + f) :: List.tl buf) n)
+  in
+
+  let arr = Array.of_list @@ List.map (fun x -> Some x) files in
+  let _, res =
+    List.fold_left
+      (fun (pos, a) (i, b, f) -> (pos + b + f, a + (i * sum_range pos b)))
+      (0, 0)
+    @@ aux files arr [] (List.length files - 1)
+  in
+  res
+
+let day09 input =
+  let files = parse input in
+  (string_of_int @@ checksum files, string_of_int @@ reorder files)
diff --git a/2024/days/Utils.ml b/2024/days/Utils.ml
index 24fc493..8a2b773 100644
--- a/2024/days/Utils.ml
+++ b/2024/days/Utils.ml
@@ -73,3 +73,5 @@ module Point = struct
     let dx, dy = (x2 - x1, y2 - y1) in
     (dx * dx) + (dy * dy)
 end
+
+let ( % ) f g x = f @@ g x
diff --git a/2024/lib/Aoc.ml b/2024/lib/Aoc.ml
index 2c1b90d..72d1db1 100644
--- a/2024/lib/Aoc.ml
+++ b/2024/lib/Aoc.ml
@@ -52,6 +52,7 @@ let days =
     (6, Day06.day06);
     (7, Day07.day07);
     (8, Day08.day08);
+    (9, Day09.day09);
   ]
 
 let run_day ctx (day, fn) =