From b9e3907e4db69724795d02920d1eddb3d5e03499 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Wed, 11 Dec 2024 08:07:26 +0000
Subject: [PATCH] Day 2024 day 11 part 1 and incredibly slow part 2

---
 bin/day2411.ml | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 bin/dune       |  6 ++++--
 2 files changed, 51 insertions(+), 2 deletions(-)
 create mode 100644 bin/day2411.ml

diff --git a/bin/day2411.ml b/bin/day2411.ml
new file mode 100644
index 0000000..7b6e737
--- /dev/null
+++ b/bin/day2411.ml
@@ -0,0 +1,47 @@
+let load_file fname =
+  match In_channel.with_open_text fname In_channel.input_line with
+  | Some x -> x
+  | None -> failwith "load_file"
+
+let log10i i =
+  let rec impl acc = function 0 -> acc | x -> impl (acc + 1) (x / 10) in
+  assert (i > 0);
+  impl ~-1 i
+
+let digits10 i = 1 + log10i i
+
+(** [pow10 n] returns [10] raised to the [n]th power. [n] must be non-negative.
+*)
+let pow10 n =
+  let rec impl acc = function 0 -> acc | x -> impl (acc * 10) (x - 1) in
+  assert (n >= 0);
+  impl 1 n
+
+let rec apply_n n fn arg = if n <= 0 then arg else apply_n (n - 1) fn (fn arg)
+
+(*
+let print_int_list lst =
+  List.iter
+    (fun i ->
+      print_int i;
+      print_char ' ')
+    lst;
+  print_newline ();
+  ()
+*)
+let calc n input =
+  let rec step_rec acc = function
+    | [] -> acc
+    | 0 :: t -> step_rec (1 :: acc) t
+    | x :: t when digits10 x mod 2 = 0 ->
+        let pow = pow10 (digits10 x / 2) in
+        let left = x / pow in
+        let right = x mod pow in
+        step_rec (right :: left :: acc) t
+    | x :: t -> step_rec ((x * 2024) :: acc) t
+  in
+  apply_n n (step_rec []) input
+
+let part1 str = Aoc.ints_of_string str |> calc 25 |> List.length
+let part2 str = Aoc.ints_of_string str |> calc 75 |> List.length
+let _ = Aoc.main load_file [ (string_of_int, part1); (string_of_int, part2) ]
diff --git a/bin/dune b/bin/dune
index a607d83..ac41748 100644
--- a/bin/dune
+++ b/bin/dune
@@ -9,7 +9,8 @@
   day2407
   day2408
   day2409
-  day2410)
+  day2410
+  day2411)
  (names
   day2401
   day2402
@@ -20,5 +21,6 @@
   day2407
   day2408
   day2409
-  day2410)
+  day2410
+  day2411)
  (libraries str aoc))