1 files changed, 76 insertions, 0 deletions
diff --git a/2025/ocaml/lib/domain/day_1.ml b/2025/ocaml/lib/domain/day_1.ml
index e69de29..6b531f4 100644
--- a/2025/ocaml/lib/domain/day_1.ml
+++ b/2025/ocaml/lib/domain/day_1.ml
@@ -0,0 +1,76 @@
+module Dial : sig
+  (** [t] is a dial between 0 and 99 inclusive *)
+  type t
+
+  (** [direction] is a direction the dial can be turned *)
+  type direction = Left of int | Right of int
+
+  val start : t
+  (** [start] is the initial state of the dial *)
+
+  val dial_out_of_range : string
+  (** [dial_out_of_range] is the error message when the dial is out of range *)
+
+  val turn : t -> direction -> t
+  (** [turn current direction] is the new state of the dial after turning [direction] from [current] *)
+
+  val to_string : t -> string
+  (** [to_string t] is the string representation of [t] *)
+
+  val to_int : t -> int
+  (** [to_int t] is the integer representation of [t] *)
+
+  val pp_direction : direction -> string
+end = struct
+  (** [t] is an int between 0 and 99 inclusive *)
+  type t = int
+
+  type direction = Left of int | Right of int
+
+  let start = 50
+
+  let dial_out_of_range = "dial out of range"
+
+  (** [next n] is the new state of the dial after turning [Right].
+      Raises: [dial_out_of_range] if [n] is greater than 99. *)
+  let next = function
+    | n when n < 99 ->
+        n + 1
+    | 99 ->
+        0
+    | _ ->
+        failwith dial_out_of_range
+
+  (** [prev n] is the new state of the dial after turning [Left].
+      Raises: [dial_out_of_range] if [n] is less than 0. *)
+  let prev = function
+    | n when n > 0 ->
+        n - 1
+    | 0 ->
+        99
+    | _ ->
+        failwith dial_out_of_range
+
+  (** [turn current direction] is the new state of the dial after turning [direction] from [current].
+      Raises: [dial_out_of_range] if the dial is moved to an invalid state. *)
+  let rec turn current direction =
+    match direction with
+    | Left 0 ->
+        current
+    | Left n ->
+        turn (prev current) (Left (n - 1))
+    | Right 0 ->
+        current
+    | Right n ->
+        turn (next current) (Right (n - 1))
+
+  let to_string = string_of_int
+
+  let to_int t = t
+
+  let pp_direction = function
+    | Left n ->
+        "Left " ^ string_of_int n
+    | Right n ->
+        "Right " ^ string_of_int n
+end