From aa617317241098f5aecbd548eb0447935fb4b789 Mon Sep 17 00:00:00 2001
From: DJ O'Leary <dijitol@proton.me>
Date: Sat, 13 Dec 2025 16:21:12 +0100
Subject: feat(2025d3): copy part 1 over to part 2

---
 2025/ocaml/lib/domain/d03p02.ml | 114 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 114 insertions(+)
 create mode 100644 2025/ocaml/lib/domain/d03p02.ml

(limited to '2025/ocaml/lib/domain')

diff --git a/2025/ocaml/lib/domain/d03p02.ml b/2025/ocaml/lib/domain/d03p02.ml
new file mode 100644
index 0000000..1367712
--- /dev/null
+++ b/2025/ocaml/lib/domain/d03p02.ml
@@ -0,0 +1,114 @@
+module Battery : sig
+  type t
+
+  val of_int : int -> t
+
+  val to_int : t -> int
+
+  val to_string : t -> string
+
+  val compare : t -> t -> int
+
+  val pp : t -> string
+end = struct
+  type t = int
+
+  let of_int i = i
+
+  let to_int b = b
+
+  let to_string = string_of_int
+
+  let compare = Int.compare
+
+  let pp = string_of_int
+end
+
+module BatteryBank : sig
+  type t
+
+  val highest_joltage : t -> int
+  (** [highest_joltage bb] is an integer created from the two highest
+      digits in [bb].
+      Raises: [Assert_failure] if [bb] is empty. *)
+
+  val of_string : string -> t
+
+  val of_list : Battery.t list -> t
+
+  val to_list : t -> Battery.t list
+
+  val pp : t -> string
+end = struct
+  type t = Battery.t list
+
+  let rec highest_joltage bb =
+    assert (List.length bb > 0) ;
+    match highest_joltage_aux (None, None) bb with
+    | None, None ->
+        0
+    | Some x, Some y ->
+        int_of_string (Battery.to_string x ^ Battery.to_string y)
+    | _ ->
+        failwith "highest_joltage must be two digits"
+
+  and highest_joltage_aux acc = function
+    | [] ->
+        acc
+    | [elt] -> (
+      match acc with
+      | None, None ->
+          (Some (Battery.of_int 0), Some elt)
+      | Some first, None ->
+          (Some first, Some elt)
+      | None, Some second ->
+          (Some second, Some elt)
+      | Some first, Some second ->
+          if elt > second then (Some first, Some elt)
+          else (Some first, Some second) )
+    | h :: t ->
+        highest_joltage_aux
+          ( match acc with
+          | None, None ->
+              (Some h, None)
+          | Some first, None ->
+              let second = Battery.of_int 0 in
+              swap_if_higher first second h
+          | Some first, Some second ->
+              swap_if_higher first second h
+          | _ ->
+              failwith "this branch should be impossible" )
+          t
+
+  and swap_if_higher first second considering =
+    assert (first >= second) ;
+    let c_gt_f = Battery.compare considering first = 1 in
+    let c_gt_s = Battery.compare considering second = 1 in
+    match (c_gt_f, c_gt_s) with
+    | true, true ->
+        (Some considering, None)
+    | false, true ->
+        (Some first, Some considering)
+    | true, false ->
+        failwith "[second] should never be greater than [first]"
+    | false, false ->
+        (Some first, Some second)
+
+  let of_string s =
+    s |> String.to_seq
+    |> Seq.map (fun c -> Char.escaped c |> int_of_string |> Battery.of_int)
+    |> List.of_seq
+
+  let of_list lst = lst
+
+  let to_list bb = bb
+
+  let pp bb =
+    "[ "
+    ^ ( bb |> List.map Battery.pp
+      |> List.fold_left
+           (fun acc b ->
+             if acc = "" then b ^ ";" else Printf.sprintf "%s %s;" acc b )
+           "" )
+    ^ " ]"
+end
-- 
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