6 files changed, 160 insertions, 0 deletions
diff --git a/ocaml/lib/year2025/day03/part02/battery.ml b/ocaml/lib/year2025/day03/part02/battery.ml
new file mode 100644
index 0000000..e8f0443
--- /dev/null
+++ b/ocaml/lib/year2025/day03/part02/battery.ml
@@ -0,0 +1,25 @@
+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
diff --git a/ocaml/lib/year2025/day03/part02/battery.mli b/ocaml/lib/year2025/day03/part02/battery.mli
new file mode 100644
index 0000000..e00940b
--- /dev/null
+++ b/ocaml/lib/year2025/day03/part02/battery.mli
@@ -0,0 +1,13 @@
+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
diff --git a/ocaml/lib/year2025/day03/part02/battery_bank.ml b/ocaml/lib/year2025/day03/part02/battery_bank.ml
new file mode 100644
index 0000000..d7dc213
--- /dev/null
+++ b/ocaml/lib/year2025/day03/part02/battery_bank.ml
@@ -0,0 +1,77 @@
+open Battery
+
+module BatteryBank = struct
+  type t = Battery.t list
+
+  type highest = {one: Battery.t option; two: Battery.t option}
+
+  let rec highest_joltage bb =
+    assert (List.length bb > 0) ;
+    match highest_joltage_aux {one= None; two= None} bb with
+    | {one= None; two= None} ->
+        0
+    | {one= Some x; two= 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
+      | {one= None; two= None} ->
+          {one= Some (Battery.of_int 0); two= Some elt}
+      | {one= Some first; two= None} ->
+          {one= Some first; two= Some elt}
+      | {one= None; two= Some second} ->
+          {one= Some second; two= Some elt}
+      | {one= Some first; two= Some second} ->
+          if elt > second then {one= Some first; two= Some elt}
+          else {one= Some first; two= Some second} )
+    | h :: t ->
+        highest_joltage_aux
+          ( match acc with
+          | {one= None; two= None} ->
+              {one= Some h; two= None}
+          | {one= Some first; two= None} ->
+              let second = Battery.of_int 0 in
+              swap_if_higher first second h
+          | {one= Some first; two= 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 ->
+        {one= Some considering; two= None}
+    | false, true ->
+        {one= Some first; two= Some considering}
+    | true, false ->
+        failwith "[second] should never be greater than [first]"
+    | false, false ->
+        {one= Some first; two= 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
diff --git a/ocaml/lib/year2025/day03/part02/battery_bank.mli b/ocaml/lib/year2025/day03/part02/battery_bank.mli
new file mode 100644
index 0000000..bffc4cb
--- /dev/null
+++ b/ocaml/lib/year2025/day03/part02/battery_bank.mli
@@ -0,0 +1,18 @@
+open Battery
+
+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
diff --git a/ocaml/lib/year2025/day03/part02/solver.ml b/ocaml/lib/year2025/day03/part02/solver.ml
new file mode 100644
index 0000000..4a06e83
--- /dev/null
+++ b/ocaml/lib/year2025/day03/part02/solver.ml
@@ -0,0 +1,23 @@
+open Battery_bank
+
+let err_empty_input = "input is empty"
+
+let to_lines s =
+  s |> String.split_on_char '\n' |> List.map String.trim
+  |> List.filter (fun l -> l <> "")
+
+let solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let answer =
+      input |> to_lines |> List.to_seq
+      |> Seq.map BatteryBank.of_string
+      |> Seq.mapi (fun idx bb ->
+          let joltage = BatteryBank.highest_joltage bb in
+          Logs.debug (fun m ->
+              m "%d line=%s highest_joltage=%d" (idx + 1) (BatteryBank.pp bb)
+                joltage ) ;
+          joltage )
+      |> Seq.fold_left ( + ) 0
+    in
+    Ok answer
diff --git a/ocaml/lib/year2025/day03/part02/solver.mli b/ocaml/lib/year2025/day03/part02/solver.mli
new file mode 100644
index 0000000..93d3b98
--- /dev/null
+++ b/ocaml/lib/year2025/day03/part02/solver.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the second part of the third day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)