refactor!: invert repo directory structure

now is language first

24 files changed, 603 insertions, 0 deletions

diff --git a/ocaml/lib/application/d01p01.ml b/ocaml/lib/application/d01p01.ml
new file mode 100644
index 0000000..206e757
--- /dev/null
+++ b/ocaml/lib/application/d01p01.ml
@@ -0,0 +1,53 @@
+open Domain.D01p01
+
+let err_empty_input = "input is empty"
+
+let rec input_to_lines input =
+  let lines =
+    String.split_on_char '\n' input |> List.filter (fun l -> l <> "")
+  in
+  pp_lines lines ; lines
+
+and pp_lines = function
+  | [] ->
+      ()
+  | h :: t ->
+      Logs.debug (fun m -> m "Line=%s" h) ;
+      pp_lines t
+
+let parse_line line =
+  Scanf.sscanf line "%c%d" (fun d n ->
+      match d with
+      | 'L' ->
+          Dial.Left n
+      | 'R' ->
+          Dial.Right n
+      | _ ->
+          failwith "invalid direction" )
+
+let lines_to_turns =
+  List.map (fun (l : string) ->
+      Logs.debug (fun m -> m "parsing %s into turn" l) ;
+      let turn = parse_line l in
+      Logs.debug (fun m -> m "%s" (Dial.pp_direction turn)) ;
+      turn )
+
+let rec turn_aux count current = function
+  | [] ->
+      count
+  | h :: t ->
+      let current' = Dial.turn current h in
+      Logs.debug (fun m -> m "dial at %d" (Dial.to_int current')) ;
+      if Dial.to_int current' = 0 then turn_aux (count + 1) current' t
+      else turn_aux count current' t
+
+let turn current = turn_aux 0 current
+
+let solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let lines = input_to_lines input in
+    let turns = lines_to_turns lines in
+    let start = Dial.start in
+    let answer = turn start turns in
+    Ok answer
diff --git a/ocaml/lib/application/d01p01.mli b/ocaml/lib/application/d01p01.mli
new file mode 100644
index 0000000..2eecf19
--- /dev/null
+++ b/ocaml/lib/application/d01p01.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the first part of the first day of advent of code 2025 for [input].
+    The solution for an empty input is [None].
+    Requires: [input] to be a multiline string using `\n` as it's line separator. *)
diff --git a/ocaml/lib/application/d01p02.ml b/ocaml/lib/application/d01p02.ml
new file mode 100644
index 0000000..7910486
--- /dev/null
+++ b/ocaml/lib/application/d01p02.ml
@@ -0,0 +1,55 @@
+open Domain.D01p02
+
+let err_empty_input = "input is empty"
+
+let rec input_to_lines input =
+  let lines =
+    String.split_on_char '\n' input |> List.filter (fun l -> l <> "")
+  in
+  pp_lines lines ; lines
+
+and pp_lines = function
+  | [] ->
+      ()
+  | h :: t ->
+      Logs.debug (fun m -> m "Line=%s" h) ;
+      pp_lines t
+
+let parse_line line =
+  Scanf.sscanf line "%c%d" (fun d n ->
+      match d with
+      | 'L' ->
+          Dial.Left n
+      | 'R' ->
+          Dial.Right n
+      | _ ->
+          failwith "invalid direction" )
+
+let lines_to_turns =
+  List.map (fun (l : string) ->
+      Logs.debug (fun m -> m "parsing %s into turn" l) ;
+      let turn = parse_line l in
+      Logs.debug (fun m -> m "%s" (Dial.pp_direction turn)) ;
+      turn )
+
+let rec turn_aux count dial = function
+  | [] ->
+      count
+  | h :: t -> (
+      let dial_and_count = Turner.turn dial count h in
+      match dial_and_count with
+      | dial', count' ->
+          Logs.debug (fun m ->
+              m "(dial=%d, count=%d)" (Dial.to_int dial') count' ) ;
+          turn_aux count' dial' t )
+
+let turn current = turn_aux 0 current
+
+let solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let lines = input_to_lines input in
+    let turns = lines_to_turns lines in
+    let start = Dial.start in
+    let answer = turn start turns in
+    Ok answer
diff --git a/ocaml/lib/application/d01p02.mli b/ocaml/lib/application/d01p02.mli
new file mode 100644
index 0000000..9e9a2f2
--- /dev/null
+++ b/ocaml/lib/application/d01p02.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the second part of the first day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a multiline string using `\n` as it's line separator. *)
diff --git a/ocaml/lib/application/d02p01.ml b/ocaml/lib/application/d02p01.ml
new file mode 100644
index 0000000..f313149
--- /dev/null
+++ b/ocaml/lib/application/d02p01.ml
@@ -0,0 +1,54 @@
+open Domain.D02p01
+
+let err_empty_input = "input is empty"
+
+(** [input_to_lines input] is a list of strings that have been split on ','.
+    Example: [input_to_lines "12-34,56-78,90-100"] becomes the list \[ "12-34"; "56-78"; "90-100" \].
+    Requires: [input] to be a string in the format "12-34,56-78,90-100". *)
+let input_to_lines input = input |> String.trim |> String.split_on_char ','
+
+let pp_lines = Seq.iter (fun l -> Logs.debug (fun m -> m "line=%s" l))
+
+(** [lines_to_ranges line] is a list of [Range]s found on [line]. *)
+let lines_to_ranges =
+  Seq.map (fun line ->
+      Scanf.sscanf line "%d-%d" (fun id1 id2 -> Range.create (id1, id2)) )
+
+let pp_ranges = Seq.iter (fun r -> Logs.debug (fun m -> m "%s" (Range.pp r)))
+
+(** [collect_all ranges] is an unsorted list of [ID]s within the [ranges] (inclusive). *)
+let collect_all =
+  Seq.concat_map (fun range -> List.to_seq (Range.collect range))
+
+let pp_collection = Seq.iter (fun c -> Logs.debug (fun m -> m "%s" (ID.pp c)))
+
+(** [rules] is a list of rules that [ID]s must pass to be considered invalid. *)
+let rules =
+  [ (fun id -> String.length (ID.to_string id) mod 2 = 0)
+  ; (fun id ->
+      let id' = ID.to_string id in
+      let id_len = String.length id' in
+      let left = String.sub id' 0 (id_len / 2) in
+      let right = String.sub id' (id_len / 2) (id_len / 2) in
+      String.equal left right ) ]
+
+(** [is_invalid id] is [true] for an [id] that passes all of the rules and is considered invalid. *)
+let is_invalid = ID.validate rules
+
+(** [passthrough_with_logs logger lst] is [lst] with the side-effects caused by calling [logger] on [lst]. *)
+let passthrough_with_logs logger lst = logger lst ; lst
+
+let solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let answer =
+      input |> input_to_lines |> List.to_seq
+      |> passthrough_with_logs pp_lines
+      |> lines_to_ranges
+      |> passthrough_with_logs pp_ranges
+      |> collect_all
+      |> passthrough_with_logs pp_collection
+      |> Seq.filter is_invalid
+      |> Seq.fold_left (fun acc id -> acc + ID.to_int id) 0
+    in
+    Ok answer
diff --git a/ocaml/lib/application/d02p01.mli b/ocaml/lib/application/d02p01.mli
new file mode 100644
index 0000000..8a8eb32
--- /dev/null
+++ b/ocaml/lib/application/d02p01.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the first part of the second day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)
diff --git a/ocaml/lib/application/d02p02.ml b/ocaml/lib/application/d02p02.ml
new file mode 100644
index 0000000..d062f7b
--- /dev/null
+++ b/ocaml/lib/application/d02p02.ml
@@ -0,0 +1,86 @@
+open Domain.D02p02
+
+(* Errors *)
+
+let err_empty_input = "input is empty"
+
+(* Logging *)
+
+(** [passthrough_with_logs logger lst] is [lst] with the side-effects caused by calling [logger] on [lst]. *)
+let passthrough_with_logs logger lst = logger lst ; lst
+
+(** [input_to_lines input] is a list of strings that have been split on ','.
+    Example: [input_to_lines "12-34,56-78,90-100"] becomes the list \[ "12-34"; "56-78"; "90-100" \].
+    Requires: [input] to be a string in the format "12-34,56-78,90-100". *)
+let input_to_lines input = input |> String.trim |> String.split_on_char ','
+
+let pp_lines = Seq.iter (fun l -> Logs.debug (fun m -> m "line=%s" l))
+
+let pp_ranges = Seq.iter (fun r -> Logs.debug (fun m -> m "%s" (Range.pp r)))
+
+let pp_collection = Seq.iter (fun c -> Logs.debug (fun m -> m "%s" (ID.pp c)))
+
+(* Core Logic *)
+
+(** [lines_to_ranges line] is a list of [Range]s found on [line]. *)
+let lines_to_ranges =
+  Seq.map (fun line ->
+      Scanf.sscanf line "%d-%d" (fun id1 id2 -> Range.create (id1, id2)) )
+
+(** [collect_all ranges] is an unsorted list of [ID]s within the [ranges] (inclusive). *)
+let collect_all =
+  Seq.concat_map (fun range -> List.to_seq (Range.collect range))
+
+(** [string_to_segments count string] is a list containing [count] string segments with of equal length.
+    Requires: length of [string] to be divisible by [count]
+    and [count] to be greater than or equal to zero *)
+let rec string_to_segments s =
+  let segment_lengths = possible_segment_lengths s in
+  List.map (fun c -> string_to_segments_aux [] c s |> List.rev) segment_lengths
+
+(** [possible_segment_lengths s] is a list of segment sizes that fit within [s]
+    that must be checked to rule out a repeating pattern. *)
+and possible_segment_lengths s =
+  List.init (String.length s) (fun x -> x + 1)
+  |> List.filter (fun x -> x != 1 && String.length s mod x = 0)
+
+and string_to_segments_aux segments count string =
+  match List.length segments = count with
+  | true ->
+      segments
+  | false ->
+      let segment_start = String.length string / count * List.length segments in
+      let segment_len = String.length string / count in
+      string_to_segments_aux
+        (String.sub string segment_start segment_len :: segments)
+        count string
+
+(** [cmp_segments segments] is true is all segments are equal *)
+let cmp_segments segments =
+  List.length (List.sort_uniq String.compare segments) = 1
+
+(** [is_invalid id] is [true] for an [id] that is considered invalid. *)
+let is_invalid id =
+  Logs.debug (fun m -> m "CHECKING ID=%s" id) ;
+  let repeating_patterns =
+    id |> string_to_segments |> List.filter cmp_segments
+  in
+  List.iter
+    (fun l -> Logs.debug (fun m -> m "HAS SUBPATTERN=%s" (List.hd l)))
+    repeating_patterns ;
+  List.length repeating_patterns > 0
+
+let solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let answer =
+      input |> input_to_lines |> List.to_seq
+      |> passthrough_with_logs pp_lines
+      |> lines_to_ranges
+      |> passthrough_with_logs pp_ranges
+      |> collect_all
+      |> passthrough_with_logs pp_collection
+      |> Seq.map ID.to_string |> Seq.filter is_invalid
+      |> Seq.fold_left (fun acc id -> acc + int_of_string id) 0
+    in
+    Ok answer
diff --git a/ocaml/lib/application/d02p02.mli b/ocaml/lib/application/d02p02.mli
new file mode 100644
index 0000000..8a8eb32
--- /dev/null
+++ b/ocaml/lib/application/d02p02.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the first part of the second day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)
diff --git a/ocaml/lib/application/d03p01.ml b/ocaml/lib/application/d03p01.ml
new file mode 100644
index 0000000..522f5d0
--- /dev/null
+++ b/ocaml/lib/application/d03p01.ml
@@ -0,0 +1,23 @@
+open Domain.D03p01
+
+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/application/d03p01.mli b/ocaml/lib/application/d03p01.mli
new file mode 100644
index 0000000..31dd0cd
--- /dev/null
+++ b/ocaml/lib/application/d03p01.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the first 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. *)
diff --git a/ocaml/lib/application/d03p02.ml b/ocaml/lib/application/d03p02.ml
new file mode 100644
index 0000000..743c658
--- /dev/null
+++ b/ocaml/lib/application/d03p02.ml
@@ -0,0 +1,23 @@
+open Domain.D03p02
+
+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/application/d03p02.mli b/ocaml/lib/application/d03p02.mli
new file mode 100644
index 0000000..93d3b98
--- /dev/null
+++ b/ocaml/lib/application/d03p02.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. *)
diff --git a/ocaml/lib/application/d04p01.ml b/ocaml/lib/application/d04p01.ml
new file mode 100644
index 0000000..7c69de4
--- /dev/null
+++ b/ocaml/lib/application/d04p01.ml
@@ -0,0 +1,48 @@
+open Domain
+
+let err_empty_input = "input is empty"
+
+let rec solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let matrix = Matrix.M.of_string input in
+    let count_paper lst = List.length @@ List.filter (fun c -> c <> ".") lst in
+    let accessible_rolls_of_paper = ref 0 in
+    for y = 0 to Array.length matrix - 1 do
+      for x = 0 to Array.length matrix.(y) - 1 do
+        if matrix.(y).(x) = "." then ()
+        else
+          let neighbour_coords = Matrix.M.get_valid_neighbours matrix x y in
+          let neighbours =
+            List.map
+              (fun (x', y') -> Matrix.M.get matrix x' y')
+              neighbour_coords
+          in
+          let paper_count = count_paper neighbours in
+          let is_accessible = paper_count < 4 in
+          if is_accessible then
+            accessible_rolls_of_paper := !accessible_rolls_of_paper + 1 ;
+          print_iteration_debug_msg x y neighbour_coords neighbours paper_count
+            is_accessible
+      done
+    done ;
+    Ok !accessible_rolls_of_paper
+
+and print_iteration_debug_msg x y neighbour_coords neighbours paper_count
+    is_accessible =
+  Logs.debug (fun m ->
+      m
+        "coords=(%d,%d) neighbour_coords=[%s] neighbours=[%s] paper=%d \
+         is_accessible=%B"
+        x y
+        (List.fold_left
+           (fun acc (x, y) ->
+             if acc = "" then
+               " (" ^ string_of_int x ^ "," ^ string_of_int y ^ "); "
+             else acc ^ "(" ^ string_of_int x ^ "," ^ string_of_int y ^ "); " )
+           "" neighbour_coords )
+        (List.fold_left
+           (fun acc neighbour ->
+             if acc = "" then " " ^ neighbour ^ "; " else acc ^ neighbour ^ "; " )
+           "" neighbours )
+        paper_count is_accessible )
diff --git a/ocaml/lib/application/d04p01.mli b/ocaml/lib/application/d04p01.mli
new file mode 100644
index 0000000..873acff
--- /dev/null
+++ b/ocaml/lib/application/d04p01.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the first part of the fourth day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)
diff --git a/ocaml/lib/application/d04p02.ml b/ocaml/lib/application/d04p02.ml
new file mode 100644
index 0000000..75b1236
--- /dev/null
+++ b/ocaml/lib/application/d04p02.ml
@@ -0,0 +1,64 @@
+open Domain
+
+let err_empty_input = "input is empty"
+
+let rec count_accessible_rolls matrix =
+  let count_paper lst = List.length @@ List.filter (fun c -> c = "@") lst in
+  let accessible_rolls_of_paper = ref [] in
+  let no_change = ref false in
+  while not !no_change do
+    let accessible_rolls_of_paper' = !accessible_rolls_of_paper in
+    count_accessible_rolls_aux matrix count_paper accessible_rolls_of_paper ;
+    if
+      List.length accessible_rolls_of_paper'
+      = List.length !accessible_rolls_of_paper
+    then no_change := true
+  done ;
+  List.length !accessible_rolls_of_paper
+
+and count_accessible_rolls_aux matrix count_paper accessible_rolls_of_paper =
+  for y = 0 to Array.length matrix - 1 do
+    for x = 0 to Array.length matrix.(y) - 1 do
+      if matrix.(y).(x) = "." then ()
+      else
+        let neighbour_coords = Matrix.M.get_valid_neighbours matrix x y in
+        let neighbours =
+          List.map (fun (nx, ny) -> Matrix.M.get matrix nx ny) neighbour_coords
+        in
+        let paper_count = count_paper neighbours in
+        let is_accessible = paper_count < 4 in
+        if is_accessible then
+          accessible_rolls_of_paper := (x, y) :: !accessible_rolls_of_paper ;
+        print_iteration_debug_msg x y neighbour_coords neighbours paper_count
+          is_accessible
+    done
+  done ;
+  List.iter
+    (fun (x', y') -> Matrix.M.set matrix x' y' ".")
+    !accessible_rolls_of_paper
+
+and print_iteration_debug_msg x y neighbour_coords neighbours paper_count
+    is_accessible =
+  Logs.debug (fun m ->
+      m
+        "coords=(%d,%d) neighbour_coords=[%s] neighbours=[%s] paper=%d \
+         is_accessible=%B"
+        x y
+        (List.fold_left
+           (fun acc (x, y) ->
+             if acc = "" then
+               " (" ^ string_of_int x ^ "," ^ string_of_int y ^ "); "
+             else acc ^ "(" ^ string_of_int x ^ "," ^ string_of_int y ^ "); " )
+           "" neighbour_coords )
+        (List.fold_left
+           (fun acc neighbour ->
+             if acc = "" then " " ^ neighbour ^ "; " else acc ^ neighbour ^ "; " )
+           "" neighbours )
+        paper_count is_accessible )
+
+let solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let matrix = Matrix.M.of_string input in
+    let accessible_rolls_of_paper = count_accessible_rolls matrix in
+    Ok accessible_rolls_of_paper
diff --git a/ocaml/lib/application/d04p02.mli b/ocaml/lib/application/d04p02.mli
new file mode 100644
index 0000000..388ec56
--- /dev/null
+++ b/ocaml/lib/application/d04p02.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the second part of the fourth day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)
diff --git a/ocaml/lib/application/d05p01.ml b/ocaml/lib/application/d05p01.ml
new file mode 100644
index 0000000..4bf79ab
--- /dev/null
+++ b/ocaml/lib/application/d05p01.ml
@@ -0,0 +1,3 @@
+let err_empty_input = "input is empty"
+
+let solve input () = if input = "" then Error err_empty_input else Ok 0
diff --git a/ocaml/lib/application/d05p01.mli b/ocaml/lib/application/d05p01.mli
new file mode 100644
index 0000000..85c3dd5
--- /dev/null
+++ b/ocaml/lib/application/d05p01.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the first part of the five day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)
diff --git a/ocaml/lib/application/d06p01.ml b/ocaml/lib/application/d06p01.ml
new file mode 100644
index 0000000..9425810
--- /dev/null
+++ b/ocaml/lib/application/d06p01.ml
@@ -0,0 +1,50 @@
+let err_empty_input = "input is empty"
+
+let log_matrix matrix =
+  let open Domain.Matrix in
+  Logs.debug (fun m -> m "%s" (M.of_list matrix |> M.pp (fun elt -> elt)))
+
+let log_list lst =
+  Logs.debug (fun m ->
+      m "[%s]" (List.fold_left (fun acc i -> acc ^ i ^ "; ") " " lst) )
+
+type operation = Mult | Add
+
+let solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let input' = Parser.columns_of_string input in
+    assert (input' <> []) ;
+    log_matrix input' ;
+    let nums =
+      List.map (List.filter (fun i -> i <> "*" && i <> "+")) input'
+      |> List.map
+         @@ List.map (fun n ->
+             Logs.debug (fun m -> m "peeking: %s" n) ;
+             int_of_string n )
+    and ops =
+      List.map (List.filter (fun i -> i = "*" || i = "+")) input'
+      |> List.concat
+      |> List.map (fun op ->
+          match op with
+          | "*" ->
+              Mult
+          | "+" ->
+              Add
+          | _ ->
+              failwith "case not covered" )
+    in
+    let answer =
+      List.map2
+        (fun op num_lst ->
+          match op with
+          | Add ->
+              List.fold_left (fun acc i -> acc + i) 0 num_lst
+          | Mult ->
+              List.fold_left (fun acc i -> acc * i) 1 num_lst )
+        ops nums
+      |> fun lst ->
+      log_list @@ List.map string_of_int lst ;
+      lst |> List.fold_left ( + ) 0
+    in
+    Ok answer
diff --git a/ocaml/lib/application/d06p01.mli b/ocaml/lib/application/d06p01.mli
new file mode 100644
index 0000000..b3abc1e
--- /dev/null
+++ b/ocaml/lib/application/d06p01.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the first part of the sixth day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)
diff --git a/ocaml/lib/application/d06p02.ml b/ocaml/lib/application/d06p02.ml
new file mode 100644
index 0000000..9425810
--- /dev/null
+++ b/ocaml/lib/application/d06p02.ml
@@ -0,0 +1,50 @@
+let err_empty_input = "input is empty"
+
+let log_matrix matrix =
+  let open Domain.Matrix in
+  Logs.debug (fun m -> m "%s" (M.of_list matrix |> M.pp (fun elt -> elt)))
+
+let log_list lst =
+  Logs.debug (fun m ->
+      m "[%s]" (List.fold_left (fun acc i -> acc ^ i ^ "; ") " " lst) )
+
+type operation = Mult | Add
+
+let solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let input' = Parser.columns_of_string input in
+    assert (input' <> []) ;
+    log_matrix input' ;
+    let nums =
+      List.map (List.filter (fun i -> i <> "*" && i <> "+")) input'
+      |> List.map
+         @@ List.map (fun n ->
+             Logs.debug (fun m -> m "peeking: %s" n) ;
+             int_of_string n )
+    and ops =
+      List.map (List.filter (fun i -> i = "*" || i = "+")) input'
+      |> List.concat
+      |> List.map (fun op ->
+          match op with
+          | "*" ->
+              Mult
+          | "+" ->
+              Add
+          | _ ->
+              failwith "case not covered" )
+    in
+    let answer =
+      List.map2
+        (fun op num_lst ->
+          match op with
+          | Add ->
+              List.fold_left (fun acc i -> acc + i) 0 num_lst
+          | Mult ->
+              List.fold_left (fun acc i -> acc * i) 1 num_lst )
+        ops nums
+      |> fun lst ->
+      log_list @@ List.map string_of_int lst ;
+      lst |> List.fold_left ( + ) 0
+    in
+    Ok answer
diff --git a/ocaml/lib/application/d06p02.mli b/ocaml/lib/application/d06p02.mli
new file mode 100644
index 0000000..49444de
--- /dev/null
+++ b/ocaml/lib/application/d06p02.mli
@@ -0,0 +1,4 @@
+val solve : string -> unit -> (int, string) result
+(** [solve input ()] is the solution for the second part of the sixth day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)
diff --git a/ocaml/lib/application/dune b/ocaml/lib/application/dune
new file mode 100644
index 0000000..b78613a
--- /dev/null
+++ b/ocaml/lib/application/dune
@@ -0,0 +1,14 @@
+(library
+  (name application)
+  (libraries
+    domain
+    fmt
+    logs)
+  (inline_tests)
+  (preprocess
+    (pps
+      ppx_deriving.show
+      ppx_deriving.eq
+      ppx_deriving.ord
+      ppx_inline_test
+      ppx_expect)))
diff --git a/ocaml/lib/application/parser.ml b/ocaml/lib/application/parser.ml
new file mode 100644
index 0000000..099900b
--- /dev/null
+++ b/ocaml/lib/application/parser.ml
@@ -0,0 +1,36 @@
+(** [rows_of_string s] is a list of strings where each element is a row of [s]. *)
+let rows_of_string s =
+  s |> String.split_on_char '\n'
+  |> List.filter_map (fun l ->
+      let trimmed = String.trim l in
+      if trimmed <> "" then Some trimmed else None )
+
+(** [columns_of_string ?sep s] is a list of strings where each element is
+    a column of [s] where each column is separated by [sep]. *)
+let rec columns_of_string ?(sep = ' ') s =
+  rows_of_string s
+  |> List.map (fun l ->
+      l |> String.split_on_char sep |> List.filter (fun l -> l <> "") )
+  |> transpose
+
+(** [group_columns lst] is a list where the first element of each
+    sublist of [lst] is grouped in [acc] and so on for the rest of the elements.
+    Example: [group_columns [] \[\["abc"; "def"\]; \["ghi"; "jkl"\]\]] is the list [\[\["abc";"ghi"\];\["def";"jkl"\]\]]. *)
+and transpose lst =
+  assert (lst <> []) ;
+  if List.mem [] lst then []
+  else List.map List.hd lst :: transpose (List.map List.tl lst)
+
+let%expect_test "rows correctly parsed" =
+  let have = {| abc def
+    ghi jkl |} in
+  let rows = rows_of_string have in
+  List.iter (fun c -> print_string (c ^ " ")) rows ;
+  [%expect {| abc def ghi jkl |}]
+
+let%expect_test "columns correctly parsed" =
+  let have = {| abc def
+    ghi jkl |} in
+  let cols = columns_of_string have in
+  List.iter (fun row -> List.iter (fun c -> print_string (c ^ " ")) row) cols ;
+  [%expect {| abc ghi def jkl |}]