feat(2025d4): solve part 1

solution: 1320

3 files changed, 75 insertions, 6 deletions

diff --git a/2025/ocaml/lib/application/d04p01.ml b/2025/ocaml/lib/application/d04p01.ml
index 4bf38ad..260c9e9 100644
--- a/2025/ocaml/lib/application/d04p01.ml
+++ b/2025/ocaml/lib/application/d04p01.ml
@@ -1,4 +1,42 @@
-let solve input () =
-  let matrix = Domain.Matrix.M.of_string input in
-  Logs.debug (fun m -> m "\n%s" @@ Domain.Matrix.M.pp (fun s -> s) matrix) ;
-  0
+open Domain
+
+let rec solve input () =
+  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 ;
+  !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/2025/ocaml/lib/domain/d04p01.ml b/2025/ocaml/lib/domain/d04p01.ml
deleted file mode 100644
index e69de29..0000000
--- a/2025/ocaml/lib/domain/d04p01.ml
+++ /dev/null
diff --git a/2025/ocaml/lib/domain/matrix.ml b/2025/ocaml/lib/domain/matrix.ml
index bb703fc..8983287 100644
--- a/2025/ocaml/lib/domain/matrix.ml
+++ b/2025/ocaml/lib/domain/matrix.ml
@@ -6,13 +6,23 @@ module M : sig
       where each element in the array is initialized to [initial]. *)
 
   val of_list : 'a list list -> 'a t
-  (** *)
+  (** [of_list lst] is a 2D array where every element in the list is an element. *)
 
   val of_string : string -> string t
   (** [of_string str] is a 2D array where every character is an element. *)
 
   val get : 'a t -> int -> int -> 'a
-  (** [get matrix x y] is an alias for [matrix.(y).(x)] *)
+  (** [get matrix x y] is an alias for [matrix.(y).(x)].
+      Requires: [x] and [y] are positive integers or 0. *)
+
+  val set : 'a t -> int -> int -> 'a -> unit
+  (** [set matrix x y v] is an alias for [matrix.(y).(x) <- v].
+      Requires: [x] and [y] are positive integers or 0. *)
+
+  val get_valid_neighbours : 'a t -> int -> int -> (int * int) list
+  (** [get_valid_neighbours matrix x y] is a list of pairs in the format
+      (x * y) that represent the valid surrounding positions of [matrix.(y).(x)].
+      Requires: [x] and [y] are positive integers or 0. *)
 
   val pp : ('a -> string) -> 'a t -> string
   (** [pp matrix string_of_elt] is the string representation of the [matrix]
@@ -40,6 +50,27 @@ end = struct
 
   let get matrix x y = matrix.(y).(x)
 
+  let set matrix x y v = matrix.(y).(x) <- v
+
+  let get_valid_neighbours matrix x y =
+    let potential_neighbours =
+      [ (x - 1, y - 1)
+      ; (x, y - 1)
+      ; (x + 1, y - 1)
+      ; (x - 1, y)
+      ; (x + 1, y)
+      ; (x - 1, y + 1)
+      ; (x, y + 1)
+      ; (x + 1, y + 1) ]
+    in
+    let min_x = 0 in
+    let max_x = Array.length matrix.(y) - 1 in
+    let min_y = 0 in
+    let max_y = Array.length matrix - 1 in
+    List.filter
+      (fun (x, y) -> min_x <= x && x <= max_x && min_y <= y && y <= max_y)
+      potential_neighbours
+
   let pp string_of_elt matrix =
     Array.fold_left
       (fun col_acc arr ->