4 files changed, 122 insertions, 0 deletions
diff --git a/ocaml/lib/year2025/day04/part01/solver.ml b/ocaml/lib/year2025/day04/part01/solver.ml
new file mode 100644
index 0000000..7cc54f6
--- /dev/null
+++ b/ocaml/lib/year2025/day04/part01/solver.ml
@@ -0,0 +1,48 @@
+let err_empty_input = "input is empty"
+
+let rec solve input () =
+  if input = "" then Error err_empty_input
+  else
+    let matrix = Shared.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 =
+            Shared.Matrix.M.get_valid_neighbours matrix x y
+          in
+          let neighbours =
+            List.map
+              (fun (x', y') -> Shared.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/year2025/day04/part01/solver.mli b/ocaml/lib/year2025/day04/part01/solver.mli
new file mode 100644
index 0000000..873acff
--- /dev/null
+++ b/ocaml/lib/year2025/day04/part01/solver.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/year2025/day04/part02/solver.ml b/ocaml/lib/year2025/day04/part02/solver.ml
new file mode 100644
index 0000000..e92c982
--- /dev/null
+++ b/ocaml/lib/year2025/day04/part02/solver.ml
@@ -0,0 +1,66 @@
+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 =
+          Shared.Matrix.M.get_valid_neighbours matrix x y
+        in
+        let neighbours =
+          List.map
+            (fun (nx, ny) -> Shared.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') -> Shared.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 = Shared.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/year2025/day04/part02/solver.mli b/ocaml/lib/year2025/day04/part02/solver.mli
new file mode 100644
index 0000000..388ec56
--- /dev/null
+++ b/ocaml/lib/year2025/day04/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 fourth day of advent of code 2025 for [input].
+    The solution for an empty input is [Error msg].
+    Requires: [input] to be a string. *)