From 0f34d9a931791406d2cabf282742530195f71927 Mon Sep 17 00:00:00 2001
From: DJ O'Leary <dijitol@proton.me>
Date: Mon, 8 Dec 2025 01:55:28 +0100
Subject: refactor(2025,ocaml): use result type for answers to allow for
 returning errors

---
 2025/ocaml/lib/application/d04p01.ml | 46 ++++++++++++++++++++----------------
 1 file changed, 26 insertions(+), 20 deletions(-)

(limited to '2025/ocaml/lib/application/d04p01.ml')

diff --git a/2025/ocaml/lib/application/d04p01.ml b/2025/ocaml/lib/application/d04p01.ml
index 260c9e9..7c69de4 100644
--- a/2025/ocaml/lib/application/d04p01.ml
+++ b/2025/ocaml/lib/application/d04p01.ml
@@ -1,26 +1,32 @@
 open Domain
 
+let err_empty_input = "input is empty"
+
 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
+  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 =
-- 
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