From 4b3c8eb99443ccaeca831bba8e6604f1036fa301 Mon Sep 17 00:00:00 2001
From: DJ O'Leary <dijitol@proton.me>
Date: Mon, 8 Dec 2025 01:28:45 +0100
Subject: feat(2025d4): solve part 1

solution: 1320
---
 2025/ocaml/lib/application/d04p01.ml | 46 ++++++++++++++++++++++++++++++++----
 1 file changed, 42 insertions(+), 4 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 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 )
-- 
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