Tidy up code for 2024 day 12.

This still has some mutable state.

lib/aoc.ml

@@ -54,12 +54,16 @@ module Grid = struct
   let length grid = String.length grid.grid
   let pos_of_idx grid idx = (idx mod grid.width, idx / grid.width)
   let idx_of_pos grid (x, y) = x + (y * grid.width)
-  let get_by_idx grid idx = grid.grid.[idx]
-  let get_by_pos grid pos = get_by_idx grid (idx_of_pos grid pos)
 
   let pos_is_valid grid (x, y) =
     x >= 0 && x < grid.width && y >= 0 && y < grid.height
 
+  let get_by_idx grid idx = grid.grid.[idx]
+  let get_by_pos grid pos = get_by_idx grid (idx_of_pos grid pos)
+
+  let get_by_pos_opt grid pos =
+    if pos_is_valid grid pos then Some (get_by_pos grid pos) else None
+
   let idx_from_opt grid = String.index_from_opt grid.grid
 
   let update_pos grid pos c =