Use same printer for both parts

We want to change Aoc.main to take a single printer parameter to
simplify the run process.

bin/day2418.ml

@@ -104,12 +104,12 @@ let find_route_length count grid =
 let part1 count rocks =
   match find_route_length count rocks with
   | None -> failwith "part1"
-  | Some (cost, _) -> cost
+  | Some (cost, _) -> string_of_int cost
 
 (** [part2 start_count grid] returns the location of the first rock to fall into
     [grid] which makes it impossible to get from the top-left to bottom-right.
 *)
-let part2 start_count grid =
+let part2 width start_count grid =
   (* Implementation notes:
   
      We do this by binary search in impl.  The left_count is a known count of
@@ -137,16 +137,11 @@ let part2 start_count grid =
   let count = impl start_count (1 + count_rocks 0 0) in
   match Array.find_index (( = ) (count - 1)) grid.grid with
   | None -> failwith "part2"
-  | Some idx -> idx
-
-(** [string_of_idx width idx] prints the (x, y) location for a given index in a
-    grid. *)
-let string_of_idx width idx =
-  Printf.sprintf "%d,%d" (idx mod width) (idx / width)
+  | Some idx -> Printf.sprintf "%d,%d" (idx mod width) (idx / width)
 
 (** Width of grid *)
 let width = 71
 
 let _ =
   Aoc.main (grid_of_file width)
-    [ (string_of_int, part1 1024); (string_of_idx width, part2 1024) ]
+    [ (Fun.id, part1 1024); (Fun.id, part2 width 1024) ]