From dcdd2bab4da6d094463fe44c467dc7baf1355322 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Wed, 18 Dec 2024 09:44:47 +0000
Subject: [PATCH] Some 2024 day 18 tidy-ups.

---
 bin/day2418.ml | 55 ++++++++++++++++++++++----------------------------
 1 file changed, 24 insertions(+), 31 deletions(-)

diff --git a/bin/day2418.ml b/bin/day2418.ml
index b01f462..460efd0 100644
--- a/bin/day2418.ml
+++ b/bin/day2418.ml
@@ -1,11 +1,13 @@
+(** [pairs_of_ints lst] returns a pair from a list of two elements. *)
 let pairs_of_ints = function
   | [ h; h' ] -> (h, h')
   | _ -> raise (Invalid_argument "pairs_of_ints")
 
+(** [pairs_of_file fname] returns a list of pairs from [fname]. *)
 let pairs_of_file fname =
   Aoc.strings_of_file fname
   |> List.map (Aoc.ints_of_string ~sep:",")
-  |> List.map pairs_of_ints
+  |> List.map pairs_of_ints |> List.to_seq
 
 (** [dijkstra visit check_end states] executes Dijkstra's algorithm.
 
@@ -50,11 +52,12 @@ let grid_get_by_pos grid p =
   let idx = grid_idx_by_pos grid p in
   grid.grid.(idx)
 
-let make_grid width rocks =
+let grid_of_rocks width rocks =
   let grid = { grid = Array.make (width * width) false; width } in
-  let rec impl = function
-    | [] -> grid
-    | p :: t ->
+  let rec impl rocks =
+    match Seq.uncons rocks with
+    | None -> grid
+    | Some (p, t) ->
         grid_set_by_pos grid p true;
         impl t
   in
@@ -74,42 +77,32 @@ let visit grid has_visited cost state =
       (cost + 1, (x, y - 1));
     ]
 
-let check_end dest state = dest = state
 let width = 71
 
-let part1 lst =
-  let count = 1024 in
-  let grid =
-    List.to_seq lst |> Seq.take count |> List.of_seq |> make_grid width
-  in
+let common count rocks =
+  let grid = rocks |> Seq.take count |> grid_of_rocks width in
   let has_visited = Array.make (width * width) false in
-  match
-    dijkstra (visit grid has_visited)
-      (check_end (width - 1, width - 1))
-      [ (0, (0, 0)) ]
-  with
-  | None -> failwith "dijkstra"
+  dijkstra (visit grid has_visited)
+    (( = ) (width - 1, width - 1))
+    [ (0, (0, 0)) ]
+
+let part1 count rocks =
+  match common count rocks with
+  | None -> failwith "part1"
   | Some (cost, _) -> cost
 
-let part2 lst =
+let part2 start_count rocks =
   let rec impl count =
-    let grid =
-      List.to_seq lst |> Seq.take count |> List.of_seq |> make_grid width
-    in
-    let has_visited = Array.make (width * width) false in
-    match
-      dijkstra (visit grid has_visited)
-        (check_end (width - 1, width - 1))
-        [ (0, (0, 0)) ]
-    with
-    | None -> grid_idx_by_pos grid (List.nth lst (count - 1))
-    | Some _ -> impl (count + 1)
+    match common count rocks with None -> count | Some _ -> impl (count + 1)
   in
-  impl 1024
+  let count = impl start_count in
+  match Seq.uncons (Seq.drop (count - 1) rocks) with
+  | None -> failwith "part2"
+  | Some ((x, y), _) -> x + (y * width)
 
 let string_of_idx width idx =
   Printf.sprintf "%d,%d" (idx mod width) (idx / width)
 
 let _ =
   Aoc.main pairs_of_file
-    [ (string_of_int, part1); (string_of_idx width, part2) ]
+    [ (string_of_int, part1 1024); (string_of_idx width, part2 1024) ]