Finish puzzle 3 of 2015 in ocaml

2015/puzzle-3.ml

@@ -0,0 +1,71 @@
+let read_all filename =
+  let ic = open_in filename in
+  let read_char () =
+    try Some (input_char ic)
+    with End_of_file -> None in
+  let rec read_chars cur =
+    match read_char () with
+    | Some s -> read_chars (s :: cur)
+    | None ->
+      close_in ic; List.rev cur in
+  read_chars []
+
+let table_inc table dir =
+  if Hashtbl.mem table dir then
+    let new_rec = (Hashtbl.find table dir) + 1 in
+    Hashtbl.replace table dir new_rec
+  else
+    Hashtbl.add table dir 1
+
+let move_direction table dir direction =
+  let (x, y) = dir in
+  let new_dir =
+    match direction with
+    | '>' -> (x + 1, y)
+    | '<' -> (x - 1, y)
+    | '^' -> (x, y + 1)
+    | 'v' -> (x, y - 1)
+    | _ -> raise (Invalid_argument "Expected one of ><^v")
+  in
+  table_inc table new_dir;
+  new_dir
+
+let rec move_all table current directions =
+  match directions with
+  | [] -> ()
+  | x::xs ->
+    let new_dir = move_direction table current x in
+    move_all table new_dir xs
+
+let table_merge t1 t2 =
+  (* Stolen from https://stackoverflow.com/a/78427785/26861165 *)
+  t2
+  |> Hashtbl.to_seq
+  |> Hashtbl.replace_seq t1;
+  ()
+
+let () =
+  let characters = read_all "3-input" in
+  let table = Hashtbl.create 2081 in
+  table_inc table (0, 0);
+  move_all table (0, 0) characters;
+  Printf.printf "Round 1: %d\n" (Hashtbl.length table);
+
+  let santa = Hashtbl.create 1024 in
+  let robosanta = Hashtbl.create 1024 in
+  table_inc santa (0, 0);
+  table_inc robosanta (0, 0);
+
+  let santa_filter i _ = i mod 2 == 0 in
+  let robosanta_filter i _ = i mod 2 == 1 in
+  let santa_dirs = List.filteri santa_filter characters in
+  let robosanta_dirs = List.filteri robosanta_filter characters in
+
+  move_all santa (0, 0) santa_dirs;
+  move_all robosanta (0, 0) robosanta_dirs;
+  table_merge santa robosanta;
+  Printf.printf "Round 2: %d\n" (Hashtbl.length santa);
+
+(* Local Variables: *)
+(* compile-command: "ocaml puzzle-3.ml" *)
+(* End: *)