From 70c53d51735feb8b8a4a82aab70d0d85766febbf Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Sun, 15 Dec 2024 08:10:42 +0000
Subject: [PATCH] 2024 day 15 part 1

---
 bin/day2415.ml | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++
 bin/dune       |  6 ++--
 2 files changed, 79 insertions(+), 2 deletions(-)
 create mode 100644 bin/day2415.ml

diff --git a/bin/day2415.ml b/bin/day2415.ml
new file mode 100644
index 0000000..98fe22c
--- /dev/null
+++ b/bin/day2415.ml
@@ -0,0 +1,75 @@
+type grid = { grid : char array; width : int; height : int }
+
+let grid_make strs =
+  let grid =
+    List.fold_left String.cat "" strs |> String.to_seq |> Array.of_seq
+  in
+  let height = List.length strs in
+  let width = Array.length grid / height in
+  { grid; width; height }
+
+let grid_copy grid = { grid with grid = Array.copy grid.grid }
+let grid_height grid = grid.height
+let[@warning "-32"] grid_length grid = grid.width * grid.height
+
+let[@warning "-32"] grid_print grid =
+  Array.iteri
+    (fun i c ->
+      print_char c;
+      if i mod grid.width = grid.width - 1 then print_newline ())
+    grid.grid
+
+let find_start_idx grid =
+  match Array.find_index (( = ) '@') grid.grid with
+  | None -> failwith "find_start_idx"
+  | Some x -> x
+
+let instrs_of_file fname =
+  let strs = Aoc.strings_of_file fname in
+  let rec impl acc = function
+    | [] | "" :: [] -> failwith "instrs_of_file.impl"
+    | "" :: t -> (List.rev acc, t)
+    | h :: t -> impl (h :: acc) t
+  in
+  let grid, moves = impl [] strs in
+  let grid = grid_make grid in
+  ( grid,
+    List.fold_left String.cat "" moves |> String.to_seq |> List.of_seq,
+    find_start_idx grid )
+
+let rec move_robot grid i di =
+  match grid.grid.(i + di) with
+  | '#' -> i
+  | 'O' ->
+      if move_robot grid (i + di) di = i + di then i else move_robot grid i di
+  | '.' ->
+      grid.grid.(i + di) <- grid.grid.(i);
+      grid.grid.(i) <- '.';
+      i + di
+  | _ -> failwith "move_robot"
+
+let process_move grid robot dir =
+  match dir with
+  | '^' -> move_robot grid robot ~-(grid_height grid)
+  | 'v' -> move_robot grid robot (grid_height grid)
+  | '<' -> move_robot grid robot ~-1
+  | '>' -> move_robot grid robot 1
+  | _ -> failwith "process_move"
+
+let rec process_moves grid robot = function
+  | [] -> grid
+  | h :: t -> process_moves grid (process_move grid robot h) t
+
+let calc_score grid =
+  Array.mapi
+    (fun idx c ->
+      if c = 'O' then (idx mod grid.width) + (100 * (idx / grid.height)) else 0)
+    grid.grid
+  |> Array.fold_left ( + ) 0
+
+let part (grid, moves, robot) =
+  let grid = grid_copy grid in
+  let _ = process_moves grid robot moves in
+  calc_score grid
+
+let _ = Aoc.main instrs_of_file [ (string_of_int, part) ]
diff --git a/bin/dune b/bin/dune
index 8bdfd57..7958c41 100644
--- a/bin/dune
+++ b/bin/dune
@@ -13,7 +13,8 @@
   day2411
   day2412
   day2413
-  day2414)
+  day2414
+  day2415)
  (names
   day2401
   day2402
@@ -28,5 +29,6 @@
   day2411
   day2412
   day2413
-  day2414)
+  day2414
+  day2415)
  (libraries str aoc))