solve day 18!

it was kinda ez

.ocamlinit

@@ -1,2 +1,2 @@
 #install_printer Aoc.Vec2.pp;;
-#install_printer Aoc.Vec2.pp_grid;;
+#install_printer Aoc.Grid.pp;;

inputs/day17.tt

@@ -0,0 +1 @@
+><<<<>>>><>>><<><<<<>>>><<>>><>>><<<<>>>><<>>>><<<>>>><<<>>>><<<>>><<<<>><>><<<<>>><><<<><><>>>><<<<><<<<>><<<>>>><<>><><>>>><<<>>><<<>>><<>>>><>>>><>><>>><<>><><<<<><>><<<>>>><<<>>><<<>><>><<<>>><<<<>>>><><<<>>><<<>>><<<<><<<<>>><<<<>>><<<>>><<<<>>><<>>><<><<<>><<<>>><<<<><<<<>>><<<><<>>>><<<>><>>><<>>><<<><<<<>>><<><<<<>>>><<>>>><<>>>><<<<>>><><<<<><>><<<>>>><<<<>><>>>><>><>><>><<<>><<<>>>><<<>><<>><<<<><<<<><<>>>><<<<><<<>>><>><<<>>><<<><<<><<<<><<><<<>>><<<<>>>><<<<><<>><<<<>>><>>><<<<><<<>>>><<><>><<<<>><<<<>>>><<<><<<<><<<<>>><<<><<>>><>>><<<<>>><<>><<<<>>><<<><<<>><<<<>>>><<<<><<><<<<>>>><>>>><<<<>>>><<<>>>><<<><<>>><>>><<<<>><<<<>>><><>><<<>>>><<<<>><<<>><<>><<<>>><<<>>>><<<<>>>><<>>>><<>>>><><<<<>><<<<>>>><<<<>>><<<<>><>><<<>>><>>>><><<>><<>>><<<>>>><>>><<>><<<>><>>><>>><><>><>><<<><<<>>>><>><<<><<<>>><<<<><<>>>><<<><<<>>>><>><<<>>>><<<>>>><>>><><>>>><>><<><<<>>>><<>>>><<<>>><<<<>>><<<>><>>>><<<<>>>><>><<>>><<<<>>>><<<<><>>>><<>><<>>>><>><<<<>>>><<<<>><<<>>>><<>><><>><<<<>><<<>>>><<<>>><<>><><<<>>>><<>><>><<>><>><<<<>><<<>>><<<<>>><<>>>><<>>><<<<>>><>><<><<<>><>>><<<>><<<<>>>><<<>>><<>>><<<>>><<<<>>>><<<<>>><<<>>><<>>>><<>>><<<>>><<><<>>>><<>>>><<><<>>><<><<<>>>><<<<><<<<>><<<>>>><>>><<<<>>><<<<>>>><<>><<<<>>>><<<><<<><<>>>><<<>>>><<<>>>><<<>><<>>><>>><<<<>>><<<<>><<<>>><<<>><<<<>>><<<<><<><>>><<<<>>><<<<>>>><>><<<<>><<<<>>>><<<<><<<>>>><>><<<<><<<<>>>><>><<><<<>>><<>>><<<<>><<<><<<><>><<>>>><<<><<<<><<<>>>><<<<>><<>><<<<>>>><><<<><<<>>>><<>>>><<>><<<<>>>><><><<<<><><<>>><<><<>>><>>><<>><><<><<>>>><<<<><<<>>>><<>><><<>>>><<<<><><<<>>>><<>>><<<><<<>>>><>><<<<>>>><>>><<><>>>><>>>><<<><>>><>>>><<<>><<<>>><<<<>>>><<>>>><<<<>>>><<><>><<<<>><<<<>>><<>>><>>>><<><<>>><>>>><<><<><<><><<<>><>><<<<>>><<<>>>><<<><<<<>><>><>>><<<<><<<>>>><<<>>><><<<<>>><<<<><<<<>>>><>><>>><<<><<>><<<><>>><<<<>><><<<<><>><<>><<>><>><<<<><<<>><>>>><<>><<<<>>><><<<>><<>><<>><>>><>>>><<><>><<<<>>><>>><<>>><<>>>><<>><<<<>>><>><<<>>><<<>>><<<<>>>><<<><<<<>><<><<<><<<>>><<><<>><>><>>><<<>>><<>><<>>><<>><<>><>><<<<>>>><<<<>>><<<>>>><<<<>>><<<>><<<>><<<<>>>><<>><<<<><<<<>><<>>><><<><<<<>>>><><>>>><<<<>>>><<<<>>>><<>><<<<>>>><<<><>><<>>>><<>>>><<<<>>>><<<>>><>><<>>>><<<>>>><<<<>>><<<<>><<<<>>>><<>>><<>>>><>>><>>>><><>><<<<>>>><<<>>>><<<>><<>>><<<<>>><>>>><<>>>><<<><>>><><<>>>><<>>>><<<>>>><<<<><<<<>>>><>>>><>><<<>>><<<>>>><<<>><<<>>><>><<>>><<><><<><<>>>><>>><<<<>><><>><>>><<<<><>>><<<>>><<<><<<<>>><<<>><<<>>><<>>><<<<><<<<><>><><<<<>>><<<>><<>><<<>><<<<>><>><>>>><<>>><<<>><<<>><<<<>><><<<<>>><<<>>>><<<<>>><<>><>><<>>><><<>>><<<>>><<<>><<>>>><<<<>><<<<>>>><<<><<><<<<>><<<<>><<<>>>><>><<<><>>>><<>>>><<><<<>>>><<<<>><<<>>>><<>>><>><<<<>>>><<<<>>><>>>><<<<><<><<>><>><>>><<<><<<<><<<<>><<<>>><<<<>><<><<<<>>>><<<>>>><<<>>>><<<<>>><<><<>><<><<<<>>>><<<><<<<>>>><<>>><<<<>><>>>><<<<>><<<>>>><<><<<<>>>><<<>><>>>><><<<>><<<><<<>>><>>>><>>><<<<><<<<>>><<>>><<<>>><<<<><<<<>>>><<>>><<<>><<>>><><<<>>>><<><>>><<>><<<>>>><<<><>><<<>>><<>>>><<<>>>><>><<><>><<>>>><<<>>>><<<<>>>><<<>>><<<>><<<<><<<<>><><<><<<>><>>><<<<>>><<<<>><<<>>><<>><<<<><<<><<<><<<>>><<<<>>><<<<>><<>>><<>><<<<>>>><<>>>><<>>>><<>><>>><>>>><<>>>><<<>>><<<<>>><>>><>>>><<<>><<>>><<>>>><<>>>><<<>>><<>>><<<><<<>><>>>><<<>><>>><<<>>><>>>><>><<>>><<<<>>><<<<>><>>><<<<>>><<<<><<<>>><><>>><<<>>><>><>>>><<>>>><<<>><<>>><<>><<<<>><<<<><>>><<>><<<<>>><<<>>><<>>>><>>>><<<><>>>><<><><>>><<>>>><<<>>>><><>><<<<>><<<>>>><<<<><<<<>>>><<<<>><>>>><<<>>>><<><<>>>><<<<>>>><<<<>>><>><>>>><>>>><>><<>>>><>>>><<>>><<<<>>>><<>>><><<>>>><<>><<<<><<><>>>><<<<>><<><<><<>><<<><<<>><<<<>>><<<<>><<>><>>><<<<>>>><<<<>>>><<<<>>><><<<<><<<<><><<<<><<<<>>>><<<<>>>><><>>><<<<><<<<><<<><><<<<>><><<<><<<<>><>>><><><<<><>>><<><<>><<<>><<<>>>><<<<>>><<>><<<<><>>><>>><<>>>><<<<>>><<<>>>><<<>>>><<<>><<<>><<<<>>><<<<>><>>><<<<><>>>><<<>><>>><<>>><<<<>><<><<<>><>>><<<>><<<<>>><<<>>>><>>><><<<><<<>>>><<>>><<><>>>><>>>><<<<>>><><><>>>><<<<>><<<<>>><<>>><<<><<<>>>><<><<<>><<<>>>><<<<>>>><>>><<<>>><><<<>><<<<>><<<>>>><>><<><><<>><<<>><<<>>><<<<>>>><<<><<<<>><<<<>>>><<<<>>>><<><<<<>>><<<<>><<<<>><<<>><<<<>>><<>>>><<>>><>><<>>>><>>><>>><<<><>>>><<<>>><<<<><<<<>>><<><><>>>><<>>>><<>>>><<<<>>>><<><<><<<>>><<<>>>><<><<<><<>>><<<><<<>>>><<<<>>><<>>>><<>><><<<><<>><<>>><<<<>><>><<<>><<<<>><<<<><<>>><<><<<<>>>><<<>><<<<>><<>>>><>>>><>>><>>>><<<>>><<<>>>><<>>>><>><>><<<>><<<>>><<>>><<<<>>><<<<>>><<>>>><>>><>><<<<>>>><<>>>><<>>>><<<<>>>><>>>><><>>><<>>>><<<<><<>>>><<<<>><<<<>>>><<>><<<<><<<<>>><<<<>><<>>>><<<>>>><<<><>><<<><<>>><<<<>><<<>>><><<<<>><<<<>><>>><<<<>>><<<<>>>><>>>><<>>>><<>>><>>>><<<<>>><<>><<<><<><<<>>>><<<>>>><<<<><<>>><>><<<<><<<>>><>><><<<><<>>>><>>>><>><<>>><<>>>><<>>>><<<<>><<>><><<>>><<>><<>><<>>><>><<<<>>>><<>>>><<<<>>><<>><><<<><>><<<>><>>><>><<>>><><>>><<>>>><<<>>><<<<>>><<<>><<>><<<<>>><<<><<<>><<<<><<>><>><<<<>><<>>>><<<>><<<>>>><<<<>>>><<<<>><<>>><<<>>><<<>>>><<<<>>><<<<>>>><>><<<<>>>><<>>><><<>>>><<<<>>>><<<<>>>><>><<>><<<<><<<<>>><><>>><<<<>>><<><>>>><<<<><<<>>>><>>>><<<>>>><<<>>>><<>>><<>>><<<<><<<>><>>><<<><<<>>><Line truncated

src/day12.ml

@@ -37,7 +37,8 @@ let reachable f t =
 let get_neighbors grid node_pos =
   let node = Grid.at_e grid node_pos in
   Vec2.directions
-  |> List.filter_map (fun dir ->
+  |> Iter.of_list
+  |> Iter.filter_map (fun dir ->
     let neighbor_pos = Vec2.(node_pos + dir) in
     match Grid.at grid neighbor_pos with
     | Some neighbor when reachable node.char neighbor.char -> Some neighbor_pos

src/day14.ml

@@ -9,7 +9,7 @@ type tile =
   | Path_L
   | Path_R
 
-let char_of_tile = function
+let char_of_tile _ = function
   | Empty -> '-'
   | Rock -> '#'
   | Sand -> 'o'

src/day16.ml

@@ -47,9 +47,10 @@ let parse_valves lines =
           ~get_weight:(fun _ _ -> 1)
           ~get_neighbors:(fun room ->
             match Hashtbl.get rooms room with
-            | Some (_, tunnels) -> tunnels
+            | Some (_, tunnels) -> Iter.of_list tunnels
             | None -> failwith "sad")
           ~start_nodes:[ from ]
+          ()
         %> Option.get_exn_or "big sad")
     in
     valves

src/day17.ml

@@ -0,0 +1,201 @@
+open Containers
+
+let cave_width = 7
+let example_lines = [ ">>><<><>><<<>><>>><<<>>><<<><<<>><>><<>>" ]
+
+let dir_of_char = function
+  | '<' -> Vec2.left
+  | '>' -> Vec2.right
+  | _ -> failwith "oof"
+;;
+
+let next_stream_idx stream idx = (idx + 1) mod Array.length stream
+
+(*[{
+                       #
+         #       #     #
+        ###      #     #     ##
+ ####    #     ###     #     ##
+
+ row    plus   angle   col   square
+
+}]*)
+type shape =
+  | Row
+  | Plus
+  | Angle
+  | Col
+  | Square
+
+let get_shape idx = [| Row; Plus; Angle; Col; Square |].(idx mod 5)
+
+let shape_points shape pos =
+  let points =
+    match shape with
+    | Row -> [ 0, 0; 1, 0; 2, 0; 3, 0 ]
+    | Plus -> [ 0, 1; 1, 0; 1, 1; 1, 2; 2, 1 ]
+    | Angle -> [ 0, 0; 1, 0; 2, 0; 2, 1; 2, 2 ]
+    | Col -> [ 0, 0; 0, 1; 0, 2; 0, 3 ]
+    | Square -> [ 0, 0; 1, 0; 0, 1; 1, 1 ]
+  in
+  points |> List.map (fun point -> Vec2.(pos + of_tuple point))
+;;
+
+type loop_key =
+  { rock : shape
+  ; stream_idx : int
+  ; normalized_topo : int array
+  }
+
+type loop =
+  { rock_idx : int
+  ; height : int
+  }
+
+type cave =
+  { grid : char Grid.t
+  ; mutable grid_topo : int array
+  ; mutable grid_height : int
+  ; total_rocks : int
+  ; mutable rocks : int
+  ; mutable stream_idx : int
+  ; loop_cache : (loop_key, loop) Hashtbl.t
+  ; mutable height : int
+  }
+
+let init_cave max_height total_rocks =
+  let grid = Grid.init ~width:7 ~height:max_height (Fun.const '.') in
+  { grid
+  ; height = -1
+  ; grid_height = -1
+  ; rocks = 0
+  ; stream_idx = 0
+  ; grid_topo = Array.init cave_width (Fun.const 0)
+  ; loop_cache = Hashtbl.create max_height
+  ; total_rocks
+  }
+;;
+
+let draw_cave cave = Grid.draw cave.grid ~rev_y:true (fun _ contents -> contents)
+
+let place_shape cave shape pos =
+  shape_points shape pos
+  |> List.iter (fun point ->
+    let delta = point.Vec2.y - cave.grid_height in
+    if delta > 0
+    then (
+      cave.height <- cave.height + delta;
+      cave.grid_height <- cave.grid_height + delta);
+    cave.grid_topo.(point.x) <- max point.y cave.grid_topo.(point.x);
+    Grid.set_e cave.grid point '#')
+;;
+
+let can_place_shape cave shape pos =
+  shape_points shape pos
+  |> List.fold_while
+       (fun _ point ->
+         match cave.grid.Grid.%(point) with
+         | Some '.' -> true, `Continue
+         | _ -> false, `Stop)
+       false
+;;
+
+let drop_shape cave stream =
+  let shape = get_shape cave.rocks in
+  let rec drop pos =
+    let stream_pos = Vec2.(pos + stream.(cave.stream_idx)) in
+    cave.stream_idx <- next_stream_idx stream cave.stream_idx;
+    let pos = if can_place_shape cave shape stream_pos then stream_pos else pos in
+    let drop_pos = Vec2.(pos + Vec2.up) in
+    if can_place_shape cave shape drop_pos
+    then drop drop_pos
+    else place_shape cave shape pos
+  in
+  cave.rocks <- cave.rocks + 1;
+  drop Vec2.{ x = 2; y = Int.(cave.grid_height + 4) }
+;;
+
+let normalize_topo topo =
+  let min = Array.min_exn Int.compare topo in
+  Array.map (fun x -> x - min) topo
+;;
+
+let rec drop_shapes cave stream =
+  let loop_key =
+    { rock = get_shape cave.rocks
+    ; stream_idx = cave.stream_idx
+    ; normalized_topo = normalize_topo cave.grid_topo
+    }
+  in
+  let remaining_rocks cave = cave.total_rocks - cave.rocks in
+  let loop_rocks loop = cave.rocks - loop.rock_idx in
+  let loop_fits loop = loop_rocks loop < remaining_rocks cave in
+  match Hashtbl.get cave.loop_cache loop_key with
+  | Some loop when loop_fits loop ->
+    let loop_height = cave.height - loop.height in
+    cave.height <- cave.height + loop_height;
+    cave.rocks <- cave.rocks + loop_rocks loop;
+    drop_shapes cave stream
+  | _ when remaining_rocks cave > 0 ->
+    let loop = { height = cave.height; rock_idx = cave.rocks } in
+    Hashtbl.add cave.loop_cache loop_key loop;
+    drop_shape cave stream;
+    drop_shapes cave stream
+  | _ -> ()
+;;
+
+let solve remaining_rocks lines =
+  let stream = lines |> List.hd |> Iter.of_str |> Iter.map dir_of_char |> Iter.to_array in
+  let cave = init_cave 100_000 remaining_rocks in
+  drop_shapes cave stream;
+  cave.height + 1
+;;
+
+let%expect_test "Day 17.1 Example" =
+  Printf.printf "height: %i\n" @@ solve 2022 example_lines;
+  [%expect {| height: 3068 |}]
+;;
+
+let%expect_test "Day 17.1" =
+  Printf.printf "height: %i\n" @@ solve 2022 (Utils.lines_of_input 17);
+  [%expect {| height: 3227 |}]
+;;
+
+let%expect_test "Day 17.2 Example" =
+  Printf.printf "height: %i\n" @@ solve 1_000_000_000_000 example_lines;
+  [%expect {| height: 1514285714288 |}]
+;;
+
+let%expect_test "Day 17.2" =
+  Printf.printf "height: %i\n" @@ solve 1_000_000_000_000 (Utils.lines_of_input 17);
+  [%expect {| height: 1597714285698 |}]
+;;
+
+let%expect_test "Smol tower" =
+  let stream =
+    example_lines |> List.hd |> Iter.of_str |> Iter.map dir_of_char |> Iter.to_array
+  in
+  let cave = init_cave 18 8 in
+  drop_shapes cave stream;
+  Printf.printf "%s" @@ draw_cave cave;
+  [%expect
+    {|
+    .......
+    .......
+    .......
+    .....#.
+    .....#.
+    ..####.
+    .###...
+    ..#....
+    .####..
+    ....##.
+    ....##.
+    ....#..
+    ..#.#..
+    ..#.#..
+    #####..
+    ..###..
+    ...#...
+    ..####. |}]
+;;

src/day18.ml

@@ -0,0 +1,121 @@
+open Containers
+
+let example_lines =
+  String.lines
+    {|2,2,2
+1,2,2
+3,2,2
+2,1,2
+2,3,2
+2,2,1
+2,2,3
+2,2,4
+2,2,6
+1,2,5
+3,2,5
+2,1,5
+2,3,5|}
+;;
+
+module Vec3 = struct
+  type t = int * int * int [@@deriving show, eq, ord, hash]
+
+  let directions = [ 1, 0, 0; -1, 0, 0; 0, 1, 0; 0, -1, 0; 0, 0, 1; 0, 0, -1 ]
+  let ( + ) (x1, y1, z1) (x2, y2, z2) = x1 + x2, y1 + y2, z1 + z2
+  let ( - ) (x1, y1, z1) (x2, y2, z2) = x1 - x2, y1 - y2, z1 - z2
+  let ( = ) = equal
+
+  let iter_region (ax, ay, az) (bx, by, bz) callback =
+    for z = az to bz do
+      for y = ay to by do
+        for x = ax to bx do
+          let p = x, y, z in
+          callback p
+        done
+      done
+    done
+  ;;
+
+  let in_bounds (lx, ly, lz) (hx, hy, hz) (x, y, z) =
+    not (x < lx || x > hx || y < ly || y > hy || z < lz || z > hz)
+  ;;
+
+  let find_bounds points =
+    let first = Iter.head_exn points in
+    let padding = 1, 1, 1 in
+    let min, max =
+      points
+      |> Iter.drop 1
+      |> Iter.fold
+           (fun ((lx, ly, lz), (hx, hy, hz)) (x, y, z) ->
+             (min lx x, min ly y, min lz z), (max hx x, max hy y, max hz z))
+           (first, first)
+    in
+    min - padding, max + padding
+  ;;
+end
+
+module Vec3Set = Set.Make (Vec3)
+
+let parse_points lines =
+  lines
+  |> List.map (fun line ->
+    match List.map Int.of_string_exn Re.(matches (Pcre.regexp {|\d+|}) line) with
+    | [ x; y; z ] -> x, y, z
+    | _ -> failwith "oopzi doopers")
+  |> Vec3Set.of_list
+;;
+
+let solve_part_1 lines =
+  let points = parse_points lines in
+  let min, max = Vec3.find_bounds @@ Vec3Set.to_iter points in
+  let surface_area = ref 0 in
+  Vec3.iter_region min max (fun point ->
+    if not @@ Vec3Set.mem point points
+    then
+      Vec3.directions
+      |> List.iter (fun dir ->
+        let n = Vec3.(point + dir) in
+        if Vec3Set.mem n points then incr surface_area));
+  !surface_area
+;;
+
+let%expect_test "Day 18.1 example" =
+  Format.printf "%i" @@ solve_part_1 example_lines;
+  [%expect {| 64 |}]
+;;
+
+let%expect_test "Day 18.1" =
+  Format.printf "%i" @@ solve_part_1 @@ Utils.lines_of_input 18;
+  [%expect {| 3662 |}]
+;;
+
+let solve_part_2 lines =
+  let points = parse_points lines in
+  let min, max = Vec3.find_bounds @@ Vec3Set.to_iter points in
+  let in_bounds = Vec3.in_bounds min max in
+  let surface_area = ref 0 in
+  let directions_iter = Iter.of_list Vec3.directions in
+  let get_neighbors point =
+    directions_iter
+    |> Iter.map Vec3.(fun dir -> point + dir)
+    |> Iter.filter (fun neighbor ->
+      let in_droplet = Vec3Set.mem neighbor points in
+      if in_droplet then incr surface_area;
+      (not in_droplet) && in_bounds neighbor)
+  in
+  let _ignore =
+    Utils.djikstra_hash ~get_weight:(fun _ _ -> 1) ~get_neighbors ~start_nodes:[ min ] ()
+  in
+  !surface_area
+;;
+
+let%expect_test "Day 18.2 example" =
+  Format.printf "%i" @@ solve_part_2 example_lines;
+  [%expect {| 58 |}]
+;;
+
+let%expect_test "Day 18.2" =
+  Format.printf "%i" @@ solve_part_2 @@ Utils.lines_of_input 18;
+  [%expect {| 2060 |}]
+;;

src/grid.ml

@@ -27,13 +27,15 @@ let at grid point =
 ;;
 
 let at_e (grid : 'a t) (point : Vec2.t) =
-  Option.get_exn_or "point out of bounds!" @@ at grid point
+  match at grid point with
+  | Some point -> point
+  | None -> failwith @@ Format.sprintf "point %a out of bounds!" Vec2.pp point
 ;;
 
 let set_e grid point item =
   if in_bounds grid point
   then Array.set grid.items (idx_of_vec2 grid.width point) item
-  else failwith "point out of bounds!"
+  else failwith (Format.sprintf "point %a out of bounds!" Vec2.pp point)
 ;;
 
 let ( .%() ) = at
@@ -82,12 +84,16 @@ let iter grid callback =
   iter_region grid a b callback
 ;;
 
-let draw grid printer =
-  let str = ref [] in
+let draw grid ?(rev_y = false) ?(rev_x = false) printer =
+  let line = ref [] in
+  let lines = ref [] in
   iter grid (fun point ->
     let item = grid.%(point) in
-    let char = printer @@ Option.get_exn_or "iter is broken" item in
-    str := !str @ [ char ];
-    if point.x = grid.width - 1 then str := !str @ [ '\n' ]);
-  String.of_list !str
+    let char = printer point (Option.get_exn_or "iter is broken" item) in
+    line := if rev_x then char :: !line else !line @ [ char ];
+    if List.length !line = grid.width
+    then (
+      lines := !lines @ [ String.of_list !line ];
+      line := []));
+  String.concat "\n" (if rev_y then List.rev !lines else !lines)
 ;;

src/grid.mli

@@ -11,4 +11,4 @@ val find : 'a t -> ('a -> bool) -> (Vec2.t * 'a) option
 val count : 'a t -> ('a -> bool) -> int
 val iter_region : 'a t -> Vec2.t -> Vec2.t -> (Vec2.t -> unit) -> unit
 val iter : 'a t -> (Vec2.t -> unit) -> unit
-val draw : 'a t -> ('a -> char) -> string
+val draw : 'a t -> ?rev_y:bool -> ?rev_x:bool -> (Vec2.t -> 'a -> char) -> string

src/utils.ml

@@ -50,7 +50,7 @@ let djikstra ~eq ~get_info ~set_info ~get_weight ~get_neighbors ~goal ~start_nod
     let c_dist, _ = get_info c_node in
     set_info c_node (c_dist, true);
     get_neighbors c_node
-    |> List.iter (fun neighbor ->
+    |> Iter.iter (fun neighbor ->
       let n_dist, n_visited = get_info neighbor in
       if not n_visited
       then (
@@ -90,6 +90,7 @@ let djikstra_hash
   ~get_weight
   ~get_neighbors
   ~start_nodes
+  ()
   =
   let module Tbl =
     CCHashtbl.Make (struct