76 lines
2.9 KiB
OCaml
76 lines
2.9 KiB
OCaml
(** [perimeter grid pos] returns the number of sides of [pos] that contribute to
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the perimeter of the region that [pos] is part of in [grid]. *)
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let perimeter grid (x, y) =
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let region = Aoc.Grid.get_by_pos grid (x, y) in
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let check (dx, dy) =
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Aoc.Grid.get_by_pos_opt grid (x + dx, y + dy) = Some region
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in
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[ (-1, 0); (1, 0); (0, 1); (0, -1) ]
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|> List.map check |> List.map Bool.to_int |> List.fold_left ( + ) 0
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(** [is_corner grid pos dir] returns true if there is a region corner on the
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[dir] side of [pos] in grid. *)
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let is_corner grid (x, y) (dx, dy) =
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let region = Aoc.Grid.get_by_pos grid (x, y) in
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if
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Aoc.Grid.get_by_pos_opt grid (x + dx, y) <> Some region
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&& Aoc.Grid.get_by_pos_opt grid (x, y + dy) <> Some region
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then true
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else if
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Aoc.Grid.get_by_pos_opt grid (x + dx, y) = Some region
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&& Aoc.Grid.get_by_pos_opt grid (x, y + dy) = Some region
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&& Aoc.Grid.get_by_pos_opt grid (x + dx, y + dy) <> Some region
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then true
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else false
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(** [corners grid pos] returns the count of the number of corners that [pos] is
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on for its region in [grid]. *)
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let corners grid pos =
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[ (-1, -1); (-1, 1); (1, 1); (1, -1) ]
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|> List.map (is_corner grid pos)
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|> List.map Bool.to_int |> List.fold_left ( + ) 0
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(** [find_regions calc grid] finds all the regions in [grid] and returns a list
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containing an element for each region. The element is a pair [(p, a)] where
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[p] is the sum of calling [calc grid pos] for every element in the region
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and [a] is the area of the region. *)
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let find_regions calc grid =
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let visited = Array.make (Aoc.Grid.length grid) false in
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let add_pos region pos lst =
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if Aoc.Grid.get_by_pos_opt grid pos = Some region then
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Aoc.Grid.idx_of_pos grid pos :: lst
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else lst
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in
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let rec scan_pos perimeter area = function
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| [] -> (perimeter, area)
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| idx :: t when visited.(idx) -> scan_pos perimeter area t
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| idx :: t (* when not visited.(idx) *) ->
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visited.(idx) <- true;
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let x, y = Aoc.Grid.pos_of_idx grid idx in
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let area = area + 1 in
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let perimeter = perimeter + calc grid (x, y) in
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let region = Aoc.Grid.get_by_idx grid idx in
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let t = add_pos region (x - 1, y) t in
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let t = add_pos region (x + 1, y) t in
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let t = add_pos region (x, y - 1) t in
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let t = add_pos region (x, y + 1) t in
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scan_pos perimeter area t
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in
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let rec impl acc idx =
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if idx >= Aoc.Grid.length grid then List.rev acc
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else
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let perimeter, area = scan_pos 0 0 [ idx ] in
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if area = 0 then impl acc (idx + 1)
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else impl ((perimeter, area) :: acc) (idx + 1)
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in
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impl [] 0
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(** [part calc grid] returns the result of Part N over [grid] where [calc] is
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the perimeter/edge counting function. *)
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let part calc grid =
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find_regions calc grid |> List.fold_left (fun acc (p, a) -> acc + (p * a)) 0
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let _ =
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Aoc.main Aoc.Grid.of_file
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[ (string_of_int, part perimeter); (string_of_int, part corners) ]
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