Advent of Code 2020 https://adventofcode.com/
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(* Common util. Should probably put this in a module or something.. *)
let get_one_line file =
try Some (input_line file) with
End_of_file -> None
let get_lines file =
let rec input lines =
match get_one_line file with
Some line -> input (line :: lines)
| None -> List.rev lines
in input []
let read_file file =
let channel = open_in(file) in
get_lines channel
let explode s = (* convert a string to a list of chars *)
let rec exp i l =
if i < 0 then l else exp (i - 1) (s.[i] :: l) in
exp (String.length s - 1) [];;
let print_grid l =
print_newline ();
List.iter (fun line ->
List.iter (Printf.printf "%c") line;
Printf.printf "\n") l
let (<<) f g x = f(g(x))
(* --- *)
let get_value l (x, y) = (* if in bounds, Some of the value, else None *)
try let row = List.nth l x in
Some (List.nth row y)
with
| Failure _ -> None
| Invalid_argument _ -> None
let dirs = [(-1, -1); (-1, 0); (-1, 1); (0, -1); (0, 1); (1, -1); (1, 0); (1, 1)]
let get_neighbors (x, y) l = (* returns list of Options. None means off the grid *)
let coords = List.map (fun (dx, dy) -> (x + dx, y + dy)) dirs in
List.map (get_value l) coords
let rec walk (x, y) l (dx, dy) = (* walk from (x, y) in direction (dx, dy) until you hit something or fall off *)
let coord = (x + dx, y + dy) in
match get_value l coord with
| Some '.' -> walk coord l (dx, dy)
| a -> a
let get_far_neighbors (x, y) l = (* returns list of Options. None means off the grid *)
List.map (walk (x, y) l) dirs
let num_occupied = (* count the number of # in a List of char options *)
List.length << List.filter (function
| None -> false
| Some v -> Char.equal v '#')
let change_seat is_far (x, y) l = (* is_far is for the 2nd problem *)
let neighbors_fun = if is_far then get_far_neighbors else get_neighbors in
let max_neighbors = if is_far then 5 else 4 in
let neighbors = neighbors_fun (x, y) l in (* make a list of all neighbors, either near or far *)
match get_value l (x, y) with
| None -> '?' (* eh? *)
| Some 'L' -> if num_occupied neighbors = 0 then '#' else 'L'
| Some '#' -> if num_occupied neighbors >= max_neighbors then 'L' else '#'
| Some s -> s
let change_seats is_far l =
List.mapi (fun x ->
List.mapi (fun y _ ->
change_seat is_far (x, y) l
)
) l
let rec start is_far grid = (* keep changing seats until the grid stops changing *)
(* print_grid grid; *)
let grid' = change_seats is_far grid in
if grid = grid'
then grid'
else start is_far grid'
let () =
let grid = read_file "day11.txt" |> List.map explode in
start false grid (* false means near-neighbors *)
|> List.flatten
|> List.filter (Char.equal '#') |> List.length (* count occupied seats *)
|> Printf.printf "near-neighbor count %i\n";
start true grid
|> List.flatten
|> List.filter (Char.equal '#') |> List.length
|> Printf.printf "far-neighbor count %i\n";