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 implode l = String.of_seq (List.to_seq l)
let (<<) f g x = f(g(x))
let (>>) f g x = g(f(x))
let rec find_idx x = function (* find index of x *)
| [] -> raise (Failure "Not Found")
| h :: t -> if x = h then 0 else 1 + find_idx x t
let abs x = if x < 0 then -x else x
(* --- *)
type instruction = { action: char; value: int }
let dirs = [('N', (0, 1)); ('E', (1, 0)); ('S', (0, -1)); ('W', (-1, 0))]
let dir_list = ['N'; 'E'; 'S'; 'W']
let parse_line = (* from string to instruction *)
explode >> function | action :: value -> { action; value = int_of_string @@ implode value }
let rec turn direction (wx, wy) = function (* returns new waypoint location *)
| 0 -> (wx, wy) (* doing this 90 degrees at a time *)
| degrees ->
let new_position = match direction with
| 'L' -> (-1 * wy, wx)
| 'R' -> (wy, -1 * wx)
in turn direction new_position (degrees - 90)
let move { action; value } (x, y) (wx, wy) = (* returns (new ship location, new waypoint location) *)
Printf.printf "@ (%i, %i). moving %c %i\n" x y action value;
match action with
| 'L' -> ((x, y), turn 'L' (wx, wy) value)
| 'R' -> ((x, y), turn 'R' (wx, wy) value)
| 'F' -> ((x + (wx * value), y + (wy * value)), (wx, wy))
| dir -> (* Either N, E, S, or W. Find the (dx, dy) and apply it *)
let (dx, dy) = List.assoc dir dirs in
((x, y), (wx + (dx * value), wy + (dy * value)))
let rec travel (x, y) (wx, wy) map =
match map with
| [] -> (x, y)
| line :: rest ->
let (new_position, waypoint_position) = move line (x, y) (wx, wy) in
travel new_position waypoint_position rest
let manhattan (x, y) = (abs x) + abs y
let () =
let map = read_file "day12.txt" |> List.map parse_line in
let (x, y) = map |> travel (0, 0) (10, 1) in
Printf.printf "ending location (%i, %i). manhattan distance: %i\n" x y (manhattan (x, y));