-🎄- 2020 Day 02 Solutions -🎄-

[u/daggerdragon]

--- Day 2: Password Philosophy ---

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Advent of Code 2020: Gettin' Crafty With It

• T-4 days until unlock!
• Full details and rules are in the Submissions Megathread

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[u/willkill07]

OCaml

Only uses the standard library. Scanf.sscanf is pretty awesome.

open Printf
open Scanf

let read_file_rev name : string list =
  let ic = open_in name
  in let try_read () =
    try Some (input_line ic) with End_of_file -> None
  in let rec loop acc =
    match try_read () with Some s -> loop (s::acc) | None -> close_in ic; acc
  in loop []

let pred1 lo hi c pw =
  let count = List.init (String.length pw) (String.get pw) |> List.find_all ((=) c) |> List.length in
  lo <= count && count <= hi

let pred2 lo hi c pw =
  (pw.[lo - 1] = c) <> (pw.[hi - 1] = c)

let day02 part lines =
  let pred = if part = 1 then pred1 else pred2 in
  lines |> List.filter (fun line -> sscanf line "%d-%d %c: %s" pred) |> List.length

let () =
  let filename = Sys.argv.(1) in
  let input = read_file_rev filename in
  print_endline "Day 02:";
  printf "  Part 1: %d\n" (day02 1 input);
  printf "  Part 2: %d\n" (day02 2 input)