Day 25

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)

Read text input from file:

public_keys <- read_lines("../input/day25.txt") |> 
  as.numeric() |> 
  set_names(c("card", "door"))

Part 1

Define a function which, given a subject number and a public key, determines the loop size:

get_loop_size <- function(public_key, subject_number) {
  
  n <- 0
  value <- 1
  
  while (value != public_key) {
    value <- (value * subject_number) %% 20201227
    n <- n + 1
  }
  
  n
}

Run on puzzle input to get the loop size for the card & door:

loop_size <- public_keys |> 
  map_int(\(x) get_loop_size(x, subject_number = 7))

Define a function to get the encryption key given a public key and loop size:

encrypt <- function(public_key, loop_size) {
  reduce(1:loop_size, \(x, n) (x * public_key) %% 20201227, .init = 1)
}

Run on puzzle input:

map2_dbl(public_keys, rev(loop_size), \(x, y) encrypt(x, y))