Day 20

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)

# Read input from file
input <- read_lines("../input/day20.txt", skip_empty_rows = TRUE) |> 
  as.numeric()

Part 1

# Shift the value at the given index by n steps
shift_value <- function(vec, idx_old, n) {
  idx_new <- (idx_old + n - 2) %% (length(vec) - 1) + 1
  value <- vec[idx_old]
  
  vec |> 
    discard_at(idx_old) |> 
    append(value, after = idx_new)
}

# Mix the given list of integers in order by shifting values one-by-one 
mix <- function(file, n = 1) {
  ids <- 1:length(file)
  
  for (rep in 1:n) {
    for (i in 1:length(file)) {
      ids <- shift_value(ids, which(ids == i), file[i])
    }
  }
  
  file[ids]
}

# Sum the grove coordinates in a given vector
grove_coords <- function(vec) {
  c(1000, 2000, 3000) |> 
    map_dbl(~ vec[(which(vec == 0) + .x) %% length(vec)]) |> 
    sum()
}

Run on puzzle input:

input |> 
  mix() |> 
  grove_coords()

[1] 2215

Part 2

Apply decryption key and mix 10 times:

(input * 811589153) |> 
  mix(n = 10) |> 
  grove_coords()

[1] 8927480683