Day 8

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)
library(unglue)

# Read input from file
input <- read_lines("../input/day08.txt", skip_empty_rows = TRUE)

Part 1

Parse input text into sets of left/right instructions and the maps of the node network:

instructions <- input |> 
  head(1) |> 
  str_split_1("")

network <- input |> 
  tail(-1) |> 
  unglue_data("{node} = ({L}, {R})") |> 
  nest(LR = c(L, R)) |> 
  mutate(LR = map(LR, as.list)) |> 
  deframe()

Starting at the AAA node, advance through the list of instructions until the ZZZ node is reached:

# Initialize
n_rep    <- length(instructions)
cur_node <- "AAA"
i        <- 0

repeat {
  if (cur_node == "ZZZ") break
  
  cur_dir  <- instructions[i %% n_rep + 1]
  cur_node <- network[[cur_node]][[cur_dir]]
  i <- i + 1
}

i

[1] 19631

Part 2

For each node ending with ‘A’, compute the number of necessary steps until it reaches a node ending with ‘Z’ and loops.

steps_to_z <- function(node_start) {

  # Initialize
  cur_node <- node_start
  i        <- 0
  
  repeat {
    if (str_ends(cur_node, "Z")) return(i)
    
    cur_dir  <- instructions[i %% n_rep + 1]
    cur_node <- network[[cur_node]][[cur_dir]]
    i <- i + 1
  }
}

cycles <- names(network) |> 
  keep(~ str_ends(.x, "A")) |> 
  map_dbl(steps_to_z)

Take the least common multiple of the result:

reduce(cycles, numbers::LCM) |> 
  format(scientific = FALSE)

[1] "21003205388413"