Day 14

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)
library(unglue)

Read input from file:

input <- read_lines("../input/day14.txt", skip_empty_rows = TRUE)

Convert input to a starting sequence and a list of pair insertion rules:

template <- head(input, 1)

rules <- tail(input, -1) |> 
  unglue_data("{pair} -> {output}")

Disable scientific formatting when displaying large numbers:

options(scipen = 999)

Part 1

Define a function to grow the polymer n times, then take the count of the most common character in the string and subtract the count of the least common character:

grow_n <- function(polymer, n) {
  
  # Save the first and last letter for later reference
  letter_start <- str_sub(polymer,  1L,  1L)
  letter_final <- str_sub(polymer, -1L, -1L)
  
  # Reformat the initial string as a dataframe of pairs and counts
  polymer <- map2_dfr(
    .x = head(str_split_1(polymer, ""), -1),
    .y = tail(str_split_1(polymer, ""), -1),
    ~ tibble(pair = str_c(.x, .y))
  ) |> 
    summarize(n = n(), .by = pair)
  
  # Grow the polymer N times, tracking its growth as compact counts of pairs
  for (i in 1:n) {
    polymer <- polymer |> 
      left_join(rules, join_by(pair)) |> 
      mutate(
        pair1 = str_replace(pair, ".$", output),
        pair2 = str_replace(pair, "^.", output),
        pair3 = case_when(is.na(output) ~ pair)
      ) |> 
      select(n, pair1, pair2, pair3) |> 
      pivot_longer(
        starts_with("pair"), 
        values_to = "pair", 
        names_to = NULL, 
        values_drop_na = TRUE
      ) |> 
      summarize(n = sum(n), .by = pair)
  }
  
  # Count final letters and de-duplicate
  counts <- polymer |> 
    mutate(letters = str_split(pair, "")) |> 
    unnest_longer(letters) |> 
    summarize(n = sum(n), .by = letters) |> 
    
    # Initial and final letters were not duplicated, so return +1 to them
    mutate(
      n = case_when(letters == letter_start ~ n + 1, .default = n),
      n = case_when(letters == letter_final ~ n + 1, .default = n),
      n = n / 2
    ) |> 
    pull(n)
  
  # Return the range of greatest - least letter counts
  max(counts) - min(counts)
}

Run for 10 steps on puzzle input:

grow_n(template, 10)

Part 2

Run for 40 steps on puzzle input:

grow_n(template, 40)