Day 20

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)

Read input from file, converting “#”/“.” characters to 1/0 integers:

input <- read_lines("../input/day20.txt", skip_empty_rows = TRUE) |> 
  str_split("") |> 
  map(~ case_match(.x, "." ~ 0, "#" ~ 1))

Separate the input into the algorithm sequence and image matrix:

alg <- pluck(input, 1)
img <- tail(input, -1) |> do.call(what = rbind)

Part 1

Print the starting image:

print_img <- \(img) {
  image(
    t(img)[, nrow(img):1],
    col = c("black", "white"), 
    asp = nrow(img) / ncol(img),
    axes = FALSE
  )
}

print_img(img)

Define a function to enhance the given image. Since the character representing the infinte expansion in all directions can change from enhancement to enhancement, we track its current value in the inf argument.

enhance <- function(mtx, inf = 0) {
  
  # Expand the borders of the image so that all 3x3 subsets are accessible
  mtx <- rbind(inf, inf, cbind(inf, inf, mtx, inf, inf), inf, inf)
  
  # Generate the full set of output coords
  mtx_out <- expand_grid(
    row = 1:(nrow(mtx) - 2),
    col = 1:(ncol(mtx) - 2) 
  ) |> 
    
    # Convert output coords to 3x3 ranges of the input and get the output value
    mutate(
      bin = map2_chr(row, col, ~ str_flatten(t(mtx[.x + 0:2, .y + 0:2]), "")),
      idx = strtoi(bin, base = 2) + 1,
      out = alg[idx]
    ) |> 
    
    # Compress back down into matrix format
    pull(out) |> 
    matrix(nrow = nrow(mtx) - 2, byrow = TRUE)
  
  # Determine the new value of the infinite grid
  inf_out <- alg[strtoi(str_flatten(rep(inf, 9), ""), base = 2) + 1]
  
  lst(mtx = mtx_out, inf = inf_out)
}

Define a function that enhances the image a given number of times:

enhance_n <- function(img, n) {
  lst(mtx = img, inf = 0) |> 
    reduce(1:n, .f = ~ do.call(what = enhance, args = .x), .init = _) |> 
    pluck("mtx")
}

Enhance the image twice, then count how many pixels are lit in the result:

result <- enhance_n(img, n = 2)

sum(result)

print_img(result)

Part 2

Enhance the image 50 times:

result <- enhance_n(img, n = 50)

sum(result)

print_img(result)