Printing Department

library(mistlecode)

options(scipen = 999)

Part 1

This is one of those ones where you do great on the test input and miss something on the real one and can’t figure it out. I was returning too early in a few edge cases. Literal edge cases. Where the toilet paper was on the edge of the grid.

mat <- mistlecode::read_matrix('input.txt')
expand.grid(
  'y' = mat |> nrow() |> seq_len(),
  'x' = mat |> ncol() |> seq_len()
) |>
  purrr::pmap(\(y, x) {
    if (mat[y, x] == '.') return(c('.' = 4))
    mistlecode::get_adjacent_values(y, x, mat) |>
      as.vector() |>
      table()
  }) |>
  unlist() |>
  (\(x) x[names(x) == '@'])() |>
  (\(x) x[x <= 4])() |>
  length()

[1] 1516

Part 2

A little bit more of me being not so bright on the execution and some waiting and there’s a solution.

mat <- mistlecode::read_matrix('input.txt')
wiped <- 0
coords <-
  expand.grid(
    'y' = mat |> nrow() |> seq_len(),
    'x' = mat |> ncol() |> seq_len()
  )
do_wipe <- function(mat) {
  current <- wiped
  mat_copy <- mat
  coords |>
    purrr::pwalk(\(y, x) {
      if (mat[y, x] == '@') {
        vals <- 
          mistlecode::get_adjacent_values(y, x, mat) |>
          as.vector() |>
          table()
        if (vals['@'] <= 4) {
          mat_copy[y, x] <<- '.'
          wiped <<- wiped + 1
        }
      }
    })
  if (current != wiped) do_wipe(mat_copy) else wiped
}
do_wipe(mat)

[1] 9122