Read in our puzzle input:

day <- 5

input <- here::here(
  "2022", "input",
  paste0("day_", stringr::str_pad(day, 2, "left", "0"))
  ) |>
  readLines()

Part 1

stacks <- head(input, 8)
orders <- tail(input, -10)

create_stack_list <- function(n) {
  start <- (4 * n) - 3
  stop <- (4 * n) - 1
  rev(substr(stacks, start, stop)) |>
    stringr::str_subset("\\[[A-Z]{1}\\]")
}

stacks_list <- purrr::map(1:9, create_stack_list)

process_order <- function(stacks_list, order) {
  nos <- stringr::str_extract_all(order, "[0-9]+", simplify = TRUE) |>
    as.numeric()
  move  <- nos[1]
  from <- nos[2]
  to <- nos[3]
  
  to_move <- utils::tail(stacks_list[[from]], move)
  stacks_list[[from]]  <- utils::head(stacks_list[[from]], (move * -1))
  stacks_list[[to]]  <- c(stacks_list[[to]], rev(to_move))

  stacks_list
}

purrr::reduce(orders, process_order, .init = stacks_list) |>
  purrr::map_chr(utils::tail, 1) |>
  stringr::str_c(collapse = "") |>
  stringr::str_remove_all("\\[|\\]")

## [1] "RFFFWBPNS"

Part 2

process_order2 <- function(stacks_list, order) {
  nos <- stringr::str_extract_all(order, "[0-9]+", simplify = TRUE) |>
    as.numeric()
  move  <- nos[1]
  from <- nos[2]
  to <- nos[3]
  
  to_move <- utils::tail(stacks_list[[from]], move)
  stacks_list[[from]]  <- utils::head(stacks_list[[from]], (move * -1))
  stacks_list[[to]]  <- c(stacks_list[[to]], to_move)

  stacks_list
}

purrr::reduce(orders, process_order2, .init = stacks_list) |>
  purrr::map_chr(utils::tail, 1) |>
  stringr::str_c(collapse = "") |>
  stringr::str_remove_all("\\[|\\]")

## [1] "CQQBBJFCS"