Lobby

library(mistlecode)

options(scipen = 999)

Part 1

Proud of this one. Just find the biggest number that isn’t the last number, then find the biggest number after the biggest number.

'input.txt' |>
  readLines() |>
  stringr::str_split('') |>
  purrr::map_int(\(x) {
    x <- as.integer(x)
    biggest <- which.max(head(x, -1))
    second_biggest <- which.max(x[(biggest+1):length(x)]) + biggest
    glue::glue('{x[biggest]}{x[second_biggest]}') |>
      as.integer()
  }) |>
  sum()

[1] 16812

Part 2

Getting the recursion right was a bit annoying. I had an off-by-one error in my which.max(). I do like these kinds of puzzles where the second part is doing the first part a more general way.

find_biggest <- function(x, value = integer(), pos = 0) {
  remaining <- 12 - length(value)
  if (remaining == 0) return(value)
  biggest <- which.max(x[(pos+1):(length(x)-remaining+1)]) + pos
  value <- c(value, x[biggest])
  pos <- biggest
  find_biggest(x, value, pos)
}

'input.txt' |>
  readLines() |>
  stringr::str_split('') |>
  purrr::map_dbl(\(x) {
    x |>
      as.integer() |>
      find_biggest() |>
      paste0(collapse = '') |>
      as.numeric()
  }) |>
  sum()

[1] 166345822896410