13: Point of Incidence

library(mistlecode)

options(scipen = 999)

dt <-
  readLines("input.txt") |>
  tibble::as_tibble() |>
  dplyr::mutate(group = cumsum(value == "")) |>
  dplyr::filter(value != "") |>
  dplyr::group_split(group) |>
  purrr::map(\(x) {
    x |>
      dplyr::pull(value) |>
      stringr::str_split("", simplify = TRUE)
  })

Part 1

Okay okay. Not too bad. Just need to make sure I get the line of reflection right.

get_up_down <- function(up, down) {
  l <- min(c(length(up), length(down)))
  up <- rev(rev(up)[seq_len(l)]); down <- down[rev(seq_len(l))];
  list('up' = up, 'down' = down)
}
get_seq <- function(s) { s |> seq_len() |> head(-1) }
check_row <- function(x) {
  x |>
    nrow() |>
    get_seq() |>
    purrr::map_int(\(r) {
      up <- seq_len(r); down <- min(c(r+1,nrow(x))):nrow(x)
      updown <- get_up_down(up, down)
      up <- x[updown$up,]; down <- x[updown$down,];
      if (all(up == down)) r * 100 else 0
    })
}
check_col <- function(x) {
  x |>
    ncol() |>
    get_seq() |>
    purrr::map_int(\(r) {
      up <- seq_len(r); down <- min(c(r+1,ncol(x))):ncol(x)
      updown <- get_up_down(up, down)
      up <- x[,updown$up]; down <- x[,updown$down];
      if (all(up == down)) r else 0
    })
}
dt |>
  purrr::map(\(x) {
    c(check_row(x), check_col(x))
  }) |>
  unlist() |>
  sum()

[1] 37718

Part 2

omg. This is gonna be easy. I just need to check if all but one are equal instead of all. I got this in well under two minutes!

check_row <- function(x) {
  x |>
    nrow() |>
    get_seq() |>
    purrr::map_int(\(r) {
      up <- seq_len(r); down <- min(c(r+1,nrow(x))):nrow(x)
      updown <- get_up_down(up, down)
      up <- x[updown$up,]; down <- x[updown$down,];
      if (sum(up == down) == length(up) - 1) r * 100 else 0
    })
}
check_col <- function(x) {
  x |>
    ncol() |>
    get_seq() |>
    purrr::map_int(\(r) {
      up <- seq_len(r); down <- min(c(r+1,ncol(x))):ncol(x)
      updown <- get_up_down(up, down)
      up <- x[,updown$up]; down <- x[,updown$down];
      if (sum(up == down) == length(up) - 1) r else 0
    })
}
dt |>
  purrr::map(\(x) {
    c(check_row(x), check_col(x))
  }) |>
  unlist() |>
  sum()

[1] 40995