14: Restroom Redoubt

library(mistlecode)

options(scipen = 999)

mdt <-
  'input.txt' |>
  readLines() |>
  stringr::str_match_all('-?\\d+') |>
  purrr::map(\(x) {
    x <- as.integer(x)
    list(
      'p' = c(x[1], x[2]) + 1, # row,col
      'v' = c(x[3], x[4])
    )
  })

Part 1

When I was almost done I realized I could’ve just multiplied the vectors by 100 then not needed a loop. Oh well. Easy enough.

m <- matrix(list(), nrow = 103, ncol = 101)
dt <- mdt
for(i in 1:100) {
  dt <- purrr::map(dt, \(x) {
      p <- x$p + x$v
      p[1] <- p[1] %% ncol(m); if (p[1] == 0) p[1] <- ncol(m);
      p[2] <- p[2] %% nrow(m); if (p[2] == 0) p[2] <- nrow(m);
      list('p' = p, 'v' = x$v)
    })
}

p <-
  dt |>
  purrr::map(\(x) x$p |> matrix(ncol = 2) |> as.data.frame()) |>
  purrr::list_rbind() |>
  setNames(c('col', 'row')) |>
  dplyr::select('row', 'col')

mid_row <- ceiling(nrow(m) / 2); mid_col <- ceiling(ncol(m) / 2);
one <- p[p$row < mid_row & p$col < mid_col,] |> nrow()
two <- p[p$row < mid_row & p$col > mid_col,] |> nrow()
three <- p[p$row > mid_row & p$col < mid_col,] |> nrow()
four <- p[p$row > mid_row & p$col > mid_col,] |> nrow()
prod(one, two, three, four)

[1] 228421332

Part 2

Ugh. mistlecode::coords_to_matrix() isn’t working. I guess I’ll use {ggplot}. Figuring out how to identify the tree was annoying. I tried clicking through the first 500 iterations with {ggplot} but that wasn’t going anywhere. I took a bit but I figured that the tree would probably have either high or low density in terms of points vs white space. Took a gander and just ran a loop until the initial state was repeated then checked the max and min densities. Sure enough. Max density it was.

m <- matrix(list(), nrow = 103, ncol = 101)
filled <- c(Inf, 0); which_filled <- c(0, 0)

dt1 <- purrr::map(mdt, \(x) {
      p <- x$p + x$v
      p[1] <- p[1] %% ncol(m); if (p[1] == 0) p[1] <- ncol(m);
      p[2] <- p[2] %% nrow(m); if (p[2] == 0) p[2] <- nrow(m);
      list('p' = p, 'v' = x$v)
    })
i <- 2
repeat {
  dt <- purrr::map(mdt, \(x) {
      p <- x$p + (x$v * i)
      p[1] <- p[1] %% ncol(m); if (p[1] == 0) p[1] <- ncol(m);
      p[2] <- p[2] %% nrow(m); if (p[2] == 0) p[2] <- nrow(m);
      list('p' = p, 'v' = x$v)
    })
  if (identical(dt, dt1)) break
  dt <-
    dt |>
    purrr::map(\(x) x$p |> matrix(ncol = 2) |> as.data.frame()) |>
    purrr::list_rbind() |>
    setNames(c('col', 'row'))
  f <- nrow(unique(dt)) / prod(dim(m))
  if (f < filled[1]) {
    filled[1] <- f 
    which_filled[1] <- i
  } else if (f > filled[2]) {
    filled[2] <- f
    which_filled[2] <- i
  }
  i <- i + 1
}

purrr::map(1:2, \(i) {
  dt <- purrr::map(mdt, \(x) {
      p <- x$p + (x$v * which_filled[i])
      p[1] <- p[1] %% ncol(m); if (p[1] == 0) p[1] <- ncol(m);
      p[2] <- p[2] %% nrow(m); if (p[2] == 0) p[2] <- nrow(m);
      list('p' = p, 'v' = x$v)
    })
  dt |>
    purrr::map(\(x) x$p |> matrix(ncol = 2) |> as.data.frame()) |>
    purrr::list_rbind() |>
    setNames(c('col', 'row')) |>
    ggplot2::ggplot() +
    ggplot2::geom_point(ggplot2::aes(x = col, y = -row)) +
    ggplot2::theme_void()
})

[[1]]

[[2]]

which_filled[2]

[1] 7790