Day 8

Data

The input is height of the trees on a grid.

input_file_path <- here("DATA", "2022", "input_day_08.txt")

no_rows <- system(paste("wc -l", input_file_path), intern = TRUE) %>%
  word(1) %>%
  as.numeric()
no_cols <- scan(input_file_path, what = "character", nmax = 1) %>% str_length()

There are 99 rows and 99 columns on the grid.

input_grid <- scan(
  input_file_path,
  what = "character"
)
# let's make a matrix out of it
input_grid <- map(
  input_grid,
  function(row){
    cur_row_trees <- str_split(row, pattern = "") %>%
      unlist()
    return(as.numeric(cur_row_trees))
  }) %>%
  do.call(rbind, args = .)

We can visualize it:

input_grid %>%
  as_tibble(rownames = "row") %>%
  pivot_longer(cols = V1:V99, names_to = "col", values_to = "val") %>%
  mutate(
    col = as.numeric(str_sub(col, start = 2, end = -1)),
    row = as.numeric(row)
  ) %>%
  ggplot(aes(col, row)) +
  geom_tile(aes(fill = as.factor(val))) +
  coord_fixed() +
  scale_fill_grey() +
  theme_minimal()

Warning: The `x` argument of `as_tibble.matrix()` must have unique column names if
`.name_repair` is omitted as of tibble 2.0.0.
ℹ Using compatibility `.name_repair`.

Functions

check_visibility <- function(sequence, last = TRUE){
  # I have a sequence of heights that ends or begins with the tree we are checking
  if(last){
    chosen_tree <- sequence[length(sequence)]
    other_trees <- sequence[-length(sequence)]
    return(all(other_trees < chosen_tree))
  }
  
  chosen_tree <- sequence[1]
  other_trees <- sequence[-1]
  return(all(other_trees < chosen_tree))
}

Check all trees

We don’t need to check trees that are on the edge.

no_visible_trees <- no_cols*2 + (no_rows - 2)*2

visibility_matrix <- matrix(
  NA, nrow = no_rows, ncol = no_cols
)
for(cur_row in 2:(no_rows - 1)){
  for(cur_col in 2:(no_cols - 1)){
    # check from top
    cur_sequence <- input_grid[1:cur_row, cur_col]
    vis_from_top <- check_visibility(cur_sequence)
    # check from bottom
    cur_sequence <- input_grid[cur_row:no_rows, cur_col]
    vis_from_bottom <- check_visibility(cur_sequence, FALSE)
    # check from left
    cur_sequence <- input_grid[cur_row, 1:cur_col]
    vis_from_left <- check_visibility(cur_sequence)
    # check from right
    cur_sequence <- input_grid[cur_row, cur_col:no_cols]
    vis_from_right <- check_visibility(cur_sequence, FALSE)
    
    cur_tree_vis <- any(
      c(vis_from_top, vis_from_bottom, vis_from_left, vis_from_right)
    )
    visibility_matrix[cur_row, cur_col] <- cur_tree_vis
    no_visible_trees <<- no_visible_trees + as.numeric(cur_tree_vis)
  }
}

That’s how looks the visibility matrix:

visibility_matrix %>%
  as_tibble(rownames = "row") %>%
  pivot_longer(cols = V1:V99, names_to = "col", values_to = "val") %>%
  mutate(
    col = as.numeric(str_sub(col, start = 2, end = -1)),
    row = as.numeric(row)
  ) %>%
  ggplot(aes(col, row)) +
  geom_tile(aes(fill = val)) +
  coord_fixed() +
  scale_fill_grey() +
  theme_minimal()

There are 1823 trees that are visible from outside of the grid! 🥳