2021 Day 9

Author

Nathan Moore

— Day 9: Smoke Basin —

Avoid the smoke!

Find all of the low points on your heightmap. What is the sum of the risk levels of all low points on your heightmap?

library(tidyverse)

my_file <- here::here("2021", "data-2021-09.txt")
x <- readLines(my_file)

Maybe write an explanation of the solution approach here

cave_width = nchar(x[[1]])
cave_length = length(x)

smokes_L = stringr::str_split_fixed(x, "", cave_width) %>% as.data.frame()
smokes_L = sapply(smokes_L, as.numeric)

smokes = array(NA, c(102, 102))
smokes[2:101, 2:101] = smokes_L

lowpoints = vector()

for (i in seq(2, cave_width+1)) {
    for (j in seq(2, cave_length+1)) {
      surr = c(smokes[i,j], smokes[i-1,j], smokes[i+1,j], 
               smokes[i,j-1], smokes[i,j+1])
      if (min(surr, na.rm=TRUE) == smokes[i,j]
          & smokes[i,j] != 9) {
            lowpoints = append(lowpoints, smokes[i,j])
            # print(i)
            # print(j)
        }
    }
}

sum(lowpoints) + length(lowpoints)

[1] 504

— Part Two —

Avoid the big basins!

What do you get if you multiply together the sizes of the three largest basins?

cave_width = nchar(x[[1]])
cave_length = length(x)

smokes_L = stringr::str_split_fixed(x, "", cave_width) %>% as.data.frame()
smokes_L = sapply(smokes_L, as.numeric)

smokes = array(NA, c(102, 102))
smokes[2:101, 2:101] = smokes_L

# basin_tracker
basin_t = array(NA, c(102, 102))

basin_num = 0

for (k in seq(40)) {
    for (i in seq(2, cave_width+1)) {
        for (j in seq(2, cave_length+1)) {
            surr = c(basin_t[i-1,j], basin_t[i+1,j], 
                     basin_t[i,j-1], basin_t[i,j+1])
            if (smokes[i,j] == 9) {
                # if it is a 9 do nothing, keep as NA
            } else if (any(surr[!is.na(surr)])) {
                # if there is a basin number in any surrounding spot, 
                # adopt that number
                basin_t[i,j] = min(surr[!is.na(surr)])
            } else {
                # if we find no surrounding basin number,
                # add one to the basin and start a new basin counter
                basin_num = basin_num + 1
                basin_t[i,j] = basin_num
            }
        }
    }
}

counter = sort(table(unlist(basin_t)), decreasing=TRUE)
counter[1][[1]] * counter[2][[1]] * counter[3][[1]]

[1] 1558722

basin_count = basin_t %>%
    as.data.frame() %>%
    mutate(rr = row_number()) %>% 
    pivot_longer(cols = -rr, names_prefix = "cols") %>% 
    count(value) %>%
    arrange(desc(n))

# part two answer
prod(basin_count$n[2:4])

[1] 1558722

Let’s make a chart for fun

cave_width = nchar(x[[1]])
cave_length = length(x)

smokes = stringr::str_split_fixed(x, "", cave_width) %>% as.data.frame()
smokes = sapply(smokes, as.numeric)

smoker = smokes %>%
    as.data.frame() %>%
    mutate(rr = row_number()) %>%
    pivot_longer(-rr, names_to = "cols") %>%
    mutate(cols = parse_number(cols))

smoker %>%
    ggplot(aes(rr, cols, colour = factor(value))) +
    geom_point() +
    scale_color_brewer(palette = "Set3")

basin_grey = c("#D9D9D9", "#D9D9D9", "#D9D9D9", "#D9D9D9", "#D9D9D9", "#D9D9D9", "#D9D9D9", "#D9D9D9", "#D9D9D9", "#222222")

smoker %>%
    ggplot(aes(rr, cols, colour = factor(value))) +
    geom_point() +
    scale_color_manual(values = basin_grey)