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— Day 24: Lobby Layout —

Fleur Kelpin Dec 24, 2020

library(tidyverse)
library(Ramble)
pDirection <- symbol("e") %alt%
  symbol("se") %alt%
  symbol("sw") %alt%
  symbol("w") %alt%
  symbol("nw") %alt%
  symbol("ne")
pDirections <- many(pDirection)
input <- read_csv("day24.txt", col_names = "directions") %>%
  rowwise() %>%
  mutate(directions = list(unlist(pDirections(directions)$result)))

Part 1

A member of the renovation crew gives you a list of the tiles that need to be flipped over (your puzzle input). Each line in the list identifies a single tile that needs to be flipped by giving a series of steps starting from a reference tile in the very center of the room. (Every line starts from the same reference tile.) Go through the renovation crew’s list and determine which tiles they need to flip. After all of the instructions have been followed, how many tiles are left with the black side up?

Let’s use Axial Coordinates, then we can sum the coordinate differences.

Axial Coordinate neighbors

diffs <- function(direction) {
  switch(direction,
    "e"= c(1L, 0L),
    "se"= c(0L, 1L),
    "sw"= c(-1L, 1L),
    "w"= c(-1L, 0L),
    "nw"= c(0L, -1L),
    "ne"= c(1L, -1L)
  )
}
input <- input %>%
  rowwise() %>%
  mutate(tile = list(reduce(
    unlist(directions), 
    function(coord, direction) { coord + diffs(direction) },
    .init=c(0,0)))) %>%
  mutate(q = tile[[1]]) %>%
  mutate(r = tile[[2]]) %>%
  select(q, r)

black_tiles <- input %>%
  group_by(q,r) %>%
  tally() %>%
  rowwise() %>%
  mutate(color = if (n %% 2 == 1) "black" else "white") %>% 
  filter(color == "black")
black_tiles %>%
  nrow()

## [1] 269

Part 2

The tile floor in the lobby is meant to be a living art exhibit. Every day, the tiles are all flipped according to the following rules:

  • Any black tile with zero or more than 2 black tiles immediately adjacent to it is flipped to white.
  • Any white tile with exactly 2 black tiles immediately adjacent to it is flipped to black.

Here, tiles immediately adjacent means the six tiles directly touching the tile in question.

Even more game of life. Yay!

floor <- matrix(data = FALSE, nrow = 150, ncol=150)
for(i in 1:nrow(black_tiles)) {
  floor[black_tiles$q[[i]] + 75,
        black_tiles$r[[i]] + 75] <- TRUE  
}
image(floor)


life_step <- function(d) {
  nrow <- dim(d)[[1]]
  ncol <- dim(d)[[2]]
  d_e <- rbind(d[-1, , drop = FALSE], 0)        #[q+1, r  ] e
  d_w <- rbind(0, d[-nrow, , drop = FALSE])     #[q-1, r  ] w
  d_se <- cbind(d[, -1, drop = FALSE], 0)       #[q  , r+1] se
  d_nw <- cbind(0, d[, -ncol, drop = FALSE])    #[q  , r-1] nw
  d_ne <- cbind(0, d_e[, -ncol, drop = FALSE])  #[q+1, r-1] ne
  d_sw <- cbind(d_w[, -1, drop = FALSE], 0)     #[q-1, r+1] sw
  pop <- d_e + d_w + d_se + d_nw + d_ne + d_sw
  (!d & pop == 2) | (d & between(pop,1,2))
}

for(i in 1:100) {
  floor <- life_step(floor)
}
image(floor)


sum(floor)

## [1] 3667