day24.Rmd

---
title: "--- Day 24: Lobby Layout ---"
author: Fleur Kelpin
date: Dec 24, 2020
output: github_document
---
```{r message=FALSE, warning=FALSE}
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](https://www.redblobgames.com/grids/hexagons/),
then we can sum the coordinate differences.

![Axial Coordinate neighbors](day24_files/coords.png)

```{r}
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)
```
```{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()
```
# 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!
```{r}
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)
```
```{r}
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))
}
```
```{r}
for(i in 1:100) {
  floor <- life_step(floor)
}
image(floor)
sum(floor)
```