day17.md

— Day 17: Conway Cubes —

Fleur Kelpin Dec 17, 2020

library(tidyverse)
n <- 8
steps <- 6
input <- readLines("day17.txt") %>%
  str_split("") %>%
  unlist() %>%
  map_lgl(~ . == "#") %>%
  matrix(nrow = n, byrow = T)

Part 1

Starting with your given initial configuration, simulate six cycles. How many cubes are left in the active state after the sixth cycle?

dim <- c(
  n + 2 * (steps + 1),
  n + 2 * (steps + 1),
  1 + 2 * (steps + 1)
)
state <- array(F, dim = dim)
state[7 + 1:n, 7 + 1:n, 8] <- input
ds <- expand.grid(dx = -1:1, dy = -1:1, dz = -1:1)
innerxy <- 2:(dim[[1]] - 1)
innerzw <- 2:(dim[[3]] - 1)

for (i in 1:6) {
  pop <- reduce(1:nrow(ds),
    function(arr, index) {
      arr + state[
        innerxy + ds$dx[[index]],
        innerxy + ds$dy[[index]],
        innerzw + ds$dz[[index]]
      ]
    },
    .init = array(0, dim = c(
      2 * steps + n,
      2 * steps + n,
      2 * steps + 1
    ))
  )
  state[innerxy, innerxy, innerzw] <-
    (!state[innerxy, innerxy, innerzw] & pop == 3) |
      (state[innerxy, innerxy, innerzw] & between(pop, 3, 4))
}
sum(state)

## [1] 388

Part 2

For some reason, your simulated results don’t match what the experimental energy source engineers expected. Apparently, the pocket dimension actually has four spatial dimensions, not three.

dim <- c(
  n + 2 * (steps + 1),
  n + 2 * (steps + 1),
  1 + 2 * (steps + 1),
  1 + 2 * (steps + 1)
)
state <- array(F, dim = dim)
state[7 + (1:n), 7 + (1:n), 8, 8] <- input
ds <- expand.grid(dx = -1:1, dy = -1:1, dz = -1:1, dw = -1:1)

for (i in 1:6) {
  pop <- reduce(1:nrow(ds),
    function(arr, index) {
      arr + state[
        innerxy + ds$dx[[index]],
        innerxy + ds$dy[[index]],
        innerzw + ds$dz[[index]],
        innerzw + ds$dw[[index]]
      ]
    },
    .init = array(0, dim = c(
      2 * steps + n,
      2 * steps + n,
      2 * steps + 1,
      2 * steps + 1
    ))
  )
  state[innerxy, innerxy, innerzw, innerzw] <-
    (!state[innerxy, innerxy, innerzw, innerzw] & pop == 3) |
      (state[innerxy, innerxy, innerzw, innerzw] & between(pop, 3, 4))
}
sum(state)

## [1] 2280