Day 17

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)

Read text input from file into a data frame of x & y coordinates with TRUE/FALSE values for active state:

input <- read_lines("../input/day17.txt") |> 
  map(~ str_split_1(.x, "") == "#") |> 
  enframe(name = "x", value = "is_active") |>   
  unnest_longer(is_active, indices_to = "y") |>
  mutate(z = 0L)

Part 1

Create a data frame of x/y/z adjustments that can join on any coordinate to get its full set of neighbors:

nbr <- expand_grid(x = -1:1, y = -1:1, z = -1:1) |> 
  filter(!(x == 0 & y == 0 & z == 0))

Define a function to cycles through the initiation process n times, iteratively comparing neighboring cubes to determine the next state with each cyle:

cycle <- function(grid, n = 6) {
  
  for (i in 1:n) {
    grid <- grid |> 
      cross_join(nbr, suffix = c("", "_nbr")) |> 
      mutate(
        x = x + x_nbr,
        y = y + y_nbr,
        z = z + z_nbr
      ) |> 
      summarize(nbrs = sum(is_active), .by = c(x, y, z)) |> 
      left_join(select(grid, x, y, z, is_active), join_by(x, y, z)) |>
      mutate(is_active = replace_na(is_active, FALSE)) |> 
      transmute(x, y, z, is_active = (nbrs == 3) | (is_active & nbrs == 2))
  }
  
  grid
  
}

Run on puzzle input:

cycle(input) |> 
  pull(is_active) |> 
  sum()

Part 2

Extend the adjustment dataframe to accommodate a fourth dimension:

nbr <- expand_grid(x = -1:1, y = -1:1, z = -1:1, w = -1:1) |> 
  filter(!(x == 0 & y == 0 & z == 0 & w == 0))

Extend the cycle function to accommodate a fourth dimension:

cycle <- function(grid, n = 6) {
  
  for (i in 1:n) {
    grid <- grid |> 
      cross_join(nbr, suffix = c("", "_nbr")) |> 
      mutate(
        x = x + x_nbr,
        y = y + y_nbr,
        z = z + z_nbr,
        w = w + w_nbr
      ) |> 
      summarize(nbrs = sum(is_active), .by = c(x, y, z, w)) |> 
      left_join(select(grid, x, y, z, w, is_active), join_by(x, y, z, w)) |>
      mutate(is_active = replace_na(is_active, FALSE)) |> 
      transmute(x, y, z, w, is_active = (nbrs == 3) | (is_active & nbrs == 2))
  }
  
  grid
  
}

Run on puzzle input:

input |> 
  mutate(w = 0L) |> 
  cycle() |> 
  pull(is_active) |> 
  sum()