Day 11

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)

Read input from plain-text file into a matrix:

input <- read_fwf(
  file = "../input/day11.txt", 
  col_positions = fwf_widths(rep(1, 10)), 
  col_types = "i",
  show_col_types = FALSE
) |> 
  as.matrix() |> 
  unname()

width  <- ncol(input)
height <- nrow(input)

Part 1

Define a function to compute the energy boost from a single flash:

n <- \(x) lag(x, default = 0)
s <- \(x) lead(x, default = 0)
w <- \(x) t(lag(t(x), default = 0))
e <- \(x) t(lead(t(x), default = 0))

add_energy <- function(x) {
  n(x) + s(x) + e(x) + w(x) + n(w(x)) + n(e(x)) + s(w(x)) + s(e(x))
}

Define a function to run a single step, which may consist of many flashes until a stable state is reached:

step <- function(mtx) {
  
  # Initialize
  mtx <- mtx + 1
  has_flashed <- matrix(FALSE, nrow = height, ncol = width)
  
  # Repeat until done flashing
  repeat {
    cur_flash <- mtx > 9 & !has_flashed
    
    if (all(cur_flash == FALSE)) break
    
    has_flashed <- has_flashed | cur_flash
    mtx <- mtx + add_energy(cur_flash)
  }
  
  # Replace all flashed values with 0
  replace(mtx, has_flashed, 0)
}

Define a function to count the total individual flashes that occur over a defined number of steps:

count_flashes <- function(mtx, n_steps) {
  
  flashes <- 0
  for (i in 1:n_steps) {
    mtx <- step(mtx)
    flashes <- flashes + sum(mtx == 0)
  }
  
  flashes
}

Run on puzzle input. Count the flashes over 100 steps:

count_flashes(input, 100) 

Part 2

Define a function to find the first step when all cells flash in unison:

first_in_unison <- function(mtx) {
  i <- 0
  
  while (any(mtx != 0)) {
     mtx <- step(mtx)
     i <- i + 1
  }
  
  i
}

Run on input:

first_in_unison(input)