Day 23

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)

# Read input from file
input <- read_lines("../input/day23.txt", skip_empty_rows = TRUE) |> 
  enframe(name = "row") |> 
  mutate(value = map(value, ~ enframe(str_split_1(.x, ""), name = "col"))) |> 
  unnest(value)

Part 1

Convert map to a list of positions for each elf, using complex numbers to store 2D coords:

elves <- input |> 
  filter(value == "#") |> 
  mutate(z = complex(real = col, imaginary = row)) |> 
  pull(z)

Define function to move elves in each round:

cardinal_dirs <- c(
  n  =  0 - 1i,
  s  =  0 + 1i,
  w  = -1 + 0i,
  e  =  1 + 0i,
  nw = -1 - 1i,
  ne =  1 - 1i,
  sw = -1 + 1i,
  se =  1 + 1i
)

adjacent_dirs <- list(
  n = c("n", "nw", "ne"),
  s = c("s", "sw", "se"),
  w = c("w", "nw", "sw"),
  e = c("e", "ne", "se")
)

move_elves <- function(elves, round_num) {

  # Determine which neighboring cells are occupied
  neighbors <- map(cardinal_dirs, \(dir) elves + dir)
  occupied  <- map(neighbors, ~ .x %in% elves)
  
  # Determine which n/s/w/e moves are valid
  valid <- map(adjacent_dirs, \(dir_set) {
    dir_set |> 
      map(~!occupied[[.x]]) |> 
      reduce(`&`)
  })
  
  # Re-order the n/s/w/e priority according to the current round number
  valid <- valid[(1:4 + round_num - 2) %% 4 + 1]
  
  # For all elves not surrounded by empty cells, determine their proposed move
  all_borders_empty <- reduce(valid, `&`)
  proposals <- valid |> 
    imap(\(vec, dir) case_when(vec ~ neighbors[[dir]])) |>
    pmap(
      ~ c(discard(c(..1, ..2, ..3, ..4), is.na), NA) |> 
        head(1)
    ) |> 
    unlist() |> 
    modify_if(all_borders_empty, ~ NA)
  
  # Nullify any colliding proposed moves
  collisions <- na.omit(proposals)[duplicated(na.omit(proposals))]
  movements <- if_else(proposals %in% collisions, NA, proposals)
  
  # Return the new elf coordinates
  coalesce(movements, elves)
}

Define a function to compute the area of the bounding box then subtract away the number of elves:

n_empty_tiles <- function(elves) {
  height <- 1 + diff(range(Im(elves)))
  width  <- 1 + diff(range(Re(elves)))
  
  height * width - length(elves)
}

Run 10 rounds on puzzle input:

reduce(1:10, move_elves, .init = elves) |> 
  n_empty_tiles()

[1] 3996

Part 2

Run until no further movements occur:

i <- 1
cur_elves <- elves

repeat {
  new_elves <- move_elves(cur_elves, i)
  
  if (all(new_elves == cur_elves)) break
  
  cur_elves <- new_elves
  i <- i + 1
}

i

[1] 908