Day 8

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)
library(unglue)

Read text input from file:

input <- read_lines("../input/day08.txt") |> 
  str_split(",") |> 
  map(as.numeric)

Part 1

Define a helper function to compute the euclidian distance between two vectors:

dist <- \(x, y) sqrt(sum(map2_dbl(x, y, ~ (.x - .y)^2)))

Construct all unique combinations of junction boxes, then compute the distance between each:

combos <- combn(1:length(input), 2, simplify = FALSE) |> 
  as_tibble_col("box") |> 
  unnest_wider(box, names_sep = "") |> 
  mutate(
    across(c(box1, box2), ~ input[.x], .names = "{.col}_coord"),
    d = map2_dbl(box1_coord, box2_coord, dist)
  ) |> 
  arrange(d)

Assign each box to its own circut, iterate through the first 1000 box pairs:

circuts <- 1:length(input)

for (i in 1:1000) {
  circut1 <- circuts[combos$box1[i]]
  circut2 <- circuts[combos$box2[i]]
  if (circut1 != circut2) {
    circuts <- replace(circuts, circuts == circut1, circut2)
  }
}

Multiply together the sizes of the 3 largest circuts:

circuts |> 
  sort() |> 
  rle() |> 
  pluck("lengths") |> 
  sort(decreasing = TRUE) |> 
  head(3) |> 
  prod()

Part 2

Iteravely make connections until all boxes are on a single circut:

circuts <- 1:length(input)
i <- 0

while(length(unique(circuts)) > 1) {
  i <- i + 1
  circut1 <- circuts[combos$box1[i]]
  circut2 <- circuts[combos$box2[i]]
  if (circut1 != circut2) {
    circuts <- replace(circuts, circuts == circut1, circut2)
  }
}

Multiply the x coordinates of the last two boxes connected:

combos$box1_coord[[i]][1] * combos$box2_coord[[i]][1]