Day 13

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)
library(unglue)

Read input from file:

input <- read_lines("../input/day13.txt", skip_empty_rows = TRUE)

Split input from plain-text strings into (1) x/y dot coordinates, and (2) a set of fold instructions.

dots <- input |> 
  unglue_data("{x},{y}", convert = TRUE) |> 
  drop_na()

folds <- input |> 
  unglue_data("fold along {axis}={value}", convert = TRUE) |> 
  drop_na()

Part 1

Define a function to fold the paper with the list of dots over a given line:

fold_paper <- function(df, axis, value) {
  df |> 
    mutate(across(all_of(axis), ~ case_when(
      .x > value ~ value - (.x - value),
      .default = .x
    ))) |> 
    distinct()
}

Count the dots after the first fold is complete:

fold_paper(dots, folds$axis[[1]], folds$value[[1]]) |> 
  nrow()

Part 2

Fold the paper completely:

folded <- reduce2(
  .x = folds$axis, 
  .y = folds$value, 
  .f = \(acc, axis, value) fold_paper(acc, axis, value), 
  .init = dots
)

Print the output:

folded |> 
  mutate(chr = '#') |> 
  complete(x = full_seq(x, 1), y = full_seq(y, 1), fill = list("chr" = " ")) |> 
  arrange(y, x) |> 
  summarize(chr = str_flatten(chr), .by = y) |> 
  pull(chr) |> 
  cat(sep = "\n")