Day 9

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)
library(ctmle, include.only = "bound")

# Read input from file
input <- read_lines("../input/day09.txt") |>
  str_split(" ")

Part 1

# Convert head movements to sequence of locations on complex plane
head_path <- input |>
  map(~ rep(.x[[1]], .x[[2]])) |>
  reduce(c) |>
  recode("R" = 1 + 0i, "L" = -1 + 0i, "U" = 0 + 1i, "D" = 0 - 1i) |>
  accumulate(.init = 0 + 0i, .f = sum)

# Find path of next knot given path of previous knot.
move_next_knot <- function(prev_knot_path) {
  accumulate(
    .x = prev_knot_path,
    .f = function(tail = .x, head = .y) {
      diff <- head - tail
      if_else(
        max(abs(Re(diff)), abs(Im(diff))) <= 1,
        tail,
        tail + bound(Re(diff), c(-1, 1)) + bound(Im(diff), c(-1, 1)) * 1i
      )
    }
  )
}

unique_tail_spots <- function(path, num_knots) {
  # Iteratively compute path of each knot from head & to tail
  move_tail <- reduce(map(1:(num_knots - 1), ~ move_next_knot), compose)

  # Find number of unique locations in the tail's path
  length(unique(move_tail(path)))
}

unique_tail_spots(head_path, num_knots = 2)

[1] 6197

Part 2

unique_tail_spots(head_path, num_knots = 10)

[1] 2562