Day 18

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)
library(sf)

# Read input from file
input <- read_lines("../input/day18.txt", skip_empty_rows = TRUE) |> 
  unglue::unglue_data("{dir} {meters} ({color})", convert = TRUE)

Part 1

Convert the instruction input into list of corners that define the trench polygon:

trench <- input |>
  mutate(
    x = case_match(dir, c('U', 'D') ~ 0, 'L' ~ -1, 'R' ~ 1) * meters,
    y = case_match(dir, c('L', 'R') ~ 0, 'D' ~ -1, 'U' ~ 1) * meters
  ) |> 
  pmap(\(x, y, ...) c(x = x, y = y)) |> 
  accumulate(`+`, .init = c(x = 0, y = 0))

Convert the polygon’s boundary to a spatial object using SF:

polygon <- trench |>
  reduce(rbind) |> 
  list() |> 
  st_polygon() |> 
  st_buffer(dist = 0.5, joinStyle = 'MITRE', mitreLimit = sqrt(2))

ggplot(polygon) + 
  geom_sf() +
  theme_void()

Compute the polygon’s area:

st_area(polygon)

[1] 53844

Part 2

Convert the hex codes to new trench instructions:

trench <- input |> 
  mutate(
    meters = strtoi(str_sub(color, 2L, -2L), base = 16L),
    dir = case_match(
      str_sub(color, start = -1L),
      '0' ~ 'R', '1' ~ 'D', '2' ~ 'L', '3' ~ 'U'
    ),
    x = case_match(dir, c('U', 'D') ~ 0, 'L' ~ -1, 'R' ~ 1) * meters,
    y = case_match(dir, c('L', 'R') ~ 0, 'D' ~ -1, 'U' ~ 1) * meters
  ) |> 
  pmap(\(x, y, ...) c(x = x, y = y)) |> 
  accumulate(`+`, .init = c(x = 0, y = 0))

Convert to a new polygon:

polygon <- trench |>
  reduce(rbind) |> 
  list() |> 
  st_polygon() |> 
  st_buffer(dist = 0.5, joinStyle = 'MITRE', mitreLimit = sqrt(2))

ggplot(polygon) + 
  geom_sf() +
  theme_void()

Compute the polygon’s area:

st_area(polygon) |> 
  format(scientific = FALSE)

[1] "42708339569950"