Day 25

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)

# Read input from file
input <- read_lines("../input/day25.txt", skip_empty_rows = FALSE)

Part 1

Convert input to lists of keys and locks by height:

groups <- input |> 
  enframe(name = NULL) |> 
  mutate(group_id = cumsum(value == ""), .before = everything()) |> 
  filter(value != "") |> 
  mutate(
    col = str_split(value, ""),
    row = row_number(),
    .by = group_id
  ) |> 
  mutate(
    type = case_when(row == 1 ~ if_else(str_detect(value, "#"), "lock", "key"))
  ) |> 
  fill(type, .direction = "down") |> 
  unnest_wider(col, names_sep = "") |> 
  mutate(group_id = str_c(type, "_", group_id)) |> 
  select(-c(value, row, type, row)) |> 
  group_split(group_id)

groups <- set_names(
  groups,
  map_chr(groups, ~ unique(pull(.x, group_id)))
)

Transpose data frames and count pin heights of each column:

pins <- groups |> 
  map(
    ~ .x |> 
      select(-group_id) |> 
      as.matrix() |> 
      t() |> 
      as_tibble() |> 
      transmute(height = rowSums(across(everything(), ~ .x == "#")) - 1) |> 
      pull(height)
  )

locks <- keep_at(pins, ~ str_starts(.x, "lock"))
keys  <- keep_at(pins, ~ str_starts(.x, "key"))

Compute maximum height for any given lock/key combo:

max_height <- nrow(groups[[1]]) - 2

Cross all locks with all keys and check for overlapping pins:

expand_grid(lock = locks, key = keys) |> 
  mutate(fits = map2_lgl(lock, key, ~ max(.x + .y) <= max_height)) |> 
  pull(fits) |> 
  sum()

[1] 3495