Day 4

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)

# Read input from file
input <- read_lines("../input/day04.txt", skip_empty_rows = FALSE)

Part 1

Reformat input into lists of winning numbers vs chosen numbers for each card, then score each card:

cards <- input |> 
  
  # Reformat as a data frame of winning vs chosen numbers per card
  str_remove("Card .*:") |> 
  enframe(name = "card_id", value = "txt") |> 
  separate(txt, into = c("winning", "chosen"), sep = "\\|") |> 
  mutate(across(c(winning, chosen), \(vec) {
    vec |> 
      trimws() |> 
      str_split("\\s+") |> 
      map(parse_number)
  })) |> 
  
  # Count the number of overlapping chosen & winning numbers per card:
  mutate(
    n_common = map2_int(winning, chosen, ~ length(intersect(.x, .y))),
    score = if_else(n_common == 0, 0, 2^(n_common - 1))
  )

Sum all cards’ scores:

sum(cards$score)

[1] 24542

Part 2

Create a reference table of the outputs received for each winning card:

rewards <- cards |> 
  mutate(
    reward_start = card_id + 1,
    reward_end = pmin(card_id + n_common, max(card_id)),
    reward = pmap(list(reward_start, reward_end, n_common), \(start, end, n) {
      if (n == 0) 
        rep(0, nrow(cards))
      else 
        rep(0, nrow(cards)) |> 
        modify_at(.at = seq(start, end), ~1)
    })
  ) |> 
  pull(reward)

Loop through each card in the inventory, accumulate rewards at each step, then sum the final total number of cards:

collect_reward <- \(inventory, i) inventory + inventory[[i]] * rewards[[i]]

reduce(1:nrow(cards), collect_reward, .init = rep(1, nrow(cards))) |> 
  sum()

[1] 8736438