2023

Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 Day 9 Day 10 Day 11

R

— R Day 4 Part 1 —

solve_day4_part1 <- function(input) {
  # split on : or |
  cards <- strsplit(input, ":|\\|")

  # numbers between : and |
  winners <- lapply(cards, \(x) x[[2]])
  winners <- stringi::stri_extract_all(winners, regex = "\\d+")

  # numbers from | to the end of the line
  playing <- lapply(cards, \(x) x[[3]])
  playing <- stringi::stri_extract_all(playing, regex = "\\d+")

  # count playing numbers in winning numbers
  win_count <- Map(\(w, p) sum(p %in% w),
                   winners,
                   playing)

  win_count <- as.numeric(unlist(win_count))
  # remove rows with no wins
  win_count <- win_count[win_count != 0]

  # start from 1 and double for each additional win
  sum(2 ^ (win_count - 1))
}

aoc_source(day = 4, part = 1)

input = aoc_read(day = 4)

aoc_run(solve_day4_part1(input))

Elapsed: 0.002 seconds
Memory:  648 KB

— R Day 4 Part 2 —

solve_day4_part2 <- function(input) {
  cards <- strsplit(input, ":|\\|")

  # numbers between : and |
  winners <- lapply(cards, \(x) x[[2]])
  winners <- stringi::stri_extract_all(winners, regex = "\\d+")

  # numbers from | to the end of the line
  playing <- lapply(cards, \(x) x[[3]])
  playing <- stringi::stri_extract_all(playing, regex = "\\d+")

  # count playing numbers in winning numbers
  win_count <- Map(\(w, p) sum(p %in% w),
                   winners,
                   playing)

  win_count <- unlist(win_count)

  copies <-  rep(1, length(cards))

  # because cards only add new copies of later cards,
  # we can sequence along them in order
  for (card in seq_along(copies)) {
    # if there are winning numbers
    if (win_count[card] > 0) {
      # we add wins to the number of following cards
      new_card_range <- (card + 1):(card + win_count[[card]])
      # the number we add is the number of copies we have of the current card
      copies[new_card_range] <- copies[new_card_range] + copies[card]
    }
  }

  sum(copies)
}

aoc_source(day = 4, part = 2)

input = aoc_read(day = 4)

aoc_run(solve_day4_part2(input))

Elapsed: 0.002 seconds
Memory:  218 KB