2023 - Day 2

You’re launched high into the atmosphere! The apex of your trajectory just barely reaches the surface of a large island floating in the sky. You gently land in a fluffy pile of leaves. It’s quite cold, but you don’t see much snow. An Elf runs over to greet you.

The Elf explains that you’ve arrived at Snow Island and apologizes for the lack of snow. He’ll be happy to explain the situation, but it’s a bit of a walk, so you have some time. They don’t get many visitors up here; would you like to play a game in the meantime?

As you walk, the Elf shows you a small bag and some cubes which are either red, green, or blue. Each time you play this game, he will hide a secret number of cubes of each color in the bag, and your goal is to figure out information about the number of cubes.

To get information, once a bag has been loaded with cubes, the Elf will reach into the bag, grab a handful of random cubes, show them to you, and then put them back in the bag. He’ll do this a few times per game.

Set-Up

Getting the games as a number was useful to get the sum of the IDs, but alternatively could have added a new column that contained the row number. Splitting the results into the separate games makes searching for colours easier.

games <- read.table("input.txt", sep = ":", col.names = c("game", "results"))
games$game <- as.numeric(sub(".* ", "", games$game))

head(games)

  game
1    1
2    2
3    3
4    4
5    5
6    6
                                                                                                                                   results
1                                                              10 green, 9 blue, 1 red; 1 red, 7 green; 11 green, 6 blue; 8 blue, 12 green
2                                        11 red, 7 green, 3 blue; 1 blue, 8 green, 5 red; 2 red, 12 green, 1 blue; 10 green, 5 blue, 7 red
3                        2 red, 7 green, 1 blue; 1 blue, 8 red; 7 green, 19 red, 5 blue; 1 blue, 10 green, 18 red; 10 red, 6 blue, 4 green
4                                                                  2 blue, 5 green, 2 red; 7 red, 3 green; 3 blue, 2 red; 16 green, 2 blue
5  1 blue, 9 red; 5 blue, 9 green, 6 red; 8 red, 10 blue, 3 green; 3 red, 13 green, 4 blue; 5 green, 9 red, 6 blue; 4 green, 8 red, 7 blue
6                                           1 red, 2 green; 2 red, 1 blue; 5 red, 10 green, 2 blue; 1 blue, 2 green, 3 red; 1 red, 6 green

games$result_split <- strsplit(games$results, ";")

Part 1

The Elf would first like to know which games would have been possible if the bag contained only 12 red cubes, 13 green cubes, and 14 blue cubes?

Having a space before the first game made things a lot easier for this part, as the regular expression could explicity look for a space before the number of blocks.

colours <- c("red", "blue", "green")

for (colour in colours) {
  games[[colour]] <- lapply(games$result_split, function(x) {
    matches <- grep(colour, x, value = TRUE)
    as.numeric(sub(paste0("^.* (\\d+) ", colour, ".*"), "\\1", matches))
  })
  games[[paste0(colour, "_max")]] <- sapply(games[[colour]], max)
}

sum(subset(games, red_max <= 12 & green_max <= 13 & blue_max <= 14)$game)

[1] 2239

Part 2

The Elf says they’ve stopped producing snow because they aren’t getting any water! He isn’t sure why the water stopped; however, he can show you how to get to the water source to check it out for yourself. It’s just up ahead!

As you continue your walk, the Elf poses a second question: in each game you played, what is the fewest number of cubes of each color that could have been in the bag to make the game possible?

Much nicer part 2 than yesterday!

sum(games$red_max * games$blue_max * games$green_max)

[1] 83435