day22.md

— Day 22: Crab Combat —

Fleur Kelpin Dec 22, 2020

library(tidyverse)
input_player1 <- read_csv("day22-player1.txt", col_names = c("card"))$card
input_player2 <- read_csv("day22-player2.txt", col_names = c("card"))$card
input_player1

##  [1] 10 39 16 32  5 46 47 45 48 26 36 27 24 37 49 25 30 13 23  1  9  3 31 14  4

input_player2

##  [1]  2 15 29 41 11 21  8 44 38 19 12 20 40 17 22 35 34 42 50  6 33  7 18 28 43

Part 1

Before the game starts, split the cards so each player has their own deck (your puzzle input). Then, the game consists of a series of rounds: both players draw their top card, and the player with the higher-valued card wins the round. The winner keeps both cards, placing them on the bottom of their own deck so that the winner’s card is above the other card. If this causes a player to have all of the cards, they win, and the game ends.

Once the game ends, you can calculate the winning player’s score. The bottom card in their deck is worth the value of the card multiplied by 1, the second-from-the-bottom card is worth the value of the card multiplied by 2, and so on. Play the small crab in a game of Combat using the two decks you just dealt. What is the winning player’s score?

p1 <- input_player1
p2 <- input_player2
num_cards <- length(input_player1) + length(input_player2)
while (between(length(p1), 1, num_cards - 1)) {
  # print(list(p1 = p1, p2 = p2))
  trick <- c(p1[[1]], p2[[1]])
  p1 <- tail(p1, n = -1)
  p2 <- tail(p2, n = -1)
  sorted <- sort(trick, decreasing = T)
  if (all(trick == sorted)) {
    p1 <- append(p1, sorted)
  } else {
    p2 <- append(p2, sorted)
  }
}
winning_hand <- c(p1, p2)
sum(winning_hand * (num_cards:1))

## [1] 31455

Part 2

You lost to the small crab! Fortunately, crabs aren’t very good at recursion. To defend your honor as a Raft Captain, you challenge the small crab to a game of Recursive Combat.

library(hash)

## hash-2.2.6.1 provided by Decision Patterns

game <- function(p1, p2) {
  seen <- hash()
  num_cards <- length(p1) + length(p2)
  i <- 1
  while (between(length(p1), 1, num_cards - 1)) {
    #print(list(turn = i, p1 = p1, p2 = p2))
    i <- i + 1
    key <- paste(c(p1, NA, p2), collapse = "_")
    if (has.key(key, seen)) {
      return(list(winner = 1, hand = p1))
    }
    seen[key] <- TRUE
    trick <- c(p1[[1]], p2[[1]])
    p1 <- tail(p1, n = -1)
    p2 <- tail(p2, n = -1)
    if (length(p1) >= trick[[1]] & length(p2) >= trick[[2]]) {
      winner <- game(p1[1:trick[[1]]], p2[1:trick[[2]]])$winner
    } else {
      winner <- if (trick[[1]] > trick[[2]]) 1 else 2
    }
    if (winner == 1) {
      p1 <- append(p1, trick)
    } else {
      p2 <- append(p2, rev(trick))
    }
  }
  if (length(p2) == 0) {
    list(winner = 1, hand = p1)
  } else {
    list(winner = 2, hand = p2)
  }
}

game(c(43, 19), c(2, 29,14))

## $winner
## [1] 1
## 
## $hand
## [1] 43 19

game(c(9, 2, 6, 3, 1), c(5, 8, 4, 7, 10))

## $winner
## [1] 2
## 
## $hand
##  [1]  7  5  6  2  4  1 10  8  9  3

result <- game(input_player1, input_player2)
result$hand

##  [1]  1  6 49 29 30 18 32 25 35 20 21  3 46 43 45  8 16  7 42 24 39 19 48 47  5
## [26]  4 44 15 22  2 38 31 34 28 41 17 37 12 27 14 26 10 50 23 36 11 40 13 33  9

sum(result$hand * (length(result$hand):1))

## [1] 32528