Day 7

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)
library(unglue)

# Read input from file
input <- read_lines("../input/day07.txt", skip_empty_rows = FALSE)

Part 1

Parse text input as a bid and set of cards for each hand:

card_types <- c(2:9, 'T', 'J', 'Q', 'K', 'A')

hands <- input |> 
  unglue_data("{card1=.}{card2=.}{card3=.}{card4=.}{card5=.} {bid}") |> 
  transmute(
    hand_id = row_number(),
    bid = as.numeric(bid),
    across(starts_with("card"), partial(factor, levels = card_types)),
    cards = pmap(list(card1, card2, card3, card4, card5), c)
  )

Define a function to compute the type of each hand:

hand_types <- c("HIGH", "PAIR", "2PAIR", "3KIND", "FULL", "4KIND", "5KIND")

hand_type <- function(cards) {
  card_counts <- sort(summary(cards), decreasing = TRUE)
  case_when(
    card_counts[1] == 3 & card_counts[2] == 2 ~ "FULL",
    card_counts[1] == 2 & card_counts[2] == 2 ~ "2PAIR",
    card_counts[1] == 5 ~ "5KIND",
    card_counts[1] == 4 ~ "4KIND",
    card_counts[1] == 3 ~ "3KIND",
    card_counts[1] == 2 ~ "PAIR",
    card_counts[1] == 1 ~ "HIGH"
  )
}

Define a function to determine the type of each hand, rank the results, and sum the total winnings:

score <- function(df) {
  df |> 
    mutate(hand_type = factor(map_chr(cards, hand_type), levels = hand_types)) |> 
    arrange(hand_type, card1, card2, card3, card4, card5) |> 
    mutate(
      rank = row_number(),
      winnings = rank * bid
    ) |> 
    pull(winnings) |> 
    sum()
}

Run on puzzle input:

score(hands)

[1] 252295678

Part 2

Redefine the hand-scoring function to allow jokers to act as any card:

hand_type <- function(cards) {
  
  counts_joker <- sum(cards == "J")
  counts_other <- cards |> 
    discard(~ .x == "J") |> 
    summary() |> 
    sort(decreasing = TRUE)
  
  top_1 <- counts_other[1] + counts_joker
  top_2 <- counts_other[2]
  
  case_when(
    top_1 == 3 & top_2 == 2 ~ "FULL",
    top_1 == 2 & top_2 == 2 ~ "2PAIR",
    top_1 == 5 ~ "5KIND",
    top_1 == 4 ~ "4KIND",
    top_1 == 3 ~ "3KIND",
    top_1 == 2 ~ "PAIR",
    top_1 == 1 ~ "HIGH"
  )
}

Re-rank the jokers to the bottom of the card heirarcy and re-score:

card_types <- c("J", discard(card_types, ~ .x == "J"))

hands |> 
  mutate(across(num_range("card", 1:5), partial(factor, levels = card_types))) |> 
  score()

[1] 250577259