Day 11

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)

# Read input from file
input <- read_lines("../input/day11.txt") |>
  trimws() |>
  discard(~ .x == "")

Part 1

# Reformat input
items <- str_match(input, "Starting items:(.*)")[,2] |>
  discard(is.na) |>
  str_split(",") |>
  map(parse_number)

operations <- input |>
  keep(~ str_detect(.x, "Operation:")) |>
  str_replace("Operation: new = ", "~ ") |>
  str_replace_all("old", ".x") |>
  map(~ rlang::as_function(as.formula(.x)))

div  <- parse_number(keep(input, ~ str_detect(.x, "Test:")))
divt <- parse_number(keep(input, ~ str_detect(.x, "If true:")))
divf <- parse_number(keep(input, ~ str_detect(.x, "If false:")))

test <- pmap(
  list(div, divt, divf),
  ~ function(x) if_else(x %% ..1 == 0, ..2 + 1, ..3 + 1)
)

num_monkeys <- length(input) / 6


compute_monkey_business <- function(num_rounds, worry_func) {
  # Initialize
  activity <- rep(0, num_monkeys)

  # Perform the tosses
  for (round in 1:num_rounds) {
    for (monkey in 1:num_monkeys) {
      for (item in items[[monkey]]) {
        worry <- worry_func(operations[[monkey]](item))
        toss  <- test[[monkey]](worry)
        items[[toss]] <- c(items[[toss]], worry)
      }
      activity[[monkey]] <- activity[[monkey]] + length(items[[monkey]])
      items[[monkey]] <- numeric(0)
    }
  }

  # Compute monkey business score
  activity |>
    sort() |>
    tail(2) |>
    reduce(`*`)
}

compute_monkey_business(num_rounds = 20, worry_func = \(x) floor(x / 3))

[1] 78678

Part 2

lcm <- DescTools::LCM(div)
compute_monkey_business(num_rounds = 10000, worry_func = \(x) x %% lcm)

[1] 15333249714