Day 18

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)

Disable scientific notation to display large integers:

options(scipen = 999)

Read text input from file:

input <- read_lines("../input/day18.txt")

Part 1

Define a function that converts the plain text to a nested list, using parentheses as signals to nest:

nest_eqn <- function(x) {
  x |> 
    str_replace_all(" ", ",") |> 
    str_replace_all("(\\d|\\+|\\*)", "'\\1'") |> 
    str_replace_all("\\(", "list(") |> 
    str_replace_all("^", "list(") |> 
    str_replace_all("$", ")") |> 
    parse(text = _) |> 
    eval()
}

Define a function which takes a nested list of single-digit integers and +/* operators and performs the operations without precedence:

parse_eqn <- function(x) {
  cur <- 0
  op  <- '+'
  
  for (item in x) {
    if (typeof(item) == "list")
      item <- parse_eqn(item)
    
    if (item %in% c("*", "+"))
      op <- item
    else
      cur <- get(op)(cur, as.numeric(item))
  }
  
  cur
}

Run on puzzle input:

input |> 
  map(nest_eqn) |> 
  map_dbl(parse_eqn) |> 
  sum()

Part 2

To apply operator precedence, we re-nest our list one level deeper whenever we encounter our character with precedence. Here, whenever we see a ‘+’ character, we nest it one level deeper alongside the items preceeding and following it.

nest_precedence <- function(x) {
  for (i in 1:length(x)) {
    if (typeof(x[[i]]) == "list")
      x[[i]] <- nest_precedence(x[[i]])
  }
  
  i <- 2

  while (i < length(x)) {
    if (typeof(x[[i]]) == "character" && x[[i]] == "+") {
      x[[i - 1]] <- list(x[[i - 1]], x[[i]], x[[i + 1]])
      x <- x[-c(i, i + 1)]
      i <- i - 1
    }
    i <- i + 1
  }
  
  x
}

Run on puzzle input:

input |> 
  map(nest_eqn) |> 
  map(nest_precedence) |> 
  map_dbl(parse_eqn) |>
  sum()