Day 10

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)

Read input from file:

input <- read_lines("../input/day10.txt") |> 
  str_split("")

Part 1

Define our open/close bracket pairs and the scores for each type:

b_open  <- c("(", "[", "{", "<")
b_close <- c(")", "]", "}", ">")
b_pairs <- c(set_names(b_close, b_open), set_names(b_open, b_close))
b_score <- set_names(c(3, 57, 1197, 25137), b_close)

Define a function to move through a line, add/remove brackets to a stack, and return the first invalid closing bracket found.

check_line <- function(x) {
  stack <- c()
  for (chr in x) {
    if (chr %in% b_open)
      stack <- c(stack, chr)
    else if (tail(stack, 1) == b_pairs[chr])
      stack <- head(stack, -1)
    else
      return(chr)
  }
}

Identify which lines have mistakes:

scanned <- input |> 
  map(check_line)

Sum the total score for all mistakes:

scanned |> 
  unlist() |> 
  map_int(~ b_score[[.x]]) |> 
  sum()

Part 2

Modify the point values for each closing bracket:

b_score <- set_names(1:4, b_close)

Alter the line-scanning function to return the autocomplete brackets:

complete_line <- function(x) {
  stack <- c()
  for (chr in x) {
    if (chr %in% b_open)
      stack <- c(stack, chr)
    else if (tail(stack, 1) == b_pairs[chr])
      stack <- head(stack, -1)
  }
  
  return(unname(b_pairs[rev(stack)]))
}

Define a function to score a given completion string:

score_autocomplete <- \(x) reduce(x, ~ .x * 5 + b_score[[.y]], .init = 0)

Autocomplete all non-error inputs, score the results, then take the median:

is_incomplete <- map_lgl(scanned, is.null)

input |> 
  keep(is_incomplete) |> 
  map(complete_line) |> 
  map_dbl(score_autocomplete) |> 
  median()