Day 6

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)
library(unglue)

Disable scientific notation to display all digits of large numbers:

options(scipen = 999)

Read text input from file:

input <- read_lines("../input/day06.txt")

Parse plain text input into a square data frame, having once cell per row per column (preserving whitespace):

df <- str_pad(input, width = max(str_length(input)), side = "right") |> 
  str_split("") |> 
  set_names(c(str_c("row", seq(length(input) - 1)), "op")) |> 
  as_tibble() |> 
  mutate(
    all_blank = rowSums(across(starts_with("row"), \(x) x != " ")) == 0,
    col_id = cumsum(all_blank),
    op = case_match(op, "+" ~ "sum", "*" ~ "prod", .default = op)
  ) |> 
  filter(!all_blank) |> 
  select(-all_blank)

Part 1

Re-combine digits row-wise into sets of numbers, apply the operation to the result, and sum:

df |> 
  summarize(
    across(starts_with("row"), \(x) parse_number(str_flatten(x))),
    op = max(op),
    .by = col_id
  ) |> 
  nest(nums = starts_with("row")) |> 
  mutate(nums = map2_dbl(op, nums, \(op, nums) get(op)(unlist(nums)))) |> 
  with(sum(nums))

Part 2

First combine all numbers column-wise instead of row-wise, then repeat the process from part 1:

df |> 
  arrange(desc(row_number())) |> 
  mutate(num_id = row_number()) |> 
  nest(num = starts_with("row")) |> 
  mutate(num = map_dbl(num, \(x) parse_number(str_flatten(unlist(x))))) |> 
  summarize(num = get(max(op))(num), .by = col_id) |> 
  with(sum(num))