Day 7

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)

Disable scientific notation to display all digits of large numbers:

options(scipen = 999)

Read text input from file into a data frame of characters with row & column positions:

input <- read_lines("../input/day07.txt") |> 
  str_split("") |> 
  map(enframe, name = "col") |> 
  list_rbind(names_to = "row") |> 
  filter(value != ".") |> 
  arrange(value != "S") |> 
  mutate(id = row_number(), .before = everything())

Part 1

Iterate through the rows of the input, tracking the current beam locations at each step, counting when a beam splits, and tracking how many timelines (for part 2) have been created so far:

ncol  <- max(input$col) + 1
beams <- rep(FALSE, ncol) |> replace(input$col[input$value == "S"], TRUE)
timelines <- as.numeric(beams)
nsplit <- 0

for (row_num in unique(input$row[-1])) {
  
  split <- input |> 
    filter(row == row_num) |> 
    pull(col) |> 
    keep(\(x) beams[x] == TRUE)
  
  newbeams <- unique(c(split - 1, split + 1))
  
  beams[split] <- FALSE
  beams[newbeams] <- TRUE
  
  timelines[split - 1] <- timelines[split - 1] + timelines[split]
  timelines[split + 1] <- timelines[split + 1] + timelines[split]
  timelines[split] <- 0
  
  nsplit <- nsplit + length(split)
  
}

Count the total number of splits that occurred:

nsplit

Part 2

Count the total timelines created:

sum(timelines)