Day 5

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)

# Read input from file
path <- "../input/day05.txt"
input <- read_lines(path)

Part 1

# Format raw input
moves <- input |>
  tail_while(~ .x != "") |>
  str_extract_all("\\d+") |>
  map(as.integer)

stacks <- read_fwf(
  path,
  n_max = length(input) - length(moves) - 2,
  col_types = "c"
) |>
  mutate(across(everything(), ~ str_extract(.x, "[A-Z]"))) |>
  as.list() |>
  map(discard, is.na) |>
  map(rev)

move_crates <- function(moves, stacks, func) {
  
  # Execute moves
  for (curr_move in moves) {
    count <- curr_move[1]
    from  <- curr_move[2]
    to    <- curr_move[3]
    
    crates <- func(tail(stacks[[from]], count))

    stacks[[to]]   <- append(stacks[[to]], crates)
    stacks[[from]] <- head(stacks[[from]], -1 * count)
  }

  # Examine final top row of crates
  stacks |>
    map(~ tail(.x, 1)) |>
    str_c(collapse = "")
}

move_crates(moves, stacks, rev)

[1] "RLFNRTNFB"

Part 2

move_crates(moves, stacks, identity)

[1] "MHQTLJRLB"