Advent of Code 2022

Mix.install([
  {:req, "~> 0.3.2"}
])

Day 5

input =
  "https://adventofcode.com/2022/day/5/input"
  |> Req.get!(headers: [cookie: "session=#{System.get_env("AOC_COOKIE")}"])
  |> Map.get(:body)

sample = """
    [D]    
[N] [C]    
[Z] [M] [P]
 1   2   3 

move 1 from 2 to 1
move 3 from 1 to 3
move 2 from 2 to 1
move 1 from 1 to 2
"""

Part 1

%{false: stacks, true: ops} =
  input
  |> String.split("\n", trim: true)
  |> Enum.filter(&(!String.starts_with?(&1, " 1")))
  |> Enum.group_by(&String.starts_with?(&1, "move"))

# |> dbg()

stacks =
  stacks
  |> Enum.map(fn line ->
    Regex.scan(~r/(   |\[\w]) ?/, line, capture: :all_but_first)
    |> Enum.map(fn
      ["   "] -> nil
      [s] -> String.replace(s, ~r"[\[\]]", "")
    end)
  end)
  # transpose
  |> Enum.zip_with(& &1)
  |> Enum.map(fn stack ->
    stack
    |> Enum.filter(& &1)
  end)
  # turn into a map
  |> Enum.with_index()
  |> Map.new(fn {v, k} -> {k + 1, v} end)

n_stacks = stacks |> Map.keys() |> Enum.count()

ops =
  Enum.map(ops, fn line ->
    Regex.run(~r/move (\d+) from (\d+) to (\d+)/, line)
    |> tl()
    |> Enum.map(&String.to_integer/1)
  end)

stacks =
  Enum.reduce(ops, stacks, fn [n, from, to], acc ->
    Enum.reduce(1..n, acc, fn _, stacks ->
      to_stack = [hd(stacks[from]) | stacks[to]]
      from_stack = tl(stacks[from])

      stacks
      |> Map.put(from, from_stack)
      |> Map.put(to, to_stack)
    end)
  end)

# list is built in reverse
n_stacks..1
|> Enum.reduce([], fn k, acc ->
  [hd(stacks[k]) | acc]
end)
|> Enum.join()

Part 2

%{false: stacks, true: ops} =
  input
  |> String.split("\n", trim: true)
  |> Enum.filter(&(!String.starts_with?(&1, " 1")))
  |> Enum.group_by(&String.starts_with?(&1, "move"))

stacks =
  stacks
  |> Enum.map(fn line ->
    Regex.scan(~r/(   |\[\w]) ?/, line, capture: :all_but_first)
    |> Enum.map(fn
      ["   "] -> nil
      [s] -> String.replace(s, ~r"[\[\]]", "")
    end)
  end)
  # transpose
  |> Enum.zip_with(& &1)
  |> Enum.map(fn stack ->
    stack
    |> Enum.filter(& &1)
  end)
  # turn into a map
  |> Enum.with_index()
  |> Map.new(fn {v, k} -> {k + 1, v} end)

n_stacks = stacks |> Map.keys() |> Enum.count()

ops =
  Enum.map(ops, fn line ->
    Regex.run(~r/move (\d+) from (\d+) to (\d+)/, line)
    |> tl()
    |> Enum.map(&String.to_integer/1)
  end)

stacks =
  Enum.reduce(ops, stacks, fn [n, from, to], acc ->
    crates = acc[from] |> Enum.take(n)
    to_stack = crates ++ acc[to]
    from_stack = acc[from] -- crates

    acc
    |> Map.put(from, from_stack)
    |> Map.put(to, to_stack)
  end)

# list is built in reverse
n_stacks..1
|> Enum.reduce([], fn k, acc ->
  [hd(stacks[k]) | acc]
end)
|> Enum.join()