Again a bit late to the party and probably more complicated than needed to be, but it feels pretty readable to me. I also had great fun exploring the Path module. That is, until I had a bug changing into parent folders that had me puzzled for easily an hour 
defmodule ExAOC2022.Day7 do
@input "./lib/day7_input.txt"
@disk_size 70_000_000
@required_space 30_000_000
def puzzle1() do
{_path, sizes} = analyze_file_system()
Enum.reduce(sizes, 0, fn {_, v}, acc -> if v <= 100_000, do: acc + v, else: acc end)
end
def puzzle2() do
{_path, sizes} = analyze_file_system()
space_left = @disk_size - sizes["/"]
space_needed = @required_space - space_left
Map.values(sizes) |> Enum.sort |> Enum.find(&(&1 >= space_needed))
end
defp analyze_file_system() do
@input
|> file_by_line
|> Enum.map(&String.split/1)
|> Enum.reduce({nil, %{}}, &run/2)
end
defp run(["$", "cd", "/"], {_, sizes}), do: {Path.rootname("/"), Map.put(sizes, "/", 0)}
defp run(["$", "cd", ".."], {path, sizes}), do: {parent_dir(path), sizes}
defp run(["$", "cd", dir], {path, sizes}) do
path = path |> Path.join(dir)
{path, Map.put(sizes, path, 0)}
end
defp run(["$", _], acc), do: acc
defp run(["dir", _], acc), do: acc
defp run([size, _file], {path, sizes}) do
sizes = for {k, _v} <- sizes, reduce: sizes do
acc ->
if String.contains?(path, k) do
Map.update!(acc, k, &(&1 + String.to_integer(size)))
else
acc
end
end
{path, sizes}
end
defp run(_, acc), do: acc
defp parent_dir(path) do
path
|> Path.split()
|> Enum.reverse()
|> tl()
|> Enum.reverse
|> Path.join()
end
defp file_by_line(file) do
file
|> File.read!()
|> String.split(~r/\R/)
end
end
