Convert a list of paths to a tree

Foldit.exs

defmodule FolditTest do
  use ExUnit.Case
  doctest Foldit

  def to_keyword([], keyword), do: keyword

  def to_keyword([val], keyword) do
    Keyword.update(keyword, nil, [val], fn existing -> [val | existing] end)
  end

  def to_keyword([key | val], keyword) do
    Keyword.update(keyword, String.to_atom(key), to_keyword(val, []), fn existing ->
      to_keyword(val, existing)
    end)
  end

  def to_tree(list) do
    list
    |> Enum.map(&String.split(&1, ~r"/"))
    |> List.foldl([], &to_keyword/2)
  end

  test "moves array into nested keyword" do
    assert to_keyword(["3.md"], []) == [nil: ["3.md"]]
    assert to_keyword(["1", "3.md"], []) == ["1": [nil: ["3.md"]]]
    assert to_keyword(["a", "b", "3.md"], []) == [a: [b: [nil: ["3.md"]]]]
  end

  test "adds to existing" do
    assert to_keyword(["3.md"], nil: ["1.md"]) == [nil: ["3.md", "1.md"]]

    assert to_keyword(["3.md"], a: [nil: "extra.md"], nil: ["1.md"]) == [
             a: [nil: "extra.md"],
             nil: ["3.md", "1.md"]
           ]
  end

  test "does multiple" do
    structure = [
      "3.md",
      "4.md"
    ]

    assert to_tree(structure) == [nil: ["4.md", "3.md"]]

    structure = [
      "3.md",
      "a/4.md"
    ]

    assert to_tree(structure) == [nil: ["3.md"], a: [nil: ["4.md"]]]

    structure = [
      "2.md",
      "3.md",
      "a/4.md",
      "a/5.md"
    ]

    assert to_tree(structure) == [nil: ["3.md", "2.md"], a: [nil: ["5.md", "4.md"]]]
  end
end