defmodule OrgGarden.Index do
  @moduledoc """
  Generates a fallback `index.md` in the content directory if none was
  exported from an `.org` file.

  The generated index lists all markdown pages alphabetically with links.
  """

  @doc """
  Generate `content_dir/index.md` if it does not already exist.

  If an `index.md` was already created by ox-hugo (from an `index.org`),
  it is left untouched.
  """
  @spec generate(String.t()) :: :ok
  def generate(content_dir) do
    index_path = Path.join(content_dir, "index.md")

    unless File.exists?(index_path) do
      IO.puts("  generating default index.md")

      pages =
        Path.join(content_dir, "**/*.md")
        |> Path.wildcard()
        |> Enum.map(fn path ->
          slug = Path.relative_to(path, content_dir) |> Path.rootname()

          title =
            path
            |> File.read!()
            |> then(fn content ->
              # Match YAML frontmatter: title: "value" or title: value
              case Regex.run(~r/^title:\s*"?(.+?)"?\s*$/m, content) do
                [_, t] -> t
                _ -> slug
              end
            end)

          {slug, title}
        end)
        |> Enum.sort_by(fn {_, title} -> title end)
        |> Enum.map(fn {slug, title} -> "- [#{title}](#{slug})" end)
        |> Enum.join("\n")

      File.write!(index_path, """
      ---
      title: Index
      ---

      #{pages}
      """)
    end

    :ok
  end

  @doc """
  Regenerate the index by removing any previously generated one first.

  Only removes the index if it was generated by us (contains `title: Index`).
  User-exported index files (from `index.org`) are left untouched.
  """
  @spec regenerate(String.t()) :: :ok
  def regenerate(content_dir) do
    index_path = Path.join(content_dir, "index.md")

    if File.exists?(index_path) do
      content = File.read!(index_path)

      if generated_index?(content) do
        File.rm!(index_path)
      end
    end

    generate(content_dir)
  end

  defp generated_index?(content) do
    # Our generated index uses "title: Index" in YAML frontmatter.
    # ox-hugo also uses YAML frontmatter now, so we check for our specific
    # title to distinguish generated vs user-exported index files.
    String.contains?(content, "title: Index")
  end
end