defmodule Mix.Tasks.Docs.RepairLinks do
  @moduledoc """
  Mix task for repairing links in HTML files generated by `mix docs`.
  """
  use Mix.Task
  require Logger

  @shortdoc "Repairs links in generated HTML files"

  def run([]) do
    Logger.error("Expected argument: path to ex_doc HTML output")
  end

  def run([path | other_args]) do
    Logger.warn("Ignoring extra arguments: #{inspect other_args}")
    do_run(path)
  end

  def do_run(doc_output_path) do
    # Relative links for files in doc/ are okay; we only need to fix the
    # links found in top-level (markdown) files.
    for fname <- Path.wildcard("./*.md") do
      html_path =
        Path.join([
          doc_output_path,
          Path.basename(fname, ".md") <> ".html"
        ])

      Logger.debug("processing #{html_path}")

      content =
        File.read!(html_path)
        |> fix_links

      File.write!(html_path, content)
    end
  end

  defp fix_links(raw_html) do
    anchor_links =
      raw_html
      |> Floki.find("a")
      |> Floki.attribute("href")

    img_links =
      raw_html
      |> Floki.find("img")
      |> Floki.attribute("src")

    bad_links =
      (anchor_links ++ img_links)
      |> Enum.reject(&String.starts_with?(&1, "http://"))
      |> Enum.reject(&String.starts_with?(&1, "https://"))
      |> Enum.reject(&String.starts_with?(&1, "#"))

    new_html =
      bad_links
      |> Enum.reduce(raw_html, fn bad_link, acc ->
           good_link = repair_link(bad_link)
           Logger.debug("repaired link: #{bad_link} => #{good_link}")
           acc
           |> String.replace(~s(href="#{bad_link}"), ~s(href="#{good_link}"))
           |> String.replace(~s(src="#{bad_link}"), ~s(src="#{good_link}"))
         end)

    new_html
  end

  defp repair_link("LICENSE") do
    # not very proud of this.
    "../LICENSE"
  end
  defp repair_link(link) do
    toplevel_file? = not String.contains?(link, "/")

    link =
      case toplevel_file? do
        true -> link
        false -> Path.join("..", link)
      end

    link =
      case Path.extname(link) do
        ".md" -> Path.basename(link, ".md") <> ".html"
        _ -> link
      end

    link
  end
end