defmodule Crawler.Snapper.LinkReplacer do
  @moduledoc """
  Replaces links found in a page so they work offline.
  """

  alias Crawler.Linker
  alias Crawler.Parser

  @doc """
  Replaces links found in a page so they work offline.

  ## Examples

      iex> LinkReplacer.replace_links(
      iex>   "<a href='http://another.domain/page.html'></a>",
      iex>   %{
      iex>     url: "http://main.domain/dir/page",
      iex>     depth: 1,
      iex>     max_depths: 2,
      iex>     html_tag: "a",
      iex>     content_type: "text/html",
      iex>   }
      iex> )
      {:ok, "<a href='../../../another.domain/page.html'></a>"}

      iex> LinkReplacer.replace_links(
      iex>   "<a href='http://another.domain/dir/page.html'></a>",
      iex>   %{
      iex>     url: "http://main.domain/page",
      iex>     depth: 1,
      iex>     max_depths: 2,
      iex>     html_tag: "a",
      iex>     content_type: "text/html",
      iex>   }
      iex> )
      {:ok, "<a href='../../another.domain/dir/page.html'></a>"}

      iex> LinkReplacer.replace_links(
      iex>   "<a href='http://another.domain/dir/page'></a>",
      iex>   %{
      iex>     url: "http://main.domain/dir/page",
      iex>     depth: 1,
      iex>     max_depths: 2,
      iex>     html_tag: "a",
      iex>     content_type: "text/html",
      iex>   }
      iex> )
      {:ok, "<a href='../../../another.domain/dir/page/index.html'></a>"}

      iex> LinkReplacer.replace_links(
      iex>   "<a href='/dir/page2.html'></a>",
      iex>   %{
      iex>     url: "http://main.domain/dir/page",
      iex>     referrer_url: "http://main.domain/dir/page",
      iex>     depth: 1,
      iex>     max_depths: 2,
      iex>     html_tag: "a",
      iex>     content_type: "text/html",
      iex>   }
      iex> )
      {:ok, "<a href='../../../main.domain/dir/page2.html'></a>"}
  """
  def replace_links(body, opts) do
    new_body =
      body
      |> Parser.parse_links(opts, &get_link/2)
      |> List.flatten()
      |> Enum.reject(&(&1 == nil))
      |> Enum.reduce(body, &modify_body(opts[:content_type], &2, opts[:url], &1))

    {:ok, new_body}
  end

  defp get_link({_, url}, _opts), do: url
  defp get_link({_, link, _, url}, _opts), do: [link, url]

  defp modify_body(content_type, body, current_url, link) do
    String.replace(
      body,
      regexes(content_type, link),
      modify_link(current_url, link)
    )
  end

  defp regexes(content_type, link) do
    case content_type do
      "text/css" -> ~r{((?!url)\(['"]?)#{link}(['"]?\))}
      _ -> ~r{((?!src|href)=['"])#{link}(['"])}
    end
  end

  defp modify_link(current_url, link) do
    "\\1" <> Linker.offline_link(current_url, link) <> "\\2"
  end
end