defmodule Fermo.Sitemap do
  require Logger

  @xml_header ~s(<?xml version="1.0" encoding="UTF-8"?>\n)
  @open_tag ~S(<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">)
  @close_tag ~S(</urlset>)

  @file_impl Application.compile_env(:fermo, :file_impl, File)
  @template Application.compile_env(:fermo, :template, Fermo.Template)

  @doc """
  The sitemap is built if config[:sitemap] is set.

  It should be a Map.

  Available options:

  * `:default_change_frequency` - see https://www.sitemaps.org/protocol.html#changefreqdef
  * `:default_priority` - a value between 0 and 1

  Other `config` options that affect the sitemap are:

  * `:base_url`
  * `:build_path`

  Use a template's frontmatter to exclude it from the sitemap:

      ---
      hide_from_sitemap: true
      ---
  """
  @callback build(map()) :: map()
  def build(config)
  def build(%{sitemap: sitemap} = config) do
    root = config.base_url
    build_path = config[:build_path] || "build"
    Fermo.File.ensure_path(build_path)
    sitemap_pathname = build_path <> "/sitemap.xml"
    datetime = DateTime.utc_now()
    lastmod = DateTime.to_iso8601(datetime, :extended)

    config_defaults = %{
      lastmod: lastmod,
      change_frequency: sitemap[:default_change_frequency] || "weekly",
      priority: sitemap[:default_priority] || 0.5
    }

    @file_impl.write!(sitemap_pathname, @xml_header)
    @file_impl.write!(sitemap_pathname, @open_tag, [:append])

    Stream.map(config.pages, fn page ->
      module = @template.module_for_template(page.template)
      page_defaults = module.defaults()
      |> Enum.into(%{}, fn {k, v} -> {String.to_atom(k), v} end)

      values = Map.merge(config_defaults, page_defaults)
      if !page_defaults[:hide_from_sitemap] do
        loc = "#{root}#{page.path}"
        ~s(
          <url>
            <loc>#{loc}</loc>
            <lastmod>#{values.lastmod}</lastmod>
            <changefreq>#{values.change_frequency}</changefreq>
            <priority>#{values.priority}</priority>
          </url>
        )
      else
        Logger.debug "[Sitemap] Excluding path '#{page.path}', as defaults include `:hide_from_sitemap`"
      end
    end)
    |> Stream.filter(&(&1))
    |> Stream.into(@file_impl.stream!(sitemap_pathname, [:append, :utf8]))
    |> Stream.run()

    @file_impl.write!(sitemap_pathname, @close_tag, [:append])
    put_in(config, [:stats, :sitemap_built], Time.utc_now)
  end
  def build(config) do
    Logger.debug "[Sitemap] Skipping sitemap generation as config[:sitemap] is missing"
    config
  end
end