{% comment %}
    This file enables manual episode ordering until
    GitHub Pages switches to Jekyll that supports it
    without any major hackery. Note, some logic will
    be required even when this transition happens
    but it won't be as involved as what we have to do
    in this file.

    To order lesson episodes or extras manually
    (instead of the default alpha-numerical order),
    create array variables 'episode_order' and
    'extras_order' in `_config.yml` like so:

    episode_order:
      - episodeA
      - episodeB

    extras_order:
      - extraA
      - extraB

    Note that "Reference" page is currently  always
    added to "Extras" as the first item.

    The main outcomes of the code in this file are:
     - 'lesson_episodes' variable that replaces
       'site.episodes' variable when manual episode
       order is defined.
     - 'lesson_extras' variable that replaces
       'site.extras' variable when manual ordering of
       files in '_extras' is used
       - 'previous_episode' and 'next_episode' objects
       that replace 'page.previous' and 'page.next' variables,
       correspondingly, and that have such properties
       as 'url' and 'title' and that are used in
       'episode_navbar.html'.

    When episode order is specified manually, the 'lesson_episodes'
    variable contains a list of episode names ("slugs", to be precise;
    "slug" is the episode name without '.md').  Therefore, when we
    iterate over 'lesson_episodes' (in syllabus.html and navbar.html) ,
    we have to check whether we use manual episode ordering and, if so,
    find the corresponding episode object. This is what we do with the
    following code in every loop over 'lesson_episodes':

        {% if site.episode_order %}
          {% assign episode = site.episodes | where: "slug", lesson_episode | first %}
        {% else %}
          {% assign episode = lesson_episode %}
        {% endif %}
{% endcomment %}

{% comment %}
Manual ordering of Episodes begins here
{% endcomment %}

{% if site.episode_order %}
    {% assign lesson_episodes = site.episode_order %}
{% else %}
    {% assign lesson_episodes = site.episodes %}
{% endif %}


{% comment %}
    If 'episode_order' is defined, we need to determine
    - previous episode object ('previous_episode')
    - and next episode object ('next_episode')
{% endcomment %}


{% if site.episode_order %}
  {% for lesson_episode in lesson_episodes %}

    {% comment %}
        We iterate over the specified lesson episodes using
        a 'for' loop because we can use
        'forloop.first', 'forloop.last', and 'forloop.index0'.
    {% endcomment %}

    {% unless lesson_episode == page.slug %} {% continue %} {% endunless %}

    {% if forloop.first %}
      {% assign previous_episode = nil %}
    {% else %}
      {% assign p_idx = forloop.index0 | minus: 1 %}
      {% assign p_name = lesson_episodes[p_idx] %}
      {% assign previous_episode = site.episodes | where: "slug", p_name | first %}
    {% endif %}

    {% if forloop.last == true %}
      {% assign next_episode = nil %}
    {% else %}
      {% assign n_idx = forloop.index0 | plus: 1 %}
      {% assign n_name = lesson_episodes[n_idx] %}
      {% assign next_episode = site.episodes | where: "slug", n_name | first %}
    {% endif %}
  {% endfor %}
{% else %}
  {% assign previous_episode = page.previous %}
  {% assign next_episode  = page.next %}
{% endif %}


{% comment %}
Manual ordering of Extras begins here
{% endcomment %}

{% if site.extras_order %}
    {% assign lesson_extras = site.extras_order %}
{% else %}
    {% assign lesson_extras = site.extras %}
{% endif %}

{% comment %}
    We do not need to determine "previous" or "next" extra.
{% endcomment %}