Feature: Allow unlimited multi-level navigation (just-the-docs#1431)

* Allow unlimited multi-level navigation

This PR supersedes just-the-docs#462.

The only user-level difference from just-the-docs#462 is that disambiguation of parent pages has to use either `grand_parent` or `ancestor` titles: the somewhat unnatural `section_id` and `in_section` fields are not supported.

The implementation has been significantly simplified by the changes introduced in v0.7.0 of the theme.

* Detect cyclic parenthood

A page should not have a parent or ancestor with the same title. If it does, the location of the repeated link is marked by ∞, to facilitate debugging the navigation (and an unbounded loop leading to a build exception is avoided).

* Add nav_error_report warning in main navigation

When activated by `nav_error_report: true` in `_config.yml`, displays warnings about pages with the same title as their parent page or an ancestral page.

* Cache site-nav with links to all pages

The extra cached site-nav is used for determining breadcrumbs and children navigation, which may involve pages that are excluded from the main navigation.

* Replace code for determining children by inclusion of components/nav/children.html

* Update CHANGELOG.md

---------

Co-authored-by: Matt Wang <[email protected]>

CHANGELOG.md

@@ -17,12 +17,16 @@ This website is built from the `HEAD` of the `main` branch of the theme reposito
 
 Code changes to `main` that are *not* in the latest release:
 
-- N/A
+### New Features
+
+- Added: Allow unlimited multi-level navigation by [@pdmosses] in [#1431]
 
 Docs changes made since the latest release:
 
 - N/A
 
+[#1431]: https://github.com/just-the-docs/just-the-docs/pull/1431
+
 ## Release v0.9.0
 
 Hi folks! This minor release adds a `nav_enabled` set of variables to enable/disable the navigation at a site, layout, and page level --- and uses that to add search and auxilary links to the `minimal` layout. In addition, it fixes `search-data.json` corruption with default layouts and some minor CSS/SCSS issues.

_config.yml

@@ -122,6 +122,9 @@ nav_external_links:
   - title: Just the Docs on GitHub
     url: https://github.com/just-the-docs/just-the-docs
 
+# Show navigation error report
+nav_error_report: true # default is false/nil.
+
 liquid:
    error_mode: strict
    strict_filters: true

_includes/components/breadcrumbs.html

@@ -1,144 +1,96 @@
 {%- comment -%}
   Include as: {%- include components/breadcrumbs.html -%}
   Depends on: page, site.
+  Includes: components/site_nav.html.
   Results in: HTML for the breadcrumbs component.
   Overwrites:
-    node, pages_list, parent_page, grandparent_page.
+    nav_list_link, site_nav, nav_list_simple, nav_list_link_class, nav_category,
+    nav_anchor_splits, nav_breadcrumbs, nav_split, nav_split_next, nav_split_test,
+    nav_breadcrumb_link, nav_list_end_less, nav_list_end_count, nav_end_index, nav_breadcrumb.
 {%- endcomment -%}
 
-{%- if page.url != "/" and page.parent -%}
+{%- if page.url != "/" and page.parent and page.title -%}
 
 {%- capture nav_list_link -%}
 <a href="{{ page.url | relative_url }}" class="nav-list-link">
 {%- endcapture -%}
 
 {%- capture site_nav -%}
-{%- include_cached components/site_nav.html -%}
+{%- include_cached components/site_nav.html all=true -%}
 {%- endcapture -%}
 
-{%- if site_nav contains nav_list_link -%}
-
-  {%- capture nav_list_simple -%}
-  <ul class="nav-list">
-  {%- endcapture -%}
-
-  {%- capture nav_list_link_class %} class="nav-list-link">
-  {%- endcapture -%}
-
-  {%- capture nav_category -%}
-  <div class="nav-category">
-  {%- endcapture -%}
-
-  {%- assign nav_anchor_splits =
-        site_nav | split: nav_list_link |
-        first | split: nav_category |
-        last | split: "</a>" -%}
-
-  {%- comment -%}
-    The ordinary pages (if any) and the collections pages (if any) are separated by
-    occurrences of nav_category.
-
-    Any ancestor nav-links of the page are contained in the last group of pages,
-    immediately preceding nav-lists. After splitting at "</a>", the anchor that
-    was split is a potential ancestor link when the following split starts with
-    a nav-list. 
-
-    The array nav_breadcrumbs is the stack of current potential ancestors of the
-    current page. A split that contains one or more "</ul>"s requires that number
-    of potential ancestors to be popped from the stack.
-
-    The number of occurrences of a string in nav_split_next is computed by removing
-    them all, then dividing the resulting size difference by the length of the string.
-  {%- endcomment %}
-
-  {%- assign nav_breadcrumbs = "" | split: "" -%}
-
-  {%- for nav_split in nav_anchor_splits -%}
-  {%- unless forloop.last -%}
-
-  {%- assign nav_split_next = nav_anchor_splits[forloop.index] | strip -%}
-
-  {%- assign nav_split_test =
-        nav_split_next | remove_first: nav_list_simple | prepend: nav_list_simple -%}
-  {%- if nav_split_test == nav_split_next -%}
-    {%- assign nav_breadcrumb_link =
-          nav_split | split: "<a " | last | prepend: "<a " |
-          replace: nav_list_link_class, ">" | append: "</a>" -%}
-    {%- assign nav_breadcrumbs = nav_breadcrumbs | push: nav_breadcrumb_link -%}
-  {%- endif -%}
-
-  {%- if nav_split_next contains "</ul>" -%}
-    {%- assign nav_list_end_less = nav_split_next | remove: "</ul>" -%}
-    {%- assign nav_list_end_count =
-          nav_split_next.size | minus: nav_list_end_less.size | divided_by: 5 -%}
-    {% for nav_end_index in (1..nav_list_end_count) %}
-      {%- assign nav_breadcrumbs = nav_breadcrumbs | pop -%}
-    {%- endfor -%}
-  {%- endif -%}
-
-  {%- endunless -%}
-  {%- endfor -%}
+{%- capture nav_list_simple -%}
+<ul class="nav-list">
+{%- endcapture -%}
 
-  {%- assign nav_parent_link = nav_breadcrumbs[-1] -%}
-  {%- assign nav_grandparent_link = nav_breadcrumbs[-2] -%}
+{%- capture nav_list_link_class %} class="nav-list-link">
+{%- endcapture -%}
 
-{%- else -%}
+{%- capture nav_category -%}
+<div class="nav-category">
+{%- endcapture -%}
 
-  {%- comment -%}
-    Pages whose links are excluded from the main navigation may still have
-    breadcrumbs. Determining them appears to require inspecting the front matter
-    of all the pages in the same group. For sites with 100s of pages, this is too
-    inefficient in Jekyll 3 (also when the for-loop is replaced by where-filters).
-  {%- endcomment -%}
+{%- assign nav_anchor_splits =
+      site_nav | split: nav_list_link |
+      first | split: nav_category |
+      last | split: "</a>" -%}
 
-  {%- assign pages_list = site[page.collection] | default: site.html_pages -%}
+{%- comment -%}
+  The ordinary pages (if any) and the collections pages (if any) are separated by
+  occurrences of nav_category.
 
-  {%- assign parent_page = nil -%}
-  {%- assign grandparent_page = nil -%}
-
-  {%- for node in pages_list -%}
+  Any ancestor nav-links of the page are contained in the last group of pages,
+  immediately preceding nav-lists. After splitting at "</a>", the anchor that
+  was split is a potential ancestor link when the following split starts with
+  a nav-list. 
+
+  The array nav_breadcrumbs is the stack of current potential ancestors of the
+  current page. A split that contains one or more "</ul>"s requires that number
+  of potential ancestors to be popped from the stack.
 
-    {%- if node.has_children and page.grand_parent -%}
+  The number of occurrences of a string in nav_split_next is computed by removing
+  them all, then dividing the resulting size difference by the length of the string.
+{%- endcomment %}
 
-      {%- if node.title == page.parent and node.parent == page.grand_parent -%}
-        {%- assign parent_page = node -%}
-      {%- endif -%}
-      {%- if node.title ==  page.grand_parent -%}
-        {%- assign grandparent_page = node -%}
-      {%- endif -%}
-      {%- if parent_page and grandparent_page -%}
-        {%- break -%}
-      {%- endif -%}
+{%- assign nav_breadcrumbs = "" | split: "" -%}
 
-    {%- elsif node.has_children and node.title == page.parent and node.parent == nil -%}
+{%- for nav_split in nav_anchor_splits -%}
+{%- unless forloop.last -%}
 
-      {%- assign parent_page = node -%}
-      {%- break -%}
+{%- assign nav_split_next = nav_anchor_splits[forloop.index] | strip -%}
 
-    {%- endif -%}
+{%- assign nav_split_test =
+      nav_split_next | remove_first: nav_list_simple | prepend: nav_list_simple -%}
+{%- if nav_split_test == nav_split_next -%}
+  {%- assign nav_breadcrumb_link =
+        nav_split | split: "<a " | last | prepend: "<a " |
+        replace: nav_list_link_class, ">" | append: "</a>" -%}
+  {%- assign nav_breadcrumbs = nav_breadcrumbs | push: nav_breadcrumb_link -%}
+{%- endif -%}
 
+{%- if nav_split_next contains "</ul>" -%}
+  {%- assign nav_list_end_less = nav_split_next | remove: "</ul>" -%}
+  {%- assign nav_list_end_count =
+        nav_split_next.size | minus: nav_list_end_less.size | divided_by: 5 -%}
+  {% for nav_end_index in (1..nav_list_end_count) %}
+    {%- assign nav_breadcrumbs = nav_breadcrumbs | pop -%}
   {%- endfor -%}
-
-  {%- capture nav_parent_link -%}
-    <a href="{{ parent_page.url | relative_url }}">{{ page.parent }}</a>
-  {%- endcapture -%}
-
-  {%- if page.grand_parent %}
-    {%- capture nav_grandparent_link -%}
-      <a href="{{ grandparent_page.url | relative_url }}">{{ page.grand_parent }}</a>
-    {%- endcapture -%}
-  {%- endif -%}
-
 {%- endif -%}
 
+{%- endunless -%}
+{%- endfor -%}
+
 <nav aria-label="Breadcrumb" class="breadcrumb-nav">
   <ol class="breadcrumb-nav-list">
-  {%- if nav_grandparent_link %}
-    <li class="breadcrumb-nav-list-item">{{ nav_grandparent_link }}</li>
-  {%- endif %}
-    <li class="breadcrumb-nav-list-item">{{ nav_parent_link }}</li>
+    {%- for nav_breadcrumb in nav_breadcrumbs %}
+    <li class="breadcrumb-nav-list-item">{{ nav_breadcrumb }}</li>
+    {%- endfor %}
     <li class="breadcrumb-nav-list-item"><span>{{ page.title }}</span></li>
   </ol>
 </nav>
 
+{% if site.nav_error_report %}
+{{ nav_error_report }}
+{% endif %}
+
 {%- endif -%}

_includes/components/children_nav.html

@@ -1,33 +1,89 @@
 {%- comment -%}
   Include as: {%- include components/children_nav.html -%}
-  Depends on: page, site.
+  Depends on: page, site, nav_breadcrumbs.
   Results in: HTML for the children-navigation component.
-  Includes:
-    sorted_pages.html
-    toc_heading_custom.html
+  Includes: components/nav/sorted.html, toc_heading_custom.html.
   Overwrites:
-    child_pages.
+    nav_ancestor_links, nav_top_node_titles, nav_child_candidates, nav_children,
+    nav_child, nav_child_ok, nav_child_ancestor, nav_sorted.
 {%- endcomment -%}
 
-{%- if page.has_children == true and page.has_toc != false -%}
-  {%- assign child_pages = site[page.collection]
-        | default: site.html_pages
-        | where: "parent", page.title
-        | where: "grand_parent", page.parent -%}
+{%- comment -%}
+  Whether a page has any children is checked efficiently by inspecting the cached
+  site_nav. If the page has no children, nav_children is set to an empty array;
+  otherwise nav_children is left unset.
+{%- endcomment -%}
 
-  {%- include sorted_pages.html pages = child_pages -%}
+{%- if page.has_children == false -%}
+  {%- assign nav_children = "" | split: "" -%}
+{%- else -%}
 
-  {%- if page.child_nav_order == 'desc' or page.child_nav_order == 'reversed' -%}
-    {%- assign sorted_pages = sorted_pages | reverse -%}
+  {%- assign nav_children = nil -%}
+
+  {%- capture nav_list_link -%}
+  <a href="{{ page.url | relative_url }}" class="nav-list-link">
+  {%- endcapture -%}
+
+  {%- capture site_nav -%}
+  {%- include_cached components/site_nav.html all=true -%}
+  {%- endcapture -%}
+
+  {%- capture nav_list_simple -%}
+  <ul class="nav-list">
+  {%- endcapture -%}
+
+  {%- assign nav_child_start = site_nav
+        | split: nav_list_link | last
+        | split: "</a>" | slice: 1 | first -%}
+
+  {%- assign nav_child_test = nav_child_start 
+        | remove_first: nav_list_simple | prepend: nav_list_simple -%}
+
+  {%- if nav_child_start != nav_child_test -%}
+    {%- assign nav_children = "" | split: "" -%}
   {%- endif -%}
+
 {%- endif -%}
 
+{%- unless nav_children -%}
+
+  {%- comment -%}
+    The layout is assumed to include components/breadcrumbs.html before this file,
+    otherwise it needs to be included here.
+  {%- endcomment -%}
+
+  {%- assign nav_ancestors = "" | split: "" -%}
+  {%- for nav_link in nav_breadcrumbs -%}
+    {%- assign nav_title = nav_link | split: ">" | slice: 1 | first | append: ">" | remove: "</a>" -%}
+    {%- assign nav_ancestors = nav_ancestors | push: nav_title -%}
+  {%- endfor -%}
+
+  {%- assign nav_parenthood = site[page.collection] | default: site.html_pages
+    | where_exp: "item", "item.title != nil" | group_by: "parent" -%}
+
+  {%- assign nav_top_nodes = nav_parenthood
+      | where_exp: "item", "item.name == ''" | map: "items" | first -%}
+
+  {% assign nav_top_node_titles = nav_top_nodes | map: "title" -%}
+
+  {%- include components/nav/children.html node=page ancestors=nav_ancestors all=true -%}
+
+{%- endunless -%}
+
+{%- if nav_children.size >= 1 -%}
+
+  {%- if page.child_nav_order == 'desc' or page.child_nav_order == 'reversed' -%}
+    {%- assign nav_children = nav_children | reverse -%}
+  {%- endif -%}
+
 <hr>
 {% include toc_heading_custom.html %}
 <ul>
-{% for child in sorted_pages %}
+  {% for nav_child in nav_children %}
   <li>
-    <a href="{{ child.url | relative_url }}">{{ child.title }}</a>{% if child.summary %} - {{ child.summary }}{% endif %}
+    <a href="{{ nav_child.url | relative_url }}">{{ nav_child.title }}</a>{% if nav_child.summary %} - {{ nav_child.summary }}{% endif %}
   </li>
-{% endfor %}
+  {% endfor %}
 </ul>
+
+{%- endif -%}