{%- comment -%}
  Include as: {%- include sorted_pages.html pages=array_of_pages -%}
  Depends on: include.pages.
  Assigns to: sorted_pages.
  Overwrites: 
    nav_order_pages, title_order_pages, double_quote, empty_array,
    nav_number_pages, nav_string_pages, nav_order_groups, group,
    title_number_pages, title_string_pages, title_order_groups.
{%- endcomment -%}

{%- comment -%}
  The `nav_order` values of pages affect the order in which they are shown in
  the navigation panel and in the automatically generated tables of contents.
  Sibling pages with the same `nav_order` value may be shown in any order.
  Sibling pages with no `nav_order` value are shown after all pages that have
  explicit `nav_order` values, ordered by their `title` values.
  
  The `nav_order` and `title` values can be numbers or strings. To avoid build
  failures, we sort numbers and strings separately. We sort numbers by their
  values, and strings lexicographically. The case-sensitivity of string sorting
  is determined by the configuration setting of `nav_sort`. Pages with no `title`
  value are excluded from the navigation.

  Note: Numbers used as `title` or `nav_order` values should not be in quotes,
  unless you intend them to be lexicographically ordered. Numbers are written
  without spaces or thousands-separators. Negative numbers are preceded by `-`.
  Floats are written with the integral and fractional parts separated by `.`.
  (Bounds on the magnitude and precision are presumably the same as in Liquid.)
{%- endcomment -%}

{%- assign nav_order_pages = include.pages
      | where_exp: "item", "item.nav_order != nil" -%}
{%- assign title_order_pages = include.pages
      | where_exp: "item", "item.nav_order == nil" -%}

{%- comment -%}
  First, filter `nav_order_pages` and `title_order_pages` according to the type
  of value to be used for sorting.
  
  The first character of the result of filtering with `jsonify` is `"` only for
  strings. Removing `"` from its `slice : 0` has size 0 for strings and 1 for
  numbers, so grouping the pages gives at most two groups.
{%- endcomment -%}

{%- assign double_quote = '"' -%}
{%- assign empty_array = "" | split: "" -%}

{%- assign nav_string_pages = empty_array -%}
{%- assign nav_number_pages = empty_array -%}
{%- unless nav_order_pages == empty -%}
  {%- assign nav_order_groups = nav_order_pages
        | group_by_exp: "item",
            "item.nav_order | jsonify | slice: 0 | remove: double_quote | size" -%}
  {%- for group in nav_order_groups -%}
    {%- if group.name == 0 -%}
      {%- assign nav_string_pages = group.items -%}
    {%- elsif group.name == 1 -%}
      {%- assign nav_number_pages = group.items -%}
    {%- endif -%}
  {%- endfor -%}
{%- endunless -%}

{%- assign title_string_pages = empty_array -%}
{%- assign title_number_pages = empty_array -%}
{%- unless title_order_pages == empty -%}
  {%- assign title_order_groups = title_order_pages
        | group_by_exp: "item",
            "item.title | jsonify | slice: 0 | remove: double_quote | size" -%}
  {%- for group in title_order_groups -%}
    {%- if group.name == 0 -%}
      {%- assign title_string_pages = group.items -%}
    {%- elsif group.name == 1 -%}
      {%- assign title_number_pages = group.items -%}
    {%- endif -%}
  {%- endfor -%}
{%- endunless -%}

{%- comment -%}
  Now sort each array of pages separately, then concatenate the sorted arrays.
{%- endcomment -%}

{%- unless nav_number_pages == empty -%}
  {%- assign nav_number_pages = nav_number_pages | sort: "nav_order" -%}
{%- endunless -%}

{%- unless nav_string_pages == empty -%}
  {%- if site.nav_sort == 'case_insensitive' -%}
    {%- assign nav_string_pages = nav_string_pages | sort_natural: "nav_order" -%}
  {%- else -%}
    {%- assign nav_string_pages = nav_string_pages | sort: "nav_order" -%}
  {%- endif -%}
{%- endunless -%}

{%- unless title_number_pages == empty -%}
  {%- assign title_number_pages = title_number_pages | sort: "title" -%}
{%- endunless -%}

{%- unless title_string_pages == empty -%}
  {%- if site.nav_sort == 'case_insensitive' -%}
    {%- assign title_string_pages = title_string_pages | sort_natural: "title" -%}
  {%- else -%}
    {%- assign title_string_pages = title_string_pages | sort: "title" -%}
  {%- endif -%}
{%- endunless -%}

{%- assign sorted_pages = nav_number_pages
      | concat: nav_string_pages
      | concat: title_number_pages
      | concat: title_string_pages -%}

{%- assign book_pages = include.pages | where_exp: "item", "item.layout == 'book'" -%}
{%- assign non_book_pages = include.pages | where_exp: "item", "item.layout != 'book'" -%}

{%- assign sorted_book_pages = book_pages | sort: "author"  -%}
{%- assign sorted_non_book_pages = non_book_pages | sort: "title" -%}

{%- assign sorted_pages = sorted_book_pages | concat: sorted_non_book_pages -%}