resolves asciidoctor#327 add customizable splitting into chapters

CHANGELOG.adoc

@@ -10,6 +10,7 @@ For a detailed view of what has changed, refer to the {uri-repo}/commits/master[
 * add basic audio and video support (#9)
 * add support for `[horizontal]` definition list (#165)
 * add proper handling of `:data-uri:` document attribute (#324)
+* add support for customizable document splitting into chapters via `epub-chapter-level` attribute (#327)
 
 == 1.5.0.alpha.15 (2020-03-11) - @slonopotamus
 

README.adoc

@@ -297,6 +297,12 @@ The recommended practice is to identify the referenced resource by means of a st
 |An optional override of the properties attribute for this document's item in the manifest.
 _Only applies to a chapter document._
 
+|epub-chapter-level
+|Specify the section level at which to split the EPUB into separate "chapter" files.
+The default is to split into chapters at level-1 sections.
+This option only affects the internal composition of the EPUB, not the way chapters and sections are displayed to users.
+Some readers may be slow if the chapter files are too large, so for large documents with few level-1 headings, one might want to use a chapter level of 2 or 3.
+
 |series-name, series-volume, series-id
 |Populates the series statements (`belongs-to-collection`) in the package metadata.
 Volume is a number, ID probably a UUID that is constant for all volumes in the series.

lib/asciidoctor-epub3/converter.rb

@@ -115,7 +115,8 @@ def get_chapter_name node
           return Asciidoctor::Document === node ? node.attr('docname') || node.id : nil
         end
         return (node.id || 'preamble') if node.context == :preamble && node.level == 0
-        Asciidoctor::Section === node && node.level <= 1 ? node.id : nil
+        chapter_level = [node.document.attr('epub-chapter-level', 1).to_i, 1].max
+        Asciidoctor::Section === node && node.level <= chapter_level ? node.id : nil
       end
 
       def get_numbered_title node
@@ -141,6 +142,14 @@ def get_numbered_title node
         title
       end
 
+      def collect_toc_items node, into
+        node.sections.each do |section|
+          next if get_chapter_name(section).nil?
+          into << section
+          collect_toc_items section, into
+        end
+      end
+
       def convert_document node
         @format = node.attr('ebook-format').to_sym
 
@@ -223,13 +232,7 @@ def convert_document node
 
         if node.doctype == 'book'
           toc_items = []
-          node.sections.each do |section|
-            toc_items << section
-            section.sections.each do |subsection|
-              next if get_chapter_name(node).nil?
-              toc_items << subsection
-            end
-          end
+          collect_toc_items node, toc_items
           node.content
         else
           toc_items = [node]

spec/converter_spec.rb

@@ -156,6 +156,31 @@
       expect(chapter_b.content).not_to include footnote
     end
 
+    it 'supports custom epub-chapter-level' do
+      book = to_epub <<~EOS
+= Book
+:epub-chapter-level: 2
+:doctype: book
+
+text0
+
+== Level 1
+
+text1
+
+=== Level 2
+
+text2
+
+==== Level 3
+
+text3
+      EOS
+
+      spine = book.spine.itemref_list
+      expect(spine).to have_size(3)
+    end
+
     it 'resolves deep includes relative to document that contains include directive' do
       book, = to_epub fixture_file('deep-include/book.adoc')
       chapter = book.item_by_href '_chapter.xhtml'