revert to iteration-based flatten in compiled JavaScript if built-in …

…flatten method on array throws range error

CHANGELOG.adoc

@@ -4,6 +4,12 @@
 This document provides a curated view of the changes to Asciidoctor Reducer in each release.
 For a detailed view of what has changed, refer to the {url-repo}/commits/main[commit history] on GitHub.
 
+== Unreleased
+
+=== Fixed
+
+* Revert to iteration-based flatten in compiled JavaScript if built-in flatten method on array throws range error
+
 == 1.1.0 (2024-11-24) - @mojavelinux
 
 === Added

lib/asciidoctor/reducer/tree_processor.rb

@@ -23,7 +23,11 @@ def process doc
           end
           target_lines[idx] = lines if target_lines
         end
-        reduced_source_lines = inc_replacements[0][:lines].flatten
+        if RUBY_ENGINE == 'opal'
+          reduced_source_lines = flatten inc_replacements[0][:lines]
+        else
+          reduced_source_lines = inc_replacements[0][:lines].flatten
+        end
         if doc.sourcemap
           logger = doc.logger
           opts = doc.options.merge logger: nil, parse: false, reduced: true
@@ -42,5 +46,31 @@ def process doc
       end
       doc
     end
+
+    private
+
+    def flatten input_list
+      input_list.flatten
+    rescue ::Exception => ex
+      raise unless `ex.name === 'RangeError'`
+      result = []
+      stack = [[0, input_list, input_list.length]]
+      until stack.empty?
+        idx, list, len = stack.pop
+        while idx < len
+          if Array === (item = list[idx])
+            if (next_idx = idx + 1) < len
+              stack << [next_idx, list, len]
+            end
+            idx = 0
+            len = (list = item).length
+          else
+            result << item
+            idx += 1
+          end
+        end
+      end
+      result
+    end
   end
 end