A bit of refactoring and added a check to make sure that any line number token must between one and the length of the source block.

Author: RayOffiah

lib/asciidoctor-external-callout.rb

@@ -146,12 +146,12 @@ def process_callouts(list, owner_block)
 
           if location.is_numeric?
 
-            numbers = location.to_i
+            line_number = location.to_i
 
-            if numbers <= owner_block.lines.length
-              line_numbers << (numbers - 1)
+            if line_number.between?(1, owner_block.lines.length)
+              line_numbers << (line_number - 1)  # Because the line number is now an array index, so -1
             else
-              warn "Out of range ==> #{numbers}"
+              warn "Line number out of range ==> #{line_number}"
             end
 
           else

test/asciidoctor-external-callout_test.rb

@@ -18,7 +18,7 @@ def test_basic_conversion
     assert document.instance_of? Document
     assert_equal 5, document.blocks.length
     assert_equal document.blocks[1].context, :paragraph
-    assert_equal 4, document.blocks[4].items.length
+    assert_equal 6, document.blocks[4].items.length
     assert_equal 'The `use_dsl` line', document.blocks[4].items[0].instance_variable_get(:@text)
     assert_equal true, document.blocks[4].has_role?('external-callout-list')
   end
@@ -73,5 +73,22 @@ def test_case_insensitive_search
 
   end
 
+  def test_for_line_zero
+
+    document = Asciidoctor.convert_file File.join(File.dirname(__FILE__), 'sample_global_search.adoc'),
+                                        safe: :unsafe, backend: :html5,
+                                        attributes: {'stylesheet' => './callout.css'}
+
+    assert document.blocks[0].context == :listing
+    assert document.blocks[0].lines.length == 30
+
+    # No line should contain a callout for 5, because the
+    # token is set for line 0
+    # No line should have a callout for 6, because the
+    # token is set for line 500.
+
+    assert_equal false,  document.blocks[0].lines.any? {|line| line.include? '<5>' or line.include? '<6>'}
+
+  end
 end
 

test/sample.adoc

@@ -55,4 +55,6 @@ How about a table?
 . The process def at @5
 . Document object used at @5 @7
 . Result first used @/result/
+. There is no line zero @0
+. There is no line five hundred @500