Prevent adding more hyperlinks than the allowed limit per worksheet in Excel of 65,530.

Exceeding this limit will result in an error when opening up the file, similar to this:

> We found a problem with some content in ‘.xlsx'. Do you want us to try to recover as much as we can? If you trust the source of this workbook, click Yes.

With this change we now prevent the creation of hyperlinks if the limit of 65,530 has been reached.

We will also output a warning to the Rails log but only once. It will show the worksheet name as well as the row and column where it was exceeded.

Fixes #475

Author: joshuapinter

lib/rubyXL/convenience_methods/cell.rb

@@ -1,5 +1,7 @@
 module RubyXL
   module CellConvenienceMethods
+    EXCEL_HYPERLINK_LIMIT_PER_WORKSHEET = 65_530 # Excel has a hard limit of 65,530 hyperlinks per Worksheet. HYPERLINK formulas are not included in this limit.
+
     def change_contents(data, formula_expression = nil)
       validate_worksheet
 
@@ -263,6 +265,15 @@ def add_hyperlink(url, tooltip = nil)
       hyperlink = RubyXL::Hyperlink.new(:ref => self.r, :r_id => r_id)
       hyperlink.tooltip = tooltip if tooltip
       worksheet.hyperlinks ||= RubyXL::Hyperlinks.new
+
+      if worksheet.hyperlinks.size >= EXCEL_HYPERLINK_LIMIT_PER_WORKSHEET # Do not create hyperlinks once the Excel limit of 65,530 is reached. Return false.
+        unless @excel_hyperlink_limit_per_worksheet_warned
+          Rails.logger.warn("Excel hyperlink limit (65,530) reached in worksheet '#{worksheet.sheet_name}' at row #{row}, column #{column}. Further hyperlinks skipped.")
+          @excel_hyperlink_limit_per_worksheet_warned = true
+        end
+        return false
+      end
+
       worksheet.hyperlinks << hyperlink
     end