Worksheet#merge_cells sometimes produced invalid merge ranges when given an array of cells.

Merging e.g. cells A9 and A10 resulted in invalid range "A10:A9".

2 files changed, 10 insertions, 2 deletions

diff --git a/lib/axlsx/workbook/worksheet/worksheet.rb b/lib/axlsx/workbook/worksheet/worksheet.rb
index 2205b97d..887968e0 100644
--- a/lib/axlsx/workbook/worksheet/worksheet.rb
+++ b/lib/axlsx/workbook/worksheet/worksheet.rb
@@ -204,7 +204,7 @@ module Axlsx
       @merged_cells << if cells.is_a?(String)
                          cells
                        elsif cells.is_a?(Array)
-                         cells = cells.sort { |x, y| x.r <=> y.r }
+                         cells = cells.sort { |x, y| [x.index, x.row.index] <=> [y.index, y.row.index] }
                          "#{cells.first.r}:#{cells.last.r}"
                        end
     end
diff --git a/test/workbook/worksheet/tc_worksheet.rb b/test/workbook/worksheet/tc_worksheet.rb
index 5366ab3d..81179dbd 100644
--- a/test/workbook/worksheet/tc_worksheet.rb
+++ b/test/workbook/worksheet/tc_worksheet.rb
@@ -382,7 +382,15 @@ class TestWorksheet < Test::Unit::TestCase
     assert_equal(@ws.merged_cells.size, 3)
     assert_equal(@ws.merged_cells.last, "A3:B3")
   end
-
+  
+  def test_merge_cells_sorts_correctly_by_row_when_given_array
+    10.times do |i|
+      @ws.add_row [i]
+    end
+    @ws.merge_cells [@ws.rows[8].cells.first, @ws.rows[9].cells.first]
+    assert_equal "A9:A10", @ws.merged_cells.first
+  end
+  
   def test_auto_filter
     assert(@ws.auto_filter.nil?)
     assert_raise(ArgumentError) { @ws.auto_filter = 123 }