add diff! method to DataTable

features/docs/defining_steps/table_diffing.feature

@@ -33,7 +33,7 @@ Feature: Table diffing
             | x | y |
             | a | b |
             | a | c |
-            Tables were not identical (Cucumber::Ast::Table::Different)
+            Tables were not identical (Cucumber::MultilineArgument::DataTable::Different)
             ./features/step_definitions/steps.rb:2:in `/the table should be:/'
             features/tables.feature:3:in `Then the table should be:'
 

lib/cucumber/multiline_argument.rb

@@ -37,6 +37,70 @@ def append_to(array)
       def to_json(options)
         raw.to_json(options)
       end
+
+      def diff!(other_table)
+        other_table = ensure_table(other_table)
+        other_table_cell_matrix = other_table.cell_matrix
+
+        ensure_green!
+
+        require_diff_lcs
+        cell_matrix.extend(Diff::LCS)
+
+        changes = cell_matrix.diff(other_table_cell_matrix).flatten
+
+        return if changes.empty?
+
+        inserted = 0
+        missing  = 0
+
+        changes.each do |change|
+          if(change.action == '-')
+            missing_row_pos = change.position + inserted
+            cell_matrix[missing_row_pos].each{|cell| cell.status = :undefined}
+
+            missing += 1
+          else # '+'
+            inserted_row = change.element
+            inserted_row.each{|cell| cell.status = :comment}
+
+            insert_row_pos = change.position + missing
+            cell_matrix.insert(insert_row_pos, inserted_row)
+
+            inserted += 1
+          end
+        end
+
+        raise Different.new(self)
+      end
+
+      def ensure_green! #:nodoc:
+        each_cell{|cell| cell.status = :passed}
+      end
+
+      private
+
+      def ensure_table(table_or_array) #:nodoc:
+        return table_or_array if DataTable === table_or_array
+        DataTable.new(table_or_array)
+      end
+
+      def require_diff_lcs #:nodoc:
+        begin
+          require 'diff/lcs'
+        rescue LoadError => e
+          e.message << "\n Please gem install diff-lcs\n"
+          raise e
+        end
+      end
+
+      class Different < StandardError
+        attr_reader :table
+        def initialize(table)
+          @table = table
+          super("Tables were not identical")
+        end
+      end
     end
 
     class None
@@ -46,7 +110,6 @@ def append_to(array)
       def describe_to(visitor)
       end
     end
-
   end
 end