class Array
    def collapse_ranges
        return self if self.length <= 2
        self.uniq!
        self.sort! rescue nil
        temp_array, return_array = [], []
        self.each_with_index do |item, i|
            if item.respond_to?(:next)
                temp_array.push item
                if item.next != self[i + 1]
                    return_array.concat 3 <= temp_array.length ?
                        [temp_array.first..temp_array.last] :
                         temp_array
                    temp_array.clear
                end
            else
                return_array.concat 3 <= temp_array.length ?
                    [temp_array.first..temp_array.last] :
                     temp_array
                temp_array.clear
                return_array.push item
            end
        end
        return return_array
    end
end

>> [1, 3, 4, 5, 7, 8].collapse_ranges.join(', ')
=> 1, 3..5, 7, 8
>> %w(a c d e g i j).collapse_ranges.join(', ')
=> a, c..e, g, i, j
>> [1, 2.5, 3, 4, 5].collapse_ranges.join(', ')
=> 1, 2.5, 3..5
>> [1, 2, 3, 4, {:test => 'value'}, 5].collapse_ranges.join(', ')
=> 1..4, testvalue, 5

class Array
    def missing_items
        return [] if self.length <= 1
        self.uniq!
        self.sort! rescue nil
        begin
            (self.first..self.last).to_a - self
        rescue
            []
        end
    end
end

>> [1, 3, 4, 10].missing_items.join(', ')
=> 2, 5, 6, 7, 8, 9
>> [1, 2, 7, 7.5, 8.2].missing_items.join(', ')
=> 3, 4, 5, 6, 8
>> %w(a b c f g j).missing_items.join(', ')
=> d, e, h, i
>> [2.5, {:test => 'value'}].missing_items.join(', ')
=>