turn a Array of Fixnums into Array of Ranges

core_extensions.rb

class Array
  # Array#to_ranges
  # Converts an array of values to an array of ranges. For example,
  # [2, 3, 33, 34, 110, 1, 111, 112, 4].to_ranges => [1..4, 33..34, 110..112]
  def to_ranges
    return nil unless self.all?{|item| item.is_a?Fixnum}
    self.sort.uniq.inject([]) do |spans, n|
      if spans.empty? || spans.last.last != n - 1
        spans + [n..n]
      else
        spans[0..-2] + [spans.last.first..n]
      end
    end
  end
end