Let's try to refactor info_string, keeping it functional and short

redis-trib-info_string.rb

# eg.: @slots = Hash[[5,19,20,32,21,28].zip([:new]*6)]
def info_string
  "#{self.to_s.ljust(25)} slots:" + @slots.keys.sort.reduce([]){|a,b|
    if a.empty? || !(a[-1][1]+1 == b)
      a << [b,b]
    else
      a[-1][1] = b
      a
    end
  }.map{|x| (x[0] == x[1]) ? x[1] : "#{x[0]}-#{x[1]}"}.join(",")
end

# another style...
def info_string
  nums = @slots.keys.sort
  start = stop = nums.first
  slots = []
  (nums[1..-1]+[nums.last+2]).each do |x|
    if x == stop + 1
      stop += 1
    else
      slots << ((start == stop) ? start : "#{start}-#{stop}")
      start = stop = x
    end
  end
  "#{self.to_s.ljust(25)} slots:" + slots.join(",")
end

Fair answer - that's what I get for throwing together something in a rush and not testing it :-)