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

This code still doesn't work. I would say that's because although it looks simpler than mine it is actually more complicated.

I prepared the array and made sure I only used integers whereas you chose to use nil and test for it at every step. This makes more operations, but that's not the problem.

1. At the end of every step, if key != max + 1, you set max and min to nil. Then at the beginning of each step you set min and max to key. So if they've been set to nil they will be set to key which is an integer, right?

Hmm... what's the last value key will take? ;)

2. There's a logic problem. You're setting min, max = nil at the end of an iteration and then to key at the beginning of the next one. But it's not the same key anymore...

So what you'd need is min = max = key. But then you have a problem with the first key, which can be added twice. So you need to skip the first iteration (hello, array slicing :p).

3. I had gotten 1) and 2) right but I actually tested my code and I had done a similar mistake, because this is unintuitive:

min = 1; max = 2; slots = []
slots << min == max ? min.to_s : "#{min}-#{max}" # yours
slots << (min == max) ? min.to_s : "#{min}-#{max}" # my 1st try
slots << ((min == max) ? min.to_s : "#{min}-#{max}") # mine above

Quizz: what's in slots now?

Answer:
[1, false, "1-2"]

When in doubt, use parentheses ;)

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