slice_before.rb

# encoding: utf-8

input = [ 1, 2, 3, 4, 5, 8, 9, 11, 12, 13, 15 ]

# Need to box the value to make it mutable
State = Struct.new(:last_value)
                                          
# divide the input into runs of consecutive numbers
slices = input.slice_before(State.new(input.first)) do |value, state|
  consecutive      = value == state.last_value.succ
  state.last_value = value

  !consecutive
end

# replace runs of 3 or more with first–last
slices.map do |runs|
  runs.size < 3 ? runs : "#{ runs.first }-#{ runs.last }"
end.join(', ')  # => 1–5, 8, 9, 11–13, 15

It worked for me without the #flatten so I opted to omit it.

Ah, right, it does. Cool. If you don't mind, I'll use a variant of this in the example

I don't mind at all. I think your use of returning in your new gist very clearly states what the code does.