gistfile1.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
s = input.slice_before(State.new(input.first)) do |value, state|
  (_, state.last_value = value != state.last_value.succ, value)[0]
end

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

that's the return value of the block, which is set to true whenever it detects a break in the sequence of input values

wow, i have a lot to learn. (yippee!)
too bad you couldn't throw in some c-style bit twiddling .

looking at this a couple months later and after reading PragProg's awesome MetaProgramming with Ruby book (even tho there is not really anything specific in the book that applies here), I can make better sense out of the original source now. Progress.