group_sequential.rb

nums = [1,2,3,5,8,9,12,13,14,15,16,19]

def numbers_grouped(nums)
  nums = nums.sort
  start, last = nums.first, nil
  sequences = []

  nums.each_with_index do |num, i|
    # Are we out of sequence?
    if last && last+1 != num
      sequences << [start, last]
      start = num
    end

    # Last iteration, still in sequence
    if (i+1) == nums.length
      sequences << [start, num]
    else
      last = num
    end
  end

  sequences
end

def group_sequential1(nums)
  nums.sort.inject([]) { |memo, num|
    if memo.last && memo.last.last.next == num
      memo.last << num
    else
      memo << [num]
    end

    memo
  }
end

def group_sequential2(nums)
  prev = nil
  # Enum#slice_before is ruby 1.9
  nums.sort.slice_before { |num|
    sequential = !prev || prev.next == num
    prev = num
    !sequential # Don't slice on sequential
  }.to_a
end

puts nums.inspect
puts numbers_grouped(nums).inspect
puts numbers_grouped(nums).map { |s| s.uniq.join('-') }.inspect

puts group_sequential1(nums).inspect
puts group_sequential1(nums).map { |a| [a.first, a.last].uniq.join('-') }.inspect

puts group_sequential2(nums).inspect
puts group_sequential2(nums).map { |a| [a.first, a.last].uniq.join('-') }.inspect
===>
[1, 2, 3, 5, 8, 9, 12, 13, 14, 15, 16, 19]
[[1, 3], [5, 5], [8, 9], [12, 16], [19, 19]]
["1-3", "5", "8-9", "12-16", "19"]
[[1, 2, 3], [5], [8, 9], [12, 13, 14, 15, 16], [19]]
["1-3", "5", "8-9", "12-16", "19"]
[[1, 2, 3], [5], [8, 9], [12, 13, 14, 15, 16], [19]]
["1-3", "5", "8-9", "12-16", "19"]