Solution suggestion for http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/370127

rangify.rb

#!/bin/env ruby19

Num = Struct.new :pre, :val do
  def succ
    self.class.new pre, val.succ
  end

  def to_s; "#{pre}#{val}" end
  def inspect; "#{self.class.name}(#{self})" end

  include Comparable
  def <=>(o) to_a <=> o.to_a end
end

def Num(s)
  %r{\A(\D*)(\d+)\z} =~ s or raise "bad arg %p" % s
  Num.new $1, $2.to_i
end

module Enumerable
  def rangify(&bl)
    bl ||= lambda {|x, y| x == y ? x : (x..y)}

    res = []
    a = b = nil

    each do |x|
      case
      when a.nil?
        a = b = x
      when x == b.succ
        b = x
      else
        res << bl[a, b]
        a = b = x
      end
    end

    res << bl[a, b] unless a.nil?
    res
  end
end

test_data = [
  [ '1', '2', '3', '4', '6', '7', '9', 'S1', 'S2' ],
  [ '1', '2', 'S3', 'S4', 'S5', 'O6' ],
]

test_data.each {|td| td.map! {|s| Num(s)} }

test_data.each do |td|
  r1 = td.rangify
  r2 = td.rangify {|x,y| x == y ? x.to_s : "#{x}-#{y}"}
  p td, r1, r2
end