Day12b.rb

require 'set'

class CaveSystem
  def initialize
    @adjs = {}
  end

  def link(a, b)
    (@adjs[a] ||= Set.new) << b
    (@adjs[b] ||= Set.new) << a
  end

  def neighbors_at(a)
    @adjs[a]
  end

  def paths(s, e, small_seen=Set.new, twice_yet=false)
    return to_enum(__method__, s, e, small_seen, twice_yet) unless block_given?
    if s == e
      yield [e]
    else
      nbs = neighbors_at(s)
      nbs -= small_seen if twice_yet
      ss = small_seen
      ss |= Set.new([s]) if s =~ /[a-z]+/
      nbs.each do |nb|
        next if nb == 'start'
        tw = twice_yet || small_seen.include?(nb)
        paths(nb, e, ss, tw) {|p| yield ([s] + p)}
      end
    end
  end
end

cs = CaveSystem.new
$<.each do |line|
  line.chomp =~ /(\w+)-(\w+)/
  cs.link $1, $2
end

p cs.paths('start', 'end').count