Day14a.rb

def expand(rules, dat)
  return to_enum(__method__, rules, dat) unless block_given?
  dat.each_cons(2).each do |c|
    yield c[0]
    yield rules[c]
  end
  yield dat[-1]
end

rules = {}

polymer = gets.chomp.chars
gets
$<.each do |line|
  line =~ /(\w\w) -> (\w)/
  rules[$1.chars] = $2
end

40.times do
  polymer = expand(rules, polymer).to_a
end

counts = Hash.new(0)
polymer.each do |c|
  counts[c] += 1
end
puts(counts.max_by(&:last)[1] - counts.min_by(&:last)[1])