Day14b.rb

def expand(rules, dat)
  next_dat = Hash.new(0)
  dat.each do |(l, r), ct|
    m = rules[[l, r]]
    next_dat[[l, m]] += ct
    next_dat[[m, r]] += ct
  end
  next_dat
end

rules = {}
polymer = Hash.new(0)
raw_polymer = gets.chomp
last = raw_polymer[-1]
raw_polymer.chars.each_cons(2) do |adj|
  polymer[adj] += 1
end
gets
$<.each do |line|
  line =~ /(\w\w) -> (\w)/
  rules[$1.chars] = $2
end

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

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