disk_map = DATA.read.chars.each_slice(2)
  .flat_map.with_index {|(a,b),i|
    [ Array.new(a.to_i, i), Array.new(b.to_i, nil) ]
  }.reject(&:empty?)

while (prev, free = disk_map.each_cons(2).find { _2.include?(nil) })
  tail = disk_map.last
  until free.empty? || tail.empty?
    prev << tail.shift
    free.pop
  end

  while disk_map.last.all?(&:nil?)
    disk_map.pop
  end
end

pp disk_map.flatten.each.with_index.sum { _1 * _2 }

__END__
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