Day16a.rb

def bits(n)
  [(n & 0b1000) >> 3,
   (n & 0b0100) >> 2,
   (n & 0b0010) >> 1,
   n & 0b0001]
end

def from_bits(bs)
  bs.each_with_index.sum {|b, i| b * (1 << (bs.length - i - 1))}
end

class Bitstream
  include Enumerable

  def initialize(io)
    @io = io
    @buffer = []
    @count = 0
  end

  def each
    return to_enum unless block_given?
    loop {yield read_bit}
  end

  def read(n)
    @count += n
    if n <= @buffer.length
      @buffer.shift(n)
    else
      bits = @buffer
      n -= bits.length

      nybbles = n / 4
      nybbles += 1 unless n % 4 == 0
      dat = @io.read(nybbles)
      @buffer = dat.chars.flat_map {|c| bits(c.to_i(16))}
      bits.concat @buffer.shift(n)
    end
  end

  def read_bit
    read(1)[0]
  end

  def read_int(n)
    from_bits(read(n))
  end

  def tell
    @count
  end
end

class Packet < Struct.new(:ver, :ty, :dat); end

def decode_packet(str)
  ver = str.read_int(3)
  ty = str.read_int(3)
  case ty
  when 4
    bs = []
    more = 1
    until more == 0
      more = str.read_bit
      bs.concat(str.read(4))
    end
    dat = from_bits(bs)
  else
    if str.read_bit == 0
      len = str.read_int(15)
      dat = []
      start = str.tell
      while str.tell - start < len
        dat << decode_packet(str)
      end
    else
      ct = str.read_int(11)
      dat = ct.times.map {decode_packet(str)}
    end
  end
  Packet.new(ver, ty, dat)
end

def ver_sum(pkt)
  if pkt.ty == 4
    pkt.ver
  else
    pkt.ver + pkt.dat.sum {|p| ver_sum(p)}
  end
end

puts ver_sum(decode_packet(Bitstream.new(STDIN)))