packedbits.py

class PackedBitsWriter:
  def __init__(self):
    self.bits_left_in_byte = 8
    self.current_byte = 0
    self.bytes = []
  
  def write(self, value, length):
    while length:
      if length >= self.bits_left_in_byte:
        bits_to_read = self.bits_left_in_byte
      else:
        bits_to_read = length
      
      mask = (1 << bits_to_read) - 1
      self.current_byte |= (value & mask) << (8 - self.bits_left_in_byte)
      
      self.bits_left_in_byte -= bits_to_read
      length -= bits_to_read
      value >>= bits_to_read
      
      if self.bits_left_in_byte:
        continue
      
      self.flush()
  
  def flush(self):
    self.bytes.append(self.current_byte)
    self.current_byte = 0
    self.bits_left_in_byte = 8

class PackedBitsReader:
  def __init__(self, bytes):
    self.bytes = bytes
    self.current_byte_index = 0
    self.current_bit_index = 0
  
  def read(self, length):
    bits_read = 0
    value = 0
    bits_left_to_read = length
    
    while bits_read != length:
      if bits_left_to_read > 8:
        bits_to_read = 8
      else:
        bits_to_read = bits_left_to_read
      if (bits_to_read + self.current_bit_index > 8):
        bits_to_read = 8 - self.current_bit_index
      
      mask = ((1 << bits_to_read) - 1) << self.current_bit_index
      current_byte = self.bytes[self.current_byte_index]
      value = ((current_byte & mask) >> self.current_bit_index) << bits_read | value
      
      self.current_bit_index += bits_to_read
      self.current_byte_index += self.current_bit_index >> 3
      self.current_bit_index %= 8
      bits_left_to_read -= bits_to_read
      bits_read += bits_to_read
    
    return value