HPACK Integer [de]compression

hpack-integer.rb

# encode int i onto n bits
def encode(i, n)
  a = []
  i = i.to_i

  # Max number that can be stored on initial bits
  max = 2**n - 1
 
  if i < max
    a << i
  else
    a << max
    i -= max

    while i >= 128
      a << (i % 128 + 128)
      i /= 128
    end

    a << i
  end
 
  a
end

p encode(1337,5)

# decode bytes into int, using last n bits of first byte
def decode(bytes, n)

  bytes = bytes.to_enum

  # How much can be stored in n bits
  max = 2**n-1

  i = bytes.next & max

  # if all bits are set, then there are more bytes to extract.
  if i == max
    shift = 0

    begin
      b = bytes.next

      # read 7 LSB
      i += (b & 127) << shift

      shift += 7

    end until b & 128 == 0 # until MSB is no longer set
  end

  i
end

p decode(encode(1337, 5), 5)

Saved two bytes! 😄