Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats: Ruby parsing library

A variable-length integer, in the format used by the 0xfe chunks in the 'dcmp' (0) and 'dcmp' (1) resource compression formats. See the dcmp_0 and dcmp_1 specs for more information about these compression formats.

This variable-length integer format can store an integer x in any of the following ways:

In a single byte, if 0 <= x <= 0x7f (7-bit unsigned integer)
In 2 bytes, if -0x4000 <= x <= 0x3eff (15-bit signed integer with the highest 0x100 values unavailable)
In 5 bytes, if -0x80000000 <= x <= 0x7fffffff (32-bit signed integer)

In practice, values are always stored in the smallest possible format, but technically any of the larger formats could be used as well.

Application

Mac OS

KS implementation details

License: MIT

Minimal Kaitai Struct required: 0.8

This page hosts a formal specification of Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Runtime library

All parsing code for Ruby generated by Kaitai Struct depends on the Ruby runtime library. You have to install it before you can parse data.

The Ruby runtime library can be installed from RubyGems:

gem install kaitai-struct

Code

Parse a local file and get structure in memory:

data = DcmpVariableLengthInteger.from_file("path/to/local/file.bin")

Or parse structure from a string of bytes:

bytes = "\x00\x01\x02..."
data = DcmpVariableLengthInteger.new(Kaitai::Struct::Stream.new(bytes))

After that, one can get various attributes from the structure by invoking getter methods like:

data.first # => The first byte of the variable-length integer.
This determines which storage format is used.

* For the 1-byte format,
  this encodes the entire value of the value.
* For the 2-byte format,
  this encodes the high 7 bits of the value,
  minus `0xc0`.
  The highest bit of the value,
  i. e. the second-highest bit of this field,
  is the sign bit.
* For the 5-byte format,
  this is always `0xff`.

data.value # => The decoded value of the variable-length integer.

Ruby source code to parse Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats

dcmp_variable_length_integer.rb

Download

# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

require 'kaitai/struct/struct'

unless Gem::Version.new(Kaitai::Struct::VERSION) >= Gem::Version.new('0.9')
  raise "Incompatible Kaitai Struct Ruby API: 0.9 or later is required, but you have #{Kaitai::Struct::VERSION}"
end


##
# A variable-length integer,
# in the format used by the 0xfe chunks in the `'dcmp' (0)` and `'dcmp' (1)` resource compression formats.
# See the dcmp_0 and dcmp_1 specs for more information about these compression formats.
# 
# This variable-length integer format can store an integer `x` in any of the following ways:
# 
# * In a single byte,
#   if `0 <= x <= 0x7f`
#   (7-bit unsigned integer)
# * In 2 bytes,
#   if `-0x4000 <= x <= 0x3eff`
#   (15-bit signed integer with the highest `0x100` values unavailable)
# * In 5 bytes, if `-0x80000000 <= x <= 0x7fffffff`
#   (32-bit signed integer)
# 
# In practice,
# values are always stored in the smallest possible format,
# but technically any of the larger formats could be used as well.
# @see https://github.com/dgelessus/python-rsrcfork/blob/f891a6e/src/rsrcfork/compress/common.py Source
class DcmpVariableLengthInteger < Kaitai::Struct::Struct
  def initialize(_io, _parent = nil, _root = self)
    super(_io, _parent, _root)
    _read
  end

  def _read
    @first = @_io.read_u1
    if first >= 128
      case first
      when 255
        @more = @_io.read_s4be
      else
        @more = @_io.read_u1
      end
    end
    self
  end

  ##
  # The decoded value of the variable-length integer.
  def value
    return @value unless @value.nil?
    @value = (first == 255 ? more : (first >= 128 ? (((first << 8) | more) - 49152) : first))
    @value
  end

  ##
  # The first byte of the variable-length integer.
  # This determines which storage format is used.
  # 
  # * For the 1-byte format,
  #   this encodes the entire value of the value.
  # * For the 2-byte format,
  #   this encodes the high 7 bits of the value,
  #   minus `0xc0`.
  #   The highest bit of the value,
  #   i. e. the second-highest bit of this field,
  #   is the sign bit.
  # * For the 5-byte format,
  #   this is always `0xff`.
  attr_reader :first

  ##
  # The remaining bytes of the variable-length integer.
  # 
  # * For the 1-byte format,
  #   this is not present.
  # * For the 2-byte format,
  #   this encodes the low 8 bits of the value.
  # * For the 5-byte format,
  #   this encodes the entire value.
  attr_reader :more
end