Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats: Lua 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.

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

dcmp_variable_length_integer.lua

-- This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
--
-- This file is compatible with Lua 5.3

local class = require("class")
require("kaitaistruct")
local utils = require("utils")

-- 
-- 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 also: Source (https://github.com/dgelessus/python-rsrcfork/tree/master/rsrcfork/compress/common.py)
DcmpVariableLengthInteger = class.class(KaitaiStruct)

function DcmpVariableLengthInteger:_init(io, parent, root)
  KaitaiStruct._init(self, io)
  self._parent = parent
  self._root = root or self
  self:_read()
end

function DcmpVariableLengthInteger:_read()
  self.first = self._io:read_u1()
  if self.first >= 128 then
    local _on = self.first
    if _on == 255 then
      self.more = self._io:read_s4be()
    else
      self.more = self._io:read_u1()
    end
  end
end

-- 
-- The decoded value of the variable-length integer.
DcmpVariableLengthInteger.property.value = {}
function DcmpVariableLengthInteger.property.value:get()
  if self._m_value ~= nil then
    return self._m_value
  end

  self._m_value = utils.box_unwrap((self.first == 255) and utils.box_wrap(self.more) or (utils.box_unwrap((self.first >= 128) and utils.box_wrap((((self.first << 8) | self.more) - 49152)) or (self.first))))
  return self._m_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`.
-- 
-- 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.