Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats: Python 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 Python generated by Kaitai Struct depends on the Python runtime library. You have to install it before you can parse data.

The Python runtime library can be installed from PyPI:

python3 -m pip install kaitaistruct

Code

Parse a local file and get structure in memory:

data = DcmpVariableLengthInteger.from_file("path/to/local/file.Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats")

Or parse structure from a bytes:

from kaitaistruct import KaitaiStream, BytesIO

raw = b"\x00\x01\x02..."
data = DcmpVariableLengthInteger(KaitaiStream(BytesIO(raw)))

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.

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

dcmp_variable_length_integer.py

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

from pkg_resources import parse_version
import kaitaistruct
from kaitaistruct import KaitaiStruct, KaitaiStream, BytesIO


if parse_version(kaitaistruct.__version__) < parse_version('0.9'):
    raise Exception("Incompatible Kaitai Struct Python API: 0.9 or later is required, but you have %s" % (kaitaistruct.__version__))

class DcmpVariableLengthInteger(KaitaiStruct):
    """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.
    
    .. seealso::
       Source - https://github.com/dgelessus/python-rsrcfork/tree/master/rsrcfork/compress/common.py
    """
    def __init__(self, _io, _parent=None, _root=None):
        self._io = _io
        self._parent = _parent
        self._root = _root if _root else self
        self._read()

    def _read(self):
        self.first = self._io.read_u1()
        if self.first >= 128:
            _on = self.first
            if _on == 255:
                self.more = self._io.read_s4be()
            else:
                self.more = self._io.read_u1()


    @property
    def value(self):
        """The decoded value of the variable-length integer.
        """
        if hasattr(self, '_m_value'):
            return self._m_value if hasattr(self, '_m_value') else None

        self._m_value = (self.more if self.first == 255 else ((((self.first << 8) | self.more) - 49152) if self.first >= 128 else self.first))
        return self._m_value if hasattr(self, '_m_value') else None