A variable-length unsigned integer using base128 encoding. 1-byte groups consist of 1-bit flag of continuation and 7-bit value chunk, and are ordered "most significant group first", i.e. in "big-endian" manner.
This particular encoding is specified and used in:
More information on this encoding is available at https://en.wikipedia.org/wiki/Variable-length_quantity
This particular implementation supports serialized values to up 8 bytes long.
This page hosts a formal specification of Variable length quantity, unsigned integer, base128, big-endian using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing 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 kaitaistructParse a local file and get structure in memory:
data = VlqBase128Be.from_file("path/to/local/file.bin")
Or parse structure from a bytes:
from kaitaistruct import KaitaiStream, BytesIO
raw = b"\x00\x01\x02..."
data = VlqBase128Be(KaitaiStream(BytesIO(raw)))
After that, one can get various attributes from the structure by invoking getter methods like:
data.value # => Resulting value as normal integer
# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
import kaitaistruct
from kaitaistruct import KaitaiStruct, KaitaiStream, BytesIO
if getattr(kaitaistruct, 'API_VERSION', (0, 9)) < (0, 9):
raise Exception("Incompatible Kaitai Struct Python API: 0.9 or later is required, but you have %s" % (kaitaistruct.__version__))
class VlqBase128Be(KaitaiStruct):
"""A variable-length unsigned integer using base128 encoding. 1-byte groups
consist of 1-bit flag of continuation and 7-bit value chunk, and are ordered
"most significant group first", i.e. in "big-endian" manner.
This particular encoding is specified and used in:
* Standard MIDI file format
* ASN.1 BER encoding
* RAR 5.0 file format
More information on this encoding is available at
<https://en.wikipedia.org/wiki/Variable-length_quantity>
This particular implementation supports serialized values to up 8 bytes long.
"""
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.groups = []
i = 0
while True:
_ = VlqBase128Be.Group(self._io, self, self._root)
self.groups.append(_)
if not (_.has_next):
break
i += 1
class Group(KaitaiStruct):
"""One byte group, clearly divided into 7-bit "value" chunk and 1-bit "continuation" flag.
"""
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.has_next = self._io.read_bits_int_be(1) != 0
self.value = self._io.read_bits_int_be(7)
@property
def last(self):
if hasattr(self, '_m_last'):
return self._m_last
self._m_last = (len(self.groups) - 1)
return getattr(self, '_m_last', None)
@property
def value(self):
"""Resulting value as normal integer."""
if hasattr(self, '_m_value'):
return self._m_value
self._m_value = (((((((self.groups[self.last].value + ((self.groups[(self.last - 1)].value << 7) if self.last >= 1 else 0)) + ((self.groups[(self.last - 2)].value << 14) if self.last >= 2 else 0)) + ((self.groups[(self.last - 3)].value << 21) if self.last >= 3 else 0)) + ((self.groups[(self.last - 4)].value << 28) if self.last >= 4 else 0)) + ((self.groups[(self.last - 5)].value << 35) if self.last >= 5 else 0)) + ((self.groups[(self.last - 6)].value << 42) if self.last >= 6 else 0)) + ((self.groups[(self.last - 7)].value << 49) if self.last >= 7 else 0))
return getattr(self, '_m_value', None)