BCD (Binary Coded Decimals) is a common way to encode integer numbers in a way that makes human-readable output somewhat simpler. In this encoding scheme, every decimal digit is encoded as either a single byte (8 bits), or a nibble (half of a byte, 4 bits). This obviously wastes a lot of bits, but it makes translation into human-readable string much easier than traditional binary-to-decimal conversion process, which includes lots of divisions by 10.
For example, encoding integer 31337 in 8-digit, 8 bits per digit, big endian order of digits BCD format yields
00 00 00 03 01 03 03 07
Encoding the same integer as 8-digit, 4 bits per digit, little endian order BCD format would yield:
73 31 30 00
Using this type of encoding in Kaitai Struct is pretty
straightforward: one calls for this type, specifying desired
encoding parameters, and gets result using either as_int or
as_str attributes.
This page hosts a formal specification of BCD (Binary Coded Decimals) 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 = Bcd.from_file("path/to/local/file.bin")
Or parse structure from a bytes:
from kaitaistruct import KaitaiStream, BytesIO
raw = b"\x00\x01\x02..."
data = Bcd(KaitaiStream(BytesIO(raw)))
After that, one can get various attributes from the structure by invoking getter methods like:
data.as_int # => Value of this BCD number as integer. Endianness would be selected based on `is_le` parameter given.
# 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 Bcd(KaitaiStruct):
"""BCD (Binary Coded Decimals) is a common way to encode integer
numbers in a way that makes human-readable output somewhat
simpler. In this encoding scheme, every decimal digit is encoded as
either a single byte (8 bits), or a nibble (half of a byte, 4
bits). This obviously wastes a lot of bits, but it makes translation
into human-readable string much easier than traditional
binary-to-decimal conversion process, which includes lots of
divisions by 10.
For example, encoding integer 31337 in 8-digit, 8 bits per digit,
big endian order of digits BCD format yields
```
00 00 00 03 01 03 03 07
```
Encoding the same integer as 8-digit, 4 bits per digit, little
endian order BCD format would yield:
```
73 31 30 00
```
Using this type of encoding in Kaitai Struct is pretty
straightforward: one calls for this type, specifying desired
encoding parameters, and gets result using either `as_int` or
`as_str` attributes.
"""
def __init__(self, num_digits, bits_per_digit, is_le, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self.num_digits = num_digits
self.bits_per_digit = bits_per_digit
self.is_le = is_le
self._read()
def _read(self):
self.digits = []
for i in range(self.num_digits):
_on = self.bits_per_digit
if _on == 4:
self.digits.append(self._io.read_bits_int_be(4))
elif _on == 8:
self.digits.append(self._io.read_u1())
@property
def as_int(self):
"""Value of this BCD number as integer. Endianness would be selected based on `is_le` parameter given."""
if hasattr(self, '_m_as_int'):
return self._m_as_int
self._m_as_int = (self.as_int_le if self.is_le else self.as_int_be)
return getattr(self, '_m_as_int', None)
@property
def as_int_le(self):
"""Value of this BCD number as integer (treating digit order as little-endian)."""
if hasattr(self, '_m_as_int_le'):
return self._m_as_int_le
self._m_as_int_le = (self.digits[0] + (0 if self.num_digits < 2 else ((self.digits[1] * 10) + (0 if self.num_digits < 3 else ((self.digits[2] * 100) + (0 if self.num_digits < 4 else ((self.digits[3] * 1000) + (0 if self.num_digits < 5 else ((self.digits[4] * 10000) + (0 if self.num_digits < 6 else ((self.digits[5] * 100000) + (0 if self.num_digits < 7 else ((self.digits[6] * 1000000) + (0 if self.num_digits < 8 else (self.digits[7] * 10000000)))))))))))))))
return getattr(self, '_m_as_int_le', None)
@property
def last_idx(self):
"""Index of last digit (0-based)."""
if hasattr(self, '_m_last_idx'):
return self._m_last_idx
self._m_last_idx = (self.num_digits - 1)
return getattr(self, '_m_last_idx', None)
@property
def as_int_be(self):
"""Value of this BCD number as integer (treating digit order as big-endian)."""
if hasattr(self, '_m_as_int_be'):
return self._m_as_int_be
self._m_as_int_be = (self.digits[self.last_idx] + (0 if self.num_digits < 2 else ((self.digits[(self.last_idx - 1)] * 10) + (0 if self.num_digits < 3 else ((self.digits[(self.last_idx - 2)] * 100) + (0 if self.num_digits < 4 else ((self.digits[(self.last_idx - 3)] * 1000) + (0 if self.num_digits < 5 else ((self.digits[(self.last_idx - 4)] * 10000) + (0 if self.num_digits < 6 else ((self.digits[(self.last_idx - 5)] * 100000) + (0 if self.num_digits < 7 else ((self.digits[(self.last_idx - 6)] * 1000000) + (0 if self.num_digits < 8 else (self.digits[(self.last_idx - 7)] * 10000000)))))))))))))))
return getattr(self, '_m_as_int_be', None)