How to rotate bits ?

[korenron]

Hello,
I have a function that take bits and cut them according to my needs

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BITS_ORDER_CHANGE = {8: 0, 7: 1, 6: 2, 5: 3, 4: 4, 3: 5, 2: 6, 1: 7}   def GetBits(Byte, start, end):  # bits order 8765 4321 , str order 0123 4567     Final_calculation = round((int(Byte, 16) * 1 + 0), 0)     bits = "{0:b}".format(Final_calculation)     Full_bits = bits.zfill(8)     print('old start', start)     start = BITS_ORDER_CHANGE[start]     print('new start', start)     print('old end', end)     end = BITS_ORDER_CHANGE[end]     print('new end', end)     Wanted_bits = Full_bits[end:start]     return Wanted_bits

my problem is that when I cut it I think is string mode , whie the bits are is in bits mode
so it's other way around

for example
if I take "CA"
in bits its - 1100 1010
I want to cut only bits 2-4
but I need to enter numbers 4-7

then I thought to use dictionary - but then it's went wrong
I need to change the order - (end - start) , and it also change the order of the bits
01 - change to 10

is there other way to do it? without overthinking what need to be cut?


Thanks,

[deanhystad]

Don't convert to a string. Use shift and bitwise and. The convention I am familiar with has bit order starting at 0, not 1.

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import math   def bit_slicer(value, lsb, msb=None):     """Slice bits[lsb:msb] from value.  Return as int"""     if msb is None:         msb = lsb     if msb < lsb:         raise ValueError("msb must be greater or equal to lsb")     mask = (2 << (msb-lsb)) - 1     return (value >> lsb) & mask   def bits(value, lsb=0, msb=None):     """Return bits[lsb:msb] from value.  Return as list"""     if msb is None:         msb = int(math.log(value, 2))     if msb < lsb:         raise ValueError("msb must be greater or equal to lsb")     return [1 if value & (1 << bit) else 0 for bit in range(lsb, msb+1)]   print(bit_slicer(0xCA, 2, 4)) print(bits(0xCA, 2, 4)) print(bits(0xCA))

Output:
2
[0, 1, 0]
[0, 1, 0, 1, 0, 0, 1, 1]

But if you want to stay with doing this the string way, just reverse the order of the bit string.

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import math   def bits(value, lsb=0, msb=None):     """Return bits[lsb:msb] from value.  Return as list"""     if msb is None:         msb = int(math.log(value, 2))     if msb < lsb:         raise ValueError("msb must be greater or equal to lsb")     bits = f"{value:b}".zfill(msb)[::-1]  # Reverse the order     return [int(bit) for bit in bits[lsb:msb+1]]   print(bits(0xCA))

Output:
[0, 1, 0, 1, 0, 0, 1, 1]

[Larz60+]

see: https://wiki.python.org/moin/BitwiseOperators
and: https://wiki.python.org/moin/BitManipulation
and: https://wiki.python.org/moin/BitArrays

The second link has many examples