python - Packing 4 Integers as ONE BYTE?

Question

I have four integers {a, b, c, d} that can have the following range of values:

a - {0 or 1} (1 bit)

b - {0 or 1} (1 bit)

c - {0, 1, 2, ..., 7} (3 bits)

d - {0, 1, 2, ..., 7} (3 bits)

at first, I would like to pack them into a one byte that can be then written to a binary file. later, I would like to unpack that one byte and get from it to a tuple of the form (a, b, c, d).

I know how to read/write a byte to a binary file in python. But how do I do the bits packing/unpacking?

Answer 1

Use shift and bitwise OR, then convert to a character to get a "byte":

x = chr(a | (b << 1) | (c << 2) | (d << 5))

To unpack this byte again, first convert to an integer, then shift and use bitwise AND:

i = ord(x)
a = i & 1
b = (i >> 1) & 1
c = (i >> 2) & 7
d = (i >> 5) & 7

Explanation: Initially, you have

0000000a
0000000b
00000ccc
00000ddd

The left-shifts give you

0000000a
000000b0
000ccc00
ddd00000

The bitwise OR results in

dddcccba

Converting to a character will convert this to a single byte.

Unpacking: The four different right-shifts result in

dddcccba
0dddcccb
00dddccc
00000ddd

Masking (bitwise AND) with 1 (0b00000001) or 7 (0b00000111) results in

0000000a
0000000b
00000ccc
00000ddd

again.