bit manipulation - Swapping bits at a given point between two bytes

Question

Let's say I have these two numbers:

x = 0xB7
y = 0xD9

Their binary representations are:

x = 1011 0111
y = 1101 1001

Now I want to crossover (GA) at a given point, say from position 4 onwards.

The expected result should be:

x = 1011 1001
y = 1101 0111

Bitwise, how can I achieve this?

Answer 1

I would just use bitwise operators:

t = (x & 0x0f)
x = (x & 0xf0) | (y & 0x0f)
y = (y & 0xf0) | t

That would work for that specific case. In order to make it more adaptable, I'd put it in a function, something like (pseudo-code, with &, | and ! representing bitwise "and", "or", and "not" respectively):

def swapBits (x, y, s, e):
    lookup = [255,127,63,31,15,7,3,1]
    mask = lookup[s] & !lookup[e]
    t = x & mask
    x = (x & !mask) | (y & mask)
    y = (y & !mask) | t
    return (x,y)

The lookup values allow you to specify which bits to swap. Let's take the values xxxxxxxx for x and yyyyyyyy for y along with start bit s of 2 and end bit e of 6 (bit numbers start at zero on the left in this scenario):

x        y        s e t        mask     !mask    execute
-------- -------- - - -------- -------- -------- -------
xxxxxxxx yyyyyyyy 2 6                   starting point
                              00111111  mask = lookup[2](00111111)
                              00111100       & !lookup[6](11111100)
                      00xxxx00          t = x & mask
xx0000xx                                x = x & !mask(11000011)
xxyyyyxx                                  | y & mask(00111100)
         yy0000yy                       y = y & !mask(11000011)
         yyxxxxyy                         | t(00xxxx00)