Reverse '|' Bitshift Or

Question

Let's say I have an 15 as c. c can be splitted into a = 7 and b = 13. That's because a | b = c. I am trying to write a function that will find one possible combination of a and b by only giving c as input.

class Program
{
    static void Main(string[] args)
    {
        int data = 560;
        int[] v = FindBitshiftOr(data);
        Console.WriteLine("{0} << {1} = {2}", v[0], v[1], v[0] | v[1]);
        Console.ReadKey();
    }


    private static Random r = new Random();
    private static int[] FindBitshiftOr(int value)
    {
        int[] d = new int[2];
        int a = r.Next(0, value);
        int c = r.Next(0, value);
        int b = ~a;
        d[0] = a;
        d[1] = b | c;
        return d;
    }
}

This was my attempt but its always returning -1, can someone explain me what's wrong?

Array(`int[]`) can't be "always -1"... Would you mind to clarify? — Alexei Levenkov, Jan 03 '15 at 07:08
You know all the bits in `c`, so the answer is every combination of those bits. But in general it's impossible to do a "reverse" bitwise OR, as if there are more than a single bit set then there are multiple combinations, and there is no single "correct" solution. — Some programmer dude, Jan 03 '15 at 07:10
Does `0, c` works as an answer? Or `c,c`? Do you have additional restrictions on `a` and `b` (like `a & b == 0`)? — Alexei Levenkov, Jan 03 '15 at 07:12
The only restrictions are: a and b shouldn't be bigger than int.maxvalue and not smaller than int.minvalue. and a | b should be == c . I have no restrictions for &? — Dark Side, Jan 03 '15 at 07:20
So answer to your task is `0, c` and no additional code is needed... To figure out why your code produces wrong results check out descriptions of corresponding operators and read something on bit-switching - like http://stackoverflow.com/questions/47981/how-do-you-set-clear-and-toggle-a-single-bit-in-c-c or http://stackoverflow.com/questions/93744/most-common-c-sharp-bitwise-operations-on-enums — Alexei Levenkov, Jan 03 '15 at 07:31

score 2 · Answer 1 · edited May 23 '17 at 11:50

You get -1 since you are setting all bits to 1 with OR'ing together number and its negation.

Essential part of your code is:

int a = 42; // you get random number, but it does not matter here.
int b = ~a | c; // "| c" part just possibly add more 1s 

result = a | b; // all 1s as it essentially  (a | ~a) | c

Proper way of separating bits would be x & ~mask - see links/samples in Most common C# bitwise operations on enums.

rakeshbs · Accepted Answer · 2015-01-03T08:07:14.817

2

Try this code. And operation is used to extract the bits that are same. I don't know C# so I can't type the exact code. The logic is take a random bit mask and AND the bits of c with that bitmask and inverse of that bitmask. The resulting two numbers will be the numbers you need.

int c = 4134;
int a = r.Next(0, value);
int first_num = c & a;
int second_num = c & ~a;
int check_c = first_num | second_num

check_c and c should be equal

edited Jan 03 '15 at 08:07

answered Jan 03 '15 at 07:46

rakeshbs

24,392
7
73
63

Great thanks, this is working just fine:) Thanks for the help!! – Dark Side Jan 03 '15 at 12:38

Reverse '|' Bitshift Or

2 Answers2