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Write an Efficient C Program to Reverse Bits of a Number?

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YasminRadwan26

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2022-06-25

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Write an Efficient C Program to Reverse Bits of a Number?

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YasminRadwan26

2022-06-25

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There are a lot of ways to reverse the bits of a number, here I am describing three general methods to reverse the bits.

Method 1:

In this method, we will check the set bits of num and run the loop through all the bits of an integer. If we find the ith bits of num is set then just put 1 at the ((INT_BITS – 1) – ith ) position of tmp, where INT_BITS is the number of bits of an integer.

#define CHAR_BITS  8  // size of character
#define INT_BITS  ( sizeof(int) * CHAR_BITS)
//bit reversal function
unsigned int ReverseTheBits(unsigned int num)
{
    unsigned int iLoop = 0;
    unsigned int tmp = 0;         //  Assign num to the tmp
    int iNumberLopp = INT_BITS;
    for(; iLoop < iNumberLopp; ++iLoop)
    {
        if((num & (1 << iLoop))) // check set bits of num
        {
            tmp |= 1 << ((INT_BITS - 1) - iLoop); //putting the set bits of num in tmp
        }
    }
    return tmp;
}

Method 2:

It is a simple algorithm to reverse bits of the 32-bit integer. This algorithm uses the eight constant value for reversing the bits and takes five simple steps.

In the below section, I am describing the functioning of each step.

Steps 1:
num = (((num & 0xaaaaaaaa) >> 1) | ((num & 0x55555555) << 1));

This expression used to swap the bits.
Let an example, suppose num is 0100, after the above expression it will be 1000.

Steps 2:
num = (((num & 0xcccccccc) >> 2) | ((num & 0x33333333) << 2));

Above expression uses to swap the 2 bits of a nibble. Suppose num is 10 00, after the above expression, it will be 00 01.

Steps 3:
num = (((num & 0xf0f0f0f0) >> 4) | ((num & 0x0f0f0f0f) << 4));

An expression used to swaps the nibbles. like if num is 0011 0010 then after the above expression it will be 0010 0011.

Steps 4:
num = (((num & 0xff00ff00) >> 8) | ((num & 0x00ff00ff) << 8));

This statement uses to swap the bytes of an integer. Let num is 00001000 00001100, after the above expression, it will be 00001100 00001000.

Steps 5:
((num >> 16) | (num << 16));

The above expression uses to swap the half-word of an integer. Means that if the num is 0000000011001110 1000100100000110 after the above result number will be 1000100100000110 0000000011001110.

//bit reversal function
unsigned int ReverseTheBits(register unsigned int x)
{
    x = (((x & 0xaaaaaaaa) >> 1) | ((x & 0x55555555) << 1));
    x = (((x & 0xcccccccc) >> 2) | ((x & 0x33333333) << 2));
    x = (((x & 0xf0f0f0f0) >> 4) | ((x & 0x0f0f0f0f) << 4));
    x = (((x & 0xff00ff00) >> 8) | ((x & 0x00ff00ff) << 8));
    return((x >> 16) | (x << 16));
}

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