How good your knowledge of bitwise tricks? Exercise. And answer.

I posted the exercise at 14-sep-2022, but no one solved it. Shame! Try to solve it now, then scroll down this page for answer.

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#include <limits.h>
#include <assert.h>
#include <stdio.h>
#include <stdint.h>

// https://blog.regehr.org/archives/1063
uint32_t rotl32 (uint32_t x, uint32_t n)
{
        assert (n<32);
        return (x<<n) | (x>>(-n&31));
}

// https://stackoverflow.com/questions/776508/best-practices-for-circular-shift-rotate-operations-in-c
static inline uint32_t rotr32 (uint32_t n, unsigned int c)
{
        const unsigned int mask = (CHAR_BIT*sizeof(n) - 1);

        // assert ( (c<=mask) &&"rotate by type width or more");
        c &= mask;
        return (n>>c) | (n<<( (-c)&mask ));
}

uint32_t rotr32_my(uint32_t x, uint32_t n)
{
        return rotl32(x, 32-n);
};

void main()
{
        for (int i=1; i<=32; i++)
                for (int j=1; j<32; j++)
                {
                        uint32_t t1=rotr32(1<<i, j);
                        uint32_t t2=rotr32_my(1<<i, j);
                        assert (t1==t2);
                };
};

Not efficient. Not to be used in practice. But correct.

(the post first published at 20230331.)

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