Bit hack: Expanding bits

Question

I am trying to convert a uint16_t input to a uint32_t bit mask. One bit in the input toggles two bits in the output bit mask. Here is an example conver

Answer 1

My solution is meant to run on mainstream x86 PCs and be simple and generic. I did not write this to compete for the fastest and/or shortest implementation. It is just another way to solve the problem submitted by OP.

#include 
#include 
#include 

#define BITS_TO_EXPAND (4U)
#define SIZE_MAX (256U)

static bool expand_uint(unsigned int *toexpand,unsigned int *expanded);

int main(void)
{
    unsigned int in = 12;
    unsigned int out = 0;
    bool success;
    char buff[SIZE_MAX];

    success = expand_uint(&in,&out);
    if(false == success)
    {
        (void) puts("Error: expand_uint failed");
        return EXIT_FAILURE;
    }
    (void) snprintf(buff, (size_t) SIZE_MAX,"%u expanded is %u\n",in,out);
    (void) fputs(buff,stdout);
    return EXIT_SUCCESS;
}
/*
** It expands an unsigned int so that every bit in a nibble is copied twice
** in the resultant number. It returns true on success, false otherwise.
*/
static bool expand_uint(unsigned int *toexpand,unsigned int *expanded)
{
    unsigned int i;
    unsigned int shifts = 0;
    unsigned int mask;

    if(NULL == toexpand || NULL == expanded)
    {
        return false;
    }
    *expanded = 0;
    for(i = 0; i < BIT_TO_EXPAND; i++)
    {
        mask = (*toexpand >> i) & 1;
        *expanded |= (mask << shifts);
        ++shifts;
        *expanded |= (mask << shifts);
        ++shifts;
    }
    return true;
}