concatenate array to bits

Question

How can I achieve the following concatenation of 1s and 0s of low the density array data into a smaller more densely filled array c

uint8_t data[16] = {1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 0, 0, 1, 1, 0, 1};
uint8_t c[2] = {0};
/*  desired result
    c[0] = 11011011
    c[1] = 10001101 
*/

I am a little bit struggeling here, up to now, I have got it like, but it does not seem to work like I expected:

static void compress(unsigned char *out, unsigned char *in, int length)
{
    for(int i=0; i<length; i++)
    {
        if((i+1)%9==0)
            out++;
        *out |= in[i]<<((length-i)%9);

    }
}  
int main(){
    compress(c,data,16);
    printf("%d",c[0]); //should be 219
    printf("%d",c[1]); //should be 177 (edit)
}

thank you for helping me out!

Answer 1

Add one line to your code printf ("%d %d\n", i,(length-i)%9 ); and you see where the problem is.

Don't use modulo operation, add variable which helds the bit-shift number (initial value is 7) and reset it when it is negative:

static void compress2(unsigned char *out, unsigned char *in, int length)
{
    int shift = 7;
    for(int i=0; i<length; i++)
    {
        *out |= in[i] << shift;
        if (--shift < 0)
        {
            ++out;
            shift = 7;
        }
    }
}