Array wraparound with modulo of unsigned

I'm trying to implement a lagged Fibonacci pseudo-random number generator for integers up to some maximum. It maintains an array of values

int values[SIZE] = { /* 55 seed values */ };

and uses the following function to return the next value

unsigned lagfib()
{
    static unsigned idx = 0;
    int r = values[idx];

    /* The following does not work: */
    values[idx] = (values[(idx-24) % SIZE] + values[(idx-55) % SIZE])
                 % MAX_VALUE;
    idx = (idx+1) % SIZE;
    return r;
}

In effect, values should be a simple ring buffer that is always full. The subtraction and modulo should wrap the index around to the end of the array. SIZE should always be at least 55, but I want to round up to 64 to speed up the modulo.

But apparently, I've got the modulo calculations wrong and I don't know how to fix them. Changing the index type to int doesn't improve things.

(PS.: Yes, static data is bad style, but I want this to be readable for both C and C++ programmers, since it pertains to both languages.)

Solution

If e.g. idx is less than 24, you'll get wraparound to the other end of the number range of unsigned int. 55 is not a divisor of e.g. 2^32, so this will not give you correct results.

I can see two options:

Maintain three separate idx variables, offset by 24 and 55 respectively.
Do e.g. (idx - 24 + SIZE) % SIZE.

Actually, I would choose the first option, and avoid the modulo entirely by rewriting the increment as:

idx = ((SIZE-1) == idx) ? 0 : (idx+1);

which will probably be way faster than calculating modulo.