linear interpolation

counter_t increment=42;
counter_t threshold=9001;
counter_t counter=0;
sample_t curSample=0,nextSample=0;

sample_t linearInterpolateNextSample() {
    counter+=increment;
    while(counter>threshold) {
        counter-=threshold;
        curSample=nextSample;
        nextSample=readNextSample();
    }

    //can be optimized using fixed point math
    //and fast division by shifting
    float t=counter/threshold;

    //lerp between the two samples
    return curSample*(1-t)+nextSample*(t);
}