if (decay > 0 && decay < decayTime) {
    // @audit if decay is close to decayTime, this fee will be zero. but below it'll be 1. the more time passes, the higher the decay. but then decay > decayTime should return 0.
    fee = lastFeePercent.sub(lastFeePercent.mul(decay).div(decayTime));
} else {
    fee = lastFeePercent;
}
// decay == 0 case should be full lastFeePercent
if(decay < decayTime) {
    fee = lastFeePercent.sub(lastFeePercent.mul(decay).div(decayTime));
} else {
    // reset to zero if decay >= decayTime
    fee = 0;
}
