code: func decayFactor(t int64) float64 {
  	if t < precomputedLen {
  		return precomputedFactor[t]
  	}
  	return math.Exp(-1.0 * float64(t) * lambda)
  }
docs: // decayFactor returns the decay factor at t seconds, using precalculated values
  // if available, or calculating the factor if needed.