t.Fatalf("want mass %g, got %g", mass, hmass)
		}

		// Check inverse cumulative sum approximations.
		for j := 0.0; j < mass; j += mass * 0.099 {
			mud.setTrackMass(j)
			l, u, ok := mud.approxInvCumulativeSum()
			inv, ok2 := mud.invCumulativeSum(j)
			if !ok || !ok2 {
				t.Fatalf("inverse cumulative sum failed: approx %v, exact %v", ok, ok2)
			}
			if !(l <= inv && inv < u) {

	}
	d.trackBucket = len(d.hist)
	d.trackSum = sum
}

// approxInvCumulativeSum is like invCumulativeSum, but specifically
// operates on the tracked mass and returns an upper and lower bound
// approximation of the inverse cumulative sum.
//
// The true inverse cumulative sum will be in the range [lower, upper).
func (d *mud) approxInvCumulativeSum() (float64, float64, bool) {
	if d.trackBucket == len(d.hist) {

		if acc.lastTime != math.MaxInt64 {
			// Update distribution.
			acc.mud.add(acc.lastMU, mu, float64(time-acc.lastTime))
		}
		acc.lastTime, acc.lastMU = time, mu
		if _, mudBound, ok := acc.mud.approxInvCumulativeSum(); ok {
			acc.bound = math.Max(acc.bound, mudBound)
		} else {
			// We haven't accumulated enough total precise
			// mass yet to even reach our goal, so keep
			// accumulating.
			acc.bound = 1
		}